Corrosion of 

Fasteners 

in Wood Treated 

with Newer Wood 

Preservatives 

Samuel L. Zelinka

United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

General

Technical 

Report

FPL–GTR–220

In cooperation 

with the

United States

Department of

Transportation

Federal 

Highway 

Administration
May 2013

Zelinka, Samuel L. 2013. Corrosion of fasteners in wood treated with 

newer wood preservatives. General Technical Report FPL-GTR-220. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products 

Laboratory. 64 p.

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Abstract

This document compiles recent research findings related 

to corrosion of metals in preservative treated wood into a 

single report on corrosion of metals in wood. The research 

was conducted as part of the Research, Technology and 

Education portion of the National Historic Covered Bridge 

Preservation (NHCBP) Program administered by the Federal 

Highway Administration. The compilation addresses: corrosion rates of metals embedded in treated wood, the mechanism of corrosion in treated wood, the effect of extractives 

on corrosion, methods for predicting the service life of 

metals in treated wood, and suitable non-metallic replacement fasteners. The document was created to serve as a desk 

reference for engineers to aid in materials selection when 

building with treated wood.

Keywords: preservative treated wood, corrosion, timber 

bridge, fastener, fiber reinforced polymers

Contents

Executive Summary............................................................1

Chapter 1— Exposure Testing of Fasteners in 

Preservative Treated Wood: Gravimetric Corrosion 

Rates and Corrosion Product Analyses...............................3

Chapter 2— Corrosion of Metals in Wood: Comparing 

the Results of a Rapid Test Method to Long-Term 

Exposure Tests Across Six Wood Treatments ..................11

Chapter 3— The Effect of Tannins and pH on the 

Corrosion of Steel in Wood Extracts.................................21

Chapter 4— Combining Hygrothermal and Corrosion 

Models To Predict Corrosion of Metal Fasteners 

Embedded in Wood...........................................................29

Chapter 5— Synthesis of Published and Unpublished 

Corrosion Data from Long-Term Tests of Fasteners 

Embedded in Wood: Calculation of Corrosion Rates 

and the Effect of Corrosion on Lateral Joint 

Strength.............................................................................41

Chapter 6— Modeling the Effect of Nail Corrosion 

on the Lateral Strength of Joints.......................................51

Chapter 7 —Environmental Degradation of FiberReinforced Polymer (FRP) Fasteners in Wood.................59

This study is part of the Research, Technology and 

Education portion of the National Historic Covered 

Bridge Preservation (NHCBP) Program administered 

by the Federal Highway Administration. The NHCBP 

program includes preservation, rehabilitation and restoration of covered bridges that are listed or are eligible 

for listing on the National Register of Historic Places; 

research for better means of restoring, and protecting 

these bridges; development of educational aids; and 

technology transfer to disseminate information on covered bridges in order to preserve the Nation’s cultural 

heritage.

This study is conducted under a joint agreement between the Federal Highway Administration–Turner 

Fairbank Highway Research Center, and the Forest 

Service – Forest Products Laboratory.

Federal Highway Administration Program Manager– 

Sheila Rimal Duwadi, P.E.

Forest Products Laboratory Program Manager– 

Michael A. Ritter, P.E.
Corrosion of Fasteners in Wood Treated 

with Newer Wood Preservatives

Samuel L. Zelinka, Materials Engineer

Forest Products Laboratory, Madison, Wisconsin

Executive Summary

Most metals are thermodynamically unstable in the environment and corrode to form the more stable ceramics (metaloxides or metal-hydroxides) from which they were at one 

time extracted. Because of the thermodynamic favorability 

of corrosion, it is a war that Nature will eventually win, but 

humankind may prolong the fight, in some cases indefinitely, through careful materials selection and design.

Metal fasteners are an essential part of modern timber 

construction; these fasteners, like all metals, are subject to 

corrosion, which may lessen the structural capacity of the 

joint. If the wood remains dry, the fasteners will not corrode. 

Timber bridges are exposed to the natural environment and 

the wood will invariably get wet during the life of the structure. It is therefore necessary in rehabilitating timber bridges 

to understand the corrosion of metals in wood so that the 

bridge can be properly inspected for corrosion and durable 

materials can be selected for replacement.

Even when wet, the wood of most species is a relatively 

benign environment for corrosion. However, wood preservatives are frequently added to wood used in exterior environments to protect it from wood decay fungi and termites. Although wood preservatives increase the service life of wood, 

in some cases these preservatives increase the corrosiveness 

of the wood toward metal fasteners. 

The corrosiveness of preservative-treated wood has been 

studied since the 1920s when the first treatments were being 

developed for railroad ties. The amount and intensity of the 

corrosion research has changed throughout time as wood 

preservatives of varying corrosivity have been developed 

and introduced. Prior to 2004, the most common waterborne 

wood preservative in the United States was Chromated Copper Arsenate (CCA). The corrosion of metals in CCAtreated wood was studied by several researchers who found 

that it was more corrosive than untreated wood and suggested that at a minimum, hot-dip galvanized fasteners should 

be used in treated wood. The hot-dip galvanized fasteners 

were widely used in CCA-treated wood for many years with 

few corrosion problems.

In 2004, CCA-treated wood was voluntarily withdrawn for 

use in residential construction in the United States and similar changes in regulation have decreased the use of CCAtreated wood in Europe and Australasia. At that time, several 

alternatives to CCA became commercial available, the most 

popular being alkaline copper quaternary (ACQ) and copper 

azole (CuAz). Like CCA, these preservatives used copper as a biocide, but in a higher concentration than CCA. 

Shortly after this transition, anecdotal evidence of corrosion 

failures was found with these new preservative treatments. 

Likewise, the results of accelerated high-temperature, highhumidity testing suggested that these new preservative treatments may be more corrosive than CCA. However, it was 

unclear how these accelerated tests and reported failures 

related to an expected corrosion lifetime for fasteners in 

treated wood. 

Although it was accepted that galvanized fasteners were 

sufficiently durable for service in CCA-treated wood, it 

was unclear how the results of the accelerated testing could 

be applied to designing durable structures in other wood 

preservatives. There were two reasons for this. First, even 

though the corrosion of metals in CCA-treated wood had 

been examined, the actual corrosion rates were not reported, 

but instead percentage weight loss was used because the 

surface area of the fasteners, which was necessary for 

calculating a corrosion rate, was, at the time, impossible to 

calculate for threaded fasteners. Second, there was also no 

way to connect the results from a metal corrosion test to the 

residual strength of the nailed connection in wood. 

The USDA Forest Service, Forest Products Laboratory 

(FPL), with funding from the Federal Highway Administration, conducted an investigation of the corrosion of fasteners 

in new wood preservatives. The goals were to understand 

and quantify the corrosion of ferrous and non-ferrous metal 

fasteners in new preservative treatments and to examine the 

feasibility and performance of non-metallic fasteners that 

eliminate the potential for corrosion altogether. Key to this 

investigation was an apparatus recently developed at FPL 

that made it possible to measure the surface area of threaded 

fasteners. 

This final report represents a single-volume compilation of 

the major research findings. The research addresses these 

pertinent questions on designing durable connections with 

new preservative treatments:

• How rapidly do embedded metals corrode in wood?

• What is the mechanism of corrosion in treated wood?

• Do extractives affect corrosion?

• How can we rapidly determine the service life of metals 

 in wood?
General Technical Report FPL–GTR–220

2

• How can we use corrosion data to predict service life of 

 metals in wood?

• Do suitable non-metallic fasteners for use in wood exist 

 and how durable are they?

These questions were explored over a period of several 

years and the findings were published as they were found. 

Papers are compiled here to have all of the pertinent data 

in a single report. Although each chapter is self-contained, 

it addresses at least one and sometimes more than one of 

these questions. This Executive Summary briefly connects 

the previously published papers to the scientific questions 

that they answer and highlight key results. Citations to the 

originally published papers are provided in this Executive 

Summary.

How Rapidly Do Embedded Metals Corrode 

in Wood? 

Arguably, the most important questions are how fast do embedded metals corrode in wood, and how do wood preservatives affect these corrosion rates. The corrosion rates of steel 

and galvanized steel were measured in a one year exposure 

test. Five different wood preservatives and untreated wood 

were tested. The results of the exposure test are given in 

Chapter 1.

Zelinka, S.L.; Sichel, R.J.; Stone, D.S. 2010. Exposure testing of fasteners in preservative treated wood: gravimetric 

corrosion rates and corrosion products analyses. Corrosion 

Science. 52(12): 3943–3948.

What Is the Mechanism for Corrosion in 

Treated Wood?

This question is more broad than the first question, but 

equally important to eventual development of non-corrosive 

preservatives and corrosion resistant fasteners. Different 

aspects of this question were explored in different papers. 

For example, corrosion products that formed in different 

preservatives are detailed in Chapter 1; the type and amount 

of corrosion products are a key part of how fast underlying 

metals corrode. The role that cupric ions in preservatives 

play on further corrosion is detailed in Chapter 2. 

Zelinka, S.L.; Stone, D.S. 2011. Corrosion of metals in 

wood: comparing the results of a rapid test method with 

long-term exposure tests across six wood treatments. 

Corrosion Science. 53(5): 1708–1714.

Do the Extractives Affect Corrosion?

Research has shown that different wood species contain different extractives that may affect the corrosion of embedded 

metals. For instance, many of the individual extractives in 

wood are being used as “green” corrosion inhibitors in the 

corrosion industry. The role of two classes of extractives, 

tannins and small organic acids, are examined in Chapter 3.

Zelinka, S.L.; Stone, D.S. 2011. The effect of tannins and 

pH on the corrosion of steel in wood extracts. Materials 

and Corrosion. 62(8): 739–744.

How Can We Rapidly Determine the Service 

Life of Metals in Wood?

A rapid test was developed at FPL to measure corrosion 

of metals in contact with wood, wherein metals are electrochemically polarized in water extracts of treated wood. 

Chapter 2 describes the rapid test and compares results from 

this test against the exposure test presented in Chapter 1. 

How Can We Use Corrosion Data to Predict 

Service Life?

One of the key aspects of applied science of any sort is 

relating laboratory measurements to the “real world.” This 

requires knowledge of the failure mechanism, models that 

describe the breakdown in service, and a failure criteria. 

Chapters 1 and 2 detail the failure mechanism. In Chapter 4, 

the corrosion mechanism is combined with a hygrothermal 

model to determine how rapidly fasteners will corrode in 

different U.S. climates. Finally, Chapters 5 and 6 show how 

corrosion affects the structural capacity of nailed joints.

Zelinka, S.L.; Derome, D.; Glass, S.V. 2011. Combining 

hygrothermal and corrosion models to predict corrosion of 

metal fasteners embedded in wood. Building and Environment 46: 2060–2068.

Zelinka, S.L.; Derome, D.; Glass, S.V. 2011. Synthesis 

of published and unpublished corrosion data from longterm tests of fasteners embedded in wood: calculation of 

corrosion rates and the effect of corrosion on lateral joint 

strength. In: NACE International Conference & Expo. 

Corrosion 2011, Houston, TX. March 13–17, 2011 (Paper 

#11163). p. 1–13.

Zelinka, S.L.; Rammer, D.R. 2012. Modeling the effect of 

nail corrosion on the lateral strength of joints. Forest Products Journal. 62(3): 160–166.

Do Suitable Non-Metallic Fasteners for Use in 

Wood Exist and How Durable Are They?

Metals corrode because they are thermodynamically unstable in nearly every environment. Ceramics and polymers 

are generally thermodynamically stable, but may have other 

challenges that make them unsuitable to replace metals. In 

the course of this research, a ceramic-fiber reinforced polymer nail for use in wood became commercially available. 

This fastener was evaluated for use in treated wood and the 

results are presented in Chapter 7.

Zelinka, S.L.; Rammer, D. R. 2013. Environmental degradation of fiber-reinforced polymer (FRP) fasteners in wood. 

ASCE Journal of Materials in Civil Engineering. 

25(5):627-631
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

3

Chapter 1

Exposure Testing of Fasteners in 

Preservative Treated Wood: Gravimetric 

Corrosion Rates and Corrosion Product 

Analyses

Samuel L. Zelinka, Materials Engineer

Forest Products Laboratory, Madison, Wisconsin

Rebecca J. Sichel, Graduate Research Assistant

College of Engineering, University of Wisconsin Madison

Donald S. Stone, Associate Professor

Department of Materials Science and Engineering, College of Engineering, University of Wisconsin-Madison

Abstract

Research was conducted to determine the corrosion rates 

of metals in preservative-treated wood and also understand 

the mechanism of metal corrosion in treated wood. Steel 

and hot-dip galvanized steel fasteners were embedded in 

wood treated with one of six preservative treatments and 

exposed to 27 °C at 100% relative humidity (RH) for 1 year. 

The corrosion rate was determined gravimetrically and the 

corrosion products were analyzed with scanning electron 

microscopy, energy dispersive x-ray spectroscopy, and x-ray 

diffraction. Although the accepted mechanism of corrosion 

in treated wood involves the reduction of cupric ions from 

the wood preservative, no reduced copper was found on the 

corrosion surfaces. The galvanized corrosion products contained sulfates, whereas the steel corrosion products consisted of iron oxides and hydroxides. The possible implications 

and limitations of this research on fasteners used in building 

applications are discussed.

Keywords: Steel, zinc, preservative-treated wood, timber, 

XRD X-ray diffraction, scanning electron microscopy 

(SEM)

Introduction

The corrosiveness of wood treated with chromated copper 

arsenate (CCA), the major wood preservative of the past 

30 years, was studied in accelerated (Simm and Button 

1985b; Dennis and others 1995) and long-term tests (Baker 

1992; Simm and Button 1985a) in the 1980s and 1990s. 

These test results led to design recommendations for minimizing metal corrosion of metals in contact with CCAtreated wood (Anon 2003). However, CCA was voluntarily 

withdrawn for use in residential applications in the United 

States on December 31, 2003; similar changes in regulation 

have taken place in Europe and Australasia. Whereas alternatives to CCA are commercially available, there has only 

been limited work studying corrosion of metals in contact 

with these alternatives. 

Recent work has investigated corrosion of metals in contact 

with alternatives to CCA using accelerated test methods. 

The Simpson Strong Tie Corporation (Pleasanton, California) published a technical bulletin (Anon 2008) based 

upon over 6,000 replicates using a high-temperature highhumidity test method (AWPA 2007a) and reported relative 

corrosion rankings of hot-dip galvanized steel exposed to 

several wood preservatives. They found that alkaline copper 

quaternary (ACQ) and copper azole (CuAz) were roughly 

twice as corrosive as CCA. However, the high-temperature 

high-humidity test method has been shown to have poor 

correlation with in-service performance (Jin and Preston 

2000). Both Zelinka and others (2007) and Kear and others 

(2008a,b) examined corrosion in solutions of the preservative chemical as opposed to treated wood and found that the 

corrosion behavior of metals in treating solution was very 

different from behavior in treated wood. The lack of success 

with these accelerated methods highlights the importance 

of long-term tests; recently, several papers have presented 

non-accelerated wood-metal corrosion data for newer wood 

preservatives. 

Thorough work by Kear and others (2009) examined the 

behavior of mild steel, hot-dip galvanized steel, and stainless steel (UNS S31600) exposed to CCA (type C), CuAz 

(type B), and ACQ (types B and C) treated at several retention levels. A unique aspect of this work was that both metal 

plates specified in the AWPA standard (AWPA 2007a) as 

well as metal fasteners were tested. The wood–metal assemblies were exposed to either accelerated (49 °C, 90% RH for 

~400 h) or nonaccelerated conditions (21°C, various RH 
General Technical Report FPL–GTR–220

4

for ~1 year). The data were presented as corrosion rate 

(mm y–1). Similar to accelerated tests, Kear and others 

(2009) found that CuAz and ACQ were more corrosive than 

CCA and attributed this to differences in fixation between 

the preservative systems. 

Zelinka and Rammer (2009) re-examined a 14-year exposure study of CCA-treated wood published by Baker (1992), 

which is the basis for the International Building Code’s 

recommendations for fasteners used in treated wood (Anon 

2003). Using archived photographs, Baker’s laboratory 

notebook, and a new surface area algorithm (Rammer and 

Zelinka 2008), Zelinka and Rammer were able to convert 

Baker’s percent weight loss data into a corrosion rate (μm 

y–1). Additional tests were conducted for 1 year in ACQtreated wood at the same temperature and RH conditions 

that Baker tested (27 °C, 100% RH). The authors found that 

ACQ-treated wood was more corrosive than CCA-treated 

wood and that the relative corrosiveness of wood treatments 

depended on the metal tested.

In addition to these long-term laboratory tests, Li (2009) 

has published results of outdoor field exposure tests of steel, 

galvanized steel, and stainless steel fasteners embedded in 

ACQ-, CuAz-, and CCA-treated wood.

The work by these authors has quantified the difference in 

corrosion rates between CCA, the historical baseline, and 

new wood preservatives such as ACQ and CuAz. These 

measured corrosion rates have practical implications for designing treated wood structures. The current work expands 

upon these earlier works with the goal of better understanding the mechanism of corrosion in treated wood. In addition 

to gravimetric tests, the current work examines corrosion 

products with several techniques to find correlations between preservative chemistry and corrosion products in both 

treated and untreated wood. 

Experimental

Southern Pine (Pinus spp.) boards, 38 mm by 140 mm by 

2.5 m from the same plantation were split into five groups 

for treatment with either chromated copper arsenate 

(CCA-C), alkaline copper quaternary with a carbonate 

formulation (ACQ-D), micronized (<1000 nm) copper 

quaternary (MCQ), didecyl dimethyl ammonium carbonate 

(DDAC), or left untreated as a control. The newer carbonate 

formulations of ACQ-D use didecyl dimethyl ammonium 

carbonate instead of didecyl dimethyl ammonium chloride; 

formulations were changed so that they would be less corrosive than the chloride formulations. Lumber treated with 

copper azole (CuAz) was also tested in this study, but the 

lumber came from a different plantation and had starting 

dimensions of 38 mm by 89 mm by 1.2 m. The retention 

of the preservatives was verified using inductively coupled 

plasma atomic emission spectroscopy (ICP-AES). The ACQ 

and MCQ formulations tested in this study are similar in 

that they both contain the same nominal amount of copper and DDAC; however, they differ in how the copper 

was applied to the wood structure. ACQ is comprised of 

soluble copper in a waterborne solution, whereas MCQ is 

comprised of sub-micron sized particles of insoluble copper 

compounds. More information on the differences between 

preservatives and a comprehensive review of the biological 

and wood-treatment aspects of these wood preservatives has 

recently been published by Freeman and McIntyre (2008). 

Details on the composition and specified and actual treatment retentions for the preservatives used in this study are 

given in Table 1-1.

Two types of box nails used for siding (8d) were tested in 

this study, a plain carbon steel nail, and a hot-dip galvanized 

nail. Both nails came from the same batch; the plain carbon 

steel nails were culled from the production line just prior to 

Table 1-1. Composition and retention of preservatives tested

Treatment 

Nominal

compositiona

(percentage weight) 

Retention (kg m–3)

Nominal Actual 

CCA (type C) 

Chromated copper arsenate 

47.5 CrO3

18.5 CuO 

34.0 As2O5

4 1.6 

ACQ (type D) 

Alkaline copper quaternary 

66.7 CuO 

33.3 DDACb

4 2.9 

CuAz (type C) 

Copper azole 

96.8 uO 

1.6 Tebuconazole 

1.6 Propiconazole 

1 0.7 

MCQ

Micronized copper quaternary 

66.7 CuO 

33.3 DDAC

4 5.0 

DDAC (carbonate)

Alkyl ammonium compound 

100.0 DDAC 1.3 n.a.c

a

Nominal compositions from AWPA Standard P5 (AWPA 2007b), with the exception 

of MCQ, which comes from ICC-ES evaluation report ESR-1980. 

b

DDAC is didecyldimethylammonium carbonate. 

c

This preservative could not be analyzed with ICP-AES.
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

5

galvanizing. The nails had a nominal length of 64 mm and a 

nominal head diameter of 7 mm; the galvanized nails had a 

nominal shank diameter of 3 mm, with the carbon steel nails 

being slightly smaller. Each fastener was digitally imaged, 

the surface area for each fastener was determined from the 

digital image using an algorithm developed previously. Alloy compositions were analyzed with a Niton XL3 x-ray 

fluorescence (XRF) analyzer (Thermo Scientific, Billerica, 

Massachusetts) (Table 1-2). Galvanized coating thicknesses 

were measured on several fasteners with eddy-current thickness gages; thicknesses ranged between 10 and 25 μm. 

Prior to exposure, the lumber was sectioned into blocks 

50 mm by 38 mm by 90 mm. A pilot hole (1.5 mm in diameter) was drilled into the center of the 50 mm by 38 mm face 

of the block. These blocks were then allowed to equilibrate 

over several weeks in a room that was kept at 27 °C, 

90% RH. 

Immediately prior to exposure, the fasteners were cleaned, 

degreased, and weighed. The fasteners were cleaned in a 

bath with soap and water that was ultrasonically agitated for 

5 min. Afterward, the fasteners were rinsed and placed in 

a bath of distilled water that was ultrasonically agitated for 

5 min. Finally, the fasteners were degreased with acetone 

and allowed to dry. Once dry, the fasteners were weighed to 

the nearest 0.1 mg and hand-driven through the pilot hole 

into a block of wood. The wood-fastener assemblies were 

then placed above a reservoir of high-purity distilled water 

inside a sealed desiccator that was kept in a room held at 

27 °C, 90% RH; the resulting environment inside the desiccator was 27 °C, 100% RH as dictated by the Gibbs phase 

rule. The average wood moisture content in this environment, determined gravimetrically from untreated specimens, 

was 24.6% ± 0.6% (standard deviation). The blocks were 

removed after 1 year of exposure (366 days) for specimens 

analyzed gravimetrically and after approximately 7 months 

for the qualitative specimens. 

Fasteners were removed from the wood in a way that minimized damage to the corrosion products. Notches were sawn 

in the wood block, and the block was then placed into a 

vise. As pressure was applied, the wood split apart and the 

fastener could be removed by hand. 

For gravimetric specimens, the corrosion products were 

removed with a combination of ultrasonic agitation and 

chemical solvents. Fasteners were placed in a 50:50 (volume 

ratio) mixture of water and Evapo-RustTM (Orison Marketing, Abilene, Texas). Ultrasonic agitation was then applied 

for 60 min. The fasteners were then wiped with a lint-free 

paper towel and allowed to dry before being weighed to the 

nearest 0.1 mg. The same process and solvents were used 

on both the galvanized and steel fasteners; this process was 

able to successfully remove both red and white rusts. The 

amount of base metal removed by the cleaning processes, 

mc, was measured by cleaning uncorroded specimens; mc

was –2.9 ± 1.4 (standard deviation) mg for the galvanized 

fasteners and –0.9 ± 0.9 mg for the steel fasteners.

To analyze the corrosion products, corroded fasteners were 

examined in a Zeiss (Oberkochen, Germany) EVO 40 scanning electron microscope (SEM) with a Thermo Noran 

Vantage (Waltham, Massachusetts) energy dispersive X-ray 

spectroscopy (EDS) analyzer. The surfaces of the corroded 

fasteners were examined with SEM and EDS after being removed from the treated wood with no further cleaning. For 

cross-sectional analysis, corroded fasteners were mounted in 

conductive phenolic resin, rough polished to 1200 grit, and 

final polished with a 1.0 micron alumina slurry. 

X-ray powder diffraction was used to qualitatively identify 

phases in the corrosion products. Powder samples were 

prepared by removing the corrosion products with a razor 

blade. An Inel (Artenay, France) CPS 120 wide angle diffractometer with a Cu K-a source was used to collect the 

powder X-ray diffraction patterns. 

Results

The corrosion rate (R in μm y–1) was calculated by the 

following: 

 (1.1)

where m is mass (g), 

t time (h), 

A surface area (cm2) calculated from the digital 

image, 

ρ density as tabulated in ASTM G1(ASTM 2005) 

and 

K constant for unit analysis (87 600 000 mm 

 cm–1 h y–1). 

Table 1-2. Elemental composition of the 

fasteners tested

Carbon steel 

Hot-dip 

galvanized 

Weight 

percentage ±2σ

Weight 

percentage ±2σ

Bi 0.004 0.001 1.67 0.060

Co ― ― 0.123 0.020

Cr 0.103 0.014 0.060 0.007

Cu 0.273 0.026 ― ―

Fe Balance 3.800 0.070

Mo 0.013 0.002 ― ―

Mn 0.729 0.039 ― ―

Ni 0.145 0.030 ― ―

Pb ― ― 0.135 0.028

Si 0.162 0.078 ― ―

Ti ― ― 0.012 0.004

Zn 0.035 0.010 Balance 

Zr ― ― 0.009 0.004
General Technical Report FPL–GTR–220

6

The subscripts i and f refer to initial, and final, respectively, 

and mc was defined earlier. The 1-year corrosion rate 

for hot-dip galvanized and steel fasteners is shown in 

Figure 1-1.

The surfaces of the corrosion products on the nail head, 

which was exposed directly to the 27 °C, 100% RH environment as well as the nail shank, which was embedded in the 

wood, were analyzed with SEM and EDS. The morphology 

and elemental composition of the corrosion products were 

similar for the head and shank and were also similar across 

preservative treatments for both the steel and galvanized 

nails. Characteristic SEM images of the corrosion products 

are given for steel and galvanized steel in Figure 1-2. Crosssectional images for steel and galvanized steel are shown in 

Figure 1-3 and Figure 1-4, respectively. The micrographs 

in most cases represent the areas with the largest corrosion 

product. Pitting was observed in the cross-sectional image 

of the steel fastener exposed to untreated wood.

The most important result from the EDS testing was that 

copper was not detected on any of the corroded fasteners. 

Carbon, iron, and oxygen were detected on all of the corroded steel fasteners. Likewise, carbon, oxygen, and zinc were 

detected on all of the corroded galvanized fasteners. For galvanized fastener that had clearly visible red rust (ACQ and 

CuAz), iron was also detected in the EDS. Finally, arsenic 

was detected on both the steel and galvanized steel fastener 

exposed to CCA-treated wood. The arsenic likely came 

from the arsenate in the preservative; Zelinka and others 

(2007) have shown that the arsenates may play a role as a 

corrosion inhibitor in the CCA preservative systems.

For the steel nails, the predominant corrosion products 

across all treatments were goethite (a-FeOOH) and magnetite (Fe3O4). The galvanized corrosion products depended on 

the treatment to which the fasteners were exposed (Fig. 1-5). 

For fasteners in CuAz-treated wood, namuwite {Zn2(SO4)

(OH)6•4H2O}, and hydrozincite {Zn5(CO3)2(OH)6} were 

clearly identifiable in the x-ray diffraction (XRD) pattern, although it is possible that smithsonite (ZnCO3) and 

simonkolleite {Zn5(OH)8Cl2·(H2O)}were also present. For 

the fasteners exposed to other treatments, namuwite and hydrozincite were the clearly identifiable phases, with possibly 

smithsonite and zincite (ZnO). 

Discussion

It is instructive to compare ACQ and MCQ to DDAC, as 

these preservatives all contain the same organic biocide, and 

differences in these treatments are due to the presence and 

type of copper used in the formulations. For both steel and 

galvanized steel, the DDAC-treated wood was only slightly 

more corrosive than the untreated wood. This suggests that 

the corrosiveness of these wood preservatives comes from 

Figure 1-1. Corrosion rate measured in the 1-y, 27 °C, 

100% RH exposure test for hot-dip galvanized (white) 

and steel (gray) fasteners for different wood preservative treatments. Error bars represent the uncertainty in 

the mean (the standard error).

Figure 1-2. Surface morphology of the corrosion products for steel (left) and galvanized steel (right) 

exposed to ACQ-treated wood (27 °C, 100% RH for 7 months). Morphologies for other treatments 

were similar and had similar compositions. The scale marker represents 100 μm.
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

7

the copper rather than the organic biocides and is congruent 

with previous theories (Baker 1988) that the cathodic corrosion reaction is the reduction of cupric ions. The MCQ and 

ACQ both have nominally the same amount of copper. In 

ACQ, the treatment solution consisted of copper solubilised 

in ammonia or ethanolamine, whereas the MCQ solution 

contains a suspension of insoluble submicron-sized copper 

compounds (Lebow 2010). The fact that the MCQ-treated 

wood is more corrosive than DDAC-treated wood suggests 

that after treatment, some of the copper becomes soluble and 

participates in the corrosion reaction. ACQ-treated wood 

was the most corrosive. Kear and others (2009) showed 

that the corrosion rate can depend on the amount of copper 

in the wood. Even though in these tests the MCQ-treated 

wood had more total copper than the ACQ-treated wood, it 

is possible that the ACQ-treated wood was more corrosive 

than the MCQ-treated wood because there was more copper 

available to participate in the corrosion reaction. This comparison of the relative corrosiveness of MCQ and ACQ is 

limited to the specific conditions of this study and does not 

Figure 1-3. Cross-section SEM images of the steel fasteners exposed to ACQ (a) CuAz (b) MCQ 

(c) CCA (d) DDAC (e) and untreated (f) wood (27 °C, 100% RH for 7 months). The scale marker 

represents 20 μm.

Figure 1-4. Cross-section SEM images of the galvanized fasteners exposed to ACQ (a) CuAz (b) MCQ 

(c) CCA (d) DDAC (e) and untreated (f) wood (27 °C, 100% RH for 7 months). The scale marker represents 20 μm.
General Technical Report FPL–GTR–220

8

necessarily reflect the relative corrosivity under a range of 

in-service exposure conditions. 

The most important results from the SEM and EDS data is 

the absence of deposited copper in the corrosion products 

because the currently accepted mechanism of corrosion in 

treated wood (Baker 1980) involves the reduction of cupric 

ions in the preservative. The absence of copper on the corroded fasteners could have two explanations: either cupric 

ions are not reduced as part of the corrosion reaction or 

cupric ions are reduced in the reaction but are not deposited 

on the surface. We believe it is the latter because several 

key experiments (Kear and others 2009; Zelinka and others 

2008; Zelinka and others 2009) have shown the importance 

of cupric ions on wood–metal corrosion. For instance, Kear 

and others (2009) found a weak correlation between the 

corrosiveness of the wood preservative and the amount of 

copper in the preservative. When polarization measurements 

were made in liquid extracts of treated wood containing 

cupric ions, reduced copper was detected with EDS on the 

corroded fasteners (Zelinka and others 2008). Furthermore, 

the extract tests have good correlation to solid wood exposure when the preservatives contain cupric ions (Zelinka and 

others 2009) but do not correlate for preservatives that do 

not contain cupric ions. The present work suggests that the 

mechanism of corrosion in both extract and solid wood is 

the reduction of cupric ions; however, this raises the question that if the cupric ions are being reduced, why are they 

not being deposited on the corroded surface? It is possible 

that the cupric ions are reduced near the metal surface, but 

instead of depositing on the fastener, the reduced copper 

remains in the wood. 

In all cases, the galvanized fasteners had a higher corrosion 

rate than the steel fasteners. This has practical applications 

because of the widespread use of galvanized fasteners. 

Although zinc coatings cathodically protect the underlying steel at coating defects, the main protection mechanism 

of the zinc coatings is barrier protection because in many 

environments, zinc corrodes more slowly than steel (Zhang 

2003). In atmospheric corrosion, zinc usually forms hydrozincite and smithsonite, which passivate the surface and 

cause the zinc to have a lower corrosion rate than steel. 

However, when exposed to certain conditions, such as air at 

100% RH (Zhang and others 1998) and in closed containers with wood vapors containing volatile acetic and formic 

acids (Knotkova-Cermakova and Vlckova 1971), zinc corrodes more rapidly than steel. Several researchers have observed that zinc fasteners have corroded more rapidly than 

carbon steel fasteners in treated wood, although the zinc 

corrosion products were not identified (Simm and Button 

1985a; Duncan 1988). We found that in addition to hydrozincite, namuwite also formed for fasteners embedded in 

treated wood. The presence of non-passivating compounds 

in zinc corrosion products exposed to treated wood may explain why galvanized fasteners corroded more rapidly than 

steel fasteners. 

Other work using different exposure conditions has found 

that galvanized fasteners corroded more slowly than steel 

fasteners in treated wood, although corrosion products were 

not identified in that study (Kear and others 2009). It is possible that different corrosion products formed under the two 

different conditions, which may explain the relative differences between steel and galvanized steel in these two studies. Comparing between these two studies, it seems that the 

type and morphology of zinc corrosion products play a large 

role in the corrosion of metals in wood products. Furthermore, corrosion products are sensitive to the test method. 

Future work should compare corrosion products from field 

exposure studies to those formed under different laboratory 

test protocols to optimize laboratory test methods. Until a 

better understanding of the relationship between laboratory 

and field measurements can be developed, caution should be 

used when extrapolating laboratory corrosion rates. 

Figure 1-5. X-ray powder diffraction pattern for galvanized fasteners exposed to MCQ-treated wood (top) and 

CuAz-treated wood (bottom). Specimens exposed to 

other treatments had similar diffraction patterns to MCQ.
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

9

Conclusions 

Corrosion rates were measured for steel and galvanized steel 

in Southern Pine treated with several wood preservatives. 

Corrosion rates over all treatments were small in absolute 

terms (~1–35 μm y–1). For each treatment, the steel fasteners had lower corrosion rates than the galvanized fasteners. 

Zinc corrosion products contained namuwite and hydrozincite. The type and morphology of corrosion products may 

explain why zinc corrosion rates were greater than steel 

corrosion rates. Future work should focus on identifying the 

phases in corrosion products for both field and laboratory 

measurements. 

Copper was not detected in any of the corrosion products. 

However, we believe that the corrosion mechanism in 

treated wood involves the reduction of cupric ions, but the 

reduced copper does not deposit on the corroding fastener.

Acknowledgments 

The authors thank Forest Products Laboratory staff Thomas 

A. Kuster and Daniel O. Foster for their assistance with the 

SEM and ICP-AES, respectively. 

References

Anon 2003. International Building Code. Florence, KY: 

Delmar Cengage Learning.

Anon 2008. Preservative treated wood. Simpson Strong-Tie 

Technical Bulletin, Pleasanton, CA.

ASTM 2005. ASTM G1: Standard practice for preparing, 

cleaning, and evaluating corrosion test specimens, West 

Conshohocken, PA, American Society for Testing and 

Materials. 

AWPA 2007a. E-12: Standard method of determining corrosion of metal in contact with treated wood, Selma, AL: 

American Wood Preservers’ Association.

AWPA 2007b. P5: Standard for waterborne preservatives. 

Granbury, TX: American Wood Preservers’ Association. 

Baker, A.J. 1980. Corrosion of metal in wood products. Durability of building materials and components. ASTM STP 

691. West Conshohocken, PA: American Society for Testing 

and Materials.

Baker, A.J. 1988. Corrosion of metals in preservative-treated 

wood. In: Hamel, M., ed. Wood protection techniques and 

the use of treated wood in construction. Madison, Wisconsin: Forest Products Society.

Baker, A.J. 1992. Corrosion of nails in CCA- and ACAtreated wood in two environments. Forest Products Journal. 

42: 39–41.

Dennis, J.K., Zou, C., Short, N.R. 1995. Corrosion behaviour of zinc and zinc alloy coated steel in preservative 

treated timber. Transactions of the Institute of Metal 

Finishing. 75: 96–101.

Duncan, J.R. 1988. Corrosion of metal fasteners in timber. 

Construction Building Materials. 2: 194–200.

Freeman, M.A., McIntyre, C.R. 2008. A comprehensive 

review of copper-based wood preservatives with a focus on 

new micronized or dispersed copper systems. Forest Products Journal. 58: 6–27.

Jin, L., Preston, A. 2000. Evaluation of the corrosivity of 

the treated wood. Laboratory vs field test methodologies. In: 

Proceedings of the 31st Annual Meeting of the International 

Group on Wood Preservation, Kona, Hawaii. 

Kear, G., Wú, H.Z., Jones, M.S. 2008a. Corrosion of ferrous- and zinc-based materials in CCA, ACQ, and CuAz 

timber preservative solutions. Materials and Structures. 

41: 1405–1417.

Kear, G., Wú, H.-Z., Jones, M.S. 2009. Weight loss studies of fastener materials corrosion in contact with timbers 

treated with copper azole and alkaline copper quaternary 

compounds. Corrosion Science. 51: 252–262.

Kear, G., Wú, H.-Z., Jones, M.S., Walsh, F. 2008b. Impedance spectroscopy studies of the dissolution of ferrous- and 

zinc-based materials in aqueous timber preservatives. Journal of Applied Electrochemistry. 38: 1599–1607.

Knotkova-Cermakova, D., Vlckova, J. 1971. Corrosive 

effect of plastics, rubber and wood on metals in confined 

spaces. British Corrosion Journal. 6: 17–22.

Lebow, S.T. 2010. Wood preservation. In: R. Ross, ed., 

Wood handbook. Centennial edition. FPL–GTR–190, U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory, 2010. 

Li, Z. 2009. Corrosion of fasteners. BUILD. August–

September: 47–48.

Rammer, D.R., Zelinka, S.L. 2008. Analytical determination 

of the surface area of a threaded fastener. ASTM Journal of 

Testing and Evaluation. 36: 80–88.

Simm, D.W., Button, H.E. 1985a. Corrosion behaviour of 

certain materials in CCA-treated timber. Environmental tests 

at 100% relative humidity. Corrosion Prevention and Control. 32: 25–35.

Simm, D.W., Button, H.E. 1985b. Electrochemical 

measurements on metals in CCA-treated wood. Corrosion 

Prevention and Control. 32: 67–74, 83.

Zelinka, S.L., Rammer, D.R. 2009. Corrosion rates of fasteners in treated wood exposed to 100% relative humidity. 

ASCE Journal of Materials in Civil Engineering. 

21: 758–763.
General Technical Report FPL–GTR–220

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Zelinka, S.L., Rammer, D.R., Stone, D.S. 2008. Electrochemical corrosion testing of fasteners in wood extract. 

Corrosion Science. 50: 1251–1257. 

Zelinka, S.L., Rammer, D.R., Stone, D. S. 2009. Evaluating 

the corrosiveness of southern pine treated with several wood 

preservatives using electrochemical techniques. Corrosion 

2009. Atlanta, GA: NACE International.

Zelinka, S.L., Rammer, D.R., Stone, D.S., Gilbertson, J.T. 

2007. Direct current testing to measure corrosiveness of 

wood preservatives. Corrosion Science. 49: 1673–1685.

Zhang, X.G. 2003. Corrosion of zinc and zinc alloys, in: 

S.D. Cramer, Covino, B.S., eds. ASM Handbook Volume 

13B-Corrosion: Materials, Materials Park, OH, ASM International: 402–417.

Zhang, X.G., Hwang, J., Wú, W.K. 1998. Corrosion testing of steel and zinc. In: 4th International Conference on 

Zinc and Zinc Alloy Coated Steel Sheet: GALVATECH ‘98, 

Chiba, Japan. 
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

11

Chapter 2

Corrosion of Metals in Wood: Comparing 

the Results of a Rapid Test Method to 

Long-Term Exposure Tests Across Six 

Wood Treatments

Samuel L. Zelinka, Materials Research Engineer

Forest Products Laboratory, Madison, Wisconsin

Donald S. Stone, Associate Professor, Department of Materials Science and Engineering

College of Engineering, University of Wisconsin-Madison

illustrated the mechanism schematically in Figure 2-1 specifically highlighting the ion transport through the wood. 

Based on Baker’s proposed corrosion mechanism and the 

assumption that migration of ions was not the rate-determining step, a new accelerated test method was developed 

at FPL (Zelinka and others 2008b). The method consisted 

of grinding wood into sawdust, steeping the sawdust in distilled water, and then running an electrochemical corrosion 

test in the resulting extract. The electrochemical extract test 

method was found to correlate well with long-term exposure 

tests of alkaline copper quaternary- (ACQ-) treated wood 

(Zelinka and others 2008b; Zelinka and Rammer 2009). The 

results suggested that the cathodic reaction is reduction of 

cupric ions as Baker (1988) hypothesized, and the reaction 

is not diffusion controlled in solid wood at the moisture of 

the solid wood exposure test. 

In untreated wood, however, the extract test produced a 

much higher corrosion rate than the exposure tests (Zelinka 

and others 2008a). Furthermore, the untreated extract was 

more corrosive than the ACQ extract—a surprising result 

considering that cupric ions increase the corrosiveness in 

solid wood. Whereas these results may partially be explained by the difference in pH between untreated (pH 4.5) 

and ACQ-treated extracts (pH 6.6), it was not possible to 

conclusively identify the reason for the differences in corrosion rate between these two extracts because both the pH 

and concentration (presence) of copper were different.

In summary, solid wood and electrochemical extract tests 

have correlated well for ACQ-treated wood but do not correlate for untreated wood. This paper is part of a broad 

examination of corrosion in wood with the ultimate aim of 

understanding the mechanism in treated wood. For this large 

investigation, six different wood treatments were chosen 

to interrogate different aspects of corrosion. Previous studies in this examination have included long-term exposure 

tests in solid wood (Zelinka and others 2010), and matched 

Abstract

This chapter compares two methods of measuring the corrosion of steel and galvanized steel in wood: a long-term 

exposure test in solid wood and a rapid test method where 

fasteners are electrochemically polarized in extracts of wood 

treated with six different treatments. For traditional wood 

preservatives, the electrochemical extract method correlates 

with solid wood exposure, which suggests that reduction of 

cupric ions is the cathodic reaction in both solid wood and 

extract. For treatments without copper, the extract method 

predicted a higher corrosion rate than the solid wood exposure. For these treatments, the cathodic reaction appears 

to be reduction of acid and dissolved oxygen. The practical 

implication of this work is that in some cases, the rapid test 

method could be used to screen new fasteners and wood 

preservatives. Scientifically, this work increases understanding of the mechanism of corrosion of fasteners in treated 

and untreated wood. 

Keywords: Steel, zinc, preservative-treated wood, timber, 

polarization resistance, weight loss 

Introduction

The current mechanism for corrosion of metals in contact 

with preservative-treated wood was proposed by Baker in 

the 1980s (1980, 1988). The wood preservatives that Baker 

studied contained cupric ions, which are thermodynamically 

unstable in the presence of steel and galvanized steel 

fasteners. Baker’s mechanism involves transport of cupric 

ions through wood to the metal surface where they are 

reduced at the expense of the fastener, which is oxidized. 

Furthermore, the mechanism is believed to be aqueous because the corrosion rate depends strongly on wood moisture 

content and there is a threshold moisture content below 

which corrosion does not occur in wood (Baker 1988; 

Dennis and others 1995; Short and Dennis 1997). We have 
General Technical Report FPL–GTR–220

12

specimens from the exposure test were also examined with 

an electrochemical method in water extracts of the treated 

wood (Zelinka and others 2009). This work examines similarities and differences between the exposure test and the 

electrochemical extract test and from these cases infers 

corrosion mechanisms. To test the inferred corrosion mechanisms, we performed additional experiments to separate the 

effects of pH and copper from other differences in preservative chemistry. In addition to having practical implications 

for accelerated corrosion testing, this study adds to the understanding of the mechanism of metal corrosion in wood.

Methods and Materials

Plain carbon steel and hot-dip galvanized steel nails (8d) 

from the same lot were examined in both the long-term exposure and the electrochemical extract test. The galvanized 

fasteners had nominal dimensions of 64 mm (length) by 

3 mm (shank diameter) with a head diameter of 7 mm. Carbon steel fasteners had the same nominal dimensions except 

for the shank diameter, which was slightly smaller. Galvanized coating thicknesses ranged between 10 and 25 μm as 

measured by an eddy-current thickness gage. Compositions 

of carbon steel nails and hot-dip galvanized coatings were 

analyzed by x-ray fluorescence. The carbon steel nail contained 0.73 wt% Mn, 0.27 wt% Cu, 0.16 wt% Si, 0.15 wt% 

Ni, 0.10 wt% Cr, 0.04 wt% Zn, and 0.01 wt% Mo, with the 

remainder as Fe. The surface of the galvanized coating contained 3.8 wt% Fe, 1.7 wt% Bi, 0.14 wt% Pb, 0.12 wt% Co, 

0.06 wt% Cr, 0.01 wt% Ti, and 0.01 wt% Zr. Prior to testing, 

the surface areas of each fastener was calculated from a high 

resolution digital photograph using an algorithm developed 

by the authors (Rammer and Zelinka 2010).

Southern Pine (Pinus spp.) lumber was treated with one of 

five wood treatments or left untreated as a control for a total 

of six different groups; the compositions and retentions are 

given in Table 2-1. Three of these preservatives, chromated 

copper arsenate (CCA), alkaline copper quaternary (ACQ), 

and copper azole (CuAz), are traditional, soluble-copper 

waterborne wood preservative systems where copper is dissolved in solution and impregnated into wood in a pressure 

vessel. In contrast, micronized copper quaternary (MCQ) 

consists of a suspension of “micronized” (<1000 nm) particles of nearly insoluble copper carbonate (Zhang and Leach 

2006); however, the total amount of copper and the ratio of 

other biocides is the same as ACQ. Though the MCQ treating solutions contain insoluble copper, research indicates 

that the copper becomes soluble in treated wood, and cupric 

ions are available (Zhang and Ziobro 2009; Stirling and 

Drummond 2009). The fifth treatment, didecyldimethylammonium carbonate (DDAC), is the cobiocide in ACQ and 

MCQ, and the nominal retention was set so that the DDACtreated wood would have the same concentration of DDAC 

as the ACQ and MCQ treatments. Finally, untreated Southern Pine was tested as a control. 

Treatments were applied to boards 38 mm by 140 mm by 

2.5 m, with the exception of the CuAz-treated wood, which 

had dimensions of 38 mm by 89 mm by 1.2 m. Specimens 

were treated and then separated for use in either the longterm exposure test or the electrochemical extract test. 

Exposure tests were conducted for 1 year in a 27 °C, 100% 

RH environment that was achieved by placing wood–metal 

assemblies over water in sealed desiccators. The average 

wood moisture content in this environment, determined 

gravimetrically on untreated specimens, was 24.6% ± 0.6% 

Figure 2-1. Schematic 

illustration of the mechanism 

in corrosion in treated wood as 

proposed by Baker (1980, 1988).
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

13

(standard deviation). Nine replicates were tested for both 

steel and galvanized steel for all six treatments. The corrosion rate was measured gravimetrically after removing the 

corrosion products, which was accomplished by placing 

the fasteners in a 50:50 mixture of water and Evapo-RustTM 

(Orison Marketing, Abilene, Texas) for 60 min while ultrasonic agitation was applied. We observed that this cleaning 

method produced a mass change on uncorroded fasteners of 

2.9 ± 1.4 (standard deviation) mg for the galvanized fasteners and –0.9 ± 0.9 mg for the steel fasteners. On additional 

specimens, corrosion products were analyzed with powder 

XRD, scanning electron microscopy (SEM), and energy 

dispersive X-ray spectroscopy (EDS). 

The extract test method involved making an extract from 

wood and then electrochemically measuring the corrosion 

rate of a fastener immersed in extract. For each treatment, a 

single batch of extract was made to reduce intra-treatment 

variability. The extract was made by grinding the wood 

into sawdust, placing sawdust in distilled water (1:10 ratio 

weight basis) for one week, at which time the sawdust removed with a Büchner funnel. Previous testing has shown 

that corrosiveness of the extract is not affected by the heat 

produced during grinding (Zelinka and others 2007). After 

it was produced, the extract was stored near 0 °C and tests 

were run within two weeks to minimize biologically induced changes in the extract. Physical properties of extracts 

are summarized in Table 2-1.

Electrochemical testing used a similar procedure to ASTM 

G59 [c], and the fasteners were machined to fit in the 

standard five-neck flask. Immediately prior to testing, the 

fasteners were cleaned with ultrasonic agitation in a soap 

solution for 5 min, then rinsed with distilled water before a 

final 5-min ultrasonic bath in distilled water. The open circuit potential (OCP) was measured for an hour; at the end 

of the hour, the OCP was recorded and the potential was 

swept from –30mV vs. OCP to +30mV vs. OCP with a scan 

rate of 0.166 mV s–1. The automatic IR correction of the 

potentiostat was applied during the test. Tafel slopes and the 

resulting corrosion rate were ascertained from the nonlinearities of the polarization resistance curve using Mansfeld’s 

method (1973a,b). Nine replicates were tested for both steel 

and galvanized steel for all six treatments. 

Results and Discussion

Corrosion product analyses from XRD, SEM, and EDS 

for the solid wood tests have been published elsewhere 

(Zelinka and others 2010), and only the pertinent details 

needed for the present discussion are included here. For 

all six treatments, the steel corrosion products consisted of 

goethite (α-FeOOH) and magnetite (Fe3O4); for galvanized 

steel namuwite {Zn2(SO4)(OH)6•4H2O}, and hydrozincite {Zn5(CO3)2(OH)6} were clearly identifiable, although 

smithsonite (ZnCO3), simonkollete {Zn5(OH)8Cl2·(H2O)}, 

and zincite (ZnO) may also have been present. Importantly, 

the EDS revealed different elemental compositions between 

the surfaces of fasteners corroded in the extract and in solid 

wood (Fig. 2-2). In every case except the CCA treatment, 

the solid wood exposure tests, EDS only detected carbon, 

oxygen, and the base metal; those exposed to CCA also contained arsenic. In contrast, for most of the electrochemical 

tests in CCA-, CuAz-, ACQ- and MCQ-treated wood, copper was detected in the corrosion products. 

Corrosion rates from the extract and solid wood tests are 

compared in Figure 2-2. There are three types of behavior 

present in Figure 2-3. Traditional, soluble copper, wood preservatives (CCA, ACQ, and CuAz) have similar corrosion 

rates in both the extract and solid wood. For both untreated 

and DDAC-treated wood, which do not contain copper 

in any form, extracts are much more corrosive than solid 

wood. Finally, MCQ-treated wood does not behave like 

either traditional wood preservatives or copper-free treatments. Differences in behavior can be ascribed to different 

reaction mechanisms occurring in these treatments. To further discuss these mechanisms, treatments will be grouped 

depending on whether or not they contain copper and what 

form it is in.

Preservatives Containing Cupric Ions

For traditional wood preservatives (ACQ, CCA, and CuAz), 

we found good correlation between the exposure test and 

the extract test. These results agree with preliminary work 

in ACQ-treated wood (Zelinka and others 2008b). In that 

work, EDS of polarized fasteners in the extract revealed the 

presence of reduced copper on the fastener surface. Zelinka 

and others (2008b) concluded that the cathodic reaction in 

the extract was the reduction of cupric ions. Furthermore, 

because the extract and exposure tests had similar corrosion 

rates across several different metals, they concluded that the 

Figure 2-2. EDS measurements of the surface of a 

galvanized fastener exposed to MCQ-treated wood 

in both the solid wood and the extract test. Note the 

presence of copper in the fastener exposed to the 

extract test.
General Technical Report FPL–GTR–220

14

mechanism in solid ACQ-treated wood also involved reduction of cupric ions and the rate-determining step was the 

same in both cases. Current results agree with these original 

conclusions and suggest that they are applicable to other 

treatments with cupric ions as well as ACQ.

It should be noted, however, that while EDS detected copper on fasteners exposed to the extract, none of the fasteners exposed to solid wood exhibited detectable copper in 

the corrosion products. This result is in agreement with the 

work of Simm and Button (1985), who examined steel, zinc, 

and aluminum fasteners exposed to CCA-treated wood. 

Whereas absence of copper in the solid wood specimens 

could mean that cupric ions are not reduced as part of the 

corrosion reaction, we believe that cupric ions are reduced 

because preservatives with cupric ions are much more corrosive than untreated wood in the 1-year exposure tests, 

and they exhibit similar corrosion rates in the extract where 

EDS confirmed reduction of cupric ions. Indeed, Simm and 

Button did not rule out the reduction of cupric ions as a possible cathodic reaction and, in more recent work, Kear and 

others (2009) found correlation between the amount of copper and corrosiveness of the wood preservative. While the 

correlation between exposure and extract tests suggests that 

reduction of cupric ions is the cathodic reaction in both the 

extract and solid wood, it is not clear what happens to reduced copper in solid wood. It is possible that reduced copper dissolves again in a different reaction or that the copper 

is reduced near the metal surface but remains on the wood 

instead of the metal, although EDS measurements taken in 

the region adjacent to the fastener showed no difference in 

copper concentration. 

DDAC and Untreated Wood

For the DDAC-treated and untreated wood, which do not 

contain copper in any form, the expected cathodic reactions are the reduction of acids and dissolved oxygen 

(Farmer 1962; Bailey and Schofield 1984; Kubler 1992; 

Baker 1980). In a previous investigation (Zelinka and others 2008a), the role of oxygen was studied in an untreated 

extract. Deaerating the solutions lowered the corrosion rate 

by approximately half, which confirmed that reduction of 

dissolved oxygen was one of the cathodic reactions. When 

sawdust is exposed to water, additional acetic acid is produced (not extracted) as the wood spontaneously undergoes 

Figure 2-3. Corrosion rates measured in the electrochemical extract test (Zelinka and others 2010) 

and a 1-y exposure test (Zelinka and others 2008) for steel (top) and galvanized steel (bottom). Note 

the difference in scale on the ordinate axis.
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

15

deacetylation (Farmer 1962). Because of the increased availability of acids and dissolved oxygen in the extract, it might 

not be surprising that the extract is more corrosive than the 

solid wood. Using the same arguments, we would expect 

extracts from the treated wood to also be much more corrosive than the extracts from untreated wood, although this 

was not observed. Some further insight to the differences 

between extracts with and without cupric ions can be gained 

by examining their chemical properties.

The major differences among all six extracts are the pH and 

the presence/amount of cupric ions. The pH of the solutions 

ranged from 4.5 for untreated wood to 6.6 for ACQ-treated 

wood. This pH range spans two regimes in the corrosion 

kinetics of steel and zinc; below pH 5, the kinetics are dominated by the hydrogen ion concentration (pH) as well as the 

counter-ion of the acid, whereas in neutral solutions, the 

corrosion of steel and zinc is nearly independent of pH 

(Silverman 2003; Andrzejaczek 1984). 

It is useful to compare the DDAC and the ACQ treatments 

as they both contain the same quaternary ammonia compound but have differences in pH and presence of copper. 

The ACQ extract had a pH of 6.6 and the corrosion rate was 

approximately 30 μm y–1 for the galvanized steel in both 

the solid wood and the extract. The DDAC treatment had 

a pH of 4.6 and galvanized steel corrosion rates of 6 and 

87 μm y–1 in solid wood and the extract, respectively. From 

the above data, the high corrosion rate in the DDAC extract 

could be due to the low pH, or due to the lack of cupric ions 

if the cupric ions reduced to form a protective corrosion 

product on the metal surface during the time the OCP was 

measured. 

To examine the effects of pH and cupric ions separately, two 

“model” extracts were made starting with the DDAC extract 

(summarized in Fig. 2-4). In the first model extract, “DDAC 

mod,” the pH of the DDAC extract was raised with sodium 

hydroxide to pH 6.4. This extract can be thought of as a 

model of the ACQ extract without cupric ions. The second 

model extract “mock ACQ” was like the first extract except 

that cupric sulphate was added so that the resultant copper 

concentration in the solution was 50 mg L–1. Cupric sulphate was used instead of copper ethanolamine, the source 

of cupric ions in ACQ-treated wood, because of problems 

with copper ethanolamine solubility at this pH (Zelinka and 

others 2007; Lee and Cooper 2010). The mock ACQ extract has the same pH and copper concentration as the ACQ 

extract and allows for the role of copper to be examined 

independently by comparing it to the pH adjusted extract 

without copper. Zinc sheet (99% pure, Alfa Aesar, Ward 

Hill, Massachusetts) polished to 600 grit was used instead of 

hot-dip galvanized fasteners to eliminate variability caused 

by surface roughness. This allowed a clearer examination of 

the effect of pH and cupric ions. 

The electrochemically determined corrosion rates of the 

polished zinc in the model extracts are summarized in 

Figure 2-5; for the DDAC extract (pH 4.6), the corrosion 

rate (in μm y–1) in zinc was 185 ± 32 (standard error) for 

ACQ 

pH - 6.6

[Cu] - 51 mg L–1

Corrosion rate - 27μm y–1

Original 

series 

with 

galvanized 

steel

DDAC 

pH - 4.6

[Cu] - 0

Corrosion rate - 87 μm y–1

Influence 

of Cu 

and pH

NaOH

“DDAC mod” 

pH - 6.4

[Cu] - 0

“Mock ACQ”

pH - 6.4

[Cu] - 50 mg L–1

CuSO4

Effect of pH Effect of Cu

Same pH and [Cu]

Same pH

Figure 2-4. Graphical explanation of the model DDAC extracts used to examine the effect of pH and 

cupric ions on the corrosion of polished zinc sheet.
General Technical Report FPL–GTR–220

16

the “DDAC mod” extract (pH 6.4) it was 21 ± 4 and for 

the “mock ACQ” extract (DDAC pH 6.4 with 50 mg L–1

CuSO4) it was 63 ± 4. These data suggest pH is responsible 

for high corrosion rates in DDAC extract because raising 

the pH to that of the ACQ greatly reduced the corrosion rate. 

The data also suggest that cupric ions increase the corrosion 

rate. The corrosion rates cannot be directly compared to 

those in Figure 2-3 because a different metal was tested to 

reduce variation between replicates. However, what can be 

compared is the ratio of corrosion rates in DDAC/ACQ (or 

mock ACQ). For galvanized fasteners, the ratio of DDAC/

ACQ corrosion rate was 3.2 and for the zinc sheet, the ratio 

of the DDAC/mock ACQ was 2.9. Similarities in ratios suggest that differences in corrosion between DDAC and ACQ 

extracts can be explained exclusively by the differences in 

pH and cupric ions. The modified extracts suggest that the 

DDAC extract is much more corrosive than ACQ because 

its pH is much lower, even though ACQ has cupric ions, 

which indeed increased the corrosion rate.

MCQ-Treated Wood

For solid wood, MCQ corrosion rates were similar to those 

measured in wood treated with soluble copper systems 

(CCA, ACQ, CuAz). However, corrosion rates measured in 

MCQ extracts exhibited unique behavior; galvanized fasteners had a higher corrosion rate in the extract than solid 

wood, whereas the corrosion rate of steel fasteners in the 

extract was so low it could barely be detected. These data 

present two main questions: (1) why did steel and galvanized steel behave so differently in the extracts, and (2) why 

was there correlation between exposure and extract tests for 

CCA-, ACQ-, and CuAz-treated wood but not for MCQtreated wood?

The large difference between the corrosion rate of steel and 

galvanized steel in the extract suggests that corrosion of 

steel may be thermodynamically unfavorable. To examine 

the thermodynamic differences between steel and galvanized steel in these solutions, we overlay the open circuit 

potentials of steel and galvanized steel in different solutions on the potential-pH diagram (Pourbaix diagram) of 

copper (Pourbaix 1973, Fig. 2-6) where the stability region 

of water is denoted by the dashed lines. The diagram was 

drawn using a copper ion solution activity of 10–4; measured 

concentrations are within an order of magnitude (Table 2-1). 

Additionally, the Pourbaix diagrams of steel and zinc are 

included in Fig. 2-7 with data from each treatment overlaid.

From the potential-pH diagram we see that the steel and galvanized steel in MCQ solutions are in two distinct regions 

of copper stability. The steel is in a region where cupric ions 

are stable, whereas the galvanized steel is in a region where 

copper metal is stable. Furthermore neither metal is immune 

from corrosion in this regime (Fig. 2-7). Assuming that the 

corrosion mechanism involves the reduction of cupric ions 

to copper metal, the potential-pH diagram predicts that steel 

fasteners will not corrode, as they are in a region where cupric ions are stable; this agrees with the corrosion rate data 

(Fig. 2-3). Furthermore, the surface of fasteners polarized 

in the extract were examined with EDS. The EDS detected 

copper on the galvanized fasteners but not on the steel 

fasteners.

When the results of the EDS are combined with the polarization resistance measurements and the potential-pH 

diagram, a coherent picture of corrosion in the MCQ extract 

Figure 2-5. Results of ancillary experiments of polished zinc sheet in model extracts used to examine 

differences between the wood preservatives which 

contain DDAC (DDAC-, ACQ-, and MCQ-treated wood).

Figure 2-6. Potential pH diagram of copper with an assumed ion solution activity of 10–4. The average opencircuit potential data of steel (filled symbols) and 

galvanized steel (open symbols). Error bars are not 

presented as they are smaller than the symbols.
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

17

emerges. These results suggest that cupric ions are available 

in MCQ extract even though the treating solution is comprised of particles of nearly insoluble copper. The open circuit potential of galvanized fasteners places them in a region 

where copper metal is the stable phase, and cupric ions are 

reduced on the metal surface as the fastener oxidizes. Conversely, steel fasteners are more noble at a potential where 

cupric ions are stable, and therefore do not corrode. 

Behavior of the steel fastener in the MCQ extract appears to 

be controlled by thermodynamics of the cupric ions in solution. One interesting aspect of this study was that the opencircuit potential (OCP) of the steel fasteners was unusually 

high when compared to the galvanized fasteners. For example, in previous work in ACQ extracts (Zelinka and others 

2008b), the average OCP for steel fasteners was –0.33 V vs. 

the standard hydrogen electrode (SHE), whereas it was 

0.19 V in the current study. For reference, OCP of galvanized fasteners was –0.62 V in the previous and –0.48 V 

in the current work. Although the OCPs were high for the 

steels in extracts with copper, this was not the case for extracts without copper; the OCP in the DDAC and untreated 

extracts were –0.38 V and –0.37 V, respectively. Interestingly, in an auxiliary experiment in this study where polished steel was tested in an ACQ extract, a high (>0.25 V vs. 

SHE) open circuit potential was measured and the steel did 

not corrode. Whatever the reason for the high open circuit 

potentials in MCQ-treated wood, it clearly limits the usefulness of electrochemical extract test in these cases.

In contrast to the steel fastener, the galvanized fastener in 

the MCQ extract has a much higher corrosion rate than in 

solid wood, and the EDS-confirmed cupric ions were present. Because the data suggest the MCQ extract contains cupric ions, and cupric ions are not stable for the open circuit 

potentials measured for galvanized steel, we would expect 

this system to behave like traditional wood preservatives 

(CCA, ACQ, CuAz), which had the same corrosion rate in 

the extract and the solid wood. This was not the case, although MCQ had the lowest pH of any of the systems with 

soluble copper. Interestingly, the corrosion rate in the MCQ 

extract was not statistically different from that measured 

in the DDAC extract, which did not contain cupric ions 

but had a slightly lower pH. To examine the role of pH and 

cupric ions in MCQ, ancillary tests were carried out on polished zinc where the pH of the MCQ extracts was adjusted.

Two pH-modified MCQ extracts were made, and the corrosion rate of zinc sheet was measured with polarization resistance (Fig. 2-8). In the first modification, the pH was raised 

with sodium hydroxide to the same pH as the ACQ extract 

(pH 6.6) “mock ACQ II”. In the second modification, the 

pH was lowered with acetic acid to nearly the same pH as 

the DDAC extract (pH 4.5). The pH of the unadjusted MCQ 

extract was 5.1, and the corrosion rate of the zinc sheet in 

this extract was 80 ± 5 (standard error) μm y–1. The pH 6.6 

(mock ACQ II) extract had a corrosion rate of 25 ± 15 and 

the pH 4.5 had a corrosion rate of 200 ± 20 (Fig. 2-5). 

Table 2-1. Type, composition, and retention of the wood preservatives tested and the physical properties of 

extracts made from these wood treatments

Nominal composition 

(weight percentage) 

Retentiona

(kg m–3) Extract physical properties 

Nominal Actual pH 

Conductivity 

(μS cm–1) 

Cu 

(mg L–1) 

Cr 

(mg L–1)

As 

(mg L–1)

ACQ (type D) 

Alkaline copper 

quaternary 

66.7 CuO 

33.3 DDACb 

4.0 1.6 6.6 596 51.0 c c

CuAz (type C) 

Copper azole 

96.8 CuO 

1.6 Tebuconazoled

1.6 Propiconazolee 

1.0 0.7 6.5 446 23.9 0.06 0.22 

MCQ

Micronized copper 

quaternary 

66.7 CuO 

33.3 DDAC 

4.0 5.0 5.1 220 32.5 f 0.09 

CCA (type C) 

Chromated copper 

arsenate 

47.5 CrO3 

18.5 CuO

34.0 As2O5 

4.0 2.9 4.9 195 33.4 6.20 9.30 

DDACb

(carbonate)

100.0 DDAC 1.3 c 4.6 173 0.2 f

 0.33 

Untreated (control) 4.5 190 0.3 c c 

a

Nominal compositions from AWPA standard P5 (AWPA 2007), with the exception of MCQ, which comes from ICC-ES evaluation report ESR1980.

b

DDAC: didecyldimethylammonium carbonate- IUPAC formula (C10H21)2(CH3)2N(CO3)H and (C10H21)2(CH3)2N(CO3) (C10H21)2(CH3)2 

(mixture). 

c

Not assayed. 

d

IUPAC formula: (RS)-1-(4-Chlorophenyl)-4,4-dimethyl-3-(1,2,4-triazol-1-ylmethyl)pentan-3-ol. e

IUPAC formula: 1-[ [2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-yl]methyl]-1,2,4-triazole. f

Assayed, but not detected. 
General Technical Report FPL–GTR–220

18

Not surprisingly, changes in pH caused large changes in the 

corrosion rate. Data from the “mock ACQ II” suggest the 

corrosion rate of the MCQ extract was higher than the ACQ 

extracts because it was more acidic. Conversely, lowering 

the pH to 4.5 more than doubled the corrosion rate. If cupric 

ions were not contributing to corrosiveness, results of the 

pH modifications suggest that the MCQ extract should be 

much lower than the DDAC extract. In summary, the high 

corrosion rate measured in the MCQ extract for galvanized 

steel appears to be caused by both the presence of cupric 

ions and the pH of the solution.

Conclusions

Steel and galvanized steel exposed to CCA, ACQ, and CuAz 

treatments have similar corrosion rates in the solid wood 

and in extracts made from the solid wood. We believe that 

the reduction of copper is the cathodic reaction in these 

solutions and this reaction proceeds at the same rate in both 

the extract and the solid wood. Conversely, for untreated 

and DDAC-treated wood, the extract is much more 

corrosive than treated wood. The cathodic reaction is the 

reduction of acids and dissolved oxygen and these proceed 

much more rapidly in the extract.

Steel fasteners did not corrode in the MCQ extract. The 

open circuit potential for steel in these extracts was in a 

region where cupric ions were stable and therefore there 

was not a thermodynamic driving force for cupric ion reduction. These results were duplicated inadvertently in an ACQ 

extract with a different type of steel. In short, if the open 

circuit potential of the metal to be tested is above cupric ion 

stability line (~0.25 V vs SHE), the electrochemical extract 

test method will not work. The galvanized fasteners in the 

MCQ extract had a much higher corrosion rate than fasteners exposed to ACQ and CuAz extracts, which was attributed to the lower pH of the MCQ extract. However, the cupric 

ions in the extract still affected corrosion.

Acknowledgments

The authors thank the assistance of USDA Forest Products 

Laboratory employees Thomas A. Kuster, Daniel. O. Foster, 

and James T. Gilbertson for their assistance with the experimental work, and Tivoli Gough for drawing Figures 2-1, 

2-4, and 2-8. 

References

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15: 239–239.

AWPA 2007. AWPA P5: Standard for waterborne 

preservatives. Granbury, TX: American Wood Preservers’ 

Association.

Bailey, G., Schofield, M.J. 1984. Corrosion of metal fastenings in CCA-treated timber—the corrosion science assessment. Journal of the Institute of Wood Science. 10: 14–18.

Baker, A.J. 1980. Corrosion of metal in wood products. Durability of building materials and components. ASTM STP 

691. West Conshohocken, PA: American Society for Testing 

and Materials.

Baker, A. J. 1988. Corrosion of metals in preservative-treated wood. In: Hamel, M., ed. Wood Protection Techniques 

and the Use of Treated Wood in Construction. Madison, WI: 

Forest Products Society.

Dennis, J.K., Zou, C., Short, N.R. 1995. Corrosion behaviour of zinc and zinc alloy coated steel in preservative 

treated timber. Transactions of the Institute of Metal Finishing. 75: 96–101.

Farmer, R.H. 1962. Corrosion of metals in association with 

wood. Part 2. Corrosion of metals in contact with wood. 

Wood. 27: 443–446.

Kear, G., Wú, H.-Z., Jones, M.S. 2009. Weight loss studies of fastener materials corrosion in contact with timbers 

Figure 2-7. Potential pH diagram of zinc and steel where 

the ion activity is assumed to be 10–4. The average pH 

and open circuit potential for each treatment overlaid.
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

19

treated with copper azole and alkaline copper quaternary 

compounds. Corrosion Science. 51: 252–262.

Kubler, H. 1992. Corrosion of nails in wood construction 

interfaces. Forest Products Journal. 42: 47–49.

Lee, M.J., Cooper, P. 2010. Effect of amine ligand, copper/

amine ratio, and pH on copper adsorption into wood. 

Holzforschung. 64: 659–665.

Mansfeld, F. 1973a. Simultaneous determination of instantaneous corrosion rates and Tafel slopes from polarization 

resistance measurements. Journal of the Electrochemical 

Society. 120: 515–518.

Mansfeld, F. 1973b. Tafel slopes and corrosion rates 

from polarization resistance measurements. Corrosion. 

29: 397–402.

Pourbaix, M. 1973. Lectures on Electrochemical Corrosion, 

New York, NY: Plenum Press.

Rammer, D.R., Zelinka, S.L. 2010. Optical method for measuring the surface area of a threaded fastener. Experimental 

Techniques. 34: 36–39.

Short, N.R., Dennis, J.K. 1997. Corrosion resistance of zincalloy coated steel in construction industry environments. 

Transactions of the Institute of Metal Finishing. 75: 47–52.

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S.D., Covino, B.S., eds. ASM handbook volume 13A 

corrosion: fundamentals, testing, and protection. Materials 

Park, OH: ASM International: 190–195.

Simm, D.W., Button, H.E. 1985. Corrosion behaviour of 

certain materials in CCA-treated timber. Environmental tests 

at 100% relative humidity. Corrosion Prevention and Control. 32: 25–35.

Stirling, R., Drummond, J. 2009. Re-distribution of copper 

in the cell walls of wood treated with micronized copper 

quat. In: The 40th Annual Meeting of the International Research Group on Wood Protection, Stockholm, Sweden, IRG 

Secretariat, 2009.

Zelinka, S.L., Rammer, D.R. 2009. Corrosion rates of fasteners in treated wood exposed to 100% relative humidity. 

ASCE Journal of Materials in Civil Engineering. 

21: 758–763.

Zelinka, S.L., Rammer, D.R., Stone, D.S. 2008a. Corrosion of metals in contact with treated wood: developing test 

methods. Paper #08403. CORROSION 2008 New Orleans, 

LA: NACE International. 

Zelinka, S.L., Rammer, D.R., Stone, D.S. 2008b. Electrochemical corrosion testing of fasteners in wood extract. 

Corrosion Science. 50: 1251–1257.

Zelinka, S.L., Rammer, D.R., Stone, D.S. 2009. Evaluating 

the corrosiveness of southern pine treated with several wood 

ACQ 

pH - 6.6

[Cu] - 51 mg L–1

Corrosion rate - 27 μm y–1

DDAC 

pH - 4.6

[Cu] - 0

Corrosion rate - 87 μm y–1

MCQ 

pH - 5.1

[Cu] - 33 mg L–1

Corrosion rate - 76 μm y–1

“Mock ACQ II”

pH - 6.6

[Cu] - 33 mg L–1

“MCQ mod” 

pH - 4.5

[Cu] - 33 mg L–1

NaOH

CH3

COOH

Effect of pH 

at fixed [Cu]

Original 

series 

with 

galvanized 

steel

Effect of Cu 

at same pH

“Micronized” vs. traditional 

copper systems

Figure 2-8. Graphical explanation of the model MCQ extracts used to examine the effect of pH and 

cupric ions on the corrosion of polished zinc sheet.
General Technical Report FPL–GTR–220

20

preservatives using electrochemical techniques. CORROSION 2009. Atlanta, GA: NACE International.

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corrosion rates and corrosion product analyses. Corrosion 

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Meeting of the International Research Group on Wood 

Protection, Stockholm, Sweden, IRG.
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

21

Chapter 3

The Effect of Tannins and pH on the 

Corrosion of Steel in Wood Extracts

Samuel L. Zelinka, Materials Research Engineer

Forest Products Laboratory, Madison, Wisconsin

Donald S. Stone, Associate Professor, Department of Materials Science and Engineering

College of Engineering, University of Wisconsin-Madison

types of extractives have been thought to affect the corrosion of metals in contact with wood or the black liquors of 

wood pulp: small organic acids (acetic and formic acid), 

tannins (or more broadly polyphenols), and phenols with 

two or three adjacent hydroxyl groups (e.g., catechol and 

pyrogallol). For solid wood, only organic acids and tannins 

have been mentioned in the literature; catechol (1,2-dihydroxybenzene) and pyrogallol (1,2,3-trihydroxybenze) are 

formed as lignin is destroyed in the pulping process (Clark 

and Green 1968; Niemelä 1989). 

Small Organic Acids

Although the pH of wood, a solid material, is not well defined, water extracts of nearly all wood species are acidic 

(Packman 1960; Bootle 1983; Balban and Uçar 2003). The 

reason for this acidity is that in the presence of water, acetyl 

and formyl groups in the hemicelluloses are hydrolyzed to 

form acetic and formic acid (Packman 1960). Research has 

shown that this process is chemical, rather than biological 

(Packman 1960; Arni and others 1965b; Smith 1982), and 

that over 90% of the acid produced is acetic (Balban and 

Uçar 2003).

The earliest papers on corrosion of metals in wood attribute 

corrosiveness of wood to its acidity. Farmer (1962) lists the 

pH of several wood species and suggests a correlation between the most corrosive wood species and the most acidic 

wood species. Farmer emphasizes corrosiveness of western 

red cedar (Thuja plicata) and lists the pH in the range 2.9 to 

4.7. Bartel-Kornacka (1967) studied the corrosiveness of 

23 tropical wood species by placing iron in contact with water-saturated sawdust and found no difference in corrosiveness for species with a pH greater than 5; some species with 

a lower pH were more corrosive. Knotkova-Cermakova and 

Vlckova (1971) cite the presence of acetic and formic acids 

as the only corrosive factors in wood. Smith (1982) ranks 

16 species of wood according to their corrosivity, lists their 

pH, and concludes that, “a strongly acid [sic] wood, pH 

less than 4.0, is potentially dangerous, and a less acid [sic]

wood, pH more than 5.0, is likely to be relatively safe.” This 

concurs with the accepted knowledge that the corrosion rate 

of carbon steel, while generally constant in the range of pH 

Abstract

Tannins and pH are often cited as factors that affect the corrosiveness of wood yet there are few data to confirm these 

statements. The purpose of this paper is to systematically 

investigate the effect of tannins and pH on corrosion of metals in wood. Four wood species known to vary in both their 

pH and extractives were chosen and extracted with water. 

The pH, tannin content, and corrosion rate of carbon steel 

were determined for each extract. To test if pH and tannin 

were the only factors influencing corrosion, “synthetic” 

extracts were made by adding tannins to distilled water and 

adjusting the pH. At a given pH, increasing the tannin content inhibited corrosion and at a given level of tannin content, lowering the pH increased corrosion. An isocorrosion 

map as a function of pH and tannins was developed by using 

simple kinetic models to extrapolate the data. The data may 

be useful in estimating the relative corrosiveness of new, 

non-metallic wood preservatives based upon the chemistry 

of naturally durable wood species. 

Keywords: wood, wood extractive, tannins, pH, corrosion, 

isocorrosion diagram 

Introduction

Recently several papers have been published on corrosion 

of metals in wood treated with copper-based wood preservatives. Although the corrosion mechanism involves reduction 

of cupric ions (Baker 1988), it also depends on the wood 

extractives. Kear and others (2008a,b) studied corrosion 

behavior of metals in solutions of wood preservatives and 

concluded that these solutions cannot be used to predict corrosion in solid wood. Zelinka and others (2008) found that 

water extracts of treated wood were much more corrosive 

than dilute solutions of the wood preservative with roughly 

the same copper concentration. These studies suggest that 

wood extractives affect corrosion of metals in treated wood; 

however, the literature on this topic is sparse, and the role of 

several extractives remains controversial.

Although a single piece of wood can contain over 700 different extractives (Hazlewood and others 2006) only three 
General Technical Report FPL–GTR–220

22

5–9, increases rapidly with decreasing pH below pH 5 

(Silverman 2003). 

To summarize, many researchers have found correlations 

between the acidity of wood and its corrosiveness, and pH 

is largely controlled by formation of acetic and formic acid. 

However pH cannot be the only variable that affects corrosion. For example, Bartel-Kornacka (1967) found differences in corrosion between species with the same pH; Cynometra ananta and Nauclea diderrichii both had a measured 

pH of 4.5, but the corrosion rate in Cynometra was twice 

as high as in Nauclea. Furthermore, when Terminalia ivorensis was extracted with a mixture of ethanol and water, it 

was much less corrosive. Similarly, Arni and others (1965a) 

removed all the volatile acids and phenolics from the wood 

and found the extracted wood was not corrosive toward mild 

steel. 

Tannins

Tannins and other polyphenolic extractives are often mentioned in the literature as compounds that affect corrosion in 

wood. Polyphenols complex with metals; adjacent hydroxyl 

groups interact with metal ions (Maclean and Gardner 1953; 

Gust and Suwalski 1994; Kannan and Kelly 1996) forming 

metal tannates that are insoluble and form a dark blue/

purple precipitate (McDonald and others 1996). However, 

researchers disagree about whether this chelation increases 

or decreases corrosion. Papers that examine the corrosion 

of metals in solid wood state that tannins increase corrosion 

(Krilov and Gref 1986; Winkelmann and others 2009a; Winkelmann and others 2009b; Pugsley and others 2001; Pugsley and others 2002), whereas studies on corrosion in the 

pulp and paper industry state that tannins are corrosion inhibitors (Maclean and Gardner 1953; Kannan and Kelly 1996; 

Hazlewood and others 2006; Singh and others 2001; Singh 

and Anaya 2001; Singh and Anaya 2007), although only one 

of these papers explicitly examined their effect (Hazlewood 

and others 2006). In the broader corrosion literature, wood 

tannins are frequently used to protect metals as either a primer or as an inhibitor (Mallow and Gardner 1953; Gust and 

Suwalski 1994; Pelikán 1966; Mammala and others 2000; 

Gust and Bobrowicz 1993; Nasrazadan 1997; Oguzie 2008; 

Rahim and others 2008).

Most of the research on the effect of tannins on corrosion 

in solid wood have examined the wear resistance of sawblades. Krilov and Gref (1986) list the pH and polyphenol 

content for 15 different hardwoods, although all but two of 

the species were from the genus Eucalyptus. No corrosion 

data were reported, but a mechanism for corrosion involving 

the chelation of ferric ions was proposed. Winklemann and 

others (2009a,b) claim that tannins accelerate the corrosion 

process by both lowering the pH and forming complexes 

with iron that do not adhere to the surface. Pugsley and 

others (2001, 2002) examined the role of tannins in the corrosion of sawblades by measuring the strength of ceramic 

reinforced tool-steels exposed to a 5 mM (8.5 g L–1) solution of tannic acid. The strength of all steels decreased with 

exposure time, and some alloys exhibited stress-corrosion 

cracking. 

In contrast to the above studies, in the broader corrosion 

literature tannins are often cited as corrosion mitigators. 

For example, Matamala and others (2000) tested tannins 

extracted from radiata pine (Pinus radiata) and black acacia (Acacia melanoxylon) as part of an anti-corrosive paint 

system and found that tannins increased the time to failure 

by over 250% when compared to an equivalent paint system 

without the tannins. Similarly, researchers have found that 

tannins extracted from radishes, honey, and various other 

plant species inhibit corrosion in acid and sodium chloride 

solutions (Oguzie 2008; Radojcic and others 2008; Rahim 

and others 2008).

The different behavior of tannins in these papers is most 

likely due to the studied application. In sawblade corrosion, 

it is possible that the iron-tannates are removed from the 

surface due to friction or heat and chelation results in a more 

active surface than normal corrosion processes, whereas in 

other situations, iron-tannates protect the surface. Regardless, the effect of tannins on the corrosion of metals in wood 

remains unclear.

Catechol and Pyrogallol

Although catechol and pyrograllol are not expected to be 

found in large quantities in solid wood, they warrant discussion in this work because they have been well studied in 

pulp and paper corrosion and are chemically similar to tannins. All of these compounds form complexes with iron at 

adjacent hydroxyl groups. Gust and Suwalski (1994) used 

Mössbauer spectroscopy to study the interaction of iron 

with catechol, pyrogallol, gallic acid, and oak tannins. They 

found that iron formed a complex with the same Mössbauer 

parameters for all compounds; that is, the interaction of iron 

and the phenolic molecule only depended on whether the 

molecule contained hydroxyl groups in adjacent positions. 

Furthermore, they found no difference in the type of complex formed between catechol and pyrogallol. These similarities suggest that catechol and pyrogallol should affect 

corrosion in the same way as tannins; however, the literature 

does not support this.

Seminal work by MacLean and Gardner (1953) found that 

catechol increased corrosion of equipment used to pulp 

western red cedar. Kannan and Kelly (1996) found that the 

critical current density for steel passivation in alkaline media was unaffected by 1,3-dihydroxybenzene and 1,4-dihydroxybenzene but was greatly increased in the presence of 

catechol and pyrogallol. Similarly, Singh and others (Singh 

and others 2001; Singh and Anaya 2001; Singh and Anaya 

2007) examined the role of catechol on corrosion and found 

that it interacted synergistically with other components of 

the black liquor to increase corrosion. 
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

23

Objectives of This Research

The literature points to the importance of pH, tannins, and 

catechols as important factors in the corrosion of metals in 

wood and wood pulp. In this work, we systematically investigate the effects of tannin content and pH in extracts of four 

wood species exhibiting a range of both pH and tannins to 

see if there is a trend. These might not be the only factors affecting corrosion. If they were, “synthetic” extracts with the 

same pH and amount of tannin content as the wood extracts 

should give the same results. To test the influence of other 

factors, corrosion rates of synthetic extracts were made and 

compared to the wood extracts.

Materials and Methods

Four wood species known to vary across a range of acidities 

and quantity of extractives were investigated in this study 

by examining their water extracts. White oak (Quercus spp.) 

was chosen, as it is very acidic (Packman 1960), one of the 

most corrosive solid woods (Smith 1982), and is also high 

in tannins (De Rosso and others 2009). Elm (Ulmus spp.) 

was chosen because it is low in extractives and is one of the 

least acidic woods; literature values of the pH range from 

6.0 to 7.2 (Farmer 1962; Smith 1982). Black locust (Robinia 

pseudoacacia, hereafter referred to as locust) was chosen 

because it is a naturally durable species (Scheffer and Hopp 

1949; Pollet and others 2008). It has been shown that the 

extractives that protect the wood are water soluble (Scheffer 

and Hopp 1949) and contain ortho-phenolic groups (Pollet 

and others 2008) so it is therefore reasonable to expect that 

these extractives may also affect the corrosiveness of locust. 

Southern Pine (Pinus spp.) was chosen to connect with 

existing work on treated wood (Zelinka and others 2008). 

Additionally, other researchers have noted the corrosivity of 

Southern Pine black liquor (Singh and others 2001; Singh 

and Anaya 2001; Singh and Anaya 2007). Elm and locust 

were harvested near Madison, Wisconsin, and stored in a 

temperature- but not humidity-controlled room for several 

years before testing. The Southern Pine and white oak were 

purchased commercially and had been kiln-dried prior to 

testing. No effort was made to separate heartwood from sapwood; rather, one homogenized extract was made for each 

wood species tested. 

Corrosion of steel in water extracts was determined by the 

method of Zelinka and others (2008). The extract was made 

by combining sawdust and distilled water in a 1:10 ratio by 

weight for one week at room temperature. After the extraction period, the sawdust was removed with vacuum filtration 

and the extract was stored at 1 °C until just before testing 

to minimize any possible biological activity. No effort was 

made to control particle size of the sawdust because in previous work we found that the corrosion rate did not depend 

on the size of particles used to form the extract (Zelinka and 

others 2008). 

Corrosion of a 1018 steel plug (UNS G10180) was 

measured electrochemically. The potential was varied from 

–30 mV to +30 mV with respect to the open circuit potential 

at a scan rate of 0.166 mV/s. Prior to polarization, the open 

circuit potential was measured for an hour. The corrosion 

rate was determined by simultaneously measuring polarization resistance and Tafel slopes, which were ascertained 

from the non-linearity in the polarization resistance curve 

using Mansfeld’s method (Mansfeld 1973a,b). 

For each extract, the pH was measured with a double junction glass bulb, and the amount of tannin was quantified 

colorimetrically with the method of Amin (1997) with minor 

modifications. In this method, tetrazolium blue is reduced 

in the presence of tannins and the absorbance at 527 nm is 

measured. The color is compared against a calibration curve 

made with tannic acid, and the tannin content is expressed 

as an equivalent concentration of tannic acid. Two modifications were made to the method. First, tetrazolium blue solution was made to a concentration of 0.005 M and twice the 

volume used because it did not readily dissolve in water at 

0.01 M. Second, when solutions were heated in 10-mL volumetric flasks, boiling over occurred, and the samples could 

not be tested quantitatively. Instead, solutions were heated in 

a 50-mL Erlynmeyer flask and after heating, transferred to a 

10-mL volumetric flask where the total volume was brought 

to 10 mL with ethanol “Synthetic” extracts were made by 

first adding tannic acid to distilled water and then adjusting 

the pH by addition of sodium hydroxide. 

Results and Discussion

The pH, tannin concentration, and corrosion rate (with 

standard error) are given in Table 3-1. Fortuitously, the pH 

of pine and oak were similar, and tannin concentrations 

were nearly equal in oak and locust. This allows a limited 

examination of the effect of pH and tannin individually 

while holding the other variable constant. Corrosion rates in 

synthetic extracts were much lower than in wood extracts, 

and they exhibited a different response to changes in pH and 

tannins. 

The steel plug exposed to oak and locust extracts formed a 

blue/black patina after the polarization resistance test. This 

behavior was replicated in the synthetic extracts, and small 

amounts of the patina could be found on the plug after it 

had been exposed to synthetic pine and elm extracts. Furthermore, in all the synthetic extracts, a blue/black cloud 

of precipitate was visible around the electrode (Fig. 3-1). 

This cloud was most visible in the synthetic oak and locust 

extract. The precipitate cloud may also have been present 

in the wood extracts but the extracts were translucent prior 

to testing so the precipitate cloud would not have been visible. It is assumed that the patina and precipitate were irontannate, since these solutions contained tannins, and irontannates are also blue/black in color (Pelikán 1966; Gust and 

Bobrowicz 1993; Matamala and others 2000). 
General Technical Report FPL–GTR–220

24

Because of the similar pH between pine and oak extracts 

and the similar tannin content in oak and locust extracts, 

ancillary tests were conducted on modified extracts of pine 

and locust. The pine extract was modified by adding tannic 

acid until the total concentration was 1,000 mg L–1 and the 

locust extract was acidified with acetic acid until the pH was 

4.3; these modified extracts both had nominally the same pH 

and tannic acid concentration as the unmodified extract of 

oak. The properties of these extracts are given in Table 3-1 

and the corrosion rates are shown graphically in Figure 3-2.

Discussion

Although only four species were tested, some general trends 

can be observed in the data. Similarity in pH between pine 

and oak allow us to examine the role of tannins holding pH 

constant. It appears that increasing tannins at a given pH 

decreases the corrosion rate; that is, tannins are acting as 

an inhibitor. Interestingly, modified pine extract, having the 

same concentration of tannins as oak extract, had the same 

corrosion rate. This suggests that the difference in corrosiveness between unmodified pine and oak is due to differences 

in tannin content, which is remarkable considering the complexity of wood chemistry. 

Likewise, the effect of pH at a given tannin concentration 

can be examined by comparing locust and oak extracts. Oak 

had a lower pH and a higher corrosion rate. When acetic 

acid was added to the locust extract until it had the same pH 

as the oak extract, the corrosion rate increased, but the modified locust was not as corrosive as oak, even though they 

had the same pH and tannin concentration. This suggests 

that locust may contain one or more compounds that act as 

corrosion inhibitors not present in either oak or pine. 

Corrosion rates in the synthetic extract are all much lower 

than in the solid wood extracts, and there is a different trend 

across the species. This can be seen most clearly by plotting 

the extract corrosion rate against the synthetic extract corrosion rate (Fig. 3-3). From the data appear two possibilities. 

The first is that oak is an outlier and all of the synthetic extracts have a corrosion rate of roughly 30 μm y–1. The other 

possibility is that pine is an outlier and there is a linear trend 

between the extract and the synthetic extract. In either case, 

one or more chemicals in the wood extract appear to affect 

corrosion in addition to tannins and the pH. 

For a clearer understanding of the effects of tannin and pH 

on corrosion rate, it is necessary to invoke kinetic models. A 

general form for the dependence of corrosion rate (r) on the 

concentration of hydrogen ions (CH+) in acidic conditions 

(below pH 5) is given by

 (3.1)

where k and n are constants; the corrosion rate of steel is 

largely independent of pH between pH 5 and 9 (Silverman 

2003). We have modeled this transition from a power-law 

dependence (CH+) to no dependence on (CH+) at pH 5 with a 

decaying exponential by adding unity to Eq. (3.2) that is

(3.2)

Figure 3-1. Blue-black precipitate forming on the 

surface of the steel plug exposed to synthetic oak 

extract.

Figure 3-2. Corrosion rates measured in modified 

extracts. Error bars represent the standard error 

(uncertainty in the mean). "Modified" extracts of 

pine and locust have the same pH and tannin content as oak.
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

25

Radojcic and others (2008) showed that the Langmuir isotherm can be used to describe the inhibitor efficiency of 

tannins as a function of concentration. Rearranging terms in 

the Langmuir equation, the corrosion rate can be written as a 

function of the concentration of tannins (c) by

(3.3)

where r0 is the corrosion rate when there are no tannins, and 

k2 is a second constant. Assuming that the effects of tannins 

and pH are orthogonal, it is possible to combine these 

models as

 (3.4)

where , , and m are fitting parameters and m > 0. Fitting the above model to our data allows us to examine more 

clearly the effects of tannin concentration and pH.

Figure 3-4 is an isocorrosion diagram created by fitting our 

model to the wood-extract data in Table 3-1. This figure was 

based on the following assumptions: (1) effects of tannin 

and pH are orthogonal; (2) literature models for the effects 

of tannins and pH are valid for wood extracts; (3) transition 

from pH-dependent to pH-independent corrosion occurs at 

pH 5 and can be modeled with a decaying exponential; and 

(4) the small data set is sufficient to fit the model. 

More data would be needed for any reliable prediction given 

the model assumptions, the possibility of synergistic effects, 

the lack of agreement between real and “synthetic” extracts, 

and the need to show correlation between corrosion in extracts and in solid wood under outdoor exposure. 

Despite its limitations, Figure 3-4 may be used for visualizing the effects of tannins and pH. One possible application 

is in development of new, completely non-metallic wood 

preservative systems. These preservatives are based on the 

chemistry of naturally durable species come to the market 

(Thevenon and others 2009; Binbuga and others 2008; 

Donoso-Fierro and others 2009) and contain antioxidants, 

including tannins, that might complex with iron. This isocorrosion map could be used to predict the relative effects of 

adding tannins or adjusting the pH of these preservatives. 

Summary and Conclusions

Corrosion of steel in wood extracts was studied. Specifically, the corrosion rate was studied as a function of pH and 

tannin content. In the extracts, at a given level of tannins, 

a lower pH increases corrosion. In contrast, at a given pH, 

a decrease in tannin content increases corrosion. It appears 

that in wood extracts, tannins act as a corrosion inhibitor. 

This is in contradiction to findings in the literature on sawblade corrosion. The difference is most likely due to the 

friction and heat produced during sawing.

By combining kinetic models in the literature, an isocorrosion map was created for wood extracts as a function of pH 

and tannins. This map is based on limited data and it does 

not explain why synthetic extracts behave differently; nevertheless, in the future with additional data such maps may 

be able to assess the relative effects of these chemicals when 

developing a new, non-metallic preservative system.

Synthetic extracts exhibited lower corrosion rates and different relative rankings between wood species. It appears there 

is one or more chemicals not accounted for in this study that 

affect corrosion. These chemicals (or chemical) may act 

synergistically with tannins and pH or also may be unevenly 

distributed among the wood species tested.

Figure 3-3. Extract corrosion rate plotted against the 

corrosion rate in the synthetic extract.

Figure 3-4. Corrosion rates in wood extracts as a function of pH and tannin content. Labeled isocorrosion 

lines are in μm y–1. The diagram was generated by fitting the model in Equation (3.4) to the data in Table 3-1.
General Technical Report FPL–GTR–220

26

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 Corrosion rate of 

steel (μm y–1) 

pH 

Tannins

(mg L–1)

a Mean 

Standard 

error 

Extract Oak 4.3 1,020 279 10

Pine 4.5 340 334 26 

Locust 5.1 990 74 24 

Elm 6.5 40 159 33 

Synthetic 

extract

Oak 4.3 1,000 49 2 

Pine 4.5 350 31 3 

Locust 5.1 1,000 27 1 

Elm 6.5 40 33 4 

Modified 

extractb 

Pine 4.2 1,000 285 24 

Locust 4.3 990 198 24 

a

Expressed as tannic acid equivalent. 

b

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Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

29

Chapter 4

Combining Hygrothermal and Corrosion 

Models To Predict Corrosion of Metal 

Fasteners Embedded in Wood

Samuel L. Zelinka, Materials Research Engineer

Forest Products Laboratory, Madison, Wisconsin

Dominique Derome, Group Leader Multiscale Modeling

Empa-Swiss Federal Laboratory for Materials Science and Technology, Dübendorf, Swizterland

Samuel V. Glass, Research Physical Scientist

Forest Products Laboratory, Madison, Wisconsin

spruce) are treated with wood preservatives to be durable 

and suitable for these exterior applications. The durability of 

treated wood depends not only on protecting the wood from 

biological attack but also on minimizing the corrosion of 

embedded fasteners. The corrosion rate of embedded fasteners depends on the moisture and temperature conditions at 

the metallic surface, which fluctuate considerably in outdoor 

wood constructions considered here. In this paper, we present a modeling approach that combines heat and mass transport in the wood exposed to environmental loading with corrosion of the metallic fasteners in contact with the wood. 

The specific context of this paper stems from a wood preservation regulation change that occurred in the United States 

in January 2004. Alkaline copper quaternary (ACQ) and 

copper azole (CuAz) replaced chromated copper arsenate 

(CCA) in residential applications (Lebow 2004). These alternative wood preservatives were suspected to be more corrosive to embedded metal fasteners than CCA (Burkholder 

2004; Anon 2008). Since the regulation change, the corrosiveness of CuAz and ACQ has been evaluated in several 

laboratory tests. Most of this work used the American Wood 

Protection Association AWPA E-12 standard, which places 

metal coupons between blocks of wood at 49 °C and 90% 

RH for at least 240 h (Freeman and McIntyre 2008; Kear 

and others 2009). Other procedures using more realistic 

temperatures were developed. Kear and others (2009) exposed fasteners embedded in treated wood at 21 °C to three 

different relative humidities ranging from 75% to 98% RH 

for 1 year. 

Zelinka and Rammer (2009) embedded fasteners in blocks 

of treated wood exposing them to 27 °C and 100% RH for 

one year. Furthermore, electrochemical tests in a water extract of treated wood were found to have the same corrosion 

rates as those measured at the 27 °C 100% RH conditions 

(Zelinka and others 2008). These tests all show that ACQ 

Abstract 

A combined heat, moisture, and corrosion model is presented and used to simulate the corrosion of metal fasteners embedded in solid wood exposed to the exterior environment. 

First, the moisture content and temperature at the wood–

fastener interface is determined at each time step. Then, the 

amount of corrosion is determined spatially using an empirical corrosion rate model and the inputs of the first step. The 

result is a corrosion profile along the length of the fastener 

generated by summing the corrosion depths determined at 

each time step. We apply the combined model to predict the 

annual corrosion depth along a metal fastener in wood decks 

situated in nine different U.S. cities. Corrosion profiles are 

found to exhibit the same general shape independently of 

climatic load, with the largest amount of corrosion occurring 

at 1 to 5 mm from the wood surface with corrosion depths 

ranging from 5 μm in Phoenix, Arizona, to 45 μm, in Hilo, 

Hawaii. Corrosion is confined to the first 7 to 20 mm of the 

fastener below the wood surface. By varying the climatic 

loads, we found that although there is a definite relation 

between total annual rain and total annual corrosion, under 

the same rain loads, corrosion is higher for a climate with 

more evenly distributed rain events. The proposed combined 

model is capable to capture corrosion behavior under varying loading. A sensitivity analysis gives guidelines for future 

corrosion modeling work for fasteners in wood.

Keywords: corrosion, hygrothermal modeling, decks, wood 

preservatives

Introduction

Wood structures that are exposed to the exterior environment, such as terrace decks, balconies, and playground 

equipment, are susceptible to moisture damage such as 

wood rot and metal corrosion. Frequently, softwoods (pine, 
General Technical Report FPL–GTR–220

30

and CuAz are more corrosive than CCA. However, mainly 

because of the variations of testing conditions, the measured 

corrosion rates from these tests have a wide range of values, 

for example, from 2 to113 μm y–1 for hot-dip galvanized 

steel in ACQ-treated wood (1-26 μm y–1 for CCA). As the 

corrosion rate of metals in contact with wood is shown to 

depend on the moisture content and temperature conditions 

at the interface, in-service conditions along the interface are 

needed for durability prediction. In-service moisture content 

(MC, the ratio of the mass of water to the mass of oven dry 

wood) of wood decks experience large fluctuations (from 

8% to 80% MC) and nonuniform distribution depending 

on the environmental conditions (Lebow and Lebow 2007; 

Glass and TenWolde 2007). Therefore, a steady-state approximation of the MC conditions or of the corrosion rate 

would not be able to capture these changes and could not 

lead to prediction of corrosion lifetime. 

For predicting the corrosion lifetime of fasteners in treated 

wood, we propose to predict the corrosion rate using the 

conditions at the wood–fastener interface determined under 

environmental conditions by combining a heat and moisture 

transport model with a corrosion model. An existing, validated heat and moisture transport model (Janssen and others 

2007a) is used to calculate the conditions at the wood–

fastener interface with a complete set of climatic data applied as boundary conditions including air temperature and 

relative humidity, rain precipitation, and wind conditions. 

The model is also able to account for solar radiation, although in this paper, its effect was not considered. The corrosion of the embedded fastener is computed from the calculated temperature and moisture content at the wood–fastener 

interface at each time step. In the next section, we describe 

this modeling approach, including the geometry of the application problem and the determination of the required material properties. We use this modeling approach in comparing 

the corrosion depth profiles of fasteners for constructions 

under nine different climatic conditions and present the results of this study. We then discuss two key aspects of this 

study: the selection of the appropriate year of climatic data 

and sensitivity of the results to variations of the three parameters of the corrosion model.

Combined Heat Moisture and 

Corrosion Model—Geometry 

of the Physical Problem

Most of the previous work on corrosion in treated wood 

has focused on residential decks; similarly, we simulate the 

corrosion of the fastener that connects the decking to the 

joists (i.e., a deck screw). The American Forest and Paper 

Association recently published a Prescriptive Residential 

Wood Deck Construction Guide based on the 2009 International Residential Code (IRC), which gives the minimum 

construction requirements for single level residential wood 

decks. The guide specifies an “8d” nail (64 mm long, 

~3 mm in diameter), 38-mm thick deck boards spaced 3.2 

mm apart, and 38-mm thick joists. 

In this work, two simplifications are made to the configuration. The geometry and meshing of the calculation domain 

are shown in Figure 4-1. The first simplification is that the 

fastener is modeled within a single wood member, which 

can be thought of as a joist with the deck board removed. 

The second simplification is that the problem can be treated 

two-dimensionally, which is equivalent to assuming that 

the fastener is in the middle of an infinitely long joist. Symmetry considerations are used so that only half of the width 

needs to be modeled. The two dimensional mesh is 20 by 

90 mm and consists of 1,965 wood elements, of exponentially decreasing size going towards the exterior boundaries 

and nail interface, 336 steel elements, and 2,400 nodes. Heat 

transport was simulated in the nail to account for possible 

thermal bridging effects; however, the nail was assumed to 

be impermeable to moisture and had no moisture capacity. 

Corrosion Model

Research indicates the corrosion mechanism for metals 

embedded in wood is aqueous (Baker 1988; Short and Dennis 1997; Dennis and others 1995; Duncan 1988) and many 

researchers have observed a threshold moisture content (between 15% and 20%) below which corrosion in wood does 

not occur (Baker 1988; Short and Dennis 1997). This parallels the mechanism in atmospheric corrosion, where a liquid 

film is required for corrosion. The simplest such model for 

calculating the risk of atmospheric corrosion is found in the 

ISO 9223 standard and assumes that a liquid film capable of 

causing corrosion forms when the relative humidity is greater than 80% and the temperature is greater than the freezing 

temperature of water (0 °C) (ISO 1992). Our empirical corrosion model maintains this temperature threshold for corrosion, but includes the dependence of the corrosion rate on 

moisture content above the corrosion temperature threshold.

Short and Dennis (1997) used electrochemical techniques to 

measure the corrosion rate of zinc as a function of moisture 

content. They placed a small strip of CCA-treated wood between a stainless steel counter electrode and a zinc-working 

electrode. Two more pieces of wood were placed on the 

outside of the working and counter electrodes to create a 

“sandwich.” The wood moisture content was adjusted by 

placing the assembly in various conditioning chambers and 

the corrosion rate was measured at 11 different moisture 

contents using polarization resistance. The relationship between polarization and the corrosion rate was explored first 

by Mansfeld (1973) and this relationship has been recently 

exploited to study a diverse range of pheonomena such 

as the corrosion of rebar in concrete (Poupard and others 

2003b; Poupard and others 2003a; Aït-Mokhtar and others 

2006). The polarization resistance data indicated a sigmoidal dependence of the corrosion rate on moisture content 

between 15 and 27% MC. In addition, below 15% MC, 
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

31

there was no corrosion and above 27% MC, the corrosion 

rate was constant at its maximum value. We have fit their 

experimental corrosion rate data (R, μm y–1) as a function 

of moisture content, M (kg kg–1 expressed as a percentage), 

with the following:

(4.1)

where A represents the maximum corrosion rate (μm y–1), 

B represents the steepness of the transition from 0 to A, and 

C represents the moisture content at which R = A/2, i.e., a 

metric of the moisture content threshold of corrosion. The 

parameters B (0.83) and C (24) are taken from our fit of the 

published data of Short and Dennis (1997). The parameter 

A, which is physically the asymptotic corrosion limit, is 

taken as 52.3 μm y–1 from recent electrochemical measurements of hot-dip galvanized steel in an extract of ACQtreated Southern Pine (Zelinka and others 2008). A graph of 

the corrosion rate as a function of moisture content is shown 

in Figure 4-2, with the data of Short and Dennis overlaid 

(Short and Dennis 1997) (scaled to the maximum corrosion 

found by Zelinka and Rammer (2008)). 

There are no data on the temperature dependence of corrosion in treated wood. However, because the corrosion 

mechanism in wood is believed to be aqueous, it is unlikely 

that corrosion occurs below freezing. Therefore our model 

enforces a “no-corrosion” condition when the wood near 

the metal surface is less than 0 °C, similar to the ISO 9223 

model (ISO 1992). 

Heat and Mass Transport Model

The coupled heat and moisture transport is calculated using 

an existing, validated heat and moisture transport model 

implemented with a finite element code (Janssen and others 2004, 2007a,b) which uses capillary pressure, pc, as 

Figure 4-1. Calculation mesh (domain) 

used in the simulations with physical 

dimensions. The elements with steel 

material properties are black to clearly 

identify the nail. Top, side, and bottom 

refer to the boundary conditions, e.g., 

exposed to environmental conditions. 

The heavy line represents the symmetry 

plane. Figure 4-2. Corrosion model in this study derived from 

Short and Dennis (1997) and Zelinka and others (2008).
General Technical Report FPL–GTR–220

32

the driving potential for moisture transport. The governing 

equations are the following:

 

(4.2)

 

(4.3)

 

(4.4)

 

(4.5)

 

(4.6)

 

(4.7)

where

w is volumetric moisture content [kg m–3]

t time [s]

T the transpose

gm moisture flux [kg m–2 s–1], and the subscripts l and 

 v stand for liquid and vapor, respectively

c specific heat [J kg–1 K–1], and the subscript 0 stands 

for dry solid

ρ density [kg m–3]

T absolute temperature [K]

gh heat flux [W m–2], and the subscripts c and a stand for 

conductive and advective, respectively

Kl liquid water permeability [s]

δv vapor permeability [s]

pv vapor pressure [Pa]

Rv the specific gas constant for water vapor [J kg–1 K–1]

Lv heat of vaporization [J kg–1]

γ the normalized thermal derivative [K–1]

λ thermal conductivity [W m–1 K–1]

The model can handle the full range of moisture content of 

wood from dry to capillary saturation as it includes liquid 

transport. The non-linearity as well as the temperature and 

moisture content dependence of the material properties are 

implemented. Time stepping is adaptive while the temperature and moisture content of the wood at each node are 

written to an output file at one hour intervals.

Boundary Conditions

To simulate the moisture conditions of a deck, atmospheric 

boundary conditions including air temperature and relative 

humidity, precipitation, and wind dependent convective heat 

and mass transfer coefficients are applied on the top surface. 

Precipitation is applied as an hourly average liquid thickness 

of rain, snow is treated as its liquid water equivalent. On the 

“side” and “bottom” surfaces (Fig. 4-1), only the air temperature and relative humidity are taken from the climatic 

data while the convective heat and mass transfer coefficients 

are taken as constant (20 W m–2 K–1 and 2.4 × 10–8 s m–1, 

respectively). 

Material Properties

The accuracy to which the hygrothermal model can predict 

the wood moisture content, and thus the corrosion rate, is 

dependent on appropriately determined material properties, 

namely, full moisture retention curve over both the hygroscopic (sorption isotherm) and overhygroscopic regimes, 

moisture diffusivity (from vapor permeability and liquid 

water conductivity), thermal conductivity, and heat capacity. 

In this work, the moisture retention curve and diffusivity are 

functions of capillary pressure, the thermal conductivity is a 

function of moisture content, and the heat capacity is a constant. No temperature dependency is included at this stage 

of the work.

Few measurements have been published on the hygrothermal properties of treated Southern Pine. Sorption isotherms 

of preservative-treated and untreated Southern Pine have 

recently been published (Zelinka and Glass 2010) and there 

were only small differences between the isotherms of ACQ, 

CCA, and untreated Southern Pine. However, the vapor permeability of treated wood, its liquid water conductivity, and 

its moisture retention curve in the overhygroscopic regime 

have not been characterized. 

In the present work, the material properties are taken from 

work on untreated Scots pine (Pinus sylvestris) (Zillig and 

others 2006). While it is likely that there are differences 

in the hygrothermal properties of Scots pine and Southern 

Pine, the data were chosen because of the quality and completeness of the data and because the data are from the same 

genus. The liquid, vapor, and total permeabilities were taken 

from measurements in the tangential direction, and utilized 

the diffusivity approach presented by Carmeliet and others 

(2007). Heat capacities and thermal conductivity values for 

loblolly pine (Pinus taeda) were taken from the Wood Handbook (Glass and Zelinka 2010). The sensitivity of the model 

to these properties has been examined previously (Zelinka 

and others 2010). 
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

33

Combining Models

Modeling the corrosion of fasteners in treated wood involves two steps: first, calculating the wood moisture 

content and temperature at the wood-metal interface; and 

second, calculating the metal corrosion rate, which is dependent on this moisture content. We calculate the corrosion rate at each output time step (every hour) at each node 

along the wood–metal interface from Equation (4.1) and the 

wood-moisture content. We assume the corrosion rate to be 

constant over the entire following hour. If the temperature 

of the wood at the interface is less than zero for a given 

hour, the corrosion rate at that node is set to zero. The total 

amount of corrosion during the simulation is found by summing the amounts determined hourly. 

Since combining the models does not introduce additional 

physical models, validation of the combined model reduces 

to a validation of the hygrothermal model and the corrosion 

model separately. The hygrothermal model has been validated previously (Janssen and others 2007a) and the corrosion 

model is empirical. 

In the next section we apply the combined models approach 

for a parametric study.

Examination of the Corrosion Potential 

in Different U.S. Climates

Corrosion in outdoor structures is expected to depend upon 

climate. Intuitively, it should depend upon the amount of 

precipitation, the way in which precipitation is distributed 

over time, drying potential, temperature, and possibly other 

climatic variables. Drying potential likely depends on temperature, humidity, solar radiation, and wind speed.

To examine the possibility for corrosion in different climates, we apply the combined models approach to nine cities in the United States. The cities are chosen based on the 

climate zones proposed by Briggs and others (2003), which 

have been adopted by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and 

the International Energy Conservation Code (IECC). The 

methodology splits the continental United States into seven 

zones (1–7) based upon temperature; each of these temperature zones is then assigned a moisture classification, “a” 

for humid, “b” for dry, or “c” for coastal. Eight cities from 

the continental United States are examined, from six of the 

seven climate zones, including all moisture subclassifications (Table 4-1 and Fig. 4-3). Additionally, Hilo, Hawaii, is 

chosen to simulate corrosion in what was expected to be an 

extreme environment (more than 5 m of annual rainfall). 

Climatic Data

The climatic data for the simulations are obtained from the 

Solar and Meteorological Surface Observation Network 

(SAMSON) dataset that includes hourly weather observations for 237 weather stations throughout the United States 

during the period 1961–1990 (NCDC 1993), including the 

nine locations selected above. It would have been possible 

to use the whole set of 30 years to perform the simulations. 

A more common approach is the selection of the dataset of 

a specific year, based on one or several criteria of particular 

relevance for the problem studies. Examples of such selection in building physics deal mainly with the hygrothermal 

behavior of wall assemblies. For example, ASHRAE 160 

recommends 10th percentile hottest and coldest years as 

moisture design reference years (ASHRAE 2009). We ran 

the combined hygrothermal/corrosion model in preliminary 

simulations with several reference years proposed for wall 

assemblies (ASHRAE 2009; Salonvaara and others 2010). 

We found that these reference years do not result in large 

amounts of corrosion compared to other years. This may be 

expected as wall assemblies and terrace decks experience 

very different boundary conditions and hygrothermal 

gradients. 

During preliminary simulations, we observed that the climatic parameter that has the most impact on the total corrosion depth is rain. Thus, the reference years are selected 

based upon precipitation, using two rain parameters as 

simplified simulations (Zelinka and others 2010) show that 

the amount of corrosion is sensitive to both the total amount 

Table 4-1. Cities chosen and summary of climatic data used in the simulations

Zone City State Year 

Maximum

predicted 

corrosion

depth 

(μm) 

Yearly 

Mean 

temperature

(°C)

Total 

rainfall

(m) 

Total number of hours 

T > 0°C Rain 

1 Hilo HI 1990 45 23.0 5.36 8760 2213 

1 Miami FL 1968 12 23.4 2.12 8760 588 

2a New Orleans LA 1961 20 19.6 2.12 8715 593 

2b Phoenix AZ 1978 05 23.3 0.39 8734 284 

3a Atlanta GA 1975 17 15.9 1.68 8440 699 

4a Baltimore MD 1989 16 12.6 1.32 8127 794 

4c Seattle WA 1971 26 10.1 1.10 8422 1205 

5a Chicago IL 1983 11 9.7 1.25 6691 739 

6a Minneapolis MN 1983 12 7.8 0.99 6217 731 
General Technical Report FPL–GTR–220

34

of annual rainfall as well as the number of hours with a rain 

event. For each city, the years 1961–1990 are ranked based 

upon the total amount of annual precipitation. From this list, 

the top 5 years are selected and the hourly data are analyzed 

for the number of hours with non-zero precipitation. From 

the five analyzed years, the year with the most hours of rain 

is selected as the reference year. The reference years selected by this method from the SAMSON data set are given 

in Table 4-1. To simulate a worst case scenario for corrosion, for example, a deck that is constantly in the shade, 

solar radiation is excluded from the climatic data. The years 

selected, along with some rainfall and temperature mean or 

total values, are included in Table 4-1.

Results of the Simulations

Simulations are run for one calendar year for each city, starting in January. The starting temperature of the wood and 

metal is set to be equal to the air temperature at the first time 

step and the starting moisture content is 13%. The results of 

the 1-year simulation for each city are shown in Figure 4-4, 

where the amount of corrosion is given as a function of the 

depth below the wood surface. 

The curves in Figure 4-4 have several features in common: 

the total corrosion penetration increases with depth for the 

first few millimeters, reaches a maximum, and then decreases with depth until it reaches zero. The similarities between 

these curves can be explained by the physics of the phenomenon. During a rain event, the wood moisture content 

at the surface is above the corrosion threshold; the depth 

of this high moisture region is dependent on the amount of 

rainfall and duration of the rain event. At depths far below 

the surface, the moisture content is not affected by the rain 

and remains below the corrosion threshold. In between these 

two locations, moisture is transported into the wood by diffusion and a gradient of moisture content is expected. Also, 

following a rain event, the wood begins to dry out from the 

surface, and surface drops quite quickly below the corrosion 

threshold. After the rainfall stops, the first few millimeters 

of the wood are at the capillary saturation threshold moisture content, which is why the maximum is not at the surface that gets wet by rain, but a short distance below it. 

An example of the relationship between moisture content 

and rain events is given in Figure 4-5, which plots the hourly precipitation and moisture content for one month (April) 

using the Baltimore climate data. The data are taken at 

1.8 mm below the fastener surface, where the maximum 

yearly average moisture content occurred for this city. At 

this depth, the moisture content reaches a maximum quickly 

after a rain event (in a few hours), and then dries out over 

tens of hours. Another illustration of the effect of precipitation on the moisture content, and therefore corrosion, is 

given in Figure 4-6, which plots the entire corrosion 

profile after the rain event corresponding to approximately 

350 hours in Figure 4-5 (time = 0 in Fig. 4-6 corresponds 

to time = 353 in Fig. 4-5). After 60 h, the peak in the moisture content graph has disappeared. 

To make comparisons between the cities easier, the maxima 

of the curves in Figure 4-5 have been tabulated in Table 4-1. 

As expected, Hilo, with over 5 m of rainfall, has the most 

corrosion and Phoenix, with less than 0.4 m, is the environment leading to the least corrosion. After Hilo, Seattle has 

the next highest corrosion rate. This city, while relatively 

Figure 4-3. Map of climate zones proposed by ASHRAE and IECC (reprinted with permission 

from the U.S. Department of Energy, Building Energy Codes Resource Center).
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

35

low in annual rainfall, has 50% more hours with a rain event 

than the next highest city (Baltimore). 

Surprisingly, Miami and New Orleans, which have the same 

total rainfall (2.12 m) and nearly the same number of hours 

(588 vs 593 h) of precipitation, have very different amounts 

of corrosion (67% difference). Since this behavior cannot 

be explained in terms of the annual totals in Figure 4-7, the 

rain events are plotted as a time series in Figure 4-7. From 

this figure, it qualitatively appears that the rain events in 

New Orleans are spread more evenly throughout the year 

5

10

15

20

25

30

35

40

45

50

0 2 4 6 8 10 12 14 16 18 20

Corrosion (μm)

Fastener depth (mm)

Hilo

Seattle

New Orleans

Atlanta

Baltimore

Minneapolis

Miami

Chicago

Phoenix

Figure 4-4. Amount of corrosion after 1 year as a function of depth below the wood surface for each 

of the climates simulated.

Figure 4-5. Hourly calculated moisture contents 1.8 mm 

below the fasteners surface as a function of time for the 

month of April using the Baltimore climatic data. Precipitation is included on the second axis.

Figure 4-6. Moisture content profile as a function of 

time after a rain event (corresponding to ~350 h in 

Fig. 4-5.).
General Technical Report FPL–GTR–220

36

whereas the Miami rain events appear more concentrated 

between hours 3000 to 8000. This difference in distribution 

of rain throughout the year could explain the differences in 

corrosion between these two cities. Likewise, Chicago and 

Minneapolis have a similar number of hours of rain, but 

Chicago has more rain (1.25 m as opposed to 0.99 m) and 

more hours with the air temperature above freezing, yet the 

simulation results indicate slightly less corrosion for Chicago than for Minneapolis. 

The corrosion profiles had a similar shape in all climates 

with a maximum 2 to 5 mm below the wood surface. The 

fact that no corrosion is observed at 10 to 20 mm below the 

surface is a result of the relatively low starting moisture content. After the lumber is preservative treated, it is completely 

saturated with water and preservative chemicals. Although 

best construction practices suggest that the wood should be 

kiln dried after treatment and before construction, in reality 

decks are frequently built from lumber that is still wet from 

treating. To simulate the effect of a “built-wet” construction, 

a simulation was run for Baltimore with the wood starting 

in a capillary saturated state. This analysis assumes that the 

freshly treated lumber is chemically the same as dried and 

retreated lumber. The corrosion profile is shown as a function of time in Figure 4-8, along with the 1-year data with 

a 13% starting moisture content shown as a baseline. The 

corrosion profile after 3 months is nearly uniform at ~3 μm 

and represents the corrosion that occurred from the initially 

high moisture content. After the first 3 months, the amount 

of corrosion remains at 3 μm, as the wood has dried to below the corrosion threshold and non-precipitation related 

corrosion is no longer occurring. This analysis suggests that 

while building with freshly treated lumber increases corrosion, it only affects the first few months after construction 

and should have a small effect compared to atmospherically 

induced corrosion.

Discussion

The combined model was used to determine the corrosion 

profile of metal fasteners embedded in wood for several different climates. Here, we discuss issues relating to the selection of a reference year for corrosion and the sensitivity of 

the corrosion model parameters.

Selection of the Reference Year

The results show that the corrosion of fasteners depends on 

the environmental loading. Although only the amount and 

number of hours of rain are examined in selecting the reference years, there is not a strict correlation between either of 

these variables and the simulated corrosion. This suggests 

that other climatic variables are also affecting the corrosion 

of metals in wood in outdoor exposure. While there has 

been much work on selecting reference years for analysis of 

building envelope assemblies (Salonvaara and others 2010; 

ASHRAE 2009; Cornick and others 2003; Sanders 1996; 

David and others 2010; Kershaw and others 2011; Zhu 

and others 1995; Blocken and Carmeliet 2010), and using 

climate year data to assess rain loads (Blocken and Carmeliet 2007; Abuku and others 2009; Blocken and Carmeliet 

2010), the moisture loads on exterior structures have not 

been assessed so systematically. Structures such as decks 

are clearly different from walls as they receive the entire 

amount of horizontal rainfall, and do not have interior/exterior temperature and moisture gradients.

The method for selecting the years is based on the results of 

a few preliminary simulations, which are used to select the 

important climatic variables, and then deduce what parameters should be used to select the climate years. However, 

the results of simulations in different climates with nearly 

the same controlled climate parameters produce different 

corrosion rates, which suggests that not all of the physics is 

0

500

1,000

1,500

2,000

2,500

1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000

Cumulative precipitation (mm)

Time (h)

Miami

New Orleans

Figure 4-7. Cumulative precipitation as a function of 

time of the year for Miami and New Orleans.

Figure 4-8. Corrosion profile for Baltimore where the 

initial moisture content was set at saturation after 3, 5, 

and 12 months compared with the baseline simulation 

with an initial moisture content of 13%.
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

37

captured in the original parameters used to select reference 

years. A more systematic approach seems warranted for the 

selection of reference years for outdoor structures.

Examination of the Sensitivity of the 

Corrosion Model

The corrosion profile is the result of two models: a hygrothermal model that determines the wood moisture content 

and temperature distribution over the calculation domain, 

and a corrosion model that translates this moisture content 

into a corrosion rate at the nail surface. While hygrothermal 

models have been developed and improved for many years, 

this work presents a first model relating wood moisture content to a corrosion rate. There are only a few examples in 

the literature where corrosion was examined as a function of 

moisture content or relative humidity. Because of the limited 

quality of the data used to construct the corrosion model, the 

sensitivity of the corrosion profile to the corrosion model 

parameters is here examined.

The sensitivity of the corrosion rate as a function of moisture content is examined by running simulations for Baltimore where only the corrosion function (Eq. (4.1)) is varied. 

The parameter A is not studied, as it changes the whole corrosion profile proportionally; that is, if A were doubled, the 

result would be doubling the entire corrosion profile. The 

parameter B, physically the width of the distribution, is 

varied from 0.1 to 2 from an original value of 0.83 

(Fig. 4-9a). The threshold value, represented by C is varied 

from 14 to 34 from an original value of 24 (Fig. 4-10a); 

results are shown in Figure 4-9b and Figure 4-10b. 

Of the two parameters, the corrosion profile is less sensitive 

to B. Increasing B from 0.83 to 2 results in nearly no change 

Figure 4-9. Effect of changing B on the corrosion rate as a function of moisture content (a) and 

the resulting corrosion depth profile (b) in a one-year simulation for Baltimore. B was varied 

from 0.1 to 2 in increments of 0.1 (compared with the original value of 0.83). The arrows point in 

the direction of increasing B.

Figure 4-10. Effect of changing C on the corrosion rate as a function of moisture content (a) and 

the resulting corrosion depth profile (b) in a one-year simulation for Baltimore. C was varied 

from 14 to 34 in increments of 1 (compared with the original value of 24). The arrows point in the 

direction of increasing C.
General Technical Report FPL–GTR–220

38

in the profile. Intuitively, this can be expected because B 

represents the steepness of the transition. Because the original function already had a steep transition, and the function 

approaches the Heaviside function as B gets arbitrarily 

large, the corrosion profile is nearly at its limiting case to 

begin with. Decreasing B causes an increase in the total corrosion and a smoothing of the transition from the corrosion 

maximum to the lowest amount of corrosion. At the lowest 

values shown, corrosion occurs along the entire length of 

the fastener. However, a close examination of Figure 4-9a 

reveals that in these cases, the corrosion rate is non-zero at 

very low moisture contents, which may be unphysical. For 

example, Baechler measured no corrosion for metals embedded in wood at 27 °C and 65% RH (~12% MC) for 

20 years (Baechler 1949). In short, it appears that within 

physical limits, the corrosion profile is not highly sensitive 

to B.

The parameter C, a measure of the transition moisture content, affects both the shape and magnitude of the corrosion 

profile. As C is increased to higher moisture contents, the 

curve retains its shape; however, the corrosion maximum 

is decreased and the depth at which there is no corrosion 

moves towards smaller values. Even with a threshold value 

of 34% MC, which is greater than the fiber saturation point 

of southern pines where free water exists in the lumina 

(Stamm 1929), the corrosion profile retains its shape. As 

C is decreased to lower moisture contents, the profile increases in magnitude and begins to broaden. The maximum 

in the corrosion profile increases rapidly as the threshold 

decreases, nearly doubling as the threshold is lowered from 

24 to 14. 

In summary, the corrosion profile is much more sensitive to 

the moisture content threshold of corrosion, i.e., parameter 

C, than to the steepness of the corrosion model, parameter 

B. As B increases, the corrosion rate as a function of moisture content approaches a Heaviside function, which would 

only require two parameters. The goal of this sensitivity 

analysis was to aid future experiments determining the corrosion rate function. From this analysis, it appears that most 

of the work should involve carefully determining the corrosion threshold by focusing the measurements around this 

value rather than taking measurements over a broad range of 

moisture contents to precisely observe the distribution.

Conclusions 

This paper presents a combined hygrothermal/corrosion 

model for simulating the corrosion of metals embedded in 

wood. The model is developed to aid in the design of wood/

metal assemblies used in exterior construction. Several U.S. 

cities are examined with the model using reference years 

based upon precipitation. Although in general cities with a 

high annual rainfall or number of hours of rain exhibit high 

corrosion, there are several exceptions. 

The simulations show that in most cases, corrosion is 

confined to the first 10 mm below the wood surface. The 

highest corrosion occurs 1–3 mm below the surface of the 

wood. For built-wet construction, the simulated wood joist 

dries to below the corrosion threshold within the first 

3 months. The sensitivity of the function that relates moisture content to corrosion is examined. We found that the 

corrosion profile is more sensitive to the corrosion threshold 

than the width of the transition from no corrosion to the 

maximum corrosion rate. 

There are several logical extensions to the currently presented work. First, the combined model could be compared 

with a field corrosion test that includes monitoring of environmental conditions and wood moisture content. Second, 

the hygrothermal properties of ACQ-treated Southern Pine 

could be better characterized, including temperature dependencies. Third, the corrosion model could be improved 

with further experiments to locate the corrosion threshold. 

Finally, it may be worthwhile to develop a more systematic 

method for selecting reference years for exterior structures.

Acknowledgments

The authors are grateful for the contributions and support 

of Jan Carmeliet and Hannelore Derluyn (Chair of Building 

Physics, ETH Zürich, Switzerland), Peter Moonen (Empa, 

Dübendorf, Switzerland), and C.R. Boardman (U.S. Forest 

Products Laboratory, Madison, WI, USA).

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38: 80–88.

Zelinka, S.L.; Rammer, D.R. 2009. Corrosion rates of fasteners in treated wood exposed to 100% relative humidity. 

ASCE Journal of Materials in Civil Engineering. 

21: 758–763.

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Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

41

Chapter 5

Synthesis of Published and Unpublished 

Corrosion Data from Long-Term Tests of 

Fasteners Embedded in Wood: Calculation 

of Corrosion Rates and the Effect of 

Corrosion on Lateral Joint Strength

Samuel L. Zelinka, Materials Research Engineer

Forest Products Laboratory, Madison, Wisconsin 

Douglas R. Rammer, Research General Engineer

Forest Products Laboratory, Madison, Wisconsin 

studies were focused on determining the corrosiveness of 

new wood preservatives that have become widely used since 

chromated copper arsenate (CCA) was voluntarily withdrawn for residential use in the United States on January 31, 

2003. These studies have shown that wood treated with new 

preservatives such as alkaline copper quaternary (ACQ) and 

copper azole (CuAz) are more corrosive than CCA-treated 

wood by a factor of between 2 and 19 depending on the metal and the environment (Kear and others 2009; Anon 2008). 

CCA-treated wood has been successfully used for many 

years with few corrosion problems (Burkholder 2004); 

however, the original research on CCA-treated wood never 

measured corrosion rates, nor was the effect of corrosion on 

joint strength analyzed. In other words, while CCA-treated 

wood may present an “acceptable” level of corrosion, this 

level is not well characterized. Furthermore, even if it were 

well characterized, knowing the difference in corrosion rates 

between ACQ and CCA lacks meaning unless it can be put 

into the context of mechanical performance.

To answer these questions, we conducted a review of 

two papers that measured long-term corrosion rates 

(14–20 years). The goal was to convert the data, which 

were originally published as a percent weight loss into corrosion rate so that it could be compared with current work 

on newer wood preservatives. On conducting this investigation, two additional unpublished reports on fastener corrosion were found in the archives at FPL. While these reports 

pre-date the use of CCA as a preservative, they contain 

valuable information about the mechanism of corrosion in 

treated wood: one report examines the effect of copper on 

the corrosion rate, and the other examines combining copper with arsenates and chromates individually (the building 

blocks of CCA). All four of these reports, with pertinent 

experimental details, are summarized here along with the 

Abstract

In the past 5 years, several accelerated test methods have 

been developed to measure the corrosion of metals in contact with wood. It is desirable to contrast these accelerated 

results against those of long-term exposure tests. While 

there have been several published long-term exposure tests 

performed on metals in treated wood, the data from these 

studies could not be used as a comparative baseline because 

the formulations and retentions of the preservatives were not 

reported or the amount corrosion was presented as percent 

mass loss instead of a true corrosion rate because the surface 

areas of the fasteners were unknown. This work reexamines 

four reports (two unpublished) that measured the corrosion 

of metals in treated wood. Where possible, the composition 

of the preservatives is reported from the original laboratory notebooks. In all cases, the percent mass loss data are 

presented in terms of a true corrosion rate by calculating the 

surface areas of the fasteners. The second part of this paper 

uses the long-term corrosion rates to calculate the reduction 

in capacity of the joint using the yield theory equations from 

the National Design Specification for Wood Construction. 

Specific examples illustrate the loss of capacity with time 

using measured corrosion rates for wood treated with chromated copper arsenate and alkaline copper quaternary.

Keywords: preservative-treated wood, corrosion, yield 

theory, chromate copper arsenate (CCA), alkaline copper 

quaternary (ACQ)

Introduction

In the past five years, there have been numerous studies on 

corrosion of metals in treated wood (Kear and others 2009; 

Zelinka and others 2008b; Kear and others 2008a,b; Zelinka 

and others 2007, 2009; Zelinka and Rammer 2009). These 
General Technical Report FPL–GTR–220

42

data, which we transformed from the originally reported 

percent weight loss to a corrosion rate. The conversion was 

accomplished by finding archived photographs, and using 

a novel method to calculate the surface area of the fastener 

from a photograph (Rammer and Zelinka 2008, 2010). To 

give a common basis to compare different treatments, the 

concentration of copper is presented as kilograms of copper 

per cubic meter of dry wood is included along with the more 

traditional “retention” of the preservative. 

The second part of this paper explores how corrosion, 

through a reduction in fastener diameter, affects the lateral 

strength of nailed connections. The reduction in capacity is 

calculated from yield theory equations given in the National 

Design Specifications for Wood Construction (NDS). Specific examples are shown comparing the reduction in capacity as a function of time for corrosion rates measured for 

ACQ and CCA-treated wood. 

Part I: Synthesis of Published 

and Unpublished Data

Corrosion of Nails in CCA- and ACA-Treated 

Wood in Two Environments (Baker 1992)

Significance and Overview

In his study,“Corrosion of Nails in CCA- and ACA-Treated 

Wood in Two Environments,” Baker (1992) conducted the 

most recent long-term corrosion study on metal fasteners in 

preservative treatments. This work has special significance 

as it strongly influenced the International Building Code’s 

statements on corrosion of metals in treated wood (Anon 

2003). Despite its widespread use, implications for design 

codes and building practices, data from this report were only 

presented in the form of percent weight loss. The original 

laboratory notebook, which contained photographs of the 

fasteners, was used to calculate a corrosion rate in μm yr–1. 

In going through the laboratory notebook, we discovered 

unpublished experimental details such as the exact compositions of the preservatives used. Additionally, uncorroded 

galvanized fasteners from the original study were found that 

have now been analyzed to obtain the coating thickness and 

elemental composition. We first present a brief summary 

of the test details, followed by the new information on the 

preservative treatment and galvanized fasteners, before finishing with the transformation of percent weight loss data 

to corrosion rates.

Experimental Details

Southern Pine (Pinus spp.) was treated to a retention of 

9.7 kg/m3 (0.6 lb/ft3) with one of three different preservatives: “CCA-I,” “CCA-II,” or ammoniacal copper arsenate 

(ACA) (Table 5-1). The resulting copper concentrations 

for CCA-I and II were 1.39 and 1.50 kg m–3, respectively. 

Nails were driven through two pieces of preservative-treated 

wood to simulate how nails are used in service. There was a 

0.8-mm (1/32-in.) spacer between the two pieces of wood so 

that all test replicates had the same availability of water and 

oxygen. Specimens were either buried underground at a test 

site near Madison, Wisconsin, or exposed to a constant 

27 °C (80 °F), 95% RH environment; replicates were removed at 1, 3, and 17 years for the underground exposure, 

and at 1, 3, and 14 years for the humidity chamber exposure. Eleven different fasteners were tested, which included 

six different materials (copper, silicon bronze, two stainless steels- UNS S30400 and S30160, a nickel alloy-UNS 

04400, and an aluminum alloy- UNS A95056) and five 

different types of coated steel (hot-dip galvanized, mechanically galvanized, electroplated galvanized, cadmium coated 

steel, and a tin-cadmium alloy coated steel). 

Treating Statistics

Official correspondence (Letter from Andrew Baker to Ray 

Lindberg of Reynolds Metal Company, Richmond, Virginia, 

dated December 13, 1973) found in Baker’s study file contained detailed information on the preservative treatments 

used in the study and are given in Table 5-1. It is instructive 

to compare these formulations to the standardized CCA 

formulations also summarized in Table 5-2 (AWPA 2007). 

The standardized formulations vary in the ratio of copper, 

chromium, and arsenic with the current formulation referred 

to as CCA-C. It is clear from the table that the composition 

of Baker’s CCA-II is the same as the formerly standardized 

CCA-B. It also appears that Baker’s CCA-I is similar to the 

formerly standardized CCA-A. However, it should be noted 

that the preservatives that Baker used were made from the 

“salt” formulation of chromic acid and arsenic acid, which is 

Table 5-1. AWPA standardized formulations (Anon 2007) of 

CCA (A-C) as well as the formulations used by Baker (1992) 

Copper

(weight percentage

as CuO) 

Chromium

(weight percentage

as CrO3)

Arsenate

(weight percentage 

as As2O5)

CCA-A 18.1 65.5 16.4 

CCA-B 19.6 35.3 45.1 

CCA-C 18.5 47.5 34.0 

CCA-Ia 17.. 61.. 22..

CCA-IIa 20.. 35.. 45..

a

Compositions used in Baker (1992). 
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

43

forbidden in the current AWPA standard (2007). It has been 

suggested that the salt formulations of preservatives may be 

more corrosive than currently used oxide formulations, although this has not been rigorously tested (Duncan 1988). 

Recent work on corrosion in treated wood has used 

CCA-C as a baseline preservative. Therefore, it is instructive to have a short discussion on the differences in these 

preservatives and how these differences may affect corrosion. It has been shown that the corrosion mechanism 

in wood involves the reduction of cupric ions and that the 

higher the copper concentration in the treated wood, the 

higher the corrosion rate (Kear and others 2009; Zelinka 

and others 2008b). On the other hand, hexavalent chromium 

is widely known as a corrosion inhibitor and it has been 

speculated that the arsenates in CCA may also act as inhibitors (Zelinka and others 2007). From this information, we 

would expect CCA-B, with the highest copper concentration 

and the lowest chromate concentration to be the most corrosive, and CCA-A with the lowest amount of copper and 

the greatest amount of chromates to be the least corrosive. 

In fact, however, the opposite was observed; based on percent weight loss, CCA-II was less corrosive than CCA-I. 

Regardless, CCA-I and CCA-II should give lower and upper 

bounds on the corrosiveness of the CCA-C used in modern 

experiments because the composition of CCA-C lies between these two extremes. 

Galvanized Fastener Details

A hot-dip galvanized nail with the same stock number as 

those used in the experimental work was found in the study 

file. To determine the galvanized coating thickness the fastener was cross sectioned, polished, and viewed in SEM. 

The coating thickness was measured at several locations 

and the average coating thickness was 24 μm (Fig. 5-1). Additionally, EDS was used to detect the composition of the 

galvanized coating (Fig. 5-2). Chromium was found on the 

fastener, which suggests that a chromate conversion coating 

was applied to the fastener post-galvanizing. 

Corrosion Rates

The study file contained several different pictures that were 

suitable for calculating the surface areas of the fasteners (for 

example, Fig. 5-3). However, because the photographs were 

not taken with a scale marker in the picture, an approximation was needed to convert the surface area (in pixels) 

to physical units. The image was calibrated by setting the 

width of the UNS30160 stainless steel fastener in Table 5-2 

to its standardized width. Combined with the starting and 

final masses recorded in the laboratory notebook, we were 

then able to calculate a corrosion rate for Baker’s data. The 

results for galvanized steel and aluminum exposed in a humidity room are shown graphically in Figure 5-4; we found 

that the corrosion rates for stainless steel and the nickel 

alloy were negligible given the uncertainties in the measurements. Baker found a linear relationship between percent 

Table 5-2. Corrosion rates measured by 

Baechler (1949) after 20 years of exposure

for carbon steel. The “—" indicates that the 

corrosion rate was much smaller than the 

uncertainty in the measurement. 

Measurements at a specified RH were 

taken at 27 °C. 

Relative humidity (%) Test

Treatment 30 65 90 fence

Untreated — — 6 <1 

11.3 kg m–3 — — 

Zn Cl (s)a 17 1 

22.6 kg m–3 — — 

Zn Cl (ns)b 21 1 

ZnCl + <1 1 21 2 

Na2Cr2O7 (ns) a

Seasoned (allowed to dry) after treatment. 

b

Not seasoned. Figure 5-1. SEM image showing measurements of the 

coating thickness of a galvanized fastener found in the 

study file of Baker.

Figure 5-2. Composition of the inner core (top) 

and coating (bottom) of the galvanized coating as 

determined by EDS.
General Technical Report FPL–GTR–220

44

weight loss and time between 1 and 17 years. Given this 

constant corrosion rate, 1-year corrosion rate data should be 

sufficient for extrapolation of long-term corrosion performance (Baechler 1949).

H2 Corrosion of Metal Fastenings in Zinc 

Chloride-Treated-Wood After Twenty Years 

(Baechler 1934)

Significance and Overview 

In his study, Baechler conducted the longest known study 

on the effect of chemical treatments on the corrosion of 

metal fasteners. This work is significant in two regards: 

(1) it contains the only long-term (>1 year) corrosion data 

on untreated wood and (2) it measures the corrosion rate at 

several moisture contents. More recent research has shown 

that the corrosion rate depends strongly on wood moisture 

content, although the only quantitative observations of this 

dependence used polarization resistance in solid wood. 

However, these measurements fail to take into account that 

the resistivity of solid wood changes by several orders of 

magnitude in this moisture content regime (Short and 

Figure 5-3. Photograph of the fasteners tested by Baker after 1-year exposure to 

soil. Several similar photographs were found and used to calculate the surface 

areas of fasteners tested to get a true corrosion rate.

Figure 5-4. Corrosion rates (in μm y–1) measured by 

Baker in a 27 °C, 95% RH environment. Error bars 

represent the uncertainties in the mean. 
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

45

Dennis 1997; Zelinka and others 2008a). As with the work 

of Baker, we discovered additional unpublished photographs 

in the study file. These photographs are included, as well 

as the corrosion rates based off of the weight loss measurements and surface areas calculated from the photographs. 

We present a brief description of the experimental testing 

before presenting the corrosion rate data. Because the two 

unpublished studies used the same fasteners and same methods for removing the corrosion products as this study, these 

details are highlighted in more thoroughly in this summary.

Experimental Details

Baechler measured the weight loss of three different fasteners in contact with treated and untreated ponderosa pine 

(Pinus ponderosa) exposed to four different environmental 

conditions. The treatments consisted of four different levels 

of zinc-chloride preservative and an untreated control. Two 

treatments, one with a net retention of 22.6 kg m–3 of 

dry zinc-chloride and the other with a net retention of 

11.8 kg m–3 were applied and then allowed to return to their 

original moisture content prior to the corrosion tests. The 

time it took for the specimens to return to their pretreatment 

moisture content was approximately 6 months. Two other 

treatments were applied shortly before the corrosion test and 

were not allowed to dry. The first of these treatments had 

a net retention of 24.2 kg m–3 zinc chloride and the second 

treatment was a mixture of two parts zinc chloride to one 

part sodium dichromate by weight, with a net retention of 

24.2 kg m–3 of total salt. Untreated pine was also tested. 

Three different fasteners were tested: a No. 4 common wire 

nail, a No. 6 brass screw, and a 38-mm long galvanized 

steel nail. The mass loss of the galvanized fasteners was not 

recorded, although visual observations were recorded. The 

corrosion products were removed by immersing the fasteners in a solution of 10% ammonium citrate in water. This 

procedure was found to reduce the mass of uncorroded nails 

in an ancillary experiment by 0.25% (a 0.04-g change on 

10 fasteners with an original weight of 16.16 g). We have 

accounted for extra mass loss due to cleaning in the corrosion rate calculations.

The outdoor exposure took place on a fence at Madison, 

Wisconsin. It is unclear whether the test pieces were exposed parallel or perpendicular to the ground but it was 

stated the specimens had a southern exposure. Photographs 

of the laboratory during this time frame show test fences on 

the second floor roofs, and we believe that the Baechler’s 

corrosion specimens may have been exposed on these racks. 

In addition to running outdoor exposure experiments, sets of 

similar specimens were exposed to a constant temperature 

of 27 °C, (80 °F) and a constant relative humidity of 30%, 

60%, or 90% for 20 years. The estimated equilibrium moisture contents associated with those conditions are 6%, 12%, 

and 20% (Glass and Zelinka 2010).

Corrosion Rates

A photograph of the fasteners after the exposure test 

(Fig. 5-5) was found in Baechler’s study file and used to 

calculate the surface area of the fasteners. Because no scale 

bar was included in the photograph, we assumed that the 

width of the carbon steel fastener was equal to its standardized width when calculating the surface area. The resulting 

corrosion rates for carbon steel nails are given in Figure 5-4; the corrosion rates for the brass screws were 

smaller than the uncertainties in the calculations for all 

conditions. The uncertainties in the measurement are large 

because the balance used by Baechler could only measure 

to the nearest 0.01 g. Corrosion rates for times fewer than 

10 years are not presented for the same reason. Although 

measurements were taken at several moisture contents, the 

trend in corrosion rate with moisture content cannot be inferred from the data because the corrosion rates are so low. 

The difference between the seasoned and unseasoned results 

most likely occurred because of a high initial corrosion rate 

during the first 6 months as the boards dried from a completely saturated condition. From the 20-year study, a baseline corrosion rate for untreated wood in a 20% moisture 

content environment is 6 μm/y, whereas the 6%, 12%, and 

exposed test fence indicated little or no corrosion. 

Significance and Overview

In his unpublished report, Tests on Stakes Treated With 

Copper Arsenate and Copper Chromate By the Double Diffusion Process, (filed March 13,1944), Baechler evaluated 

wood treated with copper arsenate and copper chromate. In 

addition to the corrosion tests, which will be presented in 

detail, the treated wood was also examined for decay and 

termite attack. One significant aspect of this work is that it is 

possible, in a limited sense, to evaluate the effects of chromates and arsenates separately. In dilute CCA studies, we 

observed that both the chromates and arsenates were acting 

as corrosion inhibitors, although no published studies have 

examined the role of the individual components on fastener 

corrosion (Zelinka and others 2007). 

Experimental Details

In addition to untreated controls, Southern Pine (Pinus spp.) 

was treated with one of two different preservative combinations; each combination was tested at two retentions 

(Table 5-3). The first combination contained copper sulfate 

and sodium arsenate, with a copper sulfate retention of 

5 kg m–3(light) or 11 kg m–3 (heavy) and a sodium arsenate 

concentration of 3 kg m–3 (light) 12 kg m–3 (heavy). The 

other combination contained copper sulfate with a retention of 5 kg m–3 (light) or 14 kg m–3 (heavy) and sodium 

chromate with a retention of 8 kg m–3 (light) or 25 kg m–3

(heavy).

Specimens were then placed outdoors in Saucier, Mississippi, and Madison, Wisconsin, although no data are reported 
General Technical Report FPL–GTR–220

46

from the specimens located at Madison, Wisconsin. Specimens were exposed for 2 years.

Three types of fasteners, nominally the same as those in 

the 20-year study, were tested: a No. 4 common wire nail, 

No. 6 brass screw and a 38 mm long galvanized steel nail 

(Baechler 1949). The same cleaning methods that were described earlier were used in this study. Weight loss was only 

determined for the carbon steel nails. All fasteners were 

graded qualitatively on a four point scale from “perfect 

condition” to “very rusty.” For the fasteners where weight 

loss was measured, individual measurements were not 

taken because the balance had a precision of only 0.01 g, 

and the average mass of an uncorroded steel fastener was

 ~1.5 g. Instead, to decrease the uncertainties in the mass 

loss measurements, the entire group of 5 fasteners was 

weighed before and after exposure to get an average percent 

weight loss. Using the surface areas calculated from the 20- 

year study, we have converted these to a corrosion rate. 

Results

The corrosion rate data are given in Table 5-3. Although 

none of these formulations are currently used as wood preservatives, they represent subcomponents of CCA. For this 

reason, it is interesting to look at differences between the 

preservatives with only chromates and those with only arsenates to better understand the corrosiveness of CCA-treated 

wood. For the treatments with the light copper retention, the 

corrosion rate of the treatment with chromate was roughly 

the same as that of the untreated wood, whereas the corrosion rate was 4 times larger with the arsenates. Similar 

trends were also seen in the data with a higher amount of 

copper. From this data it appears that the chromates are primarily responsible for mitigating the corrosiveness of CCA. 

Previously, it had been speculated that arsenates my also 

play a role in inhibition but these data suggest that at best it 

is secondary to the effect of chromates.

Significance and Overview

In their study, The Corrosion of Wire Nails in Wood Treated 

With Copper Sulphate, Bateman and Baechler (1938)

evaluated an experimental method for wood preservation 

Table 5-3. Corrosion rates for stakes 

treated with copper arsenate and 

copper chromate; exact treatment 

details are given in the text

Cu metal 

concentration 

(kg m–3) 

Corrosion 

rate 

(μm y–1) 

Untreated — 5 

Cu+Cr “light” 2.0 5 

Cu+Cr “heavy” 4.4 9 

Cu+As “light” 2.0 20 

Cu+As “heavy” 5.6 20 

Figure 5.5. Photographs of fasteners tested by Baechler (1949) after 20 years of 

exposure. The "Untreated Control" fasteners at 30% RH were used to calculate 

the surface area of the fasteners.
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

47

where the preservatives are injected into a living tree. Corrosion tests were performed on shortleaf pine (Pinus echinata) 

specimens that had been injected with copper sulfate prior to 

felling. Because of the unusual treating method, each group 

of fasteners was exposed to a different amount of copper. 

The actual copper concentration in the wood near each 

group of fasteners was measured in a separate experiment. 

No untreated controls were included. 

Experimental Details

Trees were injected with copper sulphate and then felled. 

Sections along the length of tree were then cut. The nails 

were driven directly into these sections around the circumference. No. 4 common wire nails, nominally the same as 

those used in previous studies, were used in this study and 

the corrosion products were removed with the same methods described previously (Baechler 1949). The wood–metal 

assemblies were exposed to one of two constant temperature, constant humidity conditions: 27 °C, 30% RH or 

27 °C, 75% RH and removed at intervals between 3 and 

24 months. It is not clear from the study documents whether 

the wood was conditioned prior to the test. However, data 

appear to indicate that the wood must have been green 

when the fasteners were driven; the corrosion rate decreases 

with exposure time, and even fasteners exposed to the 30% 

RH condition have a high corrosion rate (10–47 μm yr–1), 

contrasted with the negligible corrosion rates found for the 

same conditions in Table 5-2. The report includes the actual 

copper retentions in each section of wood, but the method 

used to measure the retention is not given. 

Experimental Results

Selected data from the 75% RH condition are shown in 

Figure 5-6. Two major trends can be seen in the data. The 

same trends appear in the remainder of the data, which were 

removed from the figure for clarity. The first trend is that the 

corrosion rate increases with increasing copper concentration. For example, between the extreme points on the curve, 

the copper concentration changes by a factor of ~12 and the 

corrosion rate changes by a factor of ~5. This ratio remains 

constant over all exposure times. The second trend is that 

the corrosion rate decreases with time. Two possibilities can 

explain this decrease. The first is that the nails were driven 

into green wood and the specimen was drying during the 

exposure period. An alternate explanation is that protective 

corrosion products were formed during this period that reduced the corrosion rate. However, because corrosion rates 

are so high for this environment, we believe the former 

explanation is the correct one.

Part II: Effect of Corrosion on the 

Lateral Joint Strength

The single-fastener connection performance is dependent on 

the joint geometry (thickness of main and side members), 

fastener diameter, dowel bending-yield strength, dowelbearing strength, and direction of load to the grain. Yield 

expressions relating these parameters were developed by 

Johansen using a static analysis that assumes the wood and 

the metal dowel fastener are both perfectly plastic (Johansen 1949). After nearly a decade of development, the yield 

model became the standard for dowel connection design in 

the 1991 NDS and is applicable to all types of dowel fasteners-nails, lag screws, and bolts (Aune and Patton-Mallory 

1986; McLain 1992; McLain and Thangjitham 1983; Soltis 

and Wilkinson 1987; Soltis and others 1986; Wilkinson 

1993). The yield model theory selects the worst case of yield 

modes based on different possibilities of wood bearing and 

nail bending. Mode I is a wood-bearing failure in either the 

main or side member; mode II is a rotation of the fastener in 

the joint without bending; modes III and IV are a combination of wood-bearing failure and one or more plastic hinge 

yield formations in the fastener. For a two member nailed 

joint, the lateral design load Z (N) of a joint is determined 

by the minimum of the following yield expressions:

 

 

where 

 

Figure 5-6. Corrosion rate of carbon steel nails as a 

function of copper concentration at 6 and 24 months 

of exposure at 27 °C, 75% RH.
General Technical Report FPL–GTR–220

48

 

 

 

 

where D is the dowel diameter (mm), Fe_ is the dowel bearing stress of the main (m) or side (s) member (MPa), Fyb is 

the bending yield stress of the nail (MPa), lp is the length of 

penetration in to main member, and ts is the thickness of the 

side member. For determining the effect of corrosion on the 

lateral fastener strength two assumptions are made. The first 

is that corrosion occurs uniformly along the length of the 

fastener and the corrosion rate can be treated as a constant 

reduction in diameter with time. The second assumption 

deals with the effect of moisture content on the wood dowel 

bearing strength. We assume that the wood moisture content 

at failure is greater than 19% MC; in this regime, the dowel 

bearing strength does not depend on moisture content (Rammer 2001). This is reasonable as it is likely that the moisture 

content is greater than 19% if corrosion is occurring. 

Figure 5-7 illustrates the effect of corrosion on the lateral 

strength of single shear nailed joint for a hot-dip galvanized 

8d nail (3.05 mm) for different wood preservatives using 

historical CCA data from Baker (Fig. 5-4) and recent data 

collected by Zelinka and Rammer (2009) on ACQ-treated 

wood (retention of 4 kg m–3). The below analysis assumes a 

constant corrosion rate with time, even after the galvanized 

coating has been consumed. Future work could include a 

change in corrosion rate once the reduction in diameter exceeds the coating thickness. The reduction in capacity 

depends upon the side member thickness, which, along 

with the diameter, determines the failure mode. 

For the narrow side member, the reduction is directly related 

to the reduction of nail diameter due to corrosion. As the 

side member thickness increases, the reduced diameter will 

affect both the bearing and nail bending performance. For 

large enough side member thicknesses, the joints will fail 

in Mode IV, which is independent of the wood thicknesses. 

Because most of the work on corrosion in wood has examined decking nails, and the most common decking material 

in the United States of America is “5/4 radial deck board” 

(25 mm thick), it can be expected that decking nails will 

exhibit a Mode IV failure, in which case a simpler diagram 

can be made (Fig. 5-8). In this failure mode, the capacity of 

the connection is proportional to the square of the fastener 

diameter. 

Figure 5-7. Percentage reduction of capacity as a function of time for an 8d (3.05 mm) fastener 

embedded in wood with an assumed corrosion rate of 10 (A) or 60 (B) μm y–1, which correspond 

to corrosion rates measured in hot-dip galvanized steel at 27 °C, 100% RH for CCA- and ACQtreated wood, respectively (Baker 1992, Zelinka and Rammer 2009). Each contour represents a 

10% loss in capacity; note the difference in the time axes. 

Figure 5-8. Relative capacity of an 8d decking nail exhibiting a mode IV failure. The 10 and 60 μm y–1 corrosion 

rates correspond to corrosion rates measured for hotdip galvanized fasteners at 27 °C, 100% RH in CCA- and 

ACQ-treated wood, respectively (Baker 1992; Zelinka 

and Rammer 2009).
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

49

In all cases, corrosion rates from laboratory experiments 

were used to calculate the reduction in capacity with time. 

Wood exposed in outdoor conditions will most likely experience fluctuations in moisture content, and may have higher 

or lower corrosion rates than those used in these calculations. Recent changes to ACQ formulations and treatment 

retentions may have also lessened the corrosiveness to 

treated wood.

Summary and Conclusions

This paper reevaluated two classic reports on the corrosion 

of metals in wood and presented the data as a corrosion rate. 

Two smaller, unpublished reports were also summarized 

that give insight into role of the components which comprise CCA. One of these reports highlighted the effect of 

copper concentration on the corrosion rate of carbon steel. 

The other report showed that the chromate in CCA acts as a 

corrosion inhibitor, whereas the arsenate has a smaller if any 

effect. 

From these uniform, constant corrosion rates, a reduction in 

diameter with time was calculated, and this in turn was used 

in the yield theory equations to calculate the reduction in 

lateral joint strength. For decking materials, the fasteners are 

expected to exhibit a Mode IV failure where the reduction 

in joint capacity is related to the square of the diameter. This 

dependence results in the capacity being quite sensitive to 

the corrosion rate.

References

Anon 2003. International Building Code. Florence, KY: 

Delmar Cengage Learning.

Anon 2008. Preservative treated wood. Pleasanton, CA: 

Simpson Strong-Tie Technical Bulletin.

Aune, P. Patton-Mallory, M. 1986. Lateral load-bearing capacity of nailed joints based on the yield theory: Theoretical 

development. Res. Pap. FPL–469. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.

AWPA 2007. AWPA P5: Standard for waterborne 

preservatives. Granbury, TX: American Wood Preservers’ 

Association.

Baechler, R.H. 1944. Tests on stakes treated with copper arsenate and copper chromate by the double diffusion process: 

inspection of stakes and metallic fastenings after two years’ 

exposure. Unpublished report filed March 13, 1944.

Baechler, R.H. 1949. Corrosion of metal fastenings in zincchloride treated wood after 20 years. In: Proceedings of the 

Annual Meeting of the American Wood Preservers’ Association (Granbury, TX: AWPA, 1949), St. Louis, MO: American Wood Preservers’ Association: 381–397.

Bateman, E.; Baechler, R.H. 1938. Corrosion of wire nails 

in wood treated with copper sulphate. Unpublished report 

filed September 28, 1938.

Baker, A.J. 1992. Corrosion of nails in CCA- and ACAtreated wood in two environments. Forest Products Journal. 

42: 39–41.

Burkholder, M. 2004. CCA, NFBA, and the post-frame 

building industry. Frame Building News. 16: 6–12.

Duncan, J.R. 1988. Corrosion of metal fasteners in timber. 

Construction Building Materials. 2: 194–200.

Glass, S.V.; Zelinka, S.L. 2010. Moisture relations and 

physical properties of wood. In: Ross, R.J., ed. Wood handbook: wood as an engineering material. Centennial Edition. 

Gen. Tech. Rp. FPL–GTR–190. Madison, WI: U.S. Department of Agriculture: 4-1–4-19.

Johansen, K.W. 1949. Theory of timber connections, Publication 9. Bern, Switzerland: International Association of 

Bridge and Structural Engineering: 249–262.

Kear, G.; Wú, H.-Z.; Jones, M.S. 2009. Weight loss studies of fastener materials corrosion in contact with timbers 

treated with copper azole and alkaline copper quaternary 

compounds. Corrosion Science. 51: 252–262.

Kear, G.; Wú, H.-Z.; Jones, M.S.; Walsh, F. 2008a. Impedance spectroscopy studies of the dissolution of ferrous- and 

zinc-based materials in aqueous timber preservatives. Journal of Applied Electrochemistry. 38: 1599–1607.

Kear, G.; Wú, H.Z.; Jones, M.S. 2008b. Corrosion of ferrous- and zinc-based materials in CCA, ACQ, and CuAz 

timber preservative solutions. Materials and Structures. 

41: 1405–1417.

McLain, T.E. 1992. Strength of lag-screw connections. 

ASCE Journal of Structural Engineering. 118: 2855–2871.

McLain, T.E., Thangjitham, S. 1983. Bolted connection 

design. ASCE Journal of Structural Engineering. 

109: 820–835.

Rammer, D.R. 2001. Effect of moisture content on nail 

bearing strength. Res. Pap. FPL 591. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory. 22 p.

Rammer, D.R.; Zelinka, S.L. 2008. Analytical determination 

of the surface area of a threaded fastener. ASTM Journal of 

Testing and Evaluation. 36: 80–88.

Rammer, D.R.; Zelinka, S.L. 2010. Optical method for measuring the surface area of a threaded fastener. Experimental 

Techniques. 34: 36–39.

Short, N.R.; Dennis, J.K. 1997. Corrosion resistance of zincalloy coated steel in construction industry environments. 

Transactions of the Institute of Metal Finishing. 75: 47–52.

Soltis, L.A.; Hubbard, F.K.; Wilkinson, T.L. 1986. Bearing 

strength of bolted timber joints. ASCE Journal of Structural 

Engineering. 112: 2141–2154.
General Technical Report FPL–GTR–220

50

Soltis, L.A., Wilkinson, T.L. 1987. Bolted connection design. Gen. Tech. Rep. FPL–GTR–54. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products 

Laboratory.

Wilkinson, T.L. 1993. Bolted connection design values 

based on European yield model. ASCE Journal of Structural 

Engineering. 119: 2169–2186.

Zelinka, S.; Glass, S.; Stone, D. 2008a. A percolation model 

for electrical conduction in wood with implications for 

wood-water relations. Wood and Fiber Science. 

40: 544–552.

Zelinka, S.L.; Rammer, D.R. 2009. Corrosion rates of fasteners in treated wood exposed to 100% relative humidity. 

ASCE Journal of Materials in Civil Engineering. 

21: 758–763.

Zelinka, S.L.; Rammer, D.R.; Stone, D.S. 2008b. Electrochemical corrosion testing of fasteners in wood extract. 

Corrosion Science. 50: 1251–1257.

Zelinka, S.L.; Rammer, D.R.; Stone, D.S. 2009. In: Proceedings, Evaluating the corrosiveness of southern pine treated 

with several wood preservatives using electrochemical 

techniques. CORROSION 2009. Atlanta, GA: NACE 

International.

Zelinka, S.L.; Rammer, D.R.; Stone, D.S.; Gilbertson, J.T. 

2007. Direct current testing to measure corrosiveness of 

wood preservatives. Corrosion Science. 49: 1673–1685.
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

51

Chapter 6

Modeling the Effect of Nail Corrosion on the 

Lateral Strength of Joints

Samuel L. Zelinka, Materials Research Engineer

Forest Products Laboratory, Madison, Wisconsin 

Douglas R. Rammer, Research General Engineer

Forest Products Laboratory, Madison, Wisconsin

just as important as the amount of metal that has corroded. 

Nguyen and others (2011) have highlighted the importance 

of incorporating corrosion into a reliability based timber 

engineering design code and proposed a preliminary model 

to do so based upon time of wetness for untreated and CCAtreated wood. 

This article extends previous work on the degradation of 

wood–metal connections by examining how metal corrosion 

leads to a loss of structural capacity of a laterally loaded 

nailed joint. The reduction in lateral capacity is calculated 

from yield theory equations given in the National Design 

Specifications (NDS) for Wood Construction (American 

Wood Council 2012). Specific examples are shown comparing the reduction in lateral capacity as a function of time for 

corrosion rates measured for untreated, alkaline copper quaternary (ACQ)-treated and CCA-treated wood. 

Method for Calculating the Loss 

of Lateral Capacity

Yield Theory Equations

Single-fastener connection performance is dependent on 

joint geometry, fastener diameter, dowel bending-yield 

strength, dowel-bearing strength, and direction of load to 

the grain. Yield expressions relating these parameters were 

developed by Johansen (1949) using a static analysis that 

assumes the wood and the metal dowel fastener are both 

perfectly plastic. After nearly a decade of development, the 

yield model became the standard for dowel connection design in the 1991 NDS and is applicable to all types of dowel 

fasteners—nails, lag screws, and bolts (Aune and PattonMallory 1986; McLain 1992; McLain and Thangjitham 

1983; Soltis and others 1986; Soltis and Wilkinson 1987; 

Wilkinson 1993). The yield theory model selects the worst 

case of yield modes based on different possibilities of wood 

bearing and nail bending. Mode I is a wood-bearing failure 

in either the main or side member, Mode II is a rotation of 

the fastener in the joint without bending, and Modes III and 

IV are a combination of wood-bearing failure and one or 

more plastic hinge yield formations in the fastener. In addition to yield modes, we have included the transverse shear 

Abstract

This article describes a theoretical method of linking fastener corrosion in wood connections to potential reduction 

in lateral shear strength. It builds upon published quantitative data of corrosion rates of metals in contact with treated 

wood for several different wood preservatives. These corrosion rates are then combined with yield theory equations to 

calculate a loss in lateral capacity as a function of time. The 

calculations are straightforward and can be performed in a 

spreadsheet or simple computer program. They can accommodate time-dependent and moisture-dependent corrosion 

rates. The latter of these capabilities can easily be recognized as important inasmuch as corrosion rates of fasteners are recognized as being dependent on moisture content 

of the wood in which they are embedded. The calculation 

method is dependent on corrosion rates, and the method is 

therefore limited by the lack of agreement in corrosion rates 

presented in the literature. Within these limitations, the article examines how different corrosion rates and changes in 

corrosion rates affect the mechanical properties and service 

life of nailed wood joints. 

Keywords: Corrosion, metal, wood, yield theory, 

preservative-treated wood

Introduction

Since the voluntary withdrawal of chromated copper arsenate (CCA) as a wood preservative for residential construction in January 2004, the corrosion of metals in contact with 

preservative-treated wood has been a research emphasis. In 

addition to numerous articles on the mechanism of corrosion 

in treated wood (e.g., Kear and others 2009; Zelinka and 

others 2010; Zelinka and Stone 2010, 2011), a standard has 

also been developed to measure the corrosion rates of metals 

embedded in treated wood (ASTM 2011). The emphasis of 

corrosion research has been on quantifying the metal corrosion rate in units of depth of metal wastage per unit of time. 

The previous work represents a significant improvement in 

the understanding of how metals in wood corrode. However, because metals in wood are used to fasten two wooden 

members together, the loss in joint strength with time is 
General Technical Report FPL–GTR–220

52

of the fastener as a possible failure mode. For a two-member 

nailed joint, the lateral design load Z(N) of a joint is determined by the minimum of the following yield expressions 

because a Mode II failure is not expected for smaller 

diameter fasteners such as nails:

(6.1)

where 

(6.2)

and where D is the dowel diameter (mm), Fe_ is the dowel 

bearing stress of the main (m) or side (s) member (MPa), Fyb

is the bending yield stress of the nail (MPa), lp is the length 

of penetration into the main member, and Ts is the thickness 

of the side member. For a shear failure, the yield shear stress 

is determined from Fyb because pure shear occurs on 45° 

planes and Fyb is representative of the material tensile yield 

stress for a fully plastic section.

In the following examples illustrating the effect of corrosion on the lateral fastener strength, several assumptions 

were made. The first assumption is that the corrosion only 

reduces the section modulus of the nail, not Fyb or the elastic modulus of the fastener. Practically, this assumption is 

implemented also assuming uniform corrosion over the 

entire nail and using the corrosion rate as a constant reduction in diameter with time. The other assumptions deal with 

the dowel-bearing strength. It is assumed that the corrosion 

byproducts do not affect Fe and that at failure, the wood 

moisture content is greater than 19%; in this regime, the 

NDS treats dowel-bearing strength as independent of moisture content (Rammer 2001). This is reasonable because it is 

likely that the moisture content is greater than 19% if corrosion is occurring (Baker 1988; Short and Dennis 1997). 

For a given joint geometry, wood species, and fastenerbending yield stress, the equations depend only on the 

fastener diameter, which is a function of the corrosion rate. 

Corrosion rates for metals in wood will be reviewed in the 

following section. These corrosion rates will then be used 

throughout the article to examine the relative lateral capacity of nailed joints in many different scenarios.

Corrosion Rates of Metals 

in Contact with Treated Wood

Since the voluntary withdrawal of CCA for use in residential 

construction, many experiments have measured the corrosion of metals in ACQ- and CCA-treated wood (Freeman 

and McIntyre 2008; Kear and others 2009; Zelinka and 

Rammer 2009; Zelinka and others 2008). However, there is 

little agreement between these measurements; the measured 

corrosion rates from these tests have a wide range of 

values, for example, from 2 to 113 μm y–1 and from 1 to 

26 μm y–1 for hot-dip galvanized steel in ACQ- and 

CCA-treated wood, respectively. These wide differences in 

measured corrosion rates most likely result from experimental differences in temperature, relative humidity, and wood 

moisture content. The emphasis of this article is on the 

framework for analyzing how the relative lateral capacity 

depends upon corrosion rate; therefore, only a few representative corrosion rates will be used in the ensuing analysis. 

The representative condition chosen for further analysis is 

27 °C, 100% RH, which Baechler (1949) chose to simulate 

outdoor exposures. The condition is chosen because data 

exist under these conditions for hot-dip galvanized steel in 

untreated and CCA- treated and ACQ-treated wood and it 

is one of the most tested environments for wood–metal corrosion (Baechler 1949; Baker 1992; Kear and others 2009; 

Zelinka and Rammer 2009; Zelinka and others 2010).

The corrosion rate for CCA-treated wood comes from a 

study conducted by Baker (1992), which reported the percent weight loss for “CCA-I” and “CCA-II”, which were essentially equivalent to modern CCA-A and CCA-B. Zelinka 

and Rammer (2009) later converted the percent weight loss 

to a true corrosion rate. Baker’s study involved 11 different 

types of metal fasteners embedded in CCA-treated and ammoniacal copper arsenate (ACA)-treated wood at 27 °C and 

97% to 100% RH for periods of 1, 3, and 14 years. Baker 

found a linear relationship between percent weight loss and 

time, which implies a constant corrosion rate assuming a 

constant surface area. The corrosion rate for CCA-I, which 

was found constant between 1 and 14 years, is used in the 

remainder of this article (Table 6-1).

The untreated and ACQ-treated wood corrosion rates come 

from 1-year exposure tests at 27 °C and 100% RH (Zelinka 

and Rammer 2009; Zelinka and others 2010). These corrosion rates are assumed to be constant with time, as was 

observed by Baker in CCA-I-treated wood. This assumption 
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

53

and its implications are further examined in the Section 

“Examination of a time-dependent corrosion rate.” 

The corrosion rates, treatments, and original source for the 

data are summarized in Table 6-1 and will be referred to 

throughout the paper. 

Effect of Corrosion on the Lateral 

Joint Strength

To illustrate how the yield theory equations can be used to 

calculate the reduction in lateral joint strength, we examine 

the corrosion of an 8d common nail (length, 63 mm; 

D = 3.4 mm) in treated Southern Pine (Pinus spp.; Fyb = 

620 MPa, Fem = Fes = 38 MPa). We first illustrate how these 

equations can be used to predict the loss in lateral strength 

with time for all yield theory modes. We then focus on 

Mode IV yielding and show the percentage of the original 

capacity as a function of time. If an arbitrary failure criterion (e.g., 50% of original capacity) were defined, the time 

to failure could be determined with these data. To illustrate 

the flexibility and robustness of this methodology, three 

additional cases are examined. In the first case, a coated 

fastener is examined. The final two cases examine what happens when the corrosion rate varies with time and moisture 

content.

Examination of All Failure Modes

Figure 6-1 presents the reduction in capacity of the lateral 

strength of single shear nailed joint for corrosion rates of 

10 μm y–1 (Fig. 6-1a) and 60 μm y–1 (Fig. 6-1b). These corrosion rates are roughly equivalent to those measured in 

exposure tests for hot-dip zinc galvanized fasteners in CCAand ACQ-treated wood, respectively (Table 6-1). Each line 

on the graph represents a 10% reduction in capacity, and 

the second order discontinuities are a result of a change in 

failure mode. For a narrow side member that exhibits Mode 

I behavior, the reduction is directly related to the reduction 

of nail diameter because of corrosion. As the side member 

thickness increases, the reduced diameter will affect both 

the bearing and nail bending performance. For large enough 

side member thicknesses, the joints will fail in Mode IV, 

which is independent of the wood thicknesses. Transverse 

Table 6-1. Corrosion rates used in this study and pertinent details from the original study

Corrosion rate 

(μm y1

) Reference Treatment Metal Experimental details 

5 Zelinka and others 2010 Untreated Hot-dip 

galvanized steel 

1-y exposure in a desiccator 

maintained at 27 °C, 100% RH 

10 Baker 1992; Zelinka and 

Rammer 2009 

CCA-Ia

9.6 kg m–3

Hot-dip

galvanized steel 

1, 3, and 14 y in an enclosure 

at 27 °C with a humidifier to 

maintain 97–100% RH 

60 Zelinka and Rammer 2009 ACQ-D 

4 kg m–3

Hot-dip

galvanized steel 

1-y exposure in a desiccator 

maintained at 27 °C, 100% RH 

a

CCA-I was a formulation of CCA listed in an outdated classification system. CCA-I was essentially equivalent to CCAA (“salt” formulation of CCA). CCA-A imparted greater conductivity to wood treated with it than does CCA-C (Richards 

1990). CCA-C is the only CCA formulation currently listed in AWPA standards. CCA-C has retained EPA listing as a 

restricted use preservative (Lebow 2010). 

Figure 6-1. Percentage reduction of capacity as a function of time for an 8d common (3.4 mm) fastener embedded in wood with an assumed corrosion rate of 10 (A) or 60 (B) μm y–1, which corresponds 

to corrosion rates measured for hot-dip galvanized steel at 27 °C, 100% RH.
General Technical Report FPL–GTR–220

54

fastener shear is never the controlling failure mechanism. 

Importantly, joints governed by Mode IV have the fastest 

reduction in capacity with time. Because most of the work 

on corrosion in wood has examined decking nails, and the 

most common decking material in the United States is “5/4 

radial deck board” (25 mm thick), we can expect that decking nails will most likely exhibit a Mode IV yielding in 

service. Because of its importance, and because it represents 

the quickest time to complete strength loss, only Mode IV 

failures will be considered in the next four cases. 

Examination of Mode IV Yield Mode

For Mode IV failures, which are expected in decking applications, the reduction in lateral capacity depends only upon 

the corrosion rate and the initial diameter of the fastener. 

Figure 6-2 illustrates the reduction in lateral capacity for an 

8d nail in a Mode IV failure with several different corrosion rates. The 5 μm y–1 corrosion rate corresponds to galvanized steel in untreated wood, and the 10 and 60 μm y–1

corrosion rates have been described previously (Table 6-1). 

For Mode IV failures, the yield strength is inversely related 

to the square of the diameter, which results in the lateral 

capacity decreasing rapidly with increasing corrosion rate. 

By arranging the terms, it is possible to express the time to a 

given percentage of the original capacity as a function of the 

corrosion rate, r, and the original diameter, do. For instance, 

the time to 50% of the original capacity, t50, can be given by 

the following:

 

(6.3)

This expression is shown graphically for an 8d fastener in 

the inset of Figure 6-2.

Examination of a Galvanized 

Fastener

For galvanized and other metal-coated fasteners, corrosion 

rate in the initial stages, when the coating is continuous and 

completely intact, is equivalent to the corrosion rate of the 

coating metal by itself. Once the coating has completely 

corroded away, corrosion proceeds at the rate of the base 

metal. The measured corrosion rates (Table 6-1) are high 

enough that a typical hot-dip galvanized fastener (with a 

coating on the order of 100 μm) would last only a few years 

before the base metal was completely exposed. Therefore, it 

is instructive to examine how the above calculations change 

for coated fasteners, where the corrosion rate experiences a 

change when the coating has disappeared, that is,

 

(6.4)

where tcrit is the time at which the galvanized coating has 

completely corroded away, given by

 

(6.5)

where Tcoat is the thickness of the coating. Although Equation (6.4) is generally valid, literature values on the ratio of 

corrosion rates of galvanized steel to plain steel corrosion 

rates in treated wood vary greatly. Kear and others (2009) 

and Zelinka and others (2010) both measured the corrosion 

of metals embedded in wood for 1 year in an environmental 

chamber at 100% RH. Surprisingly, Kear and others observed that the corrosion rate of galvanized steel was less 

than that of plain steel, whereas Zelinka observed that the 

corrosion rate of galvanized steel was greater than that of 

plain steel. 

Given this uncertainty in relative corrosion rates, it is possible that the corrosion rate of a galvanized fastener could 

increase or decrease once the coating has completely corroded. In either case, the relative lateral capacity as a function of time will depend upon the corrosion rates of both 

the coating and base metal. The total loss in capacity can 

be determined by Equations (6.4) and (6.1). Figure 6-3 illustrates the importance of accounting for the corrosion rate 

of the coating for an 8d nail exhibiting a Mode IV failure, 

where the galvanized coating is 100 μm thick and has a corrosion rate of 10 μm y–1 (CCA-I, Table 6-1). Along with the 

“baseline case” where the entire thickness of the fastener 

has the same corrosion rate, two different scenarios are plotted. In the first case, the ratio of the steel corrosion rate to 

the galvanized corrosion rate is assumed to be 2.5, as found 

by Kear and others (2009); in the second, it is assumed to be 

0.5, as found by Zelinka and others (2010). It is clear from 

Figure 6-3 that the corrosion rate of the base metal plays an 

Figure 6-2. Reduction in capacity of an 8d decking nail 

exhibiting a Mode IV failure. Inset is the number of years 

until a 50% reduction in capacity is reached as a function of corrosion rate.
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

55

important role in the reduction in capacity of the structure 

with time because the coating can be completely corroded 

so quickly. 

Examination of a Time-Dependent 

Corrosion Rate

The above analysis assumes that the corrosion rate is constant with time, which is consistent with observations by 

Baker (1992) for galvanized fasteners embedded in treated 

wood for times ranging between 1 and 14 years. This is 

consistent with literature observations of the corrosion rate 

of galvanized steel exposed to atmospheric corrosion; Legault and Pearson (1978) reviewed the corrosion kinetics 

of galvanized steel and found that in most industrial and 

marine environments, galvanized steel exhibited a constant 

corrosion rate because the corrosion products dissolve in 

low-pH water/rainwater. Because it is well known that wood 

is acidic, it is not surprising that galvanized steel exhibits a 

constant corrosion rate in wood. However, for most other 

metals and for galvanized steel exposed to less acidic conditions, the corrosion rate usually decreases with time (Zhang 

2003).

Many empirical sources have observed that the corrosion 

kinetics for atmospheric corrosion can be described by

 

(6.6)

where ∆W is the change in weight, K is a constant (the 

1-year corrosion rate), t is the time in years, and n is an exponent that controls the kinetics and describes the resistance 

for ion transport to the metal surface through the corrosion 

product once it has formed. The limiting case, n = 1, observed by Baker (1992), corresponds with a constant corrosion rate with time and physically represents a case where 

the corrosion products do not remain on the surface, or they 

remain but do not decrease the rate of corrosion. In theory, 

n = 0.5 should represent the lower bound, where the reaction is limited by the diffusion of ions to the metal surface. 

However, values of n as low as 0.36 have been observed in 

carbon steels exposed to atmospheric conditions (Legault 

and Preban 1975). 

Although the data suggest that metals in wood exhibits 

a constant corrosion rate with time (i.e., n = 1), only one 

article examined this behavior (Baker 1992). It is possible 

that the corrosion rate is not constant, but rather changes 

with time. If the commonly accepted atmospheric corrosion 

kinetic model holds for wood, the n = 1 case represents a 

worst-case scenario. It is possible but physically unlikely 

that the corrosion of metals in wood increases with time. 

In this case, Equation (6.6) would no longer be applicable 

and a different kinetic model would need to be developed.

By combining Equation (6.6) with the yield theory equation 

(Eq. (6.1)), it is possible to examine how a decreasing 

corrosion rate would affect the relative lateral capacity 

of nailed joints. Fig. 6-4 examines the same case as 

Fig. 6-2 with a time-dependent corrosion rate (i.e., the 

60 μm y–1 curve in Fig. 6-4 is equivalent to the n = 1 case in 

Fig. 5-4). The values of n were selected to be representative 

of those observed for different classes of materials as observed by Legault and Preban (1975), and the corrosion rate 

as a function of time is shown as an inset. Any change in n 

represents not only a decrease in the corrosion rate but also 

a deceleration with time and therefore has a large effect on 

the relative capacity with time.

Figure 6-3. Relative capacity as a function of time for an 

8d common galvanized fastener corroding at 10 microns 

per year and exhibiting a mode IV failure. Different lines 

represent different relative steel-galvanized steel corrosion rate ratios.

Figure 6-4. Relative capacity as a function of time for a 

Mode IV failure where the corrosion rate decreases with 

time according to Equation (6-6). The inset shows how 

the corrosion rate changes with time for value of n.
General Technical Report FPL–GTR–220

56

Examination of a MoistureDependent Corrosion Rate

The wood moisture content is the most important environmental variable to the corrosion of metals embedded 

in wood. Below 15% to 18% moisture content, embedded metals do not corrode (Baker 1987; Short and Dennis 

1997). Above this threshold, the corrosion rate increases 

with increasing moisture content before eventually reaching 

a plateau (Kear and others 2009; Short and Dennis 1997). 

Because of this sensitivity, most laboratory corrosion rate 

measurements have been under conditions where the wood 

moisture content is either constant or well characterized 

(e.g., Anon 2007). However, in actuality, wood exposed to 

the environment can experience large fluctuations in moisture content (Cui and Zahora 2000; Gaby and Duff 1978; 

Miller and Boxall 1987). Because of these large fluctuations 

in moisture content and the resulting fluctuations in corrosion rate, a steady-state approximation of the corrosion rate 

can be misleading.

Recently, Zelinka and others (2011) developed a combined 

hygrothermal/corrosion model to predict the cumulative 

corrosion at various points along the length of an embedded 

fastener as a function of time. The process consists of two 

steps; the first step is to calculate the wood moisture content 

profile from hourly climatic data using a hygrothermal model. The second step is to use the moisture content calculated 

from the hygrothermal model to calculate the metal corrosion rate, which is assumed to be constant over the hour. 

The hourly amounts of corrosion are summed, resulting in a 

final output of the depth of the corrosion attack (in micrometers) along the entire length of the fastener. The hygrothermal model uses an existing, validated two-dimensional finite 

element code to solve the coupled heat and mass transport 

equations using hourly climatic data as an input (Janssen 

and others 2007). It predicts moisture content and temperature at various depths within the wood. In the subsequent 

calculation of corrosion rate, two simplifying assumptions 

are made: corrosion ceases at temperatures below freezing, 

regardless of moisture content, and at temperatures above 

freezing, corrosion rate is solely dependent on moisture 

content. 

The moisture dependence of the corrosion rate was incorporated by fitting the empirical polarization resistance data of 

Short and Dennis (1997) with 

(6-7)

where A represents the maximum corrosion rate (μm y–1), B

represents the steepness of the transition from 0 to A, C represents the moisture content at which R = A/2, and changing 

C changes the corrosion threshold moisture content. The parameters B (0.83) and C (24) are taken from a fit of the published data of Short and Dennis (1997). The parameter A, 

which is physically the asymptotic corrosion limit, is taken 

as 52.3 μm y–1 from recent electrochemical measurements 

of hot-dip galvanized steel in an extract of ACQ-treated 

Southern Pine (Zelinka and others 2008).

The result of these simulations is the corrosion profile—the 

depth of corrosion penetration into the nail shank as a function of position along the length of the nail and of time. 

Results from a 1-year simulation for a specific reference 

year in Baltimore, Maryland, are included in Figure 6-5. 

The reference year was 1989. This year was chosen on the 

basis of being the 10th percentile year in terms of number of 

hours of precipitation from 1960 to 1990. For further discussion of the choice of references years see Zelinka and others 

(2011). As Figure 6-5 shows, the maximum amount of corrosion occurs 0.18 cm below the surface, and the corrosion 

profile changes throughout the year. This is consistent with 

empirical observations of corroded fasteners (Kubler 1992). 

The combined hygrothermal/corrosion model illustrates 

how fluctuations in moisture content affect the corrosion of 

embedded fasteners, and the model can be extended using 

Equation (6-1) to predict the loss in capacity as a function 

of time. 

To illustrate the importance of moisture fluctuations on 

the corrosion and the resulting loss in lateral capacity, 

Figure 6-6 was created based upon the 1-year simulation 

for Baltimore (Fig. 6-5). The analysis uses the same 

assumptions, but the corrosion rate is calculated from 

Equation (6-5). Although the moisture content (and therefore the corrosion rate) depends upon the depth below 

the surface (Fig. 6-5), the analysis assumes a worst case 

scenario and uses the moisture content at 0.18 cm. Two 

scenarios are presented in the figure. The first assumes a 

constant corrosion rate based upon the yearly mean moisture 

content (33%), and the second uses a different corrosion rate 

for every hourly output of the simulation. The two analyses 

Figure 6-5. Output (corrosion profile as a function 

of time) from the hygrothermal model, based upon 

climatic data from Baltimore, Maryland, for calendar 

year 1989.
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

57

give quite different results. This is most likely a result of the 

highly non-linear shape of Equation (6-5), which is nearly 

a Heaviside function. Because the yearly average moisture 

content was greater than C, the assumed corrosion rate was 

higher than the average of the hourly corrosion rate data. Although the average annual moisture content overestimated 

the amount of predicted corrosion, it should give a conservative bound for design in most cases.

Summary and Conclusions

Although the corrosion rates of metals embedded in treated 

wood are small in absolute terms, this work has shown 

that fastener corrosion can have a large effect on the lateral 

strength of nailed joints in outdoor structures. Although the 

major aim of the article was to present a methodology for 

practicing engineers to make informed design decisions for 

specific wood-metal combinations, several interesting conclusions can be drawn from the scenarios examined in this 

article:

•	 In most decking applications, a Mode IV yielding of 

the fastener is expected. In this case the lateral capacity 

is proportional to the square of the fastener diameter, 

resulting in the most rapid time to failure (Fig. 6-1). 

If “failure” is defined as an arbitrary percentage of the 

original capacity, the time to failure is proportional to 

the reciprocal of the corrosion rate (Fig. 6-2).

•	 For galvanized fasteners, the measured corrosion rates 

suggest that the coating will be completely corroded 

within a few years. If the base metal has a different 

corrosion rate than the coating, this results in a large 

change in the reduction of capacity as a function of 

time (Fig. 6-3).

•	 Only a few measurements have examined the corrosion rate of zinc in treated wood as a function of time. 

The results from the most comprehensive study (Baker 

1992) are consistent with a constant, time-invariant corrosion rate. If the corrosion rate decreased with time, 

this would drastically increase the service life of fasteners (Fig. 6-4). If the kinetics are indeed consistent with 

Equation (6-6), a constant corrosion rate assumption 

gives a conservative service life estimation.

•	 Hygrothermal simulations and a moisture-dependent 

corrosion rate model give practicing engineers tools to 

make more accurate predictions of the corrosion rate of 

fasteners in a given structure. Using an average yearly 

moisture content gives a conservative bound on the 

residual capacity (Fig. 6-6). 

Acknowledgments

The authors thank Charles Carll and William J. Feist for 

constructive feedback. 

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Figure 6-6. Relative capacity as a function of time 

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General Technical Report FPL–GTR–220

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Corrosion Of Metal Fasteners Embedded In Wood.” Building and Environment, 46(10), 2060-2068.

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International. 
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

59

Chapter 7

Environmental Degradation of 

Fiber-Reinforced Polymer (FRP) 

Fasteners in Wood

Samuel L. Zelinka, Materials Research Engineer

Forest Products Laboratory, Madison, Wisconsin 

Douglas R. Rammer, Research General Engineer

Forest Products Laboratory, Madison, Wisconsin

quaternary (ACQ), copper azole (CA), and their corresponding “micronized” formulations (MCQ and MCA, respectively) (Lebow 2004). 

The corrosiveness of alternatives to CCA has been researched heavily in the past 5 years as preliminary research 

(Anon 2008), and anecdotal evidence (Burkholder 2004) 

suggested that these alternatives were more corrosive than 

CCA. This recent research has confirmed that in most cases, 

these preservatives are more corrosive than CCA. For example, Kear and others (2009) tested galvanized steel in 

many different preservative formulations and exposure conditions and found that ACQ-treated wood was between 

3 and 19 times more corrosive than CCA and that CAtreated wood was between 3 and 11 times more corrosive 

(the range represents differences in retention levels and 

exposure conditions). Similarly, Zelinka and others found 

that CA and ACQ were 1.8 and 2.0 times more corrosive 

than CCA-treated wood for hot-dip galvanized fasteners 

exposed to a 100% relative humidity environment for one 

year (Zelinka and others 2010). Zelinka and Rammer have 

examined the effect of corrosion rate on the degradation in 

lateral strength of joints in decks and shown that increasing 

the corrosion rate from 10 to 60 μm yr–1 decreases the time 

to failure (defined as 50% of the original capacity) by 500%. 

(Zelinka and Rammer 2012). Because alternatives to CCA 

are more corrosive than CCA and because this difference in 

corrosion affects the usable life of structures, corrosion of 

fasteners and joist hangers must be considered when designing and building with treated wood.

There are several ways to minimize corrosion in treated 

wood. Since the mechanism of corrosion in treated wood 

involves the reduction of cupric ions, metals noble to copper, such as stainless steel, have much lower corrosion rates 

in treated wood (Zelinka and Rammer 2009). Another option for minimizing corrosion is using a polymer or ceramic 

coated fastener to chemically isolate the metal from the 

treated wood. Finally, a completely non-metallic fastener 

would be immune to traditional forms of corrosion. 

Abstract

This paper examines the durability of fiber-reinforced polymer (FRP) nails in treated wood. FRP nails were exposed 

to four conditions: (1) accelerated weathering consisting of 

exposure to UV light and condensation, (2) 100% relative 

humidity (RH), (3) driven into untreated wood and exposed 

to 100% RH, (4) driven into ACQ-treated wood and exposed to 100% RH. Changes in the physical, mechanical, 

and chemical properties were examined with weight loss, 

dynamical mechanical analysis (DMA), and near-infrared 

spectroscopy (NIR), respectively. Although fasteners exhibited changes after exposure to treated wood, the changes 

were not statistically different than the changes that occurred in untreated wood or in the humid environment. Under these three conditions, the fasteners gained 2% to 3% of 

their starting mass and the storage modulus also increased 

by 20% after exposure. The results suggest that moisture 

is more important than the wood chemistry or preservative 

chemistry in determining the durability of FRP fasteners in 

wood.

Keywords: nail, preservative-treated wood, dynamical mechanical analysis (DMA), near-infrared spectroscopy (NIR)

Introduction

Although not generally considered an aggressive environment, wood can be corrosive to metal fasteners if it is wet or 

has been treated with a wood preservative or fire retardant. 

Prior to 2004, the major wood preservative used in the United States was chromated copper arsenate (CCA) (Lebow 

2004). The corrosion of metal fasteners in CCA-treated 

wood was examined extensively by Baker (1992), who 

found that fasteners made from hot dip galvanized steel, 

stainless steel, silicon bronze, and copper gave adequate 

service life in CCA-treated wood (Anon 2003). However, in 

2004, CCA was voluntarily withdrawn by industry for use in 

residential construction, and its market share has been taken 

up by a number alternatives, such as alkaline copper 
General Technical Report FPL–GTR–220

60

Most of the focus on minimizing corrosion in wood has 

focused on either using stainless steel or coated fasteners, 

as these products dominate the market. However, there are 

disadvantages to these products. Stainless steel fasteners 

are several times more expensive than galvanized fasteners. 

Polymer-coated fasteners, while less expensive, are only as 

good as the polymer coating, which can be damaged during 

or prior to placement in wood. Zelinka and others (2009) 

have shown that in some coated wood screws, the polymer 

coating can be entirely stripped upon driving them into the 

wood. Recently, a non-metallic fastener has been introduced 

to the marketplace. This fastener, made from an FRP, by definition cannot experience corrosion because it is completely 

non-metallic and is cheaper than stainless steel. While the 

fastener is immune from corrosion, it may experience other 

forms of environmental degradation, such as crazing, embrittlement, or even possibly softening, in treated wood. It is 

important to understand the durability of these non-metallic 

fasteners as the fasteners become established in the marketplace. 

This paper examines the durability of a commercial FRP 

nail for potential use in treated wood by exposing the fastener to different environments including treated and untreated 

wood. As opposed to metals, whose weight loss is proportional to the amount of corrosion (degradation) that occurred, a simple weight loss measurement would not be able 

to capture the degradation of an FRP nail. For this reason, 

the chemical, mechanical, and physical properties of FRP 

nails were quantified before and after the exposures. While 

the interpretation of these chemical and physical changes is 

more nuanced than just a simple weight loss, these measurements give clues to the various changes that might occur to 

the FRP fasteners in wood.

Methods and Materials

Fasteners

One type of commercially available, non-metallic fastener 

was examined. The material safety data sheet that revealed 

the fastener was a composite of a high strength polymer and 

fiberglass dust. Fasteners were roughly the size of a common 8d nail and had a rectangular cross section that was 

3.2 mm by 2.6 mm by 56.9 mm in length and weighed 

0.686 g (median of 100 measurements). Ten fasteners 

(replicates) were tested in each exposure condition. 

Exposure Conditions

The fasteners were subjected to one of four different environmental exposure conditions in a laboratory setting: “accelerated weathering,” “100% relative humidity,” “untreated 

wood,” or “ACQ-treated wood.” A detailed explanation of 

these exposure conditions are given below. 

The accelerated weathering exposure test followed the 

ASTM G154 “Cycle 1” protocol with a modified 

temperature during the condensing cycle (ASTM 2006). The 

fasteners were exposed to 0.89 W/m2/nm of irradiance using UVA-340 bulbs for 8 h at 60 °C followed by a 4-h long 

dark cycle where water condensed on the fasteners at 40 °C. 

These cycles were repeated until the combined exposure 

totaled 1,000 h. Fasteners were suspended on sample racks 

and exposed directly to the UV radiation (not embedded in 

wood).

Fasteners were exposed to 100% RH to examine the moisture resistance of the fasteners. Fasteners were tested by suspending the fasteners over a reservoir of water in a sealed 

desiccator at 27 °C; the resulting relative humidity within 

the desiccator was 100% as dictated by the Gibbs phase 

rule. Fasteners were exposed for 6 months. 

The solid wood exposure tests (untreated and ACQ-treated 

wood) closely followed common procedures for measuring the corrosion of metallic fasteners embedded in treated 

wood (Zelinka 2007; Zelinka and Rammer 2009; Zelinka 

and others 2010). The fasteners were hand-driven into predrilled holes in blocks of Southern Pine (Pinus spp.) 

(38 mm by 50 mm by 90 mm) that had been equilibrated 

at 27 °C and 90% RH prior to the exposure test. Predrilled 

holes were either exactly the largest dimension of the nails 

or in some cases 0.4 mm (1/64 in.) oversized. We found that 

the fasteners would fracture if driven into undersized holes 

or directly into the pine. These wood–fastener assemblies 

were then placed in a desiccator suspended over a reservoir 

of water with a resulting relative humidity of 100% at 

27 °C. To minimize experimental differences, all the lumber 

was purchased at the same time, and half of the lumber was 

treated to 4 kg m–3 ACQ (carbonate formulation); suggested 

retention for above ground use (AWPA 2007). Fasteners 

were exposed for 6 months, after which they were carefully 

removed by sawing grooves in the wood near the fastener, 

placing the wood in the vise, and extracting the fastener by 

hand. 

Property Evaluation

Before and after the fasteners were exposed, their physical, 

mechanical, and chemical properties were evaluated to detect changes caused by the exposure. Physically, the fasteners were examined for swelling caused by water uptake by 

measuring the fasteners to the nearest 0.01 mm at four key 

points along the length of the fasteners before and after exposure; these points are illustrated in Figure 7-1. Likewise, 

the fasteners were weighed to the nearest 0.1 mg before 

and after to see if they gained weight (water uptake) or lost 

weight (environmental degradation). 

Changes in the surface chemistry were measured with 

near-infrared spectroscopy (NIR) using an ASD AgriSpec 

spectrophotometer (Analytical Spectral Devices, Boulder, 

Colorado), capable of measuring wavelengths from 350–

2500 nm with a 10-mm spot size. The light source was 

provided with a tungsten halogen bulb that was fed through 
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

61

a fiber optic sampling probe to deliver illumination to the 

physical sample. The reflected light from the sample was 

carried by a separate fiber optic bundle to the spectrophotometer. The baseline spectrum was obtained by measuring 

a white reference and this spectra was automatically applied 

and subtracted to each raw spectra. Because the fastener was 

smaller than the spot size, a special methodology was developed. The fastener was placed in a special white reference 

with a groove machined in it so that the fastener would lay 

flat. The grooved reference also assured that the spot region 

before and after exposure would be the same. Spectra were 

collected over the 10-mm spot size, which included both the 

nail and the white reference. To isolate changes in the surface chemistry of the fastener, spectra were taken before and 

after exposure using the same white reference, and relative 

differences between spectra were examined. Specific peaks 

were located and identified using the freely distributed “Extrema” program written for Matlab (Vargas-Aguilera 2007). 

Changes in the mechanical properties were measured using dynamical mechanical analysis (DMA). Fasteners were 

placed in the tension fixture of a Q800 DMA (TA Instruments, New Castle, Deleware). A frequency sweep was 

preformed from 0.1 to 100 Hz with 10 measurements per 

decade of frequency at 35 °C. The storage modulus (Eʹ, i.e., 

the in-phase component of the dynamic modulus) and the 

loss modulus (Eʺ, the out-of-phase component of the dynamic modulus) were recorded at each frequency step. 

Results and Discussion

Physical Changes

Changes in the dimensions and weights of the fasteners are 

summarized in Table 7-1. Practically speaking, the dimensions of the fasteners exposed to the 100% RH, ACQ-treated 

wood, and untreated wood conditions remained the same 

before and after exposure. For these fasteners, some of fasteners slightly elongated in one dimension, while others decreased in size. The average change in any given dimension 

was very close to the standard error of the measurements. 

Whereas in some cases the relative changes appear to be 

non-negligible (i.e., several percent), it is only because the 

starting dimensions were so small (3 mm) that slight fluctuations in the absolute dimensions cause a large percentage 

shift. It should be noted however, that the fasteners exposed 

Figure 7-1. FRP fastener with the location of the diameter measurements marked. The scale bar represents 10 mm.

Table 7-1. Absolute and percentage change in mass and dimensions of the fasteners for different exposure 

conditions. The number in parentheses represents the standard error. I, ii, iii represent groupings from theTukey 

HSD test and iirepresents no difference from the original (pre-exposure) mass.

Original values 

Mass 

(mg) 

Length 

(mm) 

Diameter (mm) at location #a

1 2 3 4 

100% RH 687 (1.2) 56.85 (0.01) 6.17 (0.004) 3.17 (0.005) 2.56 (0.007) 3.02 (0.013) 

ACQ-treated wood 687 (1.2) 56.85 (0.03) 6.17 (0.005) 3.16 (0.005) 2.56 (0.004) 3.04 (0.007) 

Untreated wood 687 (1.2) 56.87 (0.02) 6.17 (0.004) 3.16 (0.005) 2.57 (0.005) 3.04 (0.010) 

Accelerated weathering 686 (0.3) 56.92 (0.03) 6.16 (0.002) 3.16 (0.004) 2.56 (0.005) 3.03 (0.003) 

Absolute changes 

100% RH 16.3 (0.1)i –0.08 (0.04) 0.02 (0.005) –0.03 (0.01) –0.02 (0.01) 0.04 (0.02) 

ACQ-treated wood 14.4 (0.7)i –0.20 (0.17) 0.01 (0.01) –0.01 (0.02) 0.00 (0.01) –0.08 (0.05) 

Untreated wood 14.8 (0.1)i –0.27 (0.17) 0.00 (0.01) –0.02 (0.01) 0.00 (0.01) –0.12 (0.04) 

Accelerated weathering –6.2 (0.15)iii –0.17 (0.02) –0.02 (0.003) –0.02 (0.004) –0.02 (0.004) –0.03 (0.003) 

Relative changes (%) 

100% RH 2.4 –0.2 0.31 –0.84 –0.69 1.30 

ACQ-treated wood 2.1 –0.4 0.19 –0.24 0.10 –2.77 

Untreated wood 2.2 –0.5 0.02 –0.64 –0.04 –3.85 

Accelerated weathering –0.9 –0.3 –0.39 –0.72 –0.84 –0.92 

a

The locations associated with these numbers are shown in Figure 7-1. 

        
General Technical Report FPL–GTR–220

62

to accelerated weathering experienced consistent shrinkage. 

Although this change was only a few hundredths of a millimeter, it occurred nearly identically for all fasteners and 

unlike the other exposure conditions, no fasteners elongated. 

It appears therefore that the fasteners slightly shrink with 

exposure to accelerated weathering, and that the other exposure conditions cause small positive and negative fluctuations in diameter.

Although for the most part, the fasteners seem dimensionally stable, there was a noticeable and statistically significant change (α = 0.05) in mass for each of the exposure 

conditions. Differences among groups were analyzed using 

a Tukey HSD test (α = 0.05). The groupins from the Tukey 

HSD test are indicated by Roman numerals i, ii, and iii in the 

mass column of Table 7-1. The fasteners exposed to 100% 

RH, ACQ-treated and untreated wood gained more than 

2% of their starting mass (i in Table 7-1), were statistically 

different than the control (unexposed) fasteners (ii in 

Table 7-1) but differences among these three treatments 

were not statistically different. The increase in mass in 

these groups most likely arises from water ingress into the 

fasteners. In contrast, the fasteners exposed to accelerated 

weathering with UV-light lost nearly 1% of their starting 

mass (iii in Table 7-1), which suggests some UV attack of the 

fasteners. It should be noted that as part of the ASTM G154 

protocol, these fasteners were cycled through “condensing” 

periods of where the RH was 100% at 40 °C, so one would 

expect that these fasteners would have gained mass like 

those exposed to the 100% RH for 6 months, all other things 

equal. Therefore, it is possible that the fasteners would have 

lost more mass if the accelerated weathering test did not 

have the condensing periods.

Chemical Changes

The pre-test NIR spectra were similar across all specimens 

except for some small differences in intensity between spectra that manifested themselves as slight distortions along 

the y axis. To make the data analysis easier, the y data were 

scaled by the intensity of the peak at 1518 nm. Once scaled, 

the data were virtually identical; the coefficient of variation 

of the peak with the biggest variability was at most 0.04%. 

The peak at 1518 nm was chosen to normalize the other 

peaks because it was clearly identifiable and remained at 

nearly the same frequency and intensity after all exposures.

The scaled NIR spectra are shown in Figure 7-2. The NIR 

spectra for the control fasteners exhibited 8 distinctive 

peaks, which are highlighted with black dots in Figure 7-2. 

The frequency and intensity of each peak are summarized in 

Table 7-2 for each treatment. Most of the changes that occurred during the exposure test were minor; the peaks were 

either shifted in intensity or frequency by a small amount. 

The most noticeable feature from the NIR spectra was the 

peak at 1336 nm where there was distinct behavior difference between exposure conditions. The fasteners in the control group exhibited a peak at 1336 nm, which became more 

0.7

0.8

0.9

1

1.1

1.2

900 1400 1900

Intensity (relative to 1517 nm)

Wavelength (nm)

Control

A.W.

ACQ

UNT

100%

Figure 7-2. NIR spectra for the FRP nails before (control) and after being exposed to accelerated weathering 

(A.W.) driven in ACQ-treated wood (ACQ), driven into 

untreated wood (UNT) or exposed to 100% RH (100%). 

The open circles correspond with the peaks listed in 

Table 7-2.

Table 7-2. Wavelength (λ) and peak intensity (l) for the peaks in the NIR reflectance spectra. The data are 

normalized to the peak intensity at 1518 nm.

Control Accelerated weathering ACQ-treated wood Untreated wood 100% RH 

λ

(nm) 

I 

(arb) 

λ

(nm) 

I 

(arb) 

λ

(nm) 

I 

(arb) 

λ

(nm) 

I 

(arb) 

λ

(nm) 

I 

(arb) 

1 989 1.22 996 1.15 993 1.20 991 1.18 994 1.19 

2 1,044 1.16 1,044 1.10 1,042 1.15 1,042 1.13 1,042 1.14 

3 1,219 1.22 1,229 1.19 1,217 1.20 1,214 1.17 1,213 1.18 

4 1,336 1.02 1,341 1.04 

5 1,518 1.00 1,517 1.00 1,519 1.00 1,519 1.00 1,519 1.00 

6 1,578 0.86 1,579 0.86 1,578 0.90 1,578 0.90 1,578 0.90 

7 1,633 0.82 1,633 0.82 1,630 0.86 1,630 0.86 1,629 0.86 

8 1,756 0.89 1,762 0.86 1,751 0.86 1,751 0.87 1,750 0.87 
Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives

63

prominent after exposure to accelerated weathering. However, this peak was not present in those fasteners exposed to 

ACQ-treated wood, untreated wood, or 100% RH.

Mechanical Changes

The storage and loss moduli are shown in Figure 7-3 for 

typical curves for each exposure condition. The storage 

modulus was nearly constant across the frequencies tested, 

increasing very slightly with frequency; the slope was dependent on exposure condition. Likewise, the loss modulus 

decreased slightly with frequency and the amount of decrease also depended on exposure conditions. Because there 

were no peaks in the frequency spectra, the data can be compared at a given frequency to make data easier to examine. 

The storage and loss moduli (taken at 1 Hz) are presented 

in Table 7-3. Control fasteners had a storage modulus of 

8.0 GPa. Fasteners exposed to accelerated weathering had 

a lower, but not statistically different, storage modulus 

whereas fasteners exposed to treated wood, untreated wood, 

and 100% RH had a statistically significant higher storage 

modulus. 

Summary and Conclusions

The purpose of the investigation was to examine the compatibility of a non-metallic fastener for use in treated wood. 

After 6 months, exposure to treated wood, small but noticeable changes occurred to physical, chemical, and mechanical properties of the fasteners. These changes were similar 

to the changes that occurred to fasteners that were placed 

in untreated wood, and also the fasteners exposed to the 

100% RH environment that the wood was conditioned in. 

It therefore appears that most of the changes caused during 

the exposure period are caused by moisture. Wet conditions 

caused the fasteners to have a higher storage modulus and 

caused the peak at 1336 nm in the NIR spectra from the 

control group to disappear. From the perspective of using 

FRP fasteners in treated wood, the data after 6 months suggest that their performance will not be much worse than if 

the FRP fasteners were used in wet or humid environments. 

References

Anon. 2003. International Building Code. Florence, KY: 

Delmar Cengage Learning.

Anon. 2008. Preservative treated wood. Pleasanton, CA: 

Simpson Strong-Tie Technical Bulletin.

ASTM 2006. ASTM G154: Standard practice for operating 

fluorescent light apparatus for UV exposure of nonmetallic 

materials. West Conshohocken, PA: ASTM International.

AWPA 2007. AWPA U1-06: Use category system, Granbury, 

TX: American Wood Preservers’ Association.

Baker, A.J. 1992. Corrosion of nails in CCA- and ACAtreated wood in two environments. Forest Products Journal. 

42: 39–41.

Figure 7-3. Storage and loss moduli for the control and exposed fasteners.

Table 7-3. Storage and loss modulii at 1 Hz 

determined from DMA. The number in 

parentheses represents the standard error. 

Storage 

modulus

(GPa)

Loss

modulus

(MPa)

Control 8.0 (0.2)a 160 (10)

Accelerated weathering 7.6 (0.3)a 160 (20)

Untreated wood 9.0 (0.2)b

 120 (20)

100% RH 9.2 (0.2)b

 290 (20)

ACQ-treated wood 9.2 (0.2)b

 120 (10)

a,bGrouping from Tukey HSD test. 
General Technical Report FPL–GTR–220

64

Burkholder, M. 2004. CCA, NFBA, and the post-frame 

building industry. Frame Building News. 16: 6–12.

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