Review of Test Methods 

Used to Determine 

the Corrosion Rate of 

Metals in Contact With 

Treated Wood 

Samuel L. Zelinka 

Douglas R. Rammer 

United States

Department of 

Agriculture 

Forest Service 

Forest 

Products 

Laboratory 

General 

Technical 

Report 

FPL−GTR−156 

In cooperation 

with the 

United States 

Department of 

Transportation 

Federal 

Highway 

Administration 
Abstract 

Although preservative treatments prolong the life of wood, 

they can also accelerate the corrosion of fasteners. This is an 

issue that needs to be studied because corrosion of building 

materials and fasteners can negatively affect system performance. In this study, the test methods of twenty publications and one standard that examine the corrosion of metals 

in contact with wood are summarized and reviewed so that a 

comparison of their effectiveness can be made. While the 

focus is on the test methods, some qualitative results from 

these different methods are discussed to show nuances of the 

test method. Also, this literature review is intended to focus 

future research towards the use of the most promising test 

methods and eliminate overlap in research efforts. 

Keywords: corrosion, fasteners, wood preservative, 

accelerated test, corrosion rate, metals, fire retardants 

January 2005 

Zelinka, Samuel L.; Rammer, Douglas R. 2005. Review of test methods 

used to determine the corrosion rate of metals in contact with treated wood. 

Gen. Tech. Rep. FPL-GTR-156. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 23 p. 

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Contents 

 Page 

Introduction........................................................................... 1

Objective............................................................................... 2

Corrosion Background and Terminology ............................. 3

Part 1—Exposure Tests......................................................... 4

Overview........................................................................... 4

Outdoor Exposure............................................................. 4

Underground Exposure..................................................... 8

Simulated Natural Exposure Methods .............................. 9

Part 2—Accelerated Exposure Tests................................... 12

Overview......................................................................... 12

Moisture Content and Temperature ................................ 12

Damp Sawdust ................................................................ 14

Salt-Spray Tests .............................................................. 16

Part 3—Electrochemical Tests............................................ 17

Discussion........................................................................... 20

Conclusions......................................................................... 21

Acknowledgments............................................................... 21

Literature Cited ................................................................... 21
Review of Test Methods Used to 

Determine the Corrosion Rate of 

Metals in Contact With Treated Wood 

Samuel L. Zelinka, Materials Engineering Technician

Douglas R. Rammer, Research General Engineer

Forest Products Laboratory, Madison, Wisconsin 

Introduction 

Metals are used in a wide variety of applications because of 

their high strength and ductility. However, metals in most 

environments are thermodynamically unstable and corrode 

(oxidize) to a more stable state. While not generally considered an aggressive environment, wood has the possibility of 

severely corroding metal, especially when preservative or 

fire treatments are utilized. 

In almost every timber engineering application, wood is in 

intimate contact with metal. Metallic fasteners embedded in 

wood are subject to corrosion by the presence of water and 

oxygen in the cellular structure of wood. The corrosion of 

fasteners in wood is a coupled phenomenon; the corrosion 

products of the metal locally accelerate the degradation of 

the wood around the fastener (Richolson 1959, Baker 1978). 

Both the corrosion and the resulting decomposition of the 

wood significantly weaken the holding power of the fastener 

and can lead to failures in service (Zollo and Heyer 1982, 

Rossiter and others 1992, Bohnhoff 2002). 

Historically, creosote, pentachlorophenol (penta), and other 

oil-based preservatives have been used to treat wood in 

bridges (Ritter and Morrell 1992). Oil-based preservatives 

have little, if any, accelerating affect on the corrosion of 

fasteners in wood (Nelson 1950, Marian and Wissing 1960, 

Jain and others 1970, Barnes and others 1984). 

Waterborne preservatives, such as chromated copper arsenate (CCA) and ammoniacal copper arsenate (ACA), have 

also been used to treat bridges. While some of the preservative bonds to the wood and becomes “fixed,” a small percentage of the CCA or ACA remains in ionic form in the 

wood. These ionic components serve to protect the wood; 

however, they also increase the corrosiveness of the wood 

environment, especially if the wood has not been given 

ample time to fixate before being put into service. 

Recently, many designers are choosing to use alternatives to 

CCA, including ammoniacal copper quat (ACQ-B) and 

ammoniacal copper zinc arsenate (ACZA). There is very 

little published research on the effect of these ammoniabased preservatives on the corrosion rate, although there is a 

belief that ACQ and other new preservatives are much more 

corrosive that CCA. CCA contains hexavalent chromium, 

which typically acts as a corrosion inhibitor. On the other 

hand, some formulations of ACQ contain chlorides, which 

can increase the conductivity of the wood, as well as increase the corrosion rate and cause pitting corrosion in both 

carbon and stainless steels. Also, in most cases, these newer 

preservatives have more copper than CCA, which can cause 

increased galvanic corrosion. Unfortunately, there is not a 

readily available procedure to quantitatively evaluate the 

change of corrosion rate with these alternative preservatives. 

Many papers on the corrosion of metals in contact with 

wood, including several previous literature reviews, have 

been published. Chronologically, the first literature review 

was published by Graham and others (1976). It briefly summarizes more than 125 references including sources on 

general corrosion, the corrosion of metal by wood and wood 

vapors, and the affects of preservative treatments on corrosion. Because of the number of references, neither the details 

of the test procedures nor any of the results were reported. 

The next literature review was by Bailey and Schofield 

(1984). Their literature review had an emphasis on roofing 

applications and discussed the effects of metals in contact 

with wood treated with CCA. Bailey and Schofield (1984) 

split the literature surveyed into three groups based on the 

exposure conditions of the metal in contact with wood: 

outdoor conditions, underground conditions, and laboratory 

conditions. Only 10 studies were reviewed by Bailey and 

Schofield (1984), and only a brief summary of the testing 

methods and results were given for each study. The most 

current literature review was by Ruddick (1987). The purpose of Ruddick’s (1987) review was, “to summarize the 

data which is currently available on the corrosion of metal in 

contact with preservative treated wood.” The 1987 review 

focuses on comparing the results of the 34 sources surveyed. 

In addition to these literature reviews, additional, shorter 

papers give a general review of wood–metal corrosion, but 

in general, these papers are qualitative and do not critique 

test methods or numerical results (Campbell and Packman 

1944, Farmer 1962, Smith 1982, BRE Digest 1985, Duncan 

1988, Kubler 1992, Falk and Baker 1993). 
2 

Objective 

The purpose of this literature review is to give a comprehensive overview of all test methods previously used to evaluate 

the corrosion of metals in contact with wood, so that a comparison of their effectiveness can be made. Moreover, the 

literature review is intended to focus future research towards 

the use of the most promising test methods and eliminate 

overlap with previously published research. 

The research of the reviewed papers is summarized in Tables 

1 and 2. Table 1 lists the metals tested in each publication, 

and Table 2 lists the wood species tested in each publication. 

It is impossible to list the wood preservatives in a summary 

table because there are too many different variations to make 

an effective comparison across different publications. 

This paper reviews the test methods used to evaluate the 

corrosion of metals in contact with wood by breaking the 

experiments into three groups: exposure tests, accelerated 

exposure tests, and electrochemical tests. Exposure tests 

included outdoor exposure tests, underground exposure tests, 

and simulated exposure tests. Simulated exposure was used 

to describe tests that were run in a chamber with a controlled 

 Table 1—Summary of the metals tested in each experiment Mild steel Copper Zinc Monel Silicon bronze Aluminum alloys

BrassStainless steelHot-dipped galvanizedElectroplated galvanized nailMechanically galvanizedPlastic coatedNickel-plated steelChrome-plated brassChrome-plated zincCement coatedCadmium-coated steel

Baechler 1949 X X X

Farmer and Porter 1962 X

Scholten 1965 X

Wallin 1971 X X X X X X X X

Johnson 1976 X X X X

Baker and Gjovick 1977 X X X

Baker 1992 X X X X X X X X X

Laidlaw and Cox 1983 X X

Simm and Button 1985a X X X X

Davis and Allen 1993 X X X X X X X X

Wright and others 1957 X X

Doyle 1964 X X X

Richolson 1959 X X X X X

Bartel-Kornacka 1967 X

Bengelsdorf 1983 X X X X X X X X X X X

Barnes and others 1984 X X X X X

Simm and Button 1985b X X X X X

Jack and Smedley 1987 X X X

Cross 1990 X X X X X

Dennis and others 1995 X X X X

Short and Dennis 1997 X X X

Electrochemical Exposure

Wójtowicz and Piwowarczyk 2002 X X X

Accelerated

Experiment
3

Table 2—Summary of the wood species tested in each experiment Bald cypress Balsa British Columbia fir Canadian rock elm Douglas-fir English elm European beech European oak European redwood

European spruceGreenheartGum woodMahoganyObechePonderosa pineRadiata pineSlash pineSouthern PineTeakWestern redcedarWhite oak

Baechler 1949 X

Farmer and Porter 1962 X X X X X X X X X X

Scholten 1965 X

Wallin 1971 X

Johnson 1976 X

Baker and Gjovick 1977

Baker 1992 X

Laidlaw and Cox 1983 X

Simm and Button 1985a X

Davis and Allen 1993 X

Wright and others 1957 X X X X

Doyle 1964 X

Richolson 1959 X X X X X

Bartel-Kornacka 1967

Bengelsdorf 1983 X

Barnes and others 1984 X

Simm and Button 1985b X

Jack and Smedley 1987 X

Cross 1990 X X

Dennis and others 1995 X

Short and Dennis 1997 X

Wójtowicz and Piwowarczyk 2002 Unknown

Unknown

Various Tropical Species

Experiment

Exposure Accelerated Electrochemical

temperature and humidity to simulate in-service conditions. 

Accelerated exposure tests included tests that changed the 

moisture content or the temperature of the wood, or both, 

tests that used damp sawdust to accelerate corrosion, and 

salt-spray tests. Electrochemical tests accelerated the 

corrosion mechanism by sending an electrical current 

through the wood. 

To the authors’ knowledge, only one standard exists, AWPA 

E-12, that attempts to assess the corrosion of metal in wood. 

This standard, developed by the American Wood Preservers’ 

Association (AWPA 2004a), is discussed in the moisture 

content and temperature section in Part 2—Accelerated 

Exposure Tests, since it uses these factors to accelerate 

results. 

Most previous studies have focused on either how the wood 

affects the corrosion rate or how the corrosion products 

cause local degradation of the wood. Because the majority of 

experiments have attempted to measure the corrosion of 

metals in contact with wood, papers that solely discuss the 

degradation of wood in contact with metal are not included. 

Many studies used the common name to identify the wood 

species evaluated. In clear cases where the genesis could be 

easily determined, the scientific name was given. If a specific genesis was unclear, the scientific name was omitted. 

Similarly, for the preservative or fire-retardant treatments, if 

a standard name or formulization could be determined from 

the information presented in the report, it was listed; otherwise, the general or commercial name was utilized. The 

unified numbering system (UNS) designation is included 

with alloys wherever possible (ASTM 2003e). 

Corrosion Background and 

Terminology 

The corrosion of metals in an aqueous environment is an 

electrochemical process. Corrosion involves two steps: 

(1) the reactants, mainly water and oxygen, must diffuse to 

the metal surface, and (2) upon reaching the surface, the 

reactants must have enough energy to complete the reaction. 

Because these steps are in series, the slower of these two 

steps dominates the rate of corrosion. When the diffusion to 
4 

the surface is the rate-determining step of corrosion, the 

reaction is said to be “concentration controlled” or “diffusion 

controlled”. If there is an abundance of the reactants at the 

surface, the reaction rate is controlled by the number of 

atoms with enough energy to cross the energy barrier to 

complete the reaction. Because the number of atoms with 

this energy is a thermally activated process (that is, it follows 

the Arrhenius law), this situation is said to be “activation 

controlled”. When reviewing previous research, it is important to consider that accelerating tests may change corrosion 

mechanisms from a diffusion-controlled process to an activation-controlled process or vice versa. In wood near its fiber 

saturation point, there is enough water that the corrosion 

reaction is activation controlled; however, at equilibrium 

moisture contents below about 20%, the reaction appears to 

be diffusion controlled (Jack and Smedley 1987, Bailey and 

Schofield 1984). 

The defining characteristic of the corrosion rate is the mass 

loss (from metal to oxide) per unit time. If all the oxide is 

removed from the metal after corrosion has occurred, corrosion can be measured by the mass loss per unit time. However, simple mass loss measurements fail to describe the 

damage of the corrosion because they are not normalized by 

the size of the test specimen. By normalizing mass loss to 

specimen size, it is possible to compare the corrosion rates of 

two specimens of different size. Normally, corrosion is 

measured in depth of penetration per unit time, which is 

calculated from the mass loss data divided by the product of 

the density and the surface area of the corroded metal. The 

most common unit of corrosion, in both the United States 

and abroad, is mils of penetration per year (MPY), where a 

mil is one-thousandth of an inch (Jones 1996). However, 

millimeters per year (mm/y) and micrometers per year 

(μm/y) are also used. For reference, 1 MPY is equivalent to 

0.0254 mm/y or 25.4 μm/y. 

Because corrosion is an electrochemical process, the mass 

loss is directly related to a loss of electrons. In general, 

movement of charged particles, such as electrons, per unit 

time is the definition of electrical current, measured in amperes. Therefore, the electrical current produced from the 

corrosion reaction, if measured, can be related back to mass 

loss per unit time through unit analysis. Like mass loss per 

unit time, electrical current does not allow for comparisons 

between samples of different size. Therefore, electrical 

current density, measured in amperes per square centimeter, 

is the preferred unit to measure corrosion for electrochemical 

tests. Current density and depth of penetration are equivalent 

units and can be converted by unit analysis. 

Part 1—Exposure Tests 

Overview 

The simplest way to measure the corrosion of metals in 

contact with wood is to expose metal in contact with wood 

to the environment of interest. After a certain length of time, 

the metal can be removed from the wood, and both the metal 

and the wood can be visually examined for signs of corrosion. In addition to visual examination, the metal can be 

cleaned and weighed to measure the corrosion rate. In addition to being simple, the exposure method also allows the 

researcher to measure the actual corrosion rate for a given 

test set up. Careful choice of metal arrangement and environment allows the researcher to exactly replicate a given set 

of conditions that a wood–metal combination will see in 

service. However, this can also be a disadvantage. Because 

the local environment in the United States changes radically 

from a temperate rainforest in the Pacific Northwest to a 

desert environment in Arizona and New Mexico, corrosion 

data gathered in one specific environment cannot easily be 

applied to another environment. Even the same city could 

have two different corrosion environments if part of the city 

is near the seashore or contains large industrial facilities. In 

addition, there can be changes in the local environment 

during the duration of the test. Exposure tests also have the 

disadvantage that they take more time to complete than 

accelerated tests. Indeed, this is a potential problem with 

changes in preservative treatments; in the time it takes to run 

an exposure test, preservative treatments could change and 

the data that were collected might be of little value to evaluate the corrosiveness of the new preservative treatment. 

Outdoor Exposure 

To the authors’ knowledge, there have been four papers that 

have studied the corrosion of metals in contact with wood by 

exposing them to outdoor conditions. These papers will be 

discussed chronologically, starting with Baechler (1949), 

followed by Farmer and Porter (1962), Scholten (1965), and 

finishing with Wallin (1971). 

In 1949, R.H. Baechler of the USDA Forest Service, Forest 

Products Laboratory (FPL), published the results of a 

20-year investigation into the corrosion of metal fastenings. 

This publication included results of outdoor exposure tests, 

as well as indoor natural exposure tests. The indoor natural 

exposure tests will be discussed under the appropriate subject headings later in the paper. 

The purpose of Baechler’s (1949) 20-year investigation was 

to determine how zinc-chloride preservative treatments 

affect the corrosion of metals in contact with treated wood. 

Most of the experimental details were published in a 10-year 

progress report (Baechler 1939). Baechler (1939, 1949) 

measured the corrosion of three different types of metals in 

contact with one species of wood with five different levels of 

zinc-chloride preservative treatment for various exposure 

times up to 20 years. No. 4 common wire nails, No. 6 brass 

screws, and 38-mm (1.5-in.) galvanized iron nails were 

driven into the sapwood of air-dried ponderosa pine. To see 

if paint protected the fasteners from corrosion, Baechler 
5

painted half of the board after the fasteners were driven. 

Figure 1 shows an example test piece. 

Zinc-chloride preservative treatments of several different 

loadings were analyzed. Two treatments, one with a net 

retention of 22.59 kg/m3

 (1.41 lb/ft3) of dry zinc-chloride 

and the other with a net retention of 11.85 kg/m3

 (0.74 lb/ft3) 

were applied, and the specimens were then allowed to return 

to their original moisture content prior to the corrosion tests. 

It took approximately 6 months for the specimens to return 

to original moisture content. 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 24.19 kg/m3

 (1.51 lb/ft3), and the second treatment was a 

mixture of two parts zinc chloride to one part sodium dichromate by weight, with a net retention of 24.19 kg/m3

(1.51 lb/ft3

) of total salt. For comparison, untreated ponderosa pine was tested. The different treatments are summarized 

in Table 3. 

Baechler (1939, 1949) exposed 10 specimens (Fig. 1) “out 

of doors on a fence at Madison Wisconsin.” It is unclear 

whether the test pieces were exposed parallel or perpendicular to the ground. At intervals throughout the 20-year test 

period, test pieces were removed and the fasteners were 

measured for corrosion. If the fastener was damaged upon 

removal, it was excluded from the results. The fasteners 

were examined by destroying the wood around the fasteners 

and removing the corrosion products with a rubber eraser. If 

the eraser was unable to remove the corrosion products, 

sandpaper was used. Fasteners that were removed 10 years 

or more from the start of the test were cleaned with a 10% 

solution of ammonium citrate for an hour. For those nails 

that were painted over, chloroform was used to remove the 

paint. The difference of the final and initial weights was 

recorded and the mass loss reported. 

Baechler’s (1949) report remains to this day one of the 

longest running and most comprehensive exposure tests. 

Although zinc-chloride is no longer used as a preservative 

treatment, the corrosion data from the untreated replicates 

could be used as baseline numbers for corrosion of metals in 

contact with wood in a midwestern, nonurban, nonindustrial 

environment. The weight loss data reported cannot be directly compared across fastener types or applied to general 

situations since weight loss depends on the sample size and 

density. However, because all fasteners were similar in size, 

similar amounts of weight loss approximate similar amounts 

of corrosion. Another barrier to using the weight loss data is 

that Baechler (1949) did not use standard procedures 

(ASTM 2003d, NACE 2000) to remove the corrosion products. Therefore, the reported weight loss numbers may be 

incorrect and cannot be compared directly to other corrosion 

data. Moreover, Baechler (1949) did not account for galvanic corrosion that could have occurred between different 

metal types placed in the same piece of wood. While this 

would not normally be a problem if the metals were not 

electrically connected, during 20 years of exposure, it is 

very possible that the fastener heads became electrically 

 Figure 1—Fastener arrangement and test setup 

 of the 20-year exposure test of Baechler (1939). 

Table 3—Matrix of preservative treatments and 

moisture contents used by Baechler (1949)a

Moisture content Preservative treatment 

Original air-dried Untreated 

moisture content 1.41 lb/ft3 zinc chloride 

 0.74 lb/ft3

 zinc chloride 

Damp from 1.51 lb/ft3

 zinc chloride 

treatment 1.51 lb/ft3 2:1 zinc chloride to 

sodium dichromate by weight 

a

1 lb/ft3 = 16.02 kg/m3
6 

connected by rainwater or melting snow and that galvanic 

corrosion occurred, which could have skewed the results. 

Farmer and Porter (1962) conducted a 10-year investigation 

into the corrosion of aluminum in contact with wood. Commercial purity aluminum was put in contact with 10 different 

species of wood. The wood species tested were European 

spruce (Picea abies), Douglas-fir (Pseudotsuga menziesii), 

western redcedar (Thuja plicata), obeche (Triplochiton 

scleroxylon), European beech (Fagus sylvatica), European 

oak (Quercus spp.), English elm (Ulmus procera), African 

mahogany (Khaya spp.), balsa, and greenheart. Commercial 

purity aluminum was also put in contact with obeche and 

European beech treated with creosote, fluoride chrome 

arsenate, CCA, copper chrome, and copper napthenate. In 

addition to commercial purity aluminum, two different aluminum alloys were placed in contact with obeche, European 

beech, and European oak. 

The test apparatus of Farmer and Porter (1962) is shown in 

Figure 2. The wood blocks are attached to the aluminum 

sheets with stainless steel screws insulated from the aluminum by rubber washers. The wood–aluminum couple was 

exposed at a 45° angle facing south at two locations in the 

United Kingdom—Princes Risborough, a rural environment, 

and Hayling Island, a marine environment. There were 

enough replicate tests so that a wood–metal couple of each 

of the metals, wood species, and preservative treatments 

could be removed at 1, 2, 5, and 10 years. Upon removal 

from the outdoor test site, the aluminum was cleaned in an 

ultrasonic bath of nitric acid with 5% chromic acid. After 

being cleaned, the mass loss was measured and if pitting 

corrosion occurred, the pit depth was measured with a pin 

micrometer. 

One problem of the test procedure is that some of the mass 

loss of the plates might be caused by contact of the aluminum with the atmosphere, especially in the marine 

environment. It is impossible to distinguish the mass loss 

caused by the atmosphere and that caused by the wood. 

Scholten (1965), also of FPL, indirectly measured the corrosion of metals in contact with wood by measuring the nail 

withdrawal strength of wood exposed outdoors for 5 years in 

Madison, Wisconsin. The purpose of the study was to examine how different preservative treatments affected the withdrawal resistance of nails used to hold together field boxes. 

Sixpenny cement-coated box nails were used to assemble the 

field box out of ponderosa pine (pinus spp.). Seventeen 

preservative treatments were tested (Table 4). The nail withdrawal test follows the methodology of ASTM D 1761–88 

(ASTM 2003b) with the exception of pilot holes and delay 

time. While the standard (ASTM 2003b) calls for the nails to 

be withdrawn immediately after being driven, the tests were 

run 5 years after insertion so that the time and corrosion 

effects could be measured. 

Scholten (1965) makes several notes about the level of corrosion of the fasteners and even includes a photograph 

(Fig. 3). It is obvious from Figure 3 that the corrosion of the 

nail is an important factor in the lower withdrawal strength. 

Scholten (1965) speculates that, “A small amount of corrosion tends to increase the withdrawal loads; however, in 

some boxes, the corrosion had progressed to the stage where 

the nail broke off during the test.” 

While Scholten does not quantify corrosion, his test methods 

do give some insight to the corrosion of metals in contact 

with wood exposed to outdoor conditions. Because metal in 

contact with wood is often used as a fastener, the goal of any 

corrosion test should ultimately be to relate the corrosion 

back to the mechanical properties of the fastener. Although 

measuring the withdrawal strength of the fastener after 

exposure to the outdoors does relate the corrosion back to 

the mechanical properties, it does have some disadvantages. 

Firstly, the withdrawal values for nails driven into the side 

and end grain of wood decline with time even if no corrosion 

takes place (Ehlbeck 1979). Therefore, it is hard to separate 

the effects of corrosion and time delay from a simple withdrawal test. Secondly, the experiment took 5 years to run, 

which makes it inappropriate for rapid testing and evaluation 

of a new preservative treatment. In addition to these factors, 

like all other outdoor exposure tests, the results of this experiment are only valid for the climate in which the experiment was run, in this case, Madison, Wisconsin. 

Wallin (1971), of the Scandinavian Lead Zinc Association, 

also examined the corrosion of metals in contact with wood 

by exposing them to the environment. The purpose was to 

determine the corrosive effect of preservative treatments, 

including CCA-B (Boliden K 33, Osmose Utilities Services, 

Buffalo, New York) and acid copper chromate (ACC) (Celcure) of Table 5 (AWPA 1971a) on fasteners embedded in 

wood. 

Figure 2—Corrosion test of Farmer and Porter (1962). 

(Used with permission of Timber Trades Journal, 

formerly “Wood”). 
7

Table 4—Different preservative treatments used by Scholten (1965). The water repellent used in several treatments 

consisted of 6.5 wt% ester gum, 6.5 wt% raw linseed oil, and 2 wt% paraffin wax 

Treatment Preservative Solvent 

Retention

(kg/m3) 

Dip 0.5% salicylanilide Commercial aromatic solvent 28.8 

Dip 15% water repellent Commercial aromatic solvent 24.0 

Dip 5% orthophenylphenol Commercial aromatic solvent 27.2 

Dip 5% orthophenylphenol and 

15% water repellent 

Commercial aromatic solvent 27.2 

Dip 5% pentachlorophenol and 

15% water repellent 

Commercial aromatic solvent 25.6 

Dip 5% pentachlorophenol and 

5% ester gum 

Commercial aromatic solvent 38.4 

Dip 5% rosin amine D-pentachlorophenate Commercial aromatic solvent 32.0 

Dip 5% zinc alkyl sulfate Commercial aromatic solvent 32.0 

Dip Copper napthanate with 1% metalic copper Commercial aromatic solvent 30.4 

Dip 0.1% copper-8-quinolinolate Stoddard solvent and proprietary 27.2

Dip 5% copper 3 phenol salicylate Coal tar naptha 24.0 

Dip 0.94% cocoamine salt of tetrachlorophenol and

25% water repellent 

Mineral spirits 28.8 

Dip 2% sodium pentachlorophenate and 

3% borax 

Water 30.4 

Dip 3.17% ammoniacal copper arsenate Water 36.8 

Dip 5% sodium orthophenylphenate Water 35.2 

Pressure ACA Water 4.3 

Pressure Nickel sulfate 

Sodium arsenate 

Arsenic acid 

Sodium dichromate 

Water 3.4 

A unique aspect of Wallin’s (1971) test is that he set out to 

test theories on nail coatings. He speculated that electroplated galvanized fasteners do not adequately protect fasteners against corrosion in wood because the coating is too thin, 

in some cases less than 5 μm (0.0002 in.). Hot-dipped galvanized fasteners on the other hand have coatings that range 

between 40 and 80 μm (0.0016 and 0.0031 in.). Therefore, 

Wallin (1971) expected that the hot-dipped galvanized fasteners would perform better than the electroplated fasteners. 

Furthermore, Wallin (1971) suspected that nails coated with 

poly(vinylchloride) (PVC) would perform poorly because 

the PVC coating would shear off during insertion. In addition to testing PVC-coated nails, electroplated zinc nails, and 

hot-dipped galvanized nails, Wallin also tested nails made 

out of mild steel, copper, brass (63wt%Cu, 37wt%Zn), 

stainless steel (17wt%Cr, 12wt%Ni, 3wt%Mo), and an 

unspecified aluminum alloy. 

Fasteners were driven into pieces of European redwood 

(Pinus sylvestris) treated with four different preservative 

treatments. The brand names of the preservative treatments 

used, as well as their composition, are given in Table 5. In 

addition to the four preservative treatments, Wallin (1971) 

tested fasteners driven into untreated wood as a control. The 

wood–fastener couples were exposed on top of the roof of a 

sulfuric acid plant for 1 year. Although the location of the 

plant is not specified, it is assumed to be near Stockholm 

where the paper was published. Weight loss data is reported, 

although the procedure for removing the corrosion products 

is not mentioned. 

Wallin’s (1971) experiment has several factors that make it 

different from the work of Scholten (1965) and Baechler 

(1949). Data cannot be compared across the work of Wallin 

and the researchers from FPL because of the different climates in Wisconsin and Sweden. Furthermore, the industrial 

environment of the sulfuric acid plant also makes the data set 

more complicated. It is impossible to say what chemicals 

were in the local atmosphere around this plant in 1971 and 

how they changed the corrosion rate. However, since all of 

the fasteners that Wallin tested were exposed at the same 

industrial plant in Sweden, Wallin’s data can be used to 

make relative statements about how different alloys perform 

in an industrial environment. The most important result was 
8 

that hot-dipped galvanized outperformed electrodeposited 

galvanized in every situation just as Wallin had predicted. 

Underground Exposure 

Baker (1992) and Baker and Gjovick (1977) ran two experiments to estimate the amount of corrosion that could be 

expected in wood foundations for housing. One was a condition assessment of the fasteners used in wood foundations 

in Virginia and Minnesota; the other, a 17-year exposure of 

nails in treated wood buried underground. 

Baker and Gjovick (1977) presented the results of a condition assessment of fasteners used in wood foundations in 

Virginia and Minnesota in an unpublished FPL report. The 

goal was to observe the condition of fasteners in preservative-treated wood foundations and to measure the moisture 

content because the moisture content is a major factor of 

corrosion in any environment. Baker and Gjovik (1977) 

focused on galvanized steel fasteners because galvanized 

steel fasteners were recommended for use with preservativetreated lumber by the National Forest Products Association 

(now called the American Forest and Paper Association). In 

addition, there were concerns about the durability of galvanized fasteners in preservative-treated wood. A secondary 

objective was to evaluate the effectiveness of X-ray radiography as a method to nondestructively evaluate the corrosion 

of fasteners in wood foundations. 

The foundations of sixteen houses were examined. Nine of 

the sixteen house foundations were built with CCA-treated 

wood and were located in Virginia. The remaining seven 

houses were located in Minnesota, and the foundations were 

made of wood treated with ACA. The age of the foundations 

treated with CCA ranged from 9 to 38 months, and the foundations treated with ACA ranged from 22 to 50 months. At 

each location, the dirt around the foundation of the house 

was removed to a depth of 0.6 m (2 ft) so that fasteners 

could be removed and moisture measurements taken with an 

electronic moisture meter. A core of wood surrounding the 

fastener was removed, and then from that core, the fastener 

was extracted and visually examined. Radiographs were 

taken at several locations. Baker and Gjovik (1977) reported 

that the moisture content ranged from 10% to 30% in the 

foundations and also noted that several fasteners, including 

hot-dipped galvanized fasteners, had red rust. 

No quantitative measure was made of the corrosion of fasteners in preservative-treated wood foundations by Baker 

and Gjovik (1977). However, their findings give “real-life” 

proof for the need to understand and quantify the corrosion 

of fasteners in preservative-treated timber. 

min max min max min max min max min max min max

Bliden K 33 0.31 0.26 0.58 0.51 1.26 0.91 —— —— ——

Celcure 0.18 0.18 0.46 0.43 0.64 0.54 0.4 0.4 0.8 0.2 — —

KP-Cuprinola 0.4 0.06 —— —— —— —— — 0.01

BP-Hylosan —— —— —— —— —— 0.08 0.08

a

Sherwin-Williams Co., Cleveland, Ohio.

Table 5—Different preservative treatments used by Wallin (1965)

Cr SO4 Na Cl

Trade name

Composition (wt%)

Cu As2O5

Figure 3—Corrosion of fasteners exposed near Madison, 

Wisconsin, for 5 years. The unnumbered nail is a new, 

clean, cement-coated nail (Scholten 1965). 
9

Baker (1992) published the results of a 17-year study into 

the investigation of fastener corrosion in preservative-treated 

wood exposed underground to simulate the conditions of 

permanent wood foundations. Like Baker and Gjovik 

(1977), the purpose of the 1992 report by Baker was to gain 

information into the performance of fasteners exposed to 

preservative-treated wood used in wood foundations. In 

addition to the outdoor exposure, Baker (1992) ran simulated 

exposure tests by exposing fasteners embedded in preservative-treated wood to high moisture contents. The results of 

the simulated exposure test will be discussed in a later portion of the paper. 

Baker (1992) tested the sapwood of Southern Pine (Pinus 

spp.) treated with CCA Type A, CCA Type B, and ACA 

(AWPA 2004b). All preservatives were applied to a loading 

of 9.5 kg/m3

 (0.6 lb/ft3) and the wood was kiln-dried to a 

maximum 19% MC after the preservative treatment was 

applied but before fastener insertion. Eleven different types 

of nails were tested; monel (UNS N04400), stainless steel 

(American Iron and Steel Institute (AISI) 304 and AISI 316 

(UNS S30400 and UNS S31600)), copper, hot-dipped galvanized steel, mechanically coated galvanized steel, electroplated galvanized steel, cadmium-coated steel, tin-cadmium 

alloy coated steel, and aluminum (aluminum alloy (AA) 

5056 (UNS A95056)). The nails were driven in predrilled 

holes into the preservative-treated lumber. An interesting 

aspect of Baker’s test setup was that the 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 buried underground at a test site near Madison, 

Wisconsin, and replicates were removed at 1, 3, and 17 

years. Upon removal, the fasteners were washed with water 

and solvent, and if necessary, a rubber stopper was used to 

rub the corrosion product off. Baker (1992) reports the 

corrosion rate as weight loss. 

Like other exposure tests, Baker’s results are specific to the 

environment and climate in which they were run. The data 

collected on the different alloys used can be used to get a 

relative ranking of alloy performance in treated wood, assuming that the mechanism of corrosion does not change 

significantly between underground and aboveground 

exposure. 

Simulated Natural Exposure 

The outdoor environment is constantly changing. While 

many wood products are exposed to outdoor conditions, 

running a test outdoors has the disadvantage that it cannot be 

duplicated or verified by another researcher because of the 

variance in outdoor conditions. Researches such as Baechler 

(1949), Johnson (1976), Laidlaw and Cox (1983), Simm and 

Button (1985a), Han and Chung (1986), Baker (1988, 1992), 

and Davis and Allen (1993) have tried to eliminate this 

variability by running experiments in controlled conditions. 

However, there is a fine line between simulating the outdoor 

conditions and accelerating the corrosion by placing the 

specimen in too harsh of a controlled environment. 

For the purpose of comparison in this paper, experiments 

that have run controlled exposure tests for more than a year 

are grouped into the simulated natural exposure tests; even 

though some of the conditions slightly accelerate the corrosion. This classification was chosen because in most cases, 

the authors who ran experiments in accelerating conditions 

for more than a year were using it as an upper bound for 

other exposure tests. On the other hand, most tests that were 

less than a year in length were focused on accelerating the 

corrosion rate for rapid evaluation. 

In addition to running outdoor exposure experiments, 

Baechler (1939, 1949) also ran 20-year tests in humidity 

rooms to simulate different environments. 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 equilibrium moisture contents associated with those conditions are 6.1%, 11.7%, and 20.2% 

(Forest Products Laboratory 1999). The 27°C, 90% RH 

(relative humidity) represents a severe exposure condition, 

and the 27°C, 30% RH represents a lower bound for corrosion in wood used for construction. Beyond exposure conditions, the same procedure was used in both the outdoor 

exposure tests that were discussed earlier and the humidity 

room tests (Fig. 1). Baechler (1939, 1949) concluded from 

his data that the amount of corrosion on fasteners exposed to 

the outdoors was about the same as the that on fasteners 

exposed to the constant 27°C, 65% RH environment. 

Johnson (1976) ran experiments to determine the mechanical 

properties of fasteners in contact with wood treated with fire 

retardants. At that time, all mechanical fasteners were given 

a 10% reduction in design capacity when used in fireretardant-treated wood because there was little long-term 

durability information and many engineers recognized that 

hydrolytic fire-retardant treatments would accelerate 

corrosion. 

Johnson ran lateral nail tests in Douglas-fir treated with three 

different types of commercially available fire retardants: 

NON-COM1

 (Arch Wood Protection, Smyrna, Georgia), 

Pyresote (Protexol Corp., Kenilworth, New Jersey) (AWPA 

1971a,b), and FR-282

, a borax–boric acid solution. 

 

1

 NonCom is a four-part system that contains Ammonium 

sulfate, ammonium phosphate, boric acid–borax, and 

dicyandiamide. 

2

 FR 28 is a three-part system that contains boron, calcium, 

and sodium. 
10 

No information was given on the composition or retention 

on the wood treated with NON-COM or Pyresote. A nail or 

staple was driven through plywood into a piece of solid 

sawn lumber. Johnson (1976) tested assemblies where both 

the solid sawn wood and the plywood were treated or untreated, as well as running tests where either the solid sawn 

or the plywood was treated and the other piece was not. 

Eight types of fasteners were used: 6d common nail, 6d box 

nail, 6d galvanized nail, 6d galvanized-T nail, 6d nickel 

alloy 400 (monel (UNS N04400)) nail, 14 gauge galvanized 

staple, 16 gauge galvanized staple, and 16 gauge nyloncoated staple. The wood–metal couple was then exposed for 

either 2 or 7 years in one of three conditions: cold, 2°C 

(35°F) and 80% RH; standard, 21°C (70°F) and 65% RH; or 

hot, humid, 32°C (90°F) and 90% RH. 

In total, with eight types of fasteners, three treatments, three 

exposure conditions, two exposure lengths, and four methods to apply the treatments, there were 4,704 different combinations tested. The lateral nail test is shown in Figure 4. 

Unlike the ASTM (2003b) standard for determining the 

lateral load capacity of fasteners, which tests only one 

fastener at a time, Johnson’s assessment tested two fasteners 

at once. 

Johnson (1976) was the only researcher to determine the 

corrosion of metals in contact with wood by measuring the 

lateral bearing strength of the assembly. This makes it very 

difficult to compare his research with other researchers who 

have studied corrosion. Furthermore, neither the composition 

nor the retention of the proprietary treatments are given, 

which makes it impossible to replicate the experiment. 

Laidlaw and Cox (1983) also tried to simulate environments 

to which wood–metal connections are frequently exposed. 

The purpose of the study was to quantify any long-term risk 

of the corrosion of nail plates used in conjunction with preservative-treated trusses used in roof spaces. Pieces of European redwood with varying amounts of heartwood and sapwood and free of any splits or checks were held together by 

zinc-coated nail plates. The wood had been treated with 

either CCA or a proprietary organic solvent. The salt form of 

CCA was applied to a retention of between 9.5 and 18 kg/m3

(0.60 and 1.1 lb/ft3

) and was kiln-dried before testing. The 

organic solvent consisted of pentachlorophenol as well as 

tributyltin oxide and a water repellant. Auxiliary tests were 

run on radiata pine that had been treated with borates. In 

addition to the standard zinc-coated nail plates, those that 

had been coated a second time with a nonmetallic coating 

were also tested. The nonmetallic coatings tested were bitsumatic paint, hot-dipped nylon, and sintered epoxy. 

The wood joints were then exposed to one of three conditions—“damp”, “natural exposure”, or “dry”—for times 

ranging from 1 to 8 years. The damp condition corresponded 

to a humidity chamber conditioned at 27°C (81°F) and 

87% RH. The wood–metal joints that were put in the natural 

exposure condition were placed directly under a roof in a 

nonheated building in Britain. This placement sheltered the 

joints from rain, but still exposed them to the changes of 

temperature and humidity. Those specimens subjected to a 

dry condition were placed in a heated, nonhumidified 

building. 

Laidlaw and Cox (1983) measured the amount of corrosion 

by running mechanical tests. Static and fatigue tests were run 

on joints to see if the corrosion affected the fatigue strength 

more than the static strength. After the strength of the joint 

was measured, several additional tests were run on both the 

wood and the nail plate. The density, hardness, pH, and 

conductivity of the wood were measured. The conductivity 

was measured indirectly by measuring the resistance with an 

ohmmeter. The weight of the zinc coating of the nail plate 

was measured and the nail plate was examined with a 

microscope and X-ray diffraction. 

The work of Laidlaw and Cox (1983) was unique in that it 

studied the corrosion of joints connected with nail plates. 

However, they did not directly measure the corrosion rate, 

which makes it very hard to compare the data to those of 

other researchers. Additionally, it is hard to separate the 

reduction in mechanical capacity caused by time from the 

affects of corrosion. 

Figure 4—Lateral nail test of Johnson (1976) 

(1 in. = 25.4 mm) (Used with permission of 

Oregon State University). 
11

Simm and Button (1985a) classified the corrosiveness of 

CCA preservative treatments. In their introduction, Simm 

and Button (1985a) question the validity of outdoor exposure tests because wood degrades when cycled through 

changes in temperature and moisture content that occur in 

the outdoor environment; therefore “This [degradation] can 

lead to cracking and splitting of wood samples which will 

expose fasteners to corrosive conditions which may be completely different from those produced in the wood.” While 

this cracking and splitting may be seen in service, it is undesirable during testing because it adds variance to the test and 

also makes the results harder to interpret. 

To test the corrosiveness of CCA, Simm and Button (1985a) 

inserted fasteners into blocks of CCA-treated European 

redwood (Pinus sylvestris) with salt retentions ranging from 

3.0 to 9.1 kg/m3

 (0.19 to 0.56 lb/ft3) and placed them in a 

humidity chamber for 30 months. The humidity chamber 

was kept at 25°C ± 1°C (77 ± 1.8°F) with a RH that ranged 

between 95% and 100%. Four different types of metal fasteners: mild steel, electroplated galvanized steel, stainless 

steel (AISI 302 (UNS S30200)), and an aluminum alloy 

(AA 2024 (UNS A92024)), were tested (Table 6). After the 

exposure, the fasteners were placed in a steam bath, brushed 

off to remove dirt, and then placed in a chemical bath to 

remove the corrosion products. Simm and Button (1985a) 

reported the corrosion as the change in weight.

Even though the exposure methodology of Simm and Button 

(1985a) is similar to other previous research, their paper is 

filled with original ideas. Simm and Button (1985a) were the 

first researchers to apply corrosion science techniques to the 

corrosion of metals in wood. After the exposure tests, Simm 

and Button (1985a) used scanning electron microscopy and 

X-ray diffraction to analyze the fastener and the wood. By 

using these instruments, they were able to determine the 

composition of the corrosion products and monitor how far 

they traveled in the wood. With information about the corrosion products, Simm and Button (1985a) were able to speculate about the corrosion reactions that occurred at the 

wood–fastener interface and predict the behavior of fasteners 

in CCA-treated wood in other environments. Not surprisingly, Simm and Button (1985b) were also the first researchers to apply electrochemical methods used in corrosion 

science to wood. Details of this work will be discussed later. 

Han and Chung (1986) simulated the in-service corrosion 

performance by placing test blocks, containing a nail, into 

desiccators filled with K2SO4 on the bottom. Test blocks 

were treated with 2% solution of CCA-Type A, CCA-Type 

B, CCA-Type C, ACC (AWPA 2004b), and phenol flourid, 

nailed, and placed in the desiccators for 5, 10, and 20 days. 

Results showed that rate of weight loss of nail was strongly 

correlated with the type of wood preservative used to treat 

the wood. Test periods did not show any significant difference, and there was no significance between the interaction 

of test period and preservative type. 

In addition to running exposure tests in the soil, Baker 

(1988, 1992) also exposed specimens to a controlled humidity room to simulate conditions that “would have a corrosion 

rate higher than that expected in wood foundations.” The 

specimens were identical to those specimens exposed to the 

ground; they were made from preservative-treated Southern 

Pine with one of eleven different metals used to fasten the 

wood together (Table 1). The specimens were exposed to a 

humidity chamber at 27°C (80°F) and 98% ± 2% RH for 14 

years. At the end of the 14 years, the fasteners were removed, the corrosion products cleaned, by rubbing the fasteners with a rubber stopper, and weight loss data reported. 

Even though the corrosion data at 27°C (80°F) and 98% ± 

2% RH cannot be extrapolated to normal conditions, they are 

still of value. Baker’s results represent a significant database 

on the corrosion of metals embedded in CCA-treated wood. 

Even though the utilization of CCA-treated wood is decreasing, the experiment could be replicated in wood treated with 

newer waterborne preservatives. Parallel experiments with 

new preservatives could give insight into their relative corrosiveness compared with CCA.

Table 6—Aluminum alloys tested by Wright and others (1957) 

Alloying elements (weight %) 

Alloy 

U.S. 

equivalent Form Cu Fe Mg Mn Si Ti Cr Al 

65S-T6 6061-T6 Sheet 0.36 0.37 1.06 — 0.64 0.05 0.22 97.3 

65S-T7 6061-T7 Rod 0.25 0.34 0.95 — 0.55 0.01 0.24 97.66

65S-T8 6061-T8 Bolts 0.23 0.34 1.01 — 0.49 0.01 0.13 97.79

65S-T9 6061-T9 Coupons 0.29 0.4 0.94 — 0.57 — 0.2 97.6 

24S-T3 2024-T3 Sheet 4.52 0.34 1.44 0.63 0.25 0.01 — 92.81

24S-T4 2024-T4 Rod 4.64 0.27 1.52 0.64 0.2 0.01 — 92.72
12 

Davis and Allen (1993) altered outdoor conditions to obtain 

a more reproducible corrosion test. The purpose of their 

experiments were to measure how the corrosiveness of wood 

treated with CCA varies with time after treatment. Davis and 

Allen (1993) hypothesized that since freshly treated wood 

has a higher moisture content and higher conductivity because of the unfixated preservatives, it is more corrosive. To 

test this hypothesis, fasteners were inserted into slash pine 

(Pinus elliottii) treated with the salt formulation CCA at 

various times after the treatment was applied. The retention 

of CCA in the slash pine was not reported. Several different 

fastener materials were used so that a relative comparison 

could be made. The fasteners used were stainless steel (AISI 

304 (UNS S30400)), monel (UNS N04400), brass, copper, 

silicon bronze, nickel-plated steel, hot-dipped galvanized 

steel, and mild steel. Holes were predrilled in the wood for 

the fasteners to minimize splitting damage to the fastener. 

The fasteners were driven into preservative-treated wood in 

intervals after the wood had been treated. During this time, 

the wood was placed under a cover outdoors. No outdoor 

location is specified, but it is assumed to be in or near 

Queensland, Australia, where the report was published. The 

specimens were exposed either 6 or 12 months. After the 

exposure, the corrosion product was removed in accordance 

with ASTM (2003d) standard G1-81. Mass loss data were 

reported. 

While the work of Davis and Allen (1993) is interesting, it is 

important to note that their data may be slightly misleading. 

Although the wood was exposed for either 6 or 12 months, 

the actual time that the fasteners were in contact with the 

wood varied because the effects of cure time were studied by 

placing fasteners in contact with the wood at different time 

intervals after treatment. Because the fasteners that were 

driven into freshly treated wood are also the fasteners that 

were exposed the longest to the wood, the data may overemphasize the affects of cure time. 

Part 2—Accelerated Exposure 

Tests 

Overview 

Outdoor and simulated natural exposure tests have the disadvantage that they take a long time to run. In fact, in the 

time it takes for results to be gathered, the formulation of the 

preservative treatment may have changed. Moreover, if a 

company wanted to test a preservative treatment for corrosion before marketing it, it would have to delay market 

intervention several years, which would be prohibitively 

expensive. Outdoor exposure tests also have the disadvantage of low reproducibility because the conditions the wood 

is exposed to are constantly changing and unpredictable. 

Because of these disadvantages, many researchers have tried 

to accelerate the corrosion process by making the environment around the wood more conducive to corrosion. Three 

different methods have been used to increase the corrosiveness of the environment. The first method is increasing the 

moisture content and temperature of the wood. The second is 

placing the metals in contact with moist sawdust. The third 

accelerated environment is a salt-spray cabinet, which is 

commonly used outside of the wood industry to measure 

corrosion in marine environments. 

Moisture Content and Temperature 

At higher moisture contents, wood conducts electricity and 

ions better, and therefore, the corrosion reaction occurs at a 

faster rate. Because the equilibrium moisture content of 

wood is dependent on the temperature and the RH, the effects of these two variables must be examined together. 

The kinetics of corrosion (oxidation) are faster at higher 

temperatures because diffusion and other processes of 

corrosion are thermally activated. However, at higher temperatures, there is less thermodynamic tendency for corrosion (oxidation) to occur because it is an exothermic reaction. Near room temperature, the change in the reaction rate 

caused by the kinetic component dominates. Therefore, 

increasing the temperature increases the reaction rate in 

this regime. 

Several papers have tried to accelerate corrosion by increasing RH or temperature, or both. The equilibrium moisture 

content is a function of both temperature and RH. Also for 

a given moisture content, the RH itself is a function of temperature. Therefore, these factors will be discussed together. 

There have been five reports, as well as a standard test 

method, that accelerate the corrosion of metals in wood by 

changing the moisture content and temperature of the wood. 

In chronological order, they are Wright and others (1957), 

Doyle (1964), Wallin (1971), Barnes and others (1984), and 

the AWPA standard (AWPA 2004a), which was originally 

adopted in 1994. 

Wright and others (1957) were funded by the Canadian 

Navy to investigate the corrosion performance of aluminum 

alloys (Table 6) used in conjunction with woods commonly 

used in shipbuilding: mahogany, Canadian rock elm (Ulmus 

thomasii), British Colombia Douglas-fir (Pseudotsuga menziesii), and gum wood. Wright and others (1957) tested the 

corrosiveness of these species with several different methods 

including exposing the wood–aluminum to high temperature 

and moisture content as well as exposing the aluminum to 

damp sawdust. The damp sawdust experiments are discussed 

in the next section. In addition to the damp sawdust and high 

moisture content tests, Wright and others (1957) also ran 

some ancillary tests placing blocks of wood with aluminum 

dowels in the ocean for extended lengths of time. These 

ancillary tests are not discussed but can be found in the 

original publication and are not pertinent to our discussion. 
13

In addition to testing the aluminum alloys, Wright and others 

(1957) also tested commercially pure zinc as a reference. 

Dowels constructed of all alloys were placed inside holes in 

a specially machined block of wood shown in Figure 5. To 

simulate shipbuilding practice, after the metallic dowel was 

inserted, wood plugs were placed over the dowel to cover it 

from the outside environment. Wood plugs were used to 

cover half of the dowels; the other dowels were left open as 

a control. After construction, the wood block with metal 

dowels was placed in a controlled humidity chamber at 49°C 

(120°F) and 100% RH for either 2, 6, or 8 months. After the 

exposure, the dowels were removed and the corrosion products were removed. No details were reported on the methods 

used to remove the corrosion products. After the corrosion 

products were removed, the dowels were weighed and the 

corrosion was reported for weight loss per total surface area 

as mils per year (MPY). Pitting corrosion was noted on 

several of the specimens. 

The conditions chosen by Wright and others (1957) were 

very severe and would be expected to highly accelerate the 

corrosion rate. Interestingly, humidity cabinet corrosion rates 

reported by Wright and others (1957) are extremely low. 

Therefore, even in severe conditions, the corrosion of aluminum in contact with untreated wood can be ignored. However, other reports have shown that corrosion of aluminum in 

contact with preservative-treated wood is much higher than 

untreated wood (Bengelsdorf 1983, Simm and Button 1985b, 

Cross 1990, Baker 1992). Furthermore, the use of aluminum 

fasteners is not recommended for use with preservativetreated wood (Cook 2003, WTCA 2004). 

Doyle (1964) investigated the corrosion of nails and bolts in 

glue-laminated Southern Pine treated with commercial fire 

preservatives. Before lamination, a proprietary fire preservative (NON-COM) was applied to Southern Pine treated to a 

retention of 80.10 kg/m3

 (5 lb/ft3). Unlike other studies, 

Doyle (1964) tested the mechanical response of both nails 

and bolts. Doyle (1964) measured the change in withdrawal 

strength of five types of eight-penny nails; common, zinccoated, aluminum, ring-shank, and hardened ring shank as 

well as sixteen-penny nails. The dowel bearing strength was 

also tested on steel bolts with diameters of 12.7 mm (0.5 in.) 

and 19.0 mm (0.75 in.). The corrosion process was accelerated by placing the specimens in an environment of 27°C 

(80°F) and 97% RH for intervals of 3, 6, 12, 24, and 

48 weeks. After these intervals, the nail withdrawal strength, 

according to D 1761-88 (ASTM 2003b) was measured as 

well as the dowel bearing strength of the bolts. Additionally, 

nails were driven immediately into the exposed wood and 

withdrawn as a baseline. Dowel bearing strength was measured in a manner similar to the recently published ASTM 

(2003c) standard D 5764-97a for testing the dowel bearing 

strength of the full hole. Doyle also measured the weight 

loss of the nails after they had been withdrawn to measure 

the corrosion rate. The exact method for removing corrosion 

products is unclear, but it is noted in the report that the “rust 

scale” was removed. 

The work of Doyle (1964) is unique in that both nails and 

bolts were tested. Like the results of Scholten (1965) and 

Davis and Allen (1993), it is hard to separate the effects of 

corrosion and time delay from a simple withdrawal test even 

though Doyle ran immediate withdrawal tests in the exposed 

wood as a control. Doyle was the only researcher to directly 

measure the effect of treated wood on the dowel bearing 

strength. In addition, the weight loss measurements do give a 

measure of corrosion, but it is impossible to relate the corrosion measured in the experiment to the corrosion in service 

because of the accelerating techniques used, which probably 

changed the mechanism of the corrosion reaction. 

In addition to outdoor exposure tests, Wallin (1971) also ran 

accelerated tests at elevated temperature and moisture content. The same test parameters as the outdoor exposure test 

were used; seven types of nails were driven into CCAtreated European redwood (Pinus slyvestrus) and weight loss 

data were reported. The wood–metal couple was exposed in 

a greenhouse at 20°C (68°F) and 100% RH for 1 year. 

Barnes and others (1984) used heat and humidity to accelerate the corrosion of metals in preservative-treated wood. 

They tested metal coupons sandwiched between blocks of 

treated wood. According to Barnes and others, this “sandwich” test method was first used by Preston and van Poppel 

in 1978 (Preston, A.F.; van Poppel, C.M. Corrosion studies 

with copper chrome arsenate and quaternary ammonium 

preservative treatments in wood. FRI Rep. FP/WP 25. New 

Zealand Forest Service.). However, the work of Preston and 

van Poppel was never published and the authors of this 

paper were unable to procure it. Therefore, this paper is 

omitted from the literature review. 

Barnes and others (1984) tested wood treated with CCA 

(both the salt and oxide formulations), ACA, and penta. 

Figure 5—Design of the high temperature and humidity 

wood–metal exposure test of Wright and others (1957) 

(Used with permission of NACE International). 
14 

Table 7—Different preservative treatments used by 

Barnes and others (1984)

Chemical 

Retention 

(kg/m3

) 

Inorganics CCA- type C (oxide) 5.77 

CCA- type C (oxide) 7.53 

CCA- type C (oxide) 10.89 

CCA- type C (oxide) 14.10 

CCA- type C (salt) 5.93 

CCA- type C (salt) 9.61 

CCA- type C (salt) 14.42 

ACA (oxide) 7.21 

ACA (oxide) 8.49 

ACA (oxide) 13.14 

Boric acid 11.53 

CCA (oxide) and boric acid 21.62 

Organics Pentachlorophenol 7.21 

Didecyldimethyl ammonium chloride 4.32 

3-Iodo-2-propynyl butyl carbamate 2.72 

Water-dispersible pentachlorophenol 6.57 

Copper-8-quinolinolate (acid) 0.37 

Copper-8-quinolinolate (neutral) 1.28 

Copper napthenate 1.60 

Organometallics 

Zinc napthenate 1.12 

In addition to these treatments, some organic and organometallic treatments were tested (Table 7). The metal 

coupons were constructed out of carbon steel (SAE 1018 

(UNS G10180)), aluminum (AA 6061 (UNS A96061)), 

galvanized steel (20 gauge), brass (85wt%Cu, 15wt%Zn), 

and copper. The metal coupon was held between the wooden 

blocks by clamping the wood and metal together with twine. 

The metal coupons were about 1 mm (0.4 in.) thick, and the 

wood blocks were 89 mm (3.5 in.) thick. The assemblies 

were placed in a room at 38°C (100°F). Half of the specimens were exposed to 90% RH, and the other half were 

placed partially in water in a closed beaker to simulate complete water saturation. It is explicitly noted that the coupons 

were not in direct contact with the water for the saturated 

samples. The exposure period was 10 to 12 weeks. After the 

wood–metal couple was exposed, the corrosion products 

were removed according to NACE (2000) standard 

TM-01-69 and the corrosion rate was reported in MPY. 

Barnes and others (1984) is an important paper because it 

uses a similar methodology to the only standard that addresses nail corrosion (AWPA 2004a). The E12 standard 

originated in a military specification (Department of Defense 

1984) used to screen fire-retardant treatments for use in 

wood construction. In the E12-94 standard, a metal coupon 

is sandwiched between two pieces of preservative-treated 

wood. Nylon bolts are inserted through the wood to hold the 

metal coupon in place. These wood–metal assemblies are 

then placed in a conditioning chamber of 49°C ± 1°C (120°F 

± 2°F) with RH of 90% ± 1%. The standard specifies a 

minimum of 240 h of accelerated exposure, although it states 

that 366 h of exposure is more commonly used. The standard 

also specifies that the corrosion products are to be cleaned in 

accordance with ASTM (2003d) standard G-1 and that the 

corrosion rate should be reported in MPY. 

While the sandwich method used by Barnes and others 

(1984) and AWPA (2004a) is currently standardized, the 

results of the test must be interpreted carefully. Even the 

AWPA (2004a) standard cautions, “The [corrosion] rate can 

not be extrapolated to all possible scenarios.” It is unsafe to 

extrapolate the corrosion rate from these sandwich tests 

because currently, there is no way to relate the corrosion of 

metals in wood exposed to high temperature and humidity 

environments to the corrosion rate in normal service 

conditions. Therefore, the results can only be used as a 

comparison method. 

Recently, Jin and Preston (2000) compared modified E-12 

test results for nails and screws to field tests in Harrisburg, 

North Carolina. They concluded that laboratory test procedures might not provide optimal results. 

Furthermore, the sandwich method can be very sensitive to 

how tightly the assembly is held together (Personal communication, Crawford, 2003). The more tightly the assembly is 

held together, the harder it is for water and oxygen to reach 

the metal, which lowers the corrosion rate. The variability of 

results from the AWPA (2004a) standard could be greatly 

improved by specifying a torque rating on the bolts. Also, 

nylon swells when in contact with water. Therefore, the 

AWPA test can be further improved by specifying 

nonwater-soluble polymer bolts. 

In conclusion, test methods that vary moisture content or 

temperature to accelerate the corrosion rate should not be 

used as the exclusive determination of the corrosion rate—

especially for in-service conditions. These accelerated test 

methods can give rapid results. However, it is very hard to 

relate these results to in-service life. At their best, test methods that change the moisture content or temperature can give 

relative results of fastener performance. At their worst, if 

these test results are misinterpreted, they can lead to incorrect design or improper materials selection. The implication 

of critical test conditions are still as yet unknown, such as 

clamping pressure and the interaction of temperature and 

moisture content. Significant work would be needed to 

correlate real world performance to lab results. 

Damp Sawdust 

As stated many times throughout this paper, the corrosion 

rate for metals embedded in wood is partially dependent on 

the ability for water and oxygen to reach the fastener. 
15

To accelerate the rate of corrosion, researchers have tried to 

increase the moisture content of the wood to allow more 

water to reach the fastener. However, a limit exists to the 

amount of water that the wood can physically hold. To accelerate the corrosion rate even further, some researchers 

have tried placing metals in contact with sawdust suspended 

in water. Because of the similarity of the methods used by 

these researchers, all of the sawdust methods will be discussed together at the end of the section. 

In addition to exposing untreated wood–aluminum specimens to high temperature and humidity, Wright and others 

(1957) also tried to accelerate the corrosion of aluminum in 

contact with shipbuilding woods by exposing the aluminum 

to water and sawdust of the woods used to build ships. 

Sheets of zinc and aluminum alloys were placed in contact 

with 750 cm3

 (46 in3) of sawdust for 30 days at room temperature. The exact ratio of sawdust to water is not reported, 

but according to Wright and others (1957), “The sawdust 

was kept moistened with distilled water and evaporation was 

minimized by covering each dish with heavy aluminum 

foil.” After exposure to the damp sawdust, the zinc and 

aluminum sheets were cleaned and weighed. The corrosion 

was reported in MPY. 

Bartel–Kornacka (1967) investigated the corrosiveness to 

plain steel of several species of trees that are native to Ghana 

by using a sawdust method. Sawdust from several of these 

species (Table 8) was saturated with water and placed in a 

glass vile. A steel nail was then placed in contact with the 

saturated sawdust and a stopper was placed over the vial. 

The sealed vials were then placed in a chamber that was kept 

at 28°C (82°F) and 100% RH for either 28 or 62 days. Corrosion was determined by removing the corrosion product, 

igniting it, and using chemical methods to isolate and measure the iron using colorimetry. 

Bengelsdorf (1983) ran extensive sawdust tests to determine 

the corrosion of fasteners in preservative-treated wood. A 

unique aspect of Bengelsdorf’s (1983) tests compared with 

other researchers who used sawdust methods was that he 

also increased the temperature to increase the reaction rate. 

According to Bengelsdorf (1983), “Based on the accepted 

theory of doubling of the rate of a chemical reaction for each 

10°C (18°F) rise in temperature, it was anticipated that the 

elevated temperature would increase the rate of corrosion 

many [times].” While this temperature relation may be 

roughly true for biological processes around room temperature, it does not hold true for corrosion. As explained in the 

section on moisture content and temperature, the kinetics of 

corrosion under activation polarization is limited by the 

number of atoms with sufficient energy to overcome the 

energy barrier of the reaction. Therefore, the temperature 

dependence of the corrosion rate is much more complicated 

than a doubling per 10°C increase and in fact may be erroneous, as stated in the beginning of the section Moisture 

Content and Temperature. 

Table 8—Different tree species tested by 

Bartel–Kornacka (1956) 

Latin name Ghana name 

Afrormosia elata Kokrodua 

Alstonia boonei Sindru nyamedua 

Chlorophora excelsa Odum 

Cynometra avanta Ananta 

Entandrophragma angolense Edinum 

Entandrophragma candollei Omu candollei 

Entandrophragma cylindricum Penkwa 

Entandrophragma utile Efrobrodigwo 

Funtumia elastica Funtum 

Guarea cedrata Kwabohoro 

Khaya anthotheca Akwantanuro 

Khaya ivorensis Dubini mahogany 

Lannea welwitschii Kumenini 

Lova trichilioides Dubini-biri 

Mansonia altissima Pruno 

Mimusops heckelii Baku 

Nauclea diderrichii Kusia 

Piptadeniastrum africanum Dahoma 

Tarrietia utilis Nyankom 

Terminalia Iiorensis Emeri 

Terminalia superba Ofram 

Triptochiton scleroxylon Wawa 

Turreanthus vignei Opeya 

Bengelsdorf (1983) ran sawdust corrosion tests for a whole 

year at 52°C (125°F). Thirty-one different types of both 

power and hand-driven fasteners were tested including 

fasteners that used several different galvanizing techniques 

(Table 9). To simulate the way fasteners are used in service, 

the fasteners were driven into the wood and removed before 

being placed in the sawdust environment. The fasteners were 

removed from the environment, cleaned, weighed, and reinserted into the sawdust at regular intervals throughout the 

year. This process of removing, cleaning, and reinserting 

increased the measured corrosion rate because, in most 

cases, the corrosion products partially protect the metal from 

further oxidation. The fasteners were then inserted into the 

sawdust of Douglas-fir treated to a level 10% greater than 

was required by the standards. It is hard for preservative 

treatments to penetrate Douglas-fir, although ACA is more 

able to penetrate it than CCA (Ritter and Morrell 1992). This 

inability to penetrate typically results in a nonuniform zone 

of treatment near the outer edge of the member. A unique 

aspect of Bengelsdorf’s (1983) test is that instead of using 

the sawdust of treated wood, Bengelsdorf treated the sawdust. By treating sawdust, Bengelsdorf (1983) probably had 

the greatest success for a full and uniform penetration of the 

preservative in the wood. It is unclear how corrosion results 

from this uniform treatment relate to corrosion in nonuniformly treated wood. 
16 

Table 9—Fasteners tested by Bengelsdorf (1983) 

Fastener alloy/coating Type 

Application 

method 

Replicates 

S30400 stainless steel Nail Hand 1 

S31600 stainless steel Nail Hand 1 

Aluminum Nail Hand 1 

Bright steel Nail Hand 1 

Commercial bronze Nail Hand 1 

Copper Nail Hand 1 

Double hot-dipped galvanized steel Nail Hand 1 

Hot-tumbled galvanized 

steel 

Nail Hand 1 

Mechanical tin-cadmiumplated steel Nail Hand 2 

Mechanical zinc-plated and 

chromated steel 

Nail Hand 1 

Monel (UNS N04400) Nail Hand 1 

Silicon bronze Nail Hand 1 

Double-coated bright steel Nail Power 1 

Electro-galvanized and 

chromated steel 

Nail Power 1 

Galvanized steel Nail Power 1 

Hot-dipped galvanized steel Nail Power 1 

Mechanical tin-cadmiumplated steel Nail Power 3 

Mechanical zinc-plated and 

chromated steel 

Nail Power 4 

S30400 stainless steel Staple Power 2 

Aluminum (steel core) Staple Power 1 

Bronze Staple Power 1 

Electro-galvanized steel Staple Power 1 

Galvanized steel Staple Power 2 

Indeed, Bengelsdorf (1983) found that his initial results for 

corrosion of fasteners in preservative-treated lumber were 

different than he anticipated; he found that CCA was more 

corrosive than ACA, which was a misleading result more 

biased by treating higher absorbent sawdust than representative of many other lab results. 

It is unclear how much the damp sawdust methods used by 

Wright and others (1957), Bartel–Kornacka (1967), and 

Bengelsdorf (1983) accelerated the corrosion of metals in 

contact with wood. While the previously mentioned publications acknowledged that the corrosion rate of metals in 

contact with damp sawdust was accelerated in comparison to 

metals in contact with wood, there is no physical way to 

extrapolate this data back to normal, in-service conditions. 

Moreover, it is nearly impossible to compare results between 

the three damp sawdust methods because each test was 

slightly different. For instance, Bartel–Kornacka (1967) 

sealed the test vials, reducing the amount of oxygen available for the corrosion reaction and also had an unorthodox 

method to measure corrosion. Contrastingly, Bengelsdorf 

(1983) did not cover the dampened sawdust test specimens 

but increased the temperature to increase the corrosion rate. 

Similar to the accelerated tests, which increase moisture and 

humidity, damp sawdust tests are only able to give relative 

and qualitative results of corrosiveness. However, the current change in preservative treatments requires an accelerated test that can give quantitative results of the corrosion of 

fasteners in wood. 

Salt-Spray Tests 

Salt-spray or salt fog tests are a commonly used and standardized method to test metal parts that will be exposed to 

marine conditions. Richolson (1959), who worked for the 

U.S. Navy’s materials laboratory, was the only published 

researcher to apply these tests to measure the corrosion of 

metals in contact with wood. Richolson (1959) ran tests to 

determine the corrosive effects of five types of wood used in 

shipbuilding on metal fasteners by placing wood–metal 

assemblies in a salt-spray chamber that conformed to ASTM 

(2003a) standard B-117. Richolson (1959) made assemblies 

with every combination of five woods: white oak (Quercus 

abla), teak, mahogany, bald cypress (Taxodium distichum), 

and Douglas-fir (Pseudotsuga menziesii), and five types of 

38-mm (1.5-in.) No. 12 screws: bright steel, galvanized 

steel, brass, chrome-plated brass, and silicon bronze. In 

addition to these combinations, Richolson (1959) also tested 

the affect on corrosion of lubricating the screws prior to 

insertion. Before placement into the salt-spray chamber, the 

heads of the screws were covered with a wooden block in a 

similar manner to the work of Wright and others (1957). 

These wood–metal assemblies were then placed in a saltspray cabinet that was kept at 35°C ± 1°C (95°F ± 2°F) and 

95% ± 2% RH (Fig. 6). After exposure, the corrosion products were cleaned (Table 10) and the weight loss values were 

measured and reported. 

Figure 6—Salt-spray cabinet and specimens of 

Richolson (1959). 
17

Metals Solution

Bright steel 3.5% SbCl3 dissolved in 36% HCl

Brass 

Chrome-plated brass 

Silcone bronze

5% (NH4)3 C6H5O7 dissolved in 

distilled H2O

Galvanized steel None, mechanical cleaning only

Table 10—Methods of cleaning the corrosion products 

used by Richolson (1959)

Richolson (1959) is the only researcher known to the authors 

to use salt-spray methods to determine the corrosion of 

metals in contact with wood. These salt-spray methods can 

be valuable if the wood and metal will be exposed to ocean 

conditions. However; performance in a salt-spray test cannot 

be related to the corrosion performance in inland conditions 

because the addition of the chloride ion can change corrosion mechanisms. In short, care should be taken when evaluating the results of a salt-spray test to accelerate the corrosion of metals in contact with wood, and such tests methods 

should never be used to evaluate corrosion performance for 

wood that is not in contact with salt water. 

Part 3—Electrochemical Tests 

Because corrosion is an electrochemical process, applying an 

electrical current to the corrosion cell can either accelerate or 

decelerate the corrosion reaction. Through this acceleration 

and deceleration, the corrosion rate at a specific condition 

can be determined. Electrochemical methods are attractive 

because they allow for rapid testing to be done in situ; that 

is, they can be run at a temperature and moisture content of 

interest, in any desired geometry. The goal of electrochemical test methods is to measure the current density at which 

the corrosion takes place. Current density can then be converted to mass loss or depth of penetration through unit 

analysis. The corrosion current density, icorr, cannot be 

measured directly. However, several well-established techniques have been developed to measure the corrosion current 

density indirectly. Further explanation of the science and 

theory of electrochemical corrosion testing can be found in 

the third chapter of Jones (1996). 

Simm and Button (1985b) were the first researchers to use 

electrochemical methods to measure the corrosion of metals 

in contact with wood by running experiments in European 

redwood (Pinus sylvestris) treated with CCA. The treated 

wood that was used had been stored for 2 years before it was 

tested. 

Polarization resistance tests were run to measure the corrosion rate for several different metals: mild steel, stainless 

steel, pure zinc, and an aluminum alloy. In addition, several 

tests were run on zinc that had been given a chromium (VI) 

coating for passivation. Before the polarization test was run, 

the wood was soaked in a water bath at 20°C ± 2°C (68 ± 

4°F) for 2 hours to saturate the wood with water and increase 

the conductivity of the wood. Holes were drilled into the 

wood for the electrodes. The counter electrodes were placed 

12.5 mm (0.5 in.) away from the working electrode. A visual 

representation of the corrosion cell used by Simm and 

Button (1985b) is shown in Figure 7. 

Jack and Smedley (1987) ran experiments to determine how 

the corrosion rate of iron and zinc varied with moisture 

content and exposure time. They were the first researchers to 

use another corrosion science technique, electrical impedance spectroscopy (EIS) to analyze corrosion in wood. In 

EIS, a small alternating current is applied to a test cell similar to those used in direct current corrosion measurements 

(Fig. 7 and 8). The frequency of the current is changed and 

the response of the impedance is measured, which can be 

related to the corrosion rate through an equivalent circuit 

model whose impedance is the same as the corrosion cell. 

Jack and Smedley (1987) ran experiments in untreated and 

CCA-treated radiata pine (Pinus radiata). Three different 

metals were tested: copper, zinc, and iron. The wood sat for 

8 weeks after the preservative treatment was applied to 

ensure copper fixation. The wood was then ovendried at 

100°C (212°F) before being cut into slices for use in a sandwich test cell similar to that of Dennis and others (1995) 

(Fig. 8). The sandwich cell was then placed in a humidity 

chamber to achieve the desired moisture content before 

testing. The range of moisture contents tested varied from 

Figure 7—Corrosion cell of Simm and Button (1985b) 

(Used with permission of Corrosion Prevention & 

Control). 
18 

12% to 28%. Experiments were also run in wood saturated 

with water. 

In addition to running EIS experiments, Jack and Smedley 

(1987) also used other methods to measure the corrosion in 

wood including DC steady state, DC polarization, and 

weight loss methods. Insufficient details were given to 

comment on the other methods. 

Cross (1990) used EIS and direct current methods to investigate the corrosion of metals in contact with CCA-treated 

wood in roof environments. Experiments were run on two 

different species of wood, European redwood (Pinus sylvestris) and European spruce (Picea abies), treated with either 

the salt or the oxide formulation of CCA. These pieces of 

wood were exposed to temperatures ranging from 10°C to 

35°C (50°F to 95°F) with RH ranging from 87% to 100%. 

These conditions caused the moisture content of the wood to 

range between 17% and 26%. In addition to these laboratory 

conditions, specimens were also placed in four roof spaces 

in England. 

After the wood had reached equilibrium moisture content, 

metals (mild steel, hot-dipped galvanized steel, zinc electroplated steel, AISI 304 (UNS S30400) stainless steel, and AA 

6063 (UNS A96063) aluminum) were inserted into the wood 

and corrosion measurements were taken using EIS. A unique 

geometry for the working electrode was developed to simulate nail plates (Fig. 9). The data, which are reported in 

micrometers per year, can be used to estimate the relative 

life of the metals because there were no extraneous additions 

to the wood to accelerate corrosion. 

Unfortunately, Cross (1990) is a summary paper; not all of 

the experimental procedures are given, nor are all of the 

results listed. However, the data that are contained in Cross 

(1990) are worth a further review. Even though the 

publication is short on experimental details and results, 

Cross (1990) describes a very clear understanding of the 

factors that affect the corrosion of metals in contact with 

wood, and correspondingly, the experiment of Cross (1990) 

is very well designed. 

Dennis and others (1995) ran experiments to test different 

types of zinc coatings in the presences of preservativetreated wood. Although Dennis and others (1995) wanted to 

test the corrosive effect of CCA, they were unable to obtain 

the arsenic compounds, and therefore, they treated the 

specimens of European redwood with solutions of copper 

and chrome. This solution should be similar in corrosiveness 

to CCA because the arsenic does not seem to play a large 

role in the corrosion of metals in contact with CCA-treated 

wood (Simm and Button 1985a). 

Sixteen types of zinc-coated steel were tested by Dennis and 

others (1995), as well as mild steel, to determine which 

coating was superior. Details of the zinc coatings are given 

in Table 11. The metals were tested by direct current polarization methods, similar to Simm and Button (1985b). However, unlike Simm and Button (1985b), Dennis and others 

(1995) used a sandwich cell to measure the corrosion of 

metals in contact with wood (Fig. 8). Dennis and others 

(1995) were able to successfully measure the change in 

corrosion rate with moisture content. They noted that the 

corrosion rate approaches zero in copper-chrome-treated 

wood when the moisture content nears 15%. However, 

Dennis and others (1995) used direct current methods, which 

need to be corrected for the resistance of the solution. Because the resistance of the solution was changing as the 

moisture content was changing, the corrosion data plotted as 

a function of moisture content are hard to interpret. Another 

complication is that direct current methods give incorrect 

results when the corrosion reaction becomes diffusion controlled. As the moisture content falls below saturation, there 

is a higher likelihood that the reaction becomes diffusion 

controlled and the numbers may be misleading. 

Figure 8—Corrosion cell of Dennis and others (1995) 

(Used with permission of The Institute of Metal 

Finishing). 

Figure 9—Unique working electrode of Cross (1990) 

(Used with permission of E & FN Spon). 
19

Table 11—Zinc coatings tested by Dennis and 

others (1995) and Short and Dennis (1997) 

Type 

Coating composition 

(wt%) Conversion coating 

Zn 100 Chromated (0.05% Cr)

Zn 100 Chromated (0.2% Cr)

Zn 92, Fe 8 None

Zn 43.5, Al 55, Si 

1.5 Chromated 

Zn 43.5, Al 55, Si 

1.5 None 

Galvanized 

Zn 95, Al 5 None 

Zn 100 None 

Zn 100 Blue passivated (Cr) 

Zn 100 Full passivated (Cr) 

Zn 99, Co 1 None 

Zn 99, Co 1 Chromated 

Zn 87.5, Ni 12.5 None 

Zn 87.5, Ni 12.5 Chromated 

Zn 99.4, Fe 0.6 None 

Zn 99.4, Fe 0.6 Yellow passivated (Cr) 

Electroplated 

Zn 99.4, Fe 0.6 Black passivated (Cr) 

Wójtowicz and Piwowarczyk (2002) ran experiments to 

model the corrosion of galvanized metals in contact with 

fire-retardant-treated wood. Hot-dipped galvanized, electroplated zinc with a chromium (VI) coating, and electrodeposited zinc with a chromium (III) coating were tested. The 

thickness of the hot-dipped galvanized coating was 20 μm, 

(0.0008 in.) and the thickness of the electrodeposited coatings was 12 μm (0.0005 in.). Two types of treatments were 

used, which were discussed as A and B throughout the paper. Treatment A contained ammonium phosphate, ammonium sulfate, boric acid, and urea. Treatment B contained 

ammonium phosphate, ammonium sulfate, boric acid, and 

borax (hydrated sodium borate). 

To measure the corrosiveness of these wood fire retardants, 

Wójtowicz and Piwowarczyk (2002) ran direct current polarization measurements as well as EIS measurements. 

Unlike other electrochemical experiments, which ran the 

electrochemical tests in wood, Wójtowicz and Piwowarczyk 

(2002) ran the experiments in an aqueous preservative in 

which sawdust was suspended. This test setup did not allow 

for the direct measurement of the corrosion rate, but the 

purpose of the experiment was to model the performance of 

the zinc coatings, not the corrosion rate. 

Wójtowicz and Piwowarczyk (2002) found that the electrochemical tests worked very well. They were able to classify 

and model the passivation of zinc coatings in contact with 

the fire-retardant treatment and sawdust. They concluded 

that electrochemical methods are far superior to weight loss 

methods because the electrochemical methods allow the 

researcher to model the behavior and classify surface characteristics of the metal. 

Both the direct current polarization and the EIS method have 

been shown to be a viable option for measuring the instantaneous corrosion rate of metal in treated wood (Simm and 

Button 1985b, Jack and Smedley 1987, Cross 1990, Dennis 

and others 1995, Wójtowicz and Piwowarczyk 2002). However, more work is needed to further develop the methods. 

There needs to be a better understanding of ionic conduction 

and resistivity of the wood, as well as the corrosion process, 

before a meaningful EIS model can be fully developed. 

For direct current polarization methods, a direct current is 

applied to the test cell and the current density is measured. 

Because direct current is used and salt-based wood treatment 

is made up of ionic components, the direct current will drive 

the unfixated treatment chemical through the wood and 

permanently polarize it. Therefore, after direct current is 

driven through the wood one time, it is no longer possible to 

derive any useful information from it. 

An additional limitation of direct current polarization methods comes from how the corrosion process changes from 

activation to diffusion controlled in wood materials. A direct 

current polarization resistance experiment is only valid if the 

corrosion reaction is activation controlled and the resistance 

loss caused by the medium can be neglected. At fiber saturation, these assumptions probably hold, but as the moisture 

content is lowered, these assumptions become more 

questionable. 

In direct current polarization experiments, the resistivity of 

the wood fiber and multilayer cell wall structure will be 

important and should be strongly considered when designing 

the test. In a high resistivity medium like wood, the resistance loss caused by the current having to travel 12.5 mm 

(0.5 in.), as it did in the experiment of Simm and Button 

(1985b), could be quite large and dominate the measured 

resistance, which is inversely proportional to the corrosion 

current density, icorr. Grain angle also plays an important role 

because conductivity is greater along the grain than across it; 

it also differs between the radial and tangential directions 

(Forest Products Laboratory 1999). 

In our opinion, it would be experimentally desirable to run 

the experiment such that the current flows parallel to the 

grain because this represents a worst-case scenario and this 

is how most fasteners are in contact with wood. Most fasteners are inserted perpendicular to the grain so that the diffusion of water and ions to the metal surface will occur parallel 

to the wood grain. 

The EIS models derived from the experiments should be tied 

to reality. Jack and Smedley (1987) found that the equivalent 

circuit in Figure 10 fit their data very well. However, the 

paper does not discuss whether the equivalent circuit had 
20 

any physical significance, nor does it try to explain any of 

the components. It is unlikely, however, that the equivalent 

circuit in Figure 10 had any physical significance. Wood is a 

highly resistive media, and in theory, any model involving 

wood should have a resistor in series to account for this. 

Considering that the thickness between the working and 

counter electrodes in Jack and Smedley’s (1987) experiment 

is 5 mm (0.2 in.), the solution resistance term should be 

somewhere between 10 Ω and 1013 Ω. Also, to calculate the 

corrosion current density, icorr, it is necessary to know the 

polarization resistance (Rp). Neither of the resistors in the 

diagram is labeled Rp, nor do they have the resistances of Rp

that were listed in another section of the paper. 

In summary, for EIS models to be appropriate and representative, they must be developed with an understanding of the 

given corrosion process for the given environment and 

geometric parameters. Thus, while more work is needed to 

develop EIS-based methods, EIS is recognized as having 

several advantages. First, the corrosion cell can be made to 

reflect the in-service treatment environment. Second, EIS 

applies an alternating current, which eliminates any permanent polarization of the wood electrolyte or the treatment. 

Third, EIS can be used if the corrosion reaction is diffusion 

or activation controlled. In wood with moisture content 

below the fiber saturation point, the corrosion rate is controlled by diffusion and direct current measurements are no 

longer effective. Finally, it is possible to model the corrosion 

cell by an equivalent circuit whose impedance is the same as 

the corrosion cell. Components of this equivalent circuit can 

then be given physical significance such as the resistance of 

the wood or the dielectric constant of the wood. The corrosion rate in EIS is found through the polarization resistor 

component of the equivalent circuit, which is inversely 

proportional to the corrosion rate. Using the equivalent 

circuit analysis method, it is possible to correct for the resistance losses caused by the electrolyte, which in this case, is 

very important because wood conducts electricity poorly. 

There is a possibility that these models could be used to 

model the corrosion rate in different environments to predict 

the relative corrosion life of different fasteners. 

Discussion 

Exposure tests have the advantage that they give data on 

how fasteners perform in actual service conditions and the 

results are directly applicable to a specific application. However, exposure tests take a long time to complete and are 

very costly to run. A further disadvantage is that exposure 

tests are not repeatable because the weather and climate are 

always changing. 

Accelerated tests can be repeated at any laboratory and give 

results much quicker than exposure tests. However, by accelerating the test, it is possible that the mechanism of the 

corrosion reaction has changed. Even if the same mechanism 

is still occurring, there are significant voids in our ability to 

understand the factors involved, and thus, there is no way to 

develop the model and then relate the results of an accelerated test back to the corrosion rate of service conditions. 

Electrochemical methods show great promise in their ability 

to rapidly evaluate the corrosion of metals in contact with 

wood. Electrochemical tests have numerous advantages 

compared with weight loss methods: they can be run in situ, 

they can be run at any temperature or moisture content, they 

can be used to measure the corrosion rate directly. Moreover, several tests can be run in the same piece of wood 

eliminating variance between different replicates of the same 

species. Additionally, electrochemical tests allow for the 

construction of an equivalent circuit model that would be 

able to extrapolate the current database of corrosion data. 

However, electrochemical tests require expensive equipment 

and a detailed knowledge of electrochemistry. At this point 

in time, electrochemical test methods appear to be the 

best way to measure the corrosion of metals in contact 

with wood. 

The corrosion-related service life of metals in contact with 

wood is not an easy problem to understand or even estimate. 

Caution should be taken in interpreting the results from even 

the most perfectly run corrosion tests. Corrosion is a stochastic process; that is, it depends on the occurrence of random 

events. While it is possible to classify many of the variables 

that have an effect on corrosion and it is possible to design 

tests to avoid these variances, in service, these variables 

cannot be controlled. For example, splits and checks on the 

wood surface can lead to an increase in the amount of water 

and oxygen that reach the fastener, which greatly increases 

the corrosion rate. Experimenters can avert this variable by 

making sure that the wood being tested does not split or 

check during the test. However, any corrosion data from that 

experiment does not apply once a split or check develops in 

service. 

Figure 10—Equivalent circuit model of Jack and Smedley 

(1987). 
21

Conclusions 

Although preservative and fire-retardant treatments prolong 

the life or serviceability of the treated wood, they can also 

accelerate the corrosion of fasteners. This represents a liability issue that needs to be further studied. This paper reviewed the test methods used by twenty different publications and one standard to determine the corrosion rate of 

metal in contact with wood. Currently, in our opinion, electrochemical methods appear to be the most promising 

method to study the uniform corrosion of metals in contact 

with wood because they can be run in situ, they can be run at 

all temperatures and moisture contents, and they directly 

measure the corrosion rate at the condition of interest. 

Acknowledgments 

The authors thank the Federal Highway Administration for 

funding this research. In addition, the authors thank Mary 

Funmaker and Andrzej Krzysik, both of the USDA Forest 

Service, Forest Products Laboratory. Mary Funmaker processed many interlibrary loan requests, and Andrzej Krzysik 

was instrumental in translating the work of Wójtowicz and 

Piwowarczyk (2002).

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