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Reference Publl .

NATL INST. OF STAND & TECH Cat'lOIIS

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NBS TECHNICAL NOTE 1149

U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

|-QC

100

.05753

no .1149

1981

Short-Term Evaluation Procedures

for Coatings on Structural Steel
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on iMarch 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

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Absolute Physical Quantities2 — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

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Engineering and Process Technology2 — Building Technology — Fire Research —

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research and provides scientific and technical services to aid Federal agencies in the selection,

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The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. 'Some divisions within the center are located at Boulder, CO 80303.
NATIOJTAI, STOMA!

OF STANDARDS

LIHRABT Short-Term Evaluation Procedures kt 2 3 m

for Coatings on Structural Steel

'US

Paul G. Campbell

Jonathan W. Martin

Mary E. McKnight

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

Prepared for: U.S. Department of Transportation

Federal Highway Administration Washington, DC 20590

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued September 1981
National Bureau of Standards Technical Note 1149

Nat. Bur. Stand. (U.S.), Tech. Note 1149, 43 pages (Sept. 1981) CODEN: NBTNAE *

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1981

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

$3.00

(Add 25 percent for other than U.S. mailing)
ABSTRACT

This report presents the findings of the first of a two-phased study to aid

the Federal Highway Administration in evaluating and selecting protective coatings for steel. The objectives of the study are to review existing short-term

test procedures for selecting coatings, and to discuss analytical measurement

techniques for characterizing coating systems and for monitoring coating

degradation.

In assessing current accelerated aging testing procedures, several deficiencies

became apparent. These included the reported lack of reproducibility in the

rankings for different iterations of the same short-term test and the lack of

correlatability between the rankings of short-term laboratory and long-term

outdoor exposure tests.

It was concluded that, at the present time, coating manufacturers and users do

not depend heavily on accelerated test results for making durability assessments. Instead, durability assessments are based on outdoor exposure performance. Recommendations are made to design future short-term test procedures

using reliability theory.

Keywords: accelerated aging tests; coatings; corrosion; time to failure;

predictability; reproducibility.

iii

TABLE OF CONTENTS

Page

1 . INTRODU CTION 1

1 .1 Background 1 1 .2 Objectives . 1

2. ASSESSMENT CRITERIA FOR SHORT-TERM ACCELERATED AGING TESTS 3

2 .1 Proposed Criteria for Assessing Coating Short-term

Accelerated Aging Tests 3

2 .2 Criteria for Assessing Currently Available Short-term

Accelerated Aging Tests 4

3

.

REVIEW OF ACCELERATED AGING TESTS 5

3.1 Salt Spray 5 3.1.1 Description 5 3.1 .2 Extent of Usage 5 3.1.3 Time to Failure 5 3.1.4 Surface Preparation 6

3.1.5 Modes of Failure 6

3 .1 .6 Properties Measured 7 3 .1 .7 Assessment 7 3 .2 Cyclic Weathering Tests 7 3.2.1 Description 7 3 .2 .2 Extent of U sage 8

3.2.3 Time to Failure 8

3.2.4 Modes of Failure 8

3.2.5 Properties Measured 8

3.2.6 Assessment 8

3.3 Cyclic or Sequential Tests Which Include Salt Spray 10

3.3.1 Description 10

3.3.2 Extent of Usage 10

3.3.3 Time to Failure 11

3.3.4 Modes of Failure 11

3.3.5 Properties Measured 11

3.3.6 Assessment 11

3 .4 Condensation Tests 11

3.4.1 Description 11

3.4.2 Extent of Usage .... 12

3.4.3 Time to Failure 12

3.4.4 Modes of Failure 12

3.4.5 Properties Measured 12

3 . 5 Immers ion 12

3.5.1 Description 12

3.5.2 Extent of Usage 12
TABLE OF CONTENTS (cont.)

Page

3.5.3 Time to Failure 13

3.5.4 Modes of Failure 13

3.5.5 Properties Measured 13

3.6 Kesternich Test 13

3.6.1 Description 13

3.6.2 Extent of Usage 13

3.6.3 Time to Failure 13

3.6.4 Modes of Failure 14

3.6.5 Properties Measured 14

3.7 Cathodic Test 14

3.7.1 Description 14

3.7.2 Extent of Usage 14

3.7.3 Time to Failure 14

3.7.4 Modes of Failure 14

3.7.5 Properties Measured 14

4. MEASUREMENT TECHNIQUES FOR EARLY CHARACTERIZATION

OF DEGRADATION 15

4.1 Adhesion 15

4.1.1 Significance of Property 15

4. 1.

2

Measuring Techniques 15

4.

2

Electrical Measurements 16

4.2.1 Significance of Property 16

4.2.2 Measuring Techniques 17

4.2.2.1 D.C. Resistance Measurements 17

4.2.2.2 A.C. Resistance Measurements 17

4.2.2.3 Capacitance Measurements 17

4.

3

Permeability Measurements 17

4.3.1 Significance of Property 17

4.3.2 Measuring Techniques 18

4.

4

Mechanical Stress Measurements 18

4.4.1 Significance of Property 18

4.4.2 Measuring Techniques 19

4.4.2.1 Fixed Film 19

4.4.2.2 Free Films 19

4.

5

Chemical Techniques 19

4.5.1 Significance of Properties Measured 19

4.5.2 Measuring Techniques 19

4.5.2.1 Ellipsometry 19

4.5.2.2 X-Ray Photoelectron Spectroscopy 20

4.5.2.3 Infrared Spectroscopy 20

5. SUMMARY AND CONCLUSIONS 22

6

.

RECOMMENDATIONS 23

VI
TABLE OF CONTENTS (cont.)

Page

7 . REFERENCES 24

APPENDIX A — RELIABILITY-BASED ACCELERATED AGING TESTS . . . 35

Table 1. Observed Times to Failure in Weathering Machines

ACKNOWLEDGMENT

The authors gratefully acknowledge the contributions of the many researchers

that were visited or contacted in compiling information for this report.

vii

1. INTRODUCTION

1.1 BACKGROUND

The high frequency of failures of protective coatings on State and local

highway bridges, signs, barriers, etc., contributes to the large economic

losses from corrosion experienced in the United States. A recent report,

proposing a model for predicting corrosion costs on highway structural steel

[1], estimated that the total expenditure for corrosion protection of steel

bridges amounts to $130 - 160 million per year; and according to the author's

model, this expenditure should be doubled to obtain maximum bridge service

life.

An essential first step in maximizing service life is selecting the best

available coating for the expected environmental and operating conditions. In

making this selection, quantitative estimates of the service lives of candidate

systems should be available for the wide range of environmental conditions

experienced in the United States. Traditional sources for obtaining estimates

are short-term laboratory tests and outdoor exposure tests. Currently, outdoor

exposure results are heavily relied upon for service life predictions.

Over the last few years, however, changes have occurred jeopardizing the

reliance on outdoor exposure tests for durability assessments. These changes

have resulted from legislation aimed at 1) reducing solvent emissions (Federal

Clean Air Acts of 1970 and 1977) and 2) lessening worker exposure to hazardous

materials (Occupational Safety and Health Act, and Hazardous Material Acts).

The effect of this legislation, along with oil import restrictions, has been

to supplant many traditional coating systems with systems having little or no history of outdoor exposure. These new coatings systems often require improved

surface preparations and new application techniques. The quality and protection

resulting from these changes must be determined, increasing the need for quick,

reliable, durability assessment procedures. Long-term, outdoor exposure tests

cannot meet this need due to the length of time required for obtaining results.

The need can be met through the development of reliable short-term evaluation

procedures.

1.2 OBJECTIVES

The purpose of the overall study is to aid the Federal Highway Administration

in evaluating and selecting protective coatings for use on steel by identifying

or developing improved short-term test procedures.

The study was divided into two phases having the following generalized

objectives:

1. To evaluate the validity and reproducibility of existing

short-term tests for predicting the service life of coatings

on steel;

2. To develop, where needed, improved short-term tests; and
3. To develop, based upon the improved test methods, a guide

for evaluating and selecting protective coatings for steel.

Phase I of the study, which is summarized in this report, addresses the

following:

1. Development and justification of a set of criteria for evaluating

the adequacy of accelerated aging tests for coating systems;

2. Review of currently used accelerated aging procedures;

3. Assessment of these procedures relative to the proposed criteria;

and

4. Propose research needed for developing improved accelerated aging

test procedures capable of satisfying the proposed criteria.
2 . ASSESSMENT CRITERIA FOR SHORT-TERM ACCELERATED AGING TESTS

Accelerated aging tests serve two functions. For a particular end-use application, users of coatings employ them in making decisions between different coating systems. Manufacturers of coatings, on the other hand, use them in making

decisions aimed at developing more durable coating formulations. The extent to

which accelerated aging test results are useful in making these decisions depends,

to a large degree, on the criteria used in setting up the experiment. Prior to

large scale testing, therefore, it is important to state the set of criteria

used in developing an accelerated aging test and in assessing the results.

In the process of reviewing these criteria, two sets were identified. The

first is termed the proposed set. It is the set of criteria that the authors

feel best insure that the maximum benefit is derived from the accelerated aging

program. These criteria are currently used or are being considered as the

basis for accelerated aging tests in many industries [4,5,6,7,8,9]. The second

set of criteria more closely reflects the ones currently used in the coating

industry. This is the set against which the short-term tests in section 3 are assessed. In the recommendations section of this paper, it is proposed that

future accelerated aging tests be developed on the basis of the first set of

criteria.

2.1 PROPOSED CRITERIA FOR ASSESSING COATING SHORT-TERM ACCELERATED AGING TESTS

Short-term tests should 1) provide results which aid in making quantitative

service life predictions under service conditions; 2 ) be capable of distinguishing

between the durability performances of two or more coating systems under all

environments, and 3) provide a means for monitoring modifications in a coating

system performance because of changes in formulation, surface preparation, etc.

In order to meet these expectations, the following criteria are proposed:

• PREDICTABILITY - Accelerated aging procedures should provide quantitative service life predictions for the coating at its expected

environmental operating conditions (see Appendix A).

• REPRODUCIBILITY - Once an accelerated aging procedure is specified,

it should be reproducible and provide methods for testing for

reproducibility.

• DISTINGUISHABILITY - The procedure should provide a methodology for

making quantitative comparisons between the performance of different

coatings. This would be useful in determining the superiority of one coating system over another and in ascertaining if improvements

in the performance of a coating have occurred e.g., due, to changes

in formulation, surface preparation, application, etc.

• EXTENDABILITY - The analysis procedure should allow for more than

one stress type. This should provide more realistic service life

predictions, permit sensitivity analyses to be made on the effect

of each stress type, and provide an opportunity for examining

stress interactions.
One theoretical approach capable of satisfying these criteria is presented in

Appendix A.

2.2 CRITERIA FOR ASSESSING CURRENTLY AVAILABLE SHORT-TERM ACCELERATED AGING

TESTS

After reviewing the literature and after talking with personnel at many

different coating laboratories, it became apparent that the criteria stated in

section 2.1 are not applicable for assessing currently available accelerated

aging tests. Current accelerated aging tests were developed using the concept

of a "comparison" test as opposed to "predictability" tests (section 2.1). In

comparison tests, specimens from multiple different coating systems are subjected

to the same accelerated and outdoor test environments. The coating systems in

both the accelerated and outdoor tests are then ranked. A good accelerated

aging test is one in which the ranking is similar in both the accelerated and

outdoor exposures. For a new coating system, the expected life is measured

against control coatings exposed to the same environment. Control coatings are coatings having a known track record.

Comparison tests differ from predictability tests, therefore, in several key

ways. First, predictability should be replaced with the term correlatability

,

since the measure of agreement is one of association. Reproducibility has the same meaning in both contexts; but distinguishability has a more qualitative

meaning since coating systems are ranked relative to each other. In comparison testing, the term extendability has little meaning, since the objective is

to develop an accelerated test good for all outdoor environments.
3. REVIEW OF ACCELERATED AGING TESTS

A successful durability procedure has two essential components: 1) an accelerated aging test procedure, and 2) a measurement procedure for monitoring

material degradation. This chapter focuses upon currently used accelerated

aging procedures. Degradation measurement techniques are reviewed in chapter

4.

3.1 SALT SPRAY

3.1.1 Descrip tion

The salt spray test [19] was originally designed to test the durability of

metallic coatings exposed to ocean spray. Over time, it has evolved as the

most popular accelerated aging test for organic coatings covering metallic

surfaces. Basically this test entails spraying coatings housed in a special

cabinet with a 5 percent sodium chloride solution at 35 °C. The procedure is

popular because coating failures usually occur in a short time.

Many variations of this salt spray test exist. Sodium chloride is reported to

be more corrosive than seawater and to produce corrosion products different

from those formed from either seawater or calcium salts [20, 21]. Example of some variations follow. The British continuous salt spray test (BS 3900 Part

F4-1968) [22] uses a synthetic seawater solution (NaCl, 26.5 g; MgCl2 , 2.4 g;

MgS04, 3.3 g; KC1, 0.73 g; NaHC03 , 0.20 g; NaBr, 0.28 g; CaCl2 , 1.1 g; water

to 1000 ml). Other salts are also used to simulate other service environments.

The acetic acid/salt spray test, (ASTM B287-74) [23] specifies a solution of

NaCl and acetic acid (pH 3.1-3.3). The copper-accelerated-acetic-acid-salt

spray test, (CASS test, ASTM B368-68) [24] designates a 5 percent NaCl solution

containing copper (II) chloride acidified with acetic acid.

Harrison [26] utilized 3.25 percent (NH4) 2 S04 and 0.25 percent NaCl solution in

the salt spray test for simulating an industrial environment. He reports that

his results correlate better with 14 years outdoor exposure than salt spray

data resulting from a 3.5 percent NaCl solution. The ARE salt droplet test

[27] (BS 1391:1952) requires a daily spray of a synthetic sea salt solution

(NaCl, 23 g; Na2 S0 4 .10 H2 0, 8.9 g; MgCl»6H2 0, 9.8 g; CaCl2 , 12 g; in 1 liter

of solution) on coated panels. After spraying, the specimens are covered to

prevent evaporation. In the above tests, temperature is controlled.

3.1.2 Extent of Usage

Salt spray testing is widely used to test coatings for corrosion protection. A

salt spray test is included in nearly all steel coating specifications. Most

industrial laboratories utilize the salt spray testing in evaluating new

formulations

.

3.1.3 Time to Failure

Time to failure in the salt spray test depends on many factors: type of steel

(hot or cold rolled), surface preparation, coating (resin and pigment),
thickness of coating, and inclusion or exclusion of a top coat. Testing

times for alkyds are typically up to 1000 hours, longer for epoxies, and up to

7 months for inorganic zincs. Comparative performance of different binders in

the salt spray test is discussed by Appleman and Campbell [29].

3.1.4 Surface Preparation

Surface preparation is an important factor in obtaining meaningful experimental

results. Representatives from industrial laboratories commented that the ranking

of coatings after salt spray testing would agree quite well with the ranking

of exterior exposures if proper attention were given to surface preparation.

The thickness of the coating is also an important factor in establishing reliable results in the salt fog test. A critical film thickness of 2.0 mils (50

ym) is indicated in the PACE study [31] and in work reported by Desea [39].

ASTM D609-73 [29] covers procedures to be followed in the preparation of steel

panels. ASTM D1014-66 [30] describes methods to minimize the influence of

variations in steel surfaces on any series of tests.

Surface preparation depends on the type of substrate to be coated. Both cold

and hot rolled steel panels are used [31, 32, 33, 34]. Solvent cleaned cold

rolled steel is justified when cold rolled steel is the substrate to be coated.

Cold rolled steel panels are readily available and quite uniform. Hot rolled

steel should be used, however, when coatings are to be applied to structural

steel, since structured steel is hot rolled. Hot rolled steel panels typically

have non-uniform surfaces. Both blast and hand cleaned hot rolled steel panels

were used in the PACE study [31]. Hospadaruk [38], et al ., suggested a large

variation in salt spray performance of coated steel was due to carbonaceous

residues. Their report shows that failure in a 240 hour salt spray test

correlates strongly with high surface carbon concentration, substantiating the

need for controlling steel surface cleanliness.

Surface preparation is similar for all the accelerated tests discussed in this

chapter. Any requirements particular to an accelerated test will be discussed

in connection with that accelerated test.

3.1.5 Modes of Failure

A scribe mark (ASTM D1654-79a) [40] is frequently made through the cured coating

prior to testing.

Failures appear as undercutting at the scribe, rusting, and to some extent

blistering. Excess amounts of chloride can act to depassivate the surface

interfering with inhibitive pigment performance. The availability of an electrolyte permits sacrificial coatings (e.g., zinc rich coatings) to effectively protect the substrate, but at the same time a less protective film may

be formed by the reaction products. A difference in zinc reaction products

between salt spray testing and exterior exposure has been observed microscopically [41].
3.1.6 Properties Measured

Following salt spray exposure, panels are visually inspected for corrosion,

glistering, and undercutting. ASTM standards (D610-68, rusting and ASTM D714,

blistering [28]) are available for these properties. The NACE Technical Practice

Committee T-6 reports that the above ASTM methods are sufficiently standardized

to provide meaningful results. Besides visual ratings, other evaluative

procedures are sometimes used after salt spray testing. These include adhesion

testing, identification of corrosion products under the film by energy dispersive

X-ray, microscopic inspection, and weight change of the panel.

3.1.7 Assessment

Results of salt spray testing are very sensitive to operating conditions.

Variables that must be carefully controlled include uniformity of the environment throughout the cabinet, angle of the panels with respect to the vertical,

nozzle design, atomization pressure, humidity of the air used for atomization,

pH, purity of salt solution, and specimen collection rate [42]. Variations in

these operating conditions among cabinets can account for poor reproducibility.

Stone [43] reported a coefficient of variation (standard deviation/mean)

between 0.1 to 0.3 for three primers. More definite information will soon be

available from a committee of the National Association of Corrosion Engineers,

which recently conducted an interlaboratory test.

Reports vary as to whether salt spray coating performance results correlate

with exterior exposures. Variations in the rankings of coatings exposed to

both salt spray environment and exterior environment often occur. Statistical

studies aimed at elucidating these variations were not found. Most users look

for qualitative agreement between the two exposures or for dramatic reversals.

Examples of reversals are cadmium and zinc coatings [44], inorganic and organic

zinc primers [45], and several coatings tested in the PACE study [31].

Comments from the authors of the PACE report [31] express the feelings of both

users and producers in relation to the salt spray. They concluded that on the'

whole, "good" paints will usually do well in the salt fog and "poor" paints

will usually do poorly, but the exceptions are far too numerous to justify

basing decisions solely on salt spray testing methods.

3.2 CYCLIC WEATHERING TESTS

3.2.1 Description

Weathering tests, i.e., carbon or xenon arc or fluorescent lights with humidity,

temperature and wetness control, are commonly used to characterize chalking,

change in gloss, and change in color for coating systems. Typical cycles [46] are described in ASTM standards D822 , D3361-74, G53-77, and G26-77. The

development and use of weathering tests was reviewed recently by Ellinger [47].
3.2.2 Extent of Usage

Weathering tests are commonly used for gloss and chalking determinations. Few

reports have been made on the use of weathering cycles for the evaluation of

coatings for corrosion control; some doubt exists as to whether these tests are good indicators of primer corrosion control [48].

3.2.3 Time to Failure

Exposure times in most weathering cycles, excepting the Dew cycle and

fluorescent UV-condensation type (ASTM G53-77) exposure, are from 1000-2000

hours. Dew cycle exposures are often between 50-150 hours. UV-condensation

type exposure times depend on the cycle used; typical exposure times fall

between 250 to 1000 hours.

In table 1, representative failure times for coatings in several weathering

cycles are listed. Failure is designated as a 50 percent loss of gloss. While

chalking also occurs during cyclic weathering exposure, quantitative failure

times based on extent of chalking were not found in the literature.

3.2.4 Modes of Failure

Little evidence of rusting or blistering is reported after cyclic weathering.

Failure is usually reported in terms of the amount of loss of gloss and degree

of chalking. Degradation usually results from a combination of ultraviolet

light, temperature, and humidity [56]. Chalking occurs if a binder is

degraded, thereby, releasing pigment.

3.2.5 Properties Measured

Panels are examined for loss of gloss, color change, and chalking (ASTM 659-74

for chalking, ASTM 523-78 for gloss, and ASTM D1729-69 (1974) for color change)

[57]. Chalking is sometimes determined by weighing panels before and after

removal of the chalk. Color change can be measured spectrophotometrically.

Lindberg [54] examined the surfaces of several systems (including epoxy,

acrylic, polyurethane, and fluoropolymer) using scanning electron microscopy

after several hundred hours cyclic weathering exposure and several months

outdoor Florida exposure. He also made gloss (ASTM D522) and loss of weight

measurements. He concluded gloss measurement is not a good indicator of resin

degradation and that gloss measurements should be supplemented with weight loss

measurements.

3.2.6 Assessment

The reproducibility of these tests could not be ascertained. Papenroth [58]

has shown that variations in UV irradiance in a weathering machine cause differences in chalking and change of gloss. The ranking of panels exposed in

accelerated weathering cycles and outdoor environments often differ [49, 53,

54]. However, some researchers have reported good correlation [49-54].
Table 1 . Observed Times to Failure3 in Weathering Machines

Xenon Arc uv Resin Twin Carbon Arc Dew Cycle 40-45% RH cond

.

alkyd 300-2000 hrsc > d > f >8 75d >8 200-500* 240^

epoxy 400-2000 hrsc > d 50-200d >8 400-600d

polyurethane 1000- > 2000 hrs c 50-90C» d > 12 00c

acrylic 600- > 1000 hrs c > f >8 35-2 008 400-1000C 740^

(thermosetting)

vinyl, chlori- > 700 hrs f 200d 200-300d » f

nated rubber

a failure defined as 50 percent loss in gloss

b Ellinger, N., J. Oil Col. Chem. Assoc, 62_, 136 (1979) [49].

c Hoey, et al .

, J. Oil Col. Chem. Assoc, 5_7_, 151 (1974) [50].

d Lindberg, J. J. Oil Col. Chem. Assoc, 58, 399 (1975) [54].

e Oakley, E., et al., J. Oil Col. Chem. Assoc, _57_, 221 (1974) [52].

f Nowacki, L. J., Official Digest, 34_, 1191 (1962) [53].

8 Stanton, J. M., J. of Paint Technology, 39, No. 514, (1967) [55].
3.3 CYCLIC OR SEQUENTIAL TESTS WHICH INCLUDE SALT SPRAY

3.3.1 Description

Specialized tests have been designed which include exposure to salt spray as part of the cycle. The prohesion test described by Timmins [59] includes six

periods of alternating salt spray and ambient air drying. The salt spray is

Harrison's (NH4)2S04~NaCl [26] mixture. One major paint company designed

its own exposure cabinet in which the samples are exposed to one hour 5 percent

salt spray, one hour UV, and a one hour darkness cycle. The Texas Highway

Department uses a cycle of 24 hour-heat lamp exposure (60°C black panel

temperature) and 48 hour salt spray (20 percent NaCl). The Louisiana Highway

Department utilizes an 18 percent salt spray at 67 °C for 4 weeks to evaluate

coating systems on steel. The Michigan Highway Department has developed a weathering cycle of 200 hours UV-condensation exposure, 50 hours salt spray,

and 5 freeze-thaw cycles. Hospadaruk's cycle [38], which accelerates degradation

compared to salt spray testing, consists of 0.25 hour immersion in 5 percent

NaCl (air saturated), 1.25 hour at room temperature, and 22.5 hours in a humidity

cabinet at 49°C and 85 percent relative humidity. Other variations of this

test include the above plus a weekly freeze/dry cycle, or dusting the panels

with kaolin powder before testing, or a temperature of 60°C in the humidity

cabinet.

Tooke's cycle [60] consists of rotating panels in a chamber equipped with

ultraviolet lamps and nozzles for spraying synthetic seawater solution [22].

The UV light in Tooke's cycle is on for 40 minutes and off for 40 minutes.

Panels are also sprayed for 30 seconds every 15 minutes. Alumbaugh and Hearst

[61] developed a cyclic test immersing specimens in aerated synthetic seawater

for 1 1/2 hours and drying them in warm air (50°C) for 1 1/2 hours.

Another cyclic test (ASTM D2933-74) [62] combines 4 hours testing in a salt

spray, 18 hours at 37.8°C and 100 percent relative humidity and 2 hours in a freezing chamber at -28.3°C. Sakae, et. al. [63] describe several sequences of

tests for evaluating corrosion control coatings. These cycles usually include

exposure to a carbon arc or xenon arc light, exposure to a salt spray, and

exposure to an SO2 environment. The Port Authority of New York uses a nine step

cycle test which includes: 1) 168 hours in a xenon arc accelerated weathering

machine, 2) 15 freeze/thaw cycles, 3) 16.5 hours salt fog, 4) 72 hours accelerated

weathering, 5) 15 freeze/thaw cycles, 6) 5.5 hours salt fog, 7) 120 hours

accelerated weathering, 8) 5.5 hours salt fog, and 9) 120 hours accelerated

weathering. They report being able to distinguish some performance differences

not distinguishable with salt spray.

3.3.2 Extent of Usage

Most of these tests are relatively new and not standardized; usage is very

limited.

10
3.3.3 Time to Failure

In the prohesion test, primers are tested for 48 hours; complete systems are tested for 2000 hours [59]. In Tooke's [60] accelerated environment, alkyd

primers usually failed (ASTM scribe rating of 5) between 400 and 1000 hours.

Hospadaruk et al. [38] report their cycle with kaolin dust caused failure

(25 percent surface rust) in 600 hours for a maleinized oil primer and in 2000

hours for an epoxy primer. Alumbaugh and Hearst [61] report vinyls and epoxies

failed in their test cycle between 700 and 4500 hours. In the Michigan Department of Transportation, cyclic test coatings are typicaly exposed for 300 hours.

3.3.4 Modes of Failure

Since some of the cycles described in this section include ambient environmental exposure, as well as exposure to high humidity, light, and variable of

temperature, many failure modes are observed. Typical failures include blistering, rusting, undercutting at the scribe, chalking, and loss of gloss.

3.3.5 Properties Measured

Properties measured after sequential testing are similar to those measured

after salt spray and cyclic testing. Coatings and substrates are evaluated

for indication of rusting, blistering, flaking, undercutting at the scribe,

chalking, and loss of gloss.

3.3.6 Assessment

No specific information pertaining to the assessment is available for this or the following tests. The previously made comments about lack of information on reproducibility and correlatibility apply here and to the following accelerated

tests.

3.4 CONDENSATION TESTS

3.4.1 Description

In this test, specimens are exposed to 100 percent relative humidity at a constant temperature. This as often done by exposing the coated side of a test

specimen to warm, moist air while keeping the back side of the panel at ambient

temperature. Condensation must cover the coatings at all times. Corrosion

is dependent on the time of wetness [65]. The temperature prescribed in ASTM

D2247-68 (1973) [66] is 38 + 1°C. Sometimes temperatures of 49°C or 60°C are used [38, 67]. Increasing the temperature from 49°C to 60°C increases the

rate of corrosion 150 to 200 percent [38].

Van Laar [68] varied the humidity test by making the atmosphere rich in HC1,

H2SO4, or CH3COOH along with high humidity and elevated temperatures. The HC1

atmosphere was the most corrosive, followed by H2SO4 and CH3COOH.

11
3.4.2 Extent of Usage

This test is frequently used by industrial laboratories for testing coatings

for corrosion protection. Several highway departments use a humidity testing

method as part of their evaluation procedures. One highway department places

coated panels in a concrete curing chamber at a high humidity (95 percent) and

elevated temperature 65°C for 300 hours.

3.4.3 Time to Failure

For high build vinyl coatings, Pila [33] found no blistering or corrosion

after 800 hours humidity exposure for most pigments tested. Geld [69] reported

slight surface deterioration for vinyl or coal tar coatings after 1400 hours of room temperature exposure to 100 percent humidity. However, considerable rusting of substrate surfaces was found after the coating was removed. Several

experimental acrylic and alkyd thermosetting coatings failed in 300 to 400

hours [70] . A coal tar epoxy was found to protect the substrate for 1000

hours [71].

3.4.4 Modes of Failure

The conditions in a humidity cabinet are favorable for water permeation, leaching of soluble components, softening, and swelling of coatings. Diffusion of

water through coatings occurs rapidly. This reaction is driven osmotically by

the presence of soluble components at the coating/substrate interface. Soluble

components include contamination present before coating application, diffusion

of ions or gases which diffused through the coating, and trapped solvent. The

presence of those contaminants can lead to blister formation and rusting.

3.4.5 Properties Measured

The specimens are usually evaluated using ASTM procedures for blistering and

rusting. Additional information can be obtained if the coating is removed

after testing and the substrate examined.

3.5 IMMERSION

3.5.1 Description

An immersion test for organic coatings on structural steel is described in ASTM

D870-54 (1973) [73]. The test specimens are partially immersed in the test

liquid (i.e., distilled water, salt solution, acid, base, etc.) and maintained

at a temperature of 37.8 + 1°C. The test liquid is changed as required to

avoid contamination. Temperature is often a variable.

3.5.2 Extent of Usage

Immersion testing is conducted on coatings that will be subjected to immersion

in service.

12
3.5.3 Time to Failure

Some experimental one-pack epoxy primers immersed in distilled water started

blistering in less than 150 hours. However, rusting occurred only after 150

hours [32]. Two-pack epoxy primers protected the steel substrate for over

2000 hours. Vinyl coating systems showed no rusting after 6190 hours [31].

Linseed oil and linseed oil alkyd coatings have also been reported to show

little blistering after 350 hours [71]. Some phenolic and two-part epoxy

coatings showed little rusting or blistering after 600 hours [32]. Vinyl

coatings showed no blistering after 5700 hours immersion in seawater [31].

3.5.4 Modes of Failure

The modes of failure of coatings due to immersion are similar to those in the

humidity cabinet. Water permeates the coating causing swelling, leaching of

soluble components, and loss of adhesion. Rusting is probably controlled by

the availability of oxygen.

3.5.5 Properties Measured

As with corrosion tests, coatings are examined for blistering and corrosion.

Sometimes coatings are stripped so that the substrate can be examined. Tooke

and Montalvo [74] followed the loss of adhesion as a function of time of immersion. Adhesion for all primers studied (oil, oil alkyd, alkyd, and melamine

alkyd) fell off rapidly with immersion time.

3.6 KESTERNICH TEST

3.6.1 Description

This test was designed to simulate weathering in industrial environments. The

stresses include elevated temperature, 100 percent relative humidity and S02*

The Kesternich test cycle is described in standard DIN 50018 [75]. Samples

are placed in a cabinet in which water and SO2 are introduced and the cabinet

is heated to a 40°C. After eight hours of exposure, the door of the cabinet

is opened and the specimens are exposed to room conditions for 16 hours.

3.6.2 Extent of Usage

Although often reported in the European literature [76, 77], the test is not

widely used in the United States.

3.6.3 Time to Failure

Kesternich testing is usually conducted for 240 to 1000 hours, At the end

of the exposure time, the panels are evaluated. After 240 hours, Chess [76]

reported that coatings on galvanized steel showed partial to total

deterioration.

13
3.6.4 Modes of Failure

It has been shown that SO2 permeates free paint films [12]. It is, therefore,

possible that ions may be formed at the coating/substrate interface by the

reaction of water and oxidizers with SO2 . Chemically, ions on the metal surface would cause an increase in osmotic pressure and interfere with passivating

properties of inhibitive pigments, thus promoting corrosion.

3.6.5 Properties Measured

As with the salt fog test, panels are evaluated for blistering, rusting, and

undercutting of the coating at the scribe or edge.

3.7 CATHODIC TEST

3.7.1 Description

Cathodic tests, often called "bullet hole" tests [78], are designed to test

the protective qualities of sacrificial coatings. The test specimens are prepared in such a way as to leave an uncoated circular or rectangular area in

the center of the panel. Exposed edges of the panel are frequently coated with

wax to prevent corrosion. The specimen is then immersed in a liquid (such as synthetic seawater or 3 percent NaCl) at room temperature until corrosion is

observed in the uncoated area, the "bullet hole." For zinc rich coatings, the

zinc becomes the anode, while the exposed steel substrate becomes the cathode.

As long as an anodic reaction does not occur on the steel surface, corrosion

is prevented.

3.7.2 Extent of Usage

This test is sometimes used by coating manufacturers to assess the ability of

sacrificial coatings to provide cathodic protection. It is to be included in

specifications developed by the Navy and is being used in the evaluation of

zinc rich coating extenders.

3.7.3 Time to Failure

These tests are usually carried out until corrosion occurs in the"bullet hole."

The varying specification requires that coatings shall prevent corrosion in the

base metal area for a minimum of 30 days immersion in synthetic seawater.

3.7.4 Modes of Failure

Failure occurs when the zinc is consumed. The steel substrate then becomes

anodic and rusting occurs. This test does not take into account the protective

coating formed by the zinc reaction products.

3.7.5 Properties Measured

Only the time for rust to appear in the "bullet hole" is measured.

14
4. MEASUREMENT TECHNIQUES FOR EARLY CHARACTERIZATION OF DEGRADATION

The accelerated test methods described in chapter 3 have many shortcomings.

One of the most significant is that the relationship between performance in the

accelerated weathering environment and service environment is not well defined.

Currently, more sensitive evaluation techniques are being developed. These

techniques include measures of adhesion, electrical properties, permeabilities,

mechanical stress, and chemical changes.

4.1 ADHESION

4.1.1 Significance of Property

Since it is thought that corrosion does not proceed to any extent as long as the coating adheres firmly to the substrate [12, 18], changes in adhesion

should relate to durability. However, these measurements must be done carefully

and under controlled conditions as adhesion of coatings to a substrate depends

on environmental conditions [85]. Adhesive strength usually decreases when

coatings are in a humid or wet environment [81, 82] and recovers to some extent

after the coatings are placed in a dry environment [86]. The ability of a coating to maintain good adhesive properties under wet conditions is thought to

be very important with respect to its ability to provide corrosion protection

[12].

4.1.2 Measuring Techniques

Bullett and Prosser [87] have recently reviewed the available adhesion

measuring techniques. Among these, two are ASTM standard test methods — ASTM

D2197 and 3359. ASTM D2197-68 (reapproved 1979) [88] covers the determination

of adhesion by a scrape tester method or by a parallel groove adhesion tester.

In the scrape tester method, adhesion is determined by pushing the coated

panels beneath a rounded stylus or loop which is loaded in increasing amounts

until the coating is sheared from- the substrate surface. ASTM's precision

statement says that the results agree well with other qualitative evaluation

methods. In the parallel groove method, adhesion is evaluated in terms of the

closeness with which parallel grooves can be cut through the coating before

the intervening film is lifted from its substrate. ASTM's statement of

precision says that 95 percent of the time the same operator should be within

two microknife adhesion units for duplicate tests; measurements made by

different operators in different laboratories should be within four units.

Tape test methods (ASTM 3358 [89]) are commonly used in the industry for

measuring adhesion. In one, an x-cut is made through the coating over which a pressure sensitive tape is applied and removed; adhesion is assessed qualitatively on a scale of zero to five. In the other method, a matrix pattern,

using six or seven cuts in each direction is made through the film. Pressure

sensitive tape is applied over the matrix and removed. Adhesion is evaluated

by comparison with descriptions and illustrations on a zero to five scale.

Both methods are repeatable to within one rating and reproducible to within

two ratings at the 95 percent confidence level, according to the ASTM statement

of precision.

15
The most commonly used measure of adhesion, which gives a measure of the force

needed to remove a coating, is a tensile or pull-off method. Standard methods

for pulloff adhesion tests are described in ISO 4624-1978 (E) [90] and DIN

53232 [91]. In both standard test methods, two different test assemblies are described. In one test, a coating is applied to the top of a rigid cylinder

(approximately 1 cm in diameter) to which a second cylinder of the same size

is adhered. In the other test, a rigid cylinder is glued to a coated panel

having greater dimensions than the cross-sectional area of the cylinder. The

amount of force needed to pull the test assemblies apart is measured and the

mode of failure is noted. For both procedures, results are invariant with

cylinder diameter ranging from .6 to 2.5 cm.

For these two tests, the stress distributions are different [92]. Edge effects

in the second test contribute to an apparent lower adhesion value and to a greater experimental variation.

The Joint Service Research and Development Committee on Paints and Varnishes

[93] sponsored an extensive study carried out to investigate the direct pulloff techniques. Surface preparation, paint application, and temperature were

factors affecting interlaboratory reproducibility. Proper alignment and rate

of loading also affect variability. Sickfield [92] reports a range of 5 to

15 percent from the mean for coatings tested between two cylinders and a range

of 10 to 30 percent of the mean when the cylinder is glued to a panel. Others

report similar variations in test results [94].

Two field tensile testers are the Elcometer-'- and the torque wrench. The

Elcometer exerts a tensile stress through the action of a spring. Experimental

results for this tester often display high variations. The torque wrench or spanner method involves gluing a rigid test cylinder to the coating and

measuring the torque necessary to remove it [95]. Breaking strength values for

both procedures are of the same order of magnitude and have similar failure

characteristics.

4.2 ELECTRICAL MEASUREMENTS

4.2.1 Significance of Property

Electric properties relate to the corrosion protective qualities of the coatings.

Some correlation between corrosion protection qualities of the coatings and

electrical properties has been found. Limited information on the mechanism

of corrosion of a coating during immersion can be obtained from electrical

measurements [102]. Leidheiser [96] recently reviewed electrical measurement

procedures.

Elcometer is a brand name adhesion tester. Recognition of a specific com- mercial product is not meant to constitute an endorsement by the National

Bureau of Standards.

16
4.2.2 Measuring Techniques

4.2.2.1 D. C. Resistance Measurements

Bacon et. al. , [97] measured D. C. resistance of over 300 organic coatings on metal substrates immersed in electrolytes over a period of 150 days. In cases

of good protection (determined visually) , the resistance of the film exceeded

10°* ohms/cm^. When corrosion occurred, the resistance was less than 10^ ohms/

cm^. Coatings having resistances between 10° and 10° ohms/cmf were sometimes

good and sometimes not.

Mayne and Mills [98] demonstrated that the electrical properties of a film vary

greatly with location on the film. They found a correlation between film

resistance and corrosion. D.C. resistances of films were also shown to change

after immersion in KC1 [98, 103, 104]. Films increasing in resistance did not

corrode, while films decreasing in resistance did. Increased resistance was thought to be associated with increase in cross link density. The authors

suggest that improved protection may be obtained by uniformly increasing

crosslink density over the film.

Equipment needed to carry out these measurements is described by Brown [105].

Included is a calomel or Ag/AgCl electrode, an electrometer with a high input

resistance, and two potentiometers to provide added potential for high

resistance measurements or to charge panels having a long time constant.

4.2.2.2 A. C. Resistance Measurements

A.C. resistance measurements can provide the same information as that obtained

from D.C. measurements. In addition, A.C. measurements have several advantages

over D.C. measurements. The most important one is the ability of measurements

to be made as a function of frequency. Processes occuring in the coating can

be distinguished from those at the interface by obtaining data over a wide

range of frequencies [107, 108].

4.2.2.3 Capacitance Measurements

Changes in the capacitance of coatings have been associated with the penetration

of water into the film. Difference in water uptake, determined gravimetrically

and by capacitance measurements, has been used to characterize differences in

the mode of distribution of water in coatings as a function of pigment and

polymer [111]. Models have been developed relating change in capacitance with

change in depth of penetration of water into the coating [102, 112].

4.3 PERMEABILITY MEASUREMENTS

4.3.1 Significance of Property

Since water, oxygen, and ions are needed at the coating/substrate interface for

corrosion to occur, the rate at which they permeate would seem to be related to

the protective qualities of the coating. A great amount of work has recently

been done to measure permeabilities of different coating systems to do the

17
/

following: 1) water : as a function of relative humidity and temperature [113,

114]; 2) oxygen: as a function of temperature and the water absorption [115];

3) sulfur dioxide : as a function of temperature [12]; and 4) various ions : [116, 117, 118, 119, 120].

It is generally agreed that the water permeation rate through a coating does

not control the corrosion rate [12]. The rate of oxygen permeation is often a sizeable fraction of that needed for corrosion of unprotected mild steel [18,

115]; however, corrosion of coated steel, at least in the early stages of exposure, proceeds much slower. These ideas suggest that permeabilities to water

and oxygen are linked with other protective mechanisms.

— —2 +

The permeability of coatings to ions such as CI , SOa , and Na are several

orders of magnitude smaller than to water [12, 121]. Evidence exists that ion

permeability and ion exchange properties of the coating may be related directly

to the protective quality of the paint [118, 122, 123].

4.3.2 Measuring Techniques

Free films are used in obtaining measurements of permeability to oxygen and

water.

Oxygen permeabilities can be determined using a pressure differential method.

The free coating film of interest is positioned between a large and a small

chamber. The large chamber contains oxygen at a pressure of 50 or 100 mm of

mercury (6.5 x 10^ to 13 x 10^ Pa), while the small chamber is evacuated.

Pressure rises are then measured. For nine different films, oxygen permeability

was insensitive to relative humidity but very sensitive to temperature [115].

Sulfur dioxide permeabilities are measured by exposing one side of the film to

humid air containing SO 2 and titrating the water leaving the film on the downstream side of the permeation cell for its acid content [12].

The measurement of ion permeabilities and ion exchange properties of films is

much more difficult. Recent work by Ruggeri and Beck [121] identifies problems

associated with measurements for films with very low permeabilities and ion

concentrations. Typical experimental procedures include radioactive tracer

methods and use of special electrodes or specific ion titrations. These

measurements are not routinely done for organic coatings.

4.4 MECHANICAL STRESS MEASUREMENTS

4.4.1 Significance of Property

Internal stresses of coatings have a strong influence on many physical and

chemical properties of coating systems. Stresses arise during curing of the

film because of shrinkage and crosslinking [124].

Changes in temperature and relative humidity cause changes in the internal

stress. .The thermal expansion coefficients of some coatings are on the order

of two to. three times greater than that of metals. Hence, temperature

18
decreases will increase film stress. If the additional stress is sufficiently

high, the film can either become detached from the substrate or crack [129].

Water may also act as a plasticizer reducing the internal stress [126]. It can also leach out soluble products, which may result in increased internal

stress in the film on redrying [125].

4.4.2 Measuring Techniques

4.4.2.1 Fixed Film

Internal stresses in coatings can be determined using a cantilever beam

deflection method. A coating is applied to one side of a thin steel strip

and deflection is measured microscopically [127] or mechanically [128] as the

coating cures. Saarnak [127] reports that, if measurement procedures and environmental conditions are carefully controlled, satisfactory reproduction is

possible. Croll [128] reports experimental errors of about 15 percent.

Internal stresses of 5 MPa have been reported for freshly cured coatings.

4.4.2.2 Free Films

Mechanical properties of free films can be determined by stress-strain

measurements, transient, and dynamic tests. In one type of dynamic test, an oscillating strain is applied at a constant frequency and the resulting stress

and phase difference measured. Measures of both elastic and viscous response

are obtained. When these tests are performed as a function of temperature,

glass transition temperatures and brittle-ductile transition temperatures [130] are obtained. Loss tangent curves obtained from dynamic testing are related to

impact resistance of the film.

Changes in mechanical properties after aging can often be related to coating

durability [131]. Talen [132] found that alkyd resin varnish tensile results

correlated well with outdoor durability. Tensile tests after outdoor exposure

also revealed changes in properties sooner than visual changes.

4.5 CHEMICAL TECHNIQUES

4.5.1 Significance of Properties Measured

In order to improve coating systems, it is necessary to link changes in the

coating microstructure to macrostructural performance changes. By making

this connection, it is hoped that the causes of coating system degradation may

be readily identified; thus, this would provide information for coating systems

improvement. Analytical equipment capable of making microstructural change

characterizations are described below.

4.5.2 Measuring Techniques

4.5.2.1 Ellipsometry

Ellipsometry is an optical technique which has been used for observing

changes under transparent coatings at the coating/substrate interface. An

19
ellipsometer detects phase and amplitude changes in visible elliptically

polarized light when it is reflected from the metal surface. The changes can

be related to the metal oxide film thickness. This technique had previously

been applied to the study of corrosion of metals immersed in a solvent [133].

Film formation, film dissolution, and surface roughening were observed.

When electrochemical measurements are carried out simultaneously with the

optical measurements, changes in electrical potential, and pH can be determined

while observing changes in metal oxide film thickness. Ritter [134] used this

technique to study iron coated with a transparent cellulose nitrate coating and

with various inhibitors either in or under the coating. At least two stages

of activity were noted during immersion in 0.05N NaCl. During the first stage,

oxygen, water, and Cl~ permeated the coating causing localized breakdown of the

oxide layer originally present on the metal surface. Active metal sites were exposed and became anodic areas while less active areas became cathodic areas

and the pH increased gradually. During the second stage, a large decrease in one of the ellisopmetry parameters was noted as the pH increased. This

appeared to be due to both oxide film growth and surface roughening corresponding to active corrosion. Both the detection of corrosion and the kinetics of

the reactions can be determined with this technique. This technique may be useful in evaluating the performance of inhibitors as well as surface preparation

methods and pretreatments.

4.5.2.2 X-Ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy has been used to identify modes of failure at

the coating/substrate interface [80, 135, 136] as well as to characterize

surfaces of substrates before application of a coating [80]. For example,

examination of the surfaces after cathodic disbondment has shown that saponified

primer coating was present on the metal substrate [136]. Hence, failure was described as cohesive since some polymer remained on the surface.

These techniques appear to be very powerful in examining the metal surface and

the metal/coating interface. The instruments are expensive and technical

knowledge and skill is required for using these instruments, designing

experiments, and analyzing the results.

4.5.2.3 Infrared Spectroscopy

Infrared spectroscopy is used to examine the top surface of coatings. Hearst

[137] examined the surfaces of several kinds of coatings before and after atmospheric, xenon-arc, and mercury-arc weathering using a dispersive infrared

spectrophotometer. The effect of exposures on the coatings was an increase in

carbonyl content. However, the changes he observed in the surface did not

correspond to durability in service. This is not completely unexpected since

it is generally felt that changes in the surface of coatings do not influence

performance unless the coating becomes very thin.

20
Very small differences in chemical composition can be determined using a Fourier transform infrared spectrophotometer. It is possible to determine

the combined or individual effect of both heat and moisture on coatings by

subtracting spectra. It is also possible to obtain spectra at varying depths

from the surface and observe the changes in composition as a function of depth.

The potential of this instrument is very great.

21
5. SUMMARY AND CONCLUSIONS

Prior to reviewing the literature, four criteria were established for evaluating

accelerated aging tests for coatings over steel substrates. The four criteria

were predictability, reproducibility, distinguishability, and extendability; it was felt, at the time, all accelerated aging tests should satisfy these criteria.

After a cursory review of the literature, it became apparent however, that

these are not the criteria upon which currently used accelerated aging tests were based. The predominant philosophy of current procedures is that of

association or correlatability as opposed to predictability. Tests based on correlatability are termed comparison tests.

The most common comparison tests found in the literature and reported by

coating laboratories were the salt fog, cyclic or sequential weathering, high

relative humidity, cathodic, and the Kesternich tests. The salt fog is the

most frequently used. Although the stress conditions in the above vary from

test to test, a certain commonality exists. Each of these tests is designed

to subject the coatings to a set of stresses or a sequence of stresses thought

to be similar to those encountered in service. Typically, multiple coatings

are subjected to a test and some of these coatings are controls (controls are coatings having whose durabilities are well known). After exposure to the test

conditions, the performance of each coating is assessed and ranked against that

of the controls. Correlations are then made relating the accelerated test

rankings against those of the same coatings exposed outdoors.

In assessing these comparison tests, several serious problems have been noted.

First, results from the analysis are measures of association, not quantitative

service life predictions. This makes it difficult to distinguish between the

merits of different coatings, makes the results difficult to extend to other

possible service environments, and requires reliable long-term performance data

for the control coatings. Second, the rankings from different iterations of

the same accelerated aging tests are seldom reproducible. Finally, poor correlations exist between accelerated and outdoor exposure data. The end result

of these deficiencies is that users and manufacturers of coating systems tend

not to rely on the results of their short-term tests; instead, they depend

heavily upon long-term outdoor exposure results and use their short-term

results as supporting evidence. The obvious result of this lack of confidence

is that the qualification times for new coatings are very long.

In conclusion, existing short-term test methods do not satisfy the four

criteria outlined in chapter 2. Consequently, the methods are not fully

adequate for reliably predicting service life of coatings.

22
6. RECOMMENDATIONS

The ultimate objectives of any accelerated aging procedure are to form the

basis for the selection of the best coating for a given environmental and

operating condition and to provide a method for assessing durability improvement for a coating system. These objectives can best be met if quantitative

service life predictions can be made from the short-term or accelerated test

data. It has been concluded that current short-term tests do not meet the

need for quantitative service life data and that accelerated aging procedures

having a more scientific base are needed. Specifically, we recommend:

1. Use of accelerated test procedures in which stresses are varied

singly, but at multiple levels; e.g., exposing specimens to 95

percent relative humidity at each of four temperature levels —

30°C, 50°C, 70°C, and 90°C. Initially, the response of the coatings

to constant stress conditions should be accessed. Once this is

accomplished, cyclic changes in a stress should be examined.

2. Application of analytical tools for quantitative characterization

of the materials and material degradation processes as a function

of exposure time. Included in this are non-destructive evaluation

procedures useful in quantifying coating degradation.

3. Application of mathematical methods capable of relating material

response to a wide variety of environmental and operating conditions,

Reliability theory, as outlined in Appendix A, seems suitable.

23
7 . REFERENCES

1. Frondistou-iY annas, S., "Coating and Corrosion Costs of Highway Structural

Steel," Federal Highway Administration Report No. FHWA-RD-79-121, 1980.

2. Mann, N. R., Shafer, R. E., and Singpurwalla, N. D., Methods for Statistical Analys is of Reliability and Life Data , John Wiley, 1974.

3. Bain, L. J., Statistical Analysis of Reliability and Life Testing Models:

Theory and Methods, Marcel-Dekker, 1978.

4. Lieblein, J. and Zelen, M.,

" Statistical Investigation of the Fatigue L ife

of Deep Groove Ball Bearing ," Res. Paper 2719. J. Res. Nat. Bur . Stand.

,

57, 273 (1956).

5. Nelson, W., "Accelerate Life Testing — Step Stress Models and Data

Analyses," IEEE Trans. Reliability R-19, 103 (1980).

6. Shooman, M. L., Probabilistic Reliability: An Engineering Approach,

McGraw-Hill, New York, 1968.

7. Whittaker, I. C. and Besumer, P. M., "A Reliability Analysis Approach to

Fatigue Life Variability of Aircraft Structures," Boeing Comp . Tech. Rep.,

AFMC-TR-G9-65, 1969.

8. Cinlar, E. and Bazant, Z. P., "Stochastic Process for Extrapolating

Concrete Creep," Journal of the Engineering Mechanics Division, ASCE,

1069-1088 (1977).

9. Martin, J. W., "The Analysis of Life Data for Wood in the Bending Mode,"

Wood Science and Technology, 14_, 187 (1980).

10. Hare, C. H., "Anti-Corrosive Barrier and Inhibitive Primers," Federation

Series on Coatings Technology, Unit 27, Federation of Societies for

Coatings Technology, 1979.

11. Yassen, M., and Funke, W., "Effect of Temperature on Water Adsorption

and Permeation Properties of Coatings," J. Oil Col. Chem. Assoc, 62_, 284

(1979).

12. Funke, W., "Corrosion Tests for Organic Coatings - A Review of Their

Usefulness and Limitations," J. Oil Col. Chem. Assoc, 62_, 63 (1979).

13. Walker, P., "The Effect of Water Upon the Adhesion of Surface Coatings,"

Paint Technology, 31_, No. 8, 22 (1967).

14. McCafferty, E., "Mechanisms of Corrosion Control by Inhibitors," Corrosion

Control by Coatings , Leidheiser H., ed., p. 276, Science Press, Princeton,

1979.

24
15. Stern, M. , "The Mechanism of Passivating-type Inhibitors," J. Electrochem

Soc. , 105 , 638 (1958).

16. Thomas, J. G. N. , "Uptake of Phosphate Ions by Oxide Films on Mild Steel,"

Brit. Corros. J., 5_, 41 (1970).

17. Lumsden, J. B. , "The Properties of Passive Films on Metals," p. 173 in

Corrosion Control by Coatings , Leidheiser, H. , ed. , Science Press, 1979.

18. Mayne, J. E. 0., "The Mechanism of the Protective Action of Paints,"

Corrosion, Vol. 2 , 2nd ed. , L. L. Sheir, ed. , Newnes-Butterworths, London

1976, 15, 24.

19. "Standard Method of Salt Spray (fog) Testing", B117-73 (1979), Part 27,

1980 Annual Book of ASTM Standards, American Society for Testing and

Materials, Philadelphia, PA.

20. Champion, F. A. , Corrosion Testing Procedures , 2nd ed. , John Wiley, New

York, 1965.

21. Brasher, D. M. and Nurse, T. J. , "Electrical Measurements of Immersed

Paint Coatings on Metal. II. , Effect of Osmotic Pressure and Ionic Concentration of Solution on Paint Breakdown," J. Appl. Chem. , 9_, 96 (1959).

22. "Resistance to Continuous Salt Spray," BS 3900: Part 4; British Standards

Institution, 2 Part Street, London WIA 285, 1968.

23. "Standard Method of Acetic Acid-Salt Spray (fog) Testing," B287-74, Part

27, 1980 Annual Book of ASTM Standards, American Society for Testing and

Materials, Philadelphia, PA.

24. "Standard Method for Copper-Accelerated Acetic Acid-Salt Spray (fog)

Testing (Cass Test)," B368-68, Part 27, 1980 Annual Book of ASTM

Standards, American Society for Testing and Materials, Philadelphia, PA.

25. "Salt Spray (fog) Test, Method 6061, Federal Test Method Standard No.

141A, Paint, Varnish, Lacquers, and Related Materials; Methods of

Inspection, Sampling and Testing," Change Notice 3, July 1976.

26. Harrison, J. B. , "The Realistic Prediction of Primer Performance, A Case

History," J. Oil Col. Chem. Assoc, 62_, 18 (1979).

27. Performance Tests for Protection Schemes Used in the Protection of Lightguage Steel and Wrought Iron Against Corrosion," BS 1391: 1952, British

Standards Institute, 2 Part St., London WIA 285, 1960.

28. "Evaluating Degree of Rusting on Painted Steel Surfaces," D610-68 and

"Evaluating Degree of Blistering of Paints," D714-56, Part 27, 1980,

Annual Book of ASTM Standards, American Society for Testing and Materials,

Philadelphia, PA.

25
29. Appleman, B. R. and Campbell, P. G., "Salt Spray Testing for Short-Term

Evaluation of Coatings," presented at the Annual Meeting of the Federation of Societies for Coating Technology, Atlanta, GA, October 29, 1980.

30. "Standard Method of Conducting Exterior Exposure Tests of Paints on

Steel," D1014-66 (1973), Part 27, Annual Book of ASTM Standards, American

Society for Testing and Materials, Philadelphia, PA.

31. Bruno, J. A., Keane, J. D., and Weaver, R. E. F., "Performance of

Alternate Coatings in the Environment (PACE)." Report FHWA-PA-79-05, May,

1979.

32. Lapasin, R., Papo, A., and Torriano, G., "New Shop Primers for Steel Based

on Inhibitive Pigments of Low Toxicity," Br. Corros, J., 12_, No. 2, 91,

(1977).

33. Pila, S., "Factors Involved in the Formulation of Anti-Corrosive High

Build Vinyl Coatings," J. Oil Col. Chem. Assoc, 5_6, 195 (1973).

34. Boxall, J., Von Fraunhofer, J. A. and Werren, S. C, "Investigation of the

Water and Ion Permeation Characteristics of Certain Polymers," J. Oil Col.

Chem. Assoc, 55, 24 (1972).

35. Emch, D., "Water Borne, Air Dry, Force Dry, Lead and Chromate-Free Metal

Primers," Presentation at May, 1980 Symposium of Baltimore Society for

Coatings Technology.

36. Van Westrenen, W. J., "The Behavior in Salt-Spray Exposure of WaterSoluble Epoxy Ester Coatings," Paint Technology, 27, November 1968.

37. Cook, A. R., "Galvanized Steel and Zinc Pigments, Paints," in Corrosion

Control by Coatings , Leidheiser, H., ed., Science Press, 1979, p. 49.

38. Hospadaruk, V., Huff, J., Zurilla, R. W. and Greenwood, H. T., "Paint

Failure, Steel Surface Quality and Accelerated Corrosion Testing,"

Society of Automotive Engineers, Technical Paper Series, No. 780186, 1978.

39. Desea, R. I., "Corrosion Resistance of Waterborne Industrial Force Dry

and Bake Coatings," Presentation at May, 1980 Symposium of Baltimore

Society for Coating Technology.

40. "Standard Evaluation of Painted or Coated Specimen Subjected to Corrosive

Environment," D1654-79, Part 27, 1980 Annual Book of ASTM Standards,

American Society for Testing and Materials, Philadelphia, PA.

41. Newton, D. S. and Guest, B. M., "The Natural and Accelerated Weathering

of Fine Dust Primers," J. Oil Col. Chem. Assoc, 47_, 942 (1964).

42. Gaynes, N. I., "The Organic Corner, The Salt Fog Test on Organic

Coatings," Metal Finishing, 72_, No. 7, 41 (1974).

26
43. Stone, J., "Paint Adhesion on a Scribed Surface: The PASS Test," J. of

Paint Technology, 4^, No. 539, 661 (1969).

44. Mazia, J., "The Organic Corner, More Metal Finishing Myths (MFM's),"

Metal Finishing, 75, No. 5, 77 (1977).

45. Morrison, J., National Aeronautics and Space Administration, John F.

Kennedy Space Center, FL., private communication, 1980.

46. a. "Standard Practice for Operating Light-and Water-Exposure Apparatus

(Carbon-Arc type) for Testing Paint, Varnish, Lacquer, and Related Products," ASTM D822-79, Part 27, 1980 Annual Book of ASTM Standards,

American Society for Testing and Materials, Philadelphia, PA.

b. "Standard Recommended Practice for Operating Light-Exposure Apparatus

(Unfiltered Carbon-arc type), for Testing Paint, Varnish, Lacquer and

Related Products Using the Dew Cycle," ASTM D3361-74, Part 27, 1980

Annual Book of ASTM Standards, American Society for Testing and Materials,

Philadelphia, PA.

c. "Standard Recommended Practice for Operating Light-Exposure Apparatus

(Xenon-Arc type) with and without Water for Exposure of Nonmetallic Materials," ASTM G26-77, Part 27, 1980 Annual Book of ASTM Standards, American

Society for Testing and Materials, Philadelphia, PA.

d. "Standard Recommended Practice for Operating Light- and Water-Exposure

Apparatus (Fluorescent UV -condensation type) for Exposure of Non-metallic

Materials," ASTM G53-77, Part 41, 1977 Annual Book of ASTM Standards,

American Society for Testing and Materials, Philadelphia, PA.

47. Ellinger, M., "Weathering Tests," Prog, in Org. Coatings, _5, 21 (1977).

48. McMichael, K. L. and Azar, D., "Thin Films of Paint on Steel,"

FHWA-LA-7 7-108, May, 1977.

49. Ellinger, M. , "Accelerated Weathering Tests," J. Oil Col. Chem. Assoc,

62_, 136 (1979).

50. Hoey, C. E. and Hipwood, H. A., "An Appraisal of Analytical Weathering

Methods for Assessment of the Durability of Paint Film," J. Oil. Col.

Chem. Assoc, 57_, 151 (1974).

51. Berger, W., "Light Stability and Outdoor Resistance of Paint Binders,"

Paint Manufacture, 30, November, 1972.

52. Oakley, E. and Marron, J. J., "Accelerated Testing of Durable Coating,"

J. Oil Col. Chem. Assoc, 5_7_, 221 (1974).

53. Nowacki , L. J., "An Evaluation of Various Weatherometers for Determining

the Service Life of Organic Coatings," Official Digest 34, 1191, (1962).

27
54. Lindberg, B., "Theories-Laboratory Investigation-Practical Performance,"

J. Oil Col. Chera. Assoc. 58, 399 (1975).

55. Stanton, J. M.

,' "Evaluation of the Dew Cycle Method of Accelerated

Testing," J. of Paint Technology, 3_9, No. 514, (1967).

56. Hoffmann, E. and Saracz, A., "Weathering of Paint Film IV: Chalking in

Emulsion Paint Films Caused by Silicone-Coated Anatase Titanium Dioxide

and Zinc Oxide of Varying Humidities," J. Oil Chem. Assoc. 55, 1079,

(1972).

57. Part 27, 1980 Annual Book of ASTM Standards, American Society for Testing

and Materials, Philadelphia, PA.

a. "Standard Method of Evaluating Degree of Chalking of Exterior Paints,"

D659-74.

b. "Standard Test Method for Specular Gloss," ASTM D523-78.

c. "Standard Method for Visual Evaluation of Color Differences of Opaque

Materials," ASTM D1729-69 (1974).

58. Papenroath, W., Atlas Sunspots, 2_, No. 4, 1 (1973).

59. Timmins, F. D., "Avoiding Paint Failures by Prohesion," J. Oil Col. Chem.

Assoc, 62_, 131 (1979).

60. Tooke, W. R. and Hurst, D. R., "Accelerated Environmental Testing," Georgia

Department of Transportation, Project No. 6301, November, 1973.

61. Alumbaugh, R. L. and Hearst, P. J., "Prediction of Paint Performance from

a Combination of Accelerated Laboratory Tests," Civil Engineering

Laboratory, Tech. Note N-1412 , November, 1975.

62. "Standard Method of Testing Coated Steel Specimens Dynamically for

Resistance to Corrosion," ASTM D2 933-74, Part 27, 1980 Annual book of

ASTM Standards, American Society for Testing and Materials, Philadelphia,

PA.

63. Sakae, Y., Kikuchi, M. and Najimak, "One Proposal of Accelerated

Weathering Tests," Proc. Int. Congr. Met. Corros., 5th, 1044, 1972

(Pub. 1974), Ed. by Sato, N., National Assoc. Corros. Eng . , Houston, TX.

64. Burnside, G. L. and Brewer, G. E. F., "Five Year Use, Test Track Run, and

Salt Spray Test, Show Excellent Correlation in Automotive Primer Coats,"

Metal Finishing, 71, No. 11, 62 (1973).

65. Grossman, P. R., "Investigation of Atmospheric Exposure Factors that

Determine Time of Wetness of Outdoor Structures," Special Technical

Publication 646, American Society for Testing and Materials, Philadelphia,

PA, 1978.

28
66. "Standard Method for Testing Coated Metal Specimen at 100% Relative

Humidity," D2247-68 (1973), Part 27, 1980 Annual Book of ASTM Standards,

American Society for Testing and Materials, Philadelphia, PA.

67. Jedlicka, W. W. and Geschke, T. H. , "Rapid and Continuous Evaluation of

Corrosion Using Extremity Thin Metal Substrates," J. Paint Technology,

41, 680 (1969).

68. Van Laar, J. A. W. , "The Durability of Lacquered Steel at Various Relative

Humidities," Paint Technology, 30, No. 12, 36 (1966).

69. Geld, I. and McCaul, C. , "Corrosion and Coatings Test Program of the NYC

Board of Water Supply," Materials Performance, 11_, No. 2, 41 (1972).

70. Wampner, H. L. , "Acrylic Resins for Industrial Chemical Coatings," J. Oil

Col. Chem. Assoc, 5_2_, 309 (1969).

71. Guruviah, S. and Rajagopalan, K. S. , "Evaluation of Primer Paint by

Accelerated Test," Paintindia, 19_, No. 2, 17 (1969).

72. Mazia, J., "The Organic Corner," Metal Finishing, 77, No. 6, 63 (1979).

73. "Standard Method of Water Immersion Test of Organic Coatings on Steel,"

ASTM D870-54 (1973), Part 27, 1980 Annual Book of ASTM Standards, American

Society for Testing and Materials, Philadelphia, PA.

74. Tooke, W. R. and Montalvo, J. R. , "Coatings Adherence Measurement by an Angular Scribe-Stripping Technique," J. Paint Technology, 38_, 18 (1966).

75. "Method of Test in Damp Heat Alternating Atmosphere Containing Sulphur

Dioxide (Kesternich Type Apparatus)," DIN 50018, December, 1963.

76. Chess, J. A., Hastings, M. R. and Paolini, Jr., A., "Accelerated Corrosion

Testing," J. Coating Technology, 49_, 55 (1977).

77. Leclercq, M. , "New Instrumentation for Accelerated Corrosion Tests in a Damp Sulfur Dioxide-Containing Atmosphere," FATIPEC Congress, 14 , 371,

(1978).

78. Metil, I., "Ferrophosphrous Extenders," Modern Paint and Coatings,

October, 1977.

79. Metil, I., "Recent Developments in Organic Zinc-rich Primers," Modern

Paint and Coatings, 39, December, 1979.

80. Castle, J. E. and Watts, J. F. , "Cathode Disbondment of Well Characterized Steel Interfaces," Corrosion Control by Organic Coatings Conference,

Lehigh, PA, August, 1980, p. 252.

81. McGill, W. J., "Adhesion and Failure of Organic Coatings," J. Oil Col.

Chem. Assoc, 60, 121 (1977).

29
82. Kumins, Charles A. , "Physical Chemical Models for Organic Protective

Coatings," J. Coatings Technology, 52_, No. 664, 39 (1980).

83. Bullett, T. R. , "Film Structure and Adhesion," J. Adhesion, 4_, 73 (1972).

84. Croll, S. G. , "Adhesion Loss Due to Internal Strain," J. Coatings

Technology, 52, No. 665, 35 (1980).

85. Walker, P. , "Long Term Adhesion Studies on Multicoat Systems Exposed to

Natural Weathering," J. Oil Col. Chem. Assoc, _52, 516 (1969).

86. Walker, P., "The Effect of Water Upon the Adhesion of Surface Coatings,"

Paint Technology, 31, No. 8, 22 (1967).

87. Bullett, T. R. and Prosser, J. L. , "The Measurement of Adhesion," Prog,

in Org. Coatings, 1_, 45 (1972).

88. "Standard Test Methods for Adhesion of Organic Coatings," ASTM D2197-68

(1979), Part 27, 1980 Annual Book of ASTM Standards, American Society for

Testing and Materials, Philadelphia, PA.

89. "Standard Methods for Measuring Adhesion by the Tape Test," ASTM D3359-78,

Part 27, 1980 Annual Book of ASTM Standards, American Society for Testing

and Materials, Philadelphia, PA.

90. "Paints and Varnishes - Pull-off Test for Adhesion," ISO 4624-1978(E)

,

International Organization for Standardization, 1978.

91. "Messung der Haf tfestigkeit von Anstrichen und ahnlichen Beschichtungen

nach der Abreissmethode, " DIN 53232, April 1973, Alleinverkauf der

Normblatter durch Beuth-Vertrieb GmbH, Berlin.

92. Sickfield, J., "Experimental Aspects of Adhesion Testing, An IUPAC Study,"

J. Oil Col. Chem. Assoc, 61_, 292 (1978).

93. Joint Services Research and Development Committee on Paints and Varnishes,

"An Investigation into a Method of Test for Assessing the Adhesion of

Paint Coatings: Part II," J. Oil Col. Chem. Assoc. 49_, 17 (1966).

94. Schwab, L. K. and Drisko, R. W. , "Relation of Steel Surface Profile to

Coating Performance," Corrosion Control by Organic Coatings Conference,

Lehigh, PA, August, 1980.

95. Walker, P., "Factors Affecting the Performance of Etch Primers," Paint

Technology, 33_, No. 2, 18 (1968).

96. Leidheiser, H. , "Electrical and Electrochemical Measurements as Predictors

of Corrosion at the Metal-Organic Coating Interface," Progress in Org.

Coatings, 7, 79 (1979).

30
97. Bacon, R. C. , Smith, J. J., and Rugg, F. M. , "Electrolytic Resistance in

Evaluating Protective Merit of Coating on Metals," Ind. Eng. Chem. , 40_,

161 (1948).

98. Mayne, J. E. 0. and Mills, D. J., "The Effect of the Substrate on the

Electrical Resistance of Polymer Films," J. Oil Col. Chem. Assoc, 58,

155 (1975). ,

99. Kinsella, E. M. and Mayne, J. E. 0. , "Ionic Conduction in Polymer Films.

I. Influence of Electrolyte on Resistance," Br. Polym. J., 1_, 173 (1969).

LOO. Rajagopalan, K. S. , Guruviah, S. and Rajagopalan, C. S. , "Evaluation of

Varnishes by Capacitance and Resistance Measurements," J. Oil Col. Chem.

Assoc, 63_, 144 (1980).

.01. Brasher, D. M. and Nurse, T. J. , "Electrical Measurements of Immersed

Paint Coatings on Metal. II. Effect of Osmotic Pressure and Ionic

Concentration of Solution on Paint Breakdown," J. Appl. Chem., 9_, 96

(1959).

102. Kendig, M. W. , and Leidheiser, Jr., H. , "The Electrical Properties of

Protective Polymer Coatings as Related to Corrosion of the Substrate,"

J. Electrochem. Soc: Electrochemical Science and Technology, 123 , No. 7, 982, (1976).

L03. Maitland, C. C. , and Mayne, J. E. 0., "Factors Affecting the Electrolytic

Resistance of Polymer Films," Official Digest, 34_, 972 (1962).

L04. Mills, D. J., and Mayne, J. E. 0., "The Inhomogeneous Nature of Polymer

Films and Its Affect on Resistance Inhibition," Corrosion Control by

Organic Coatings Conference, Lehigh, PA, August, 1980.

L05. Brown, J. R. , "Evaluation of Protective Coatings for Ship Bottoms,"

Corrosion, 15_, No. 6, 49 (1959).

L06. Hearst, P. J., "Electrical Properties of Coatings as Related to

Performance. II. Experiments with Seven Additional Immersed Coatings

Systems," U. S. Naval Civil Engineering Laboratory, Port Hueneme, CA,

Tech. Note N-885, 1967.

L07. Piens, M. and Verbist, R. , "Electrochemical Values - Their Significance

When Applied to a Coated Substrate," Corrosion Control by Organic Coatings

Conference, Lehigh, PA, August 1980.

108. Scantlebury, J. D. and Sussex, G. A. M. , "Impedance Techniques for the

Study of Organic Coatings," Corrosion Control by Organic Coatings

Conference, Lehigh, PA, August 1980.

31
109. Standish, J. and Leidheiser, Jr. , H. , "Properties and Behavior of

Corrosion-Protective Organic Coatings Determined by Electrical Impedance

Measurements," Corrosion Control by Organic Coatings Conference, Lehigh,

PA, August 1980.

110. Devay, J., Meszaros, L. , and Janaszik, F. , "Electro-Chemical Techniques

to Monitor Performance of Polymer Coatings in Corrosion Protection," Ind.

Eng. Prod. Res. Dev. 28, 13 (1979).

111. Gentles, J. K. , "The Use of the Capacitance Cell in the Prediction of

Paint Performance," J. Oil Col. Chem. Assoc, 46, 850 (1963).

112. O'Brien, H. C. "Some Laboratory Evaluations of Bituminous Coatings," Ind.

Eng. Chem., 58, No. 6, 45 (1966).

113. Yassen, M. and Funke, W. , "Effect of Temperature on Water Adsorption and

Permeation Properties of Coatings," J. Oil Col. Chem. Assoc, 62, 284

(1979).

114. Guruviah, S. , "The Relationship Between the Permeation of Oxygen and Water

Through Paint Films and Corrosion of Painted Mild Steel," J. Oil Col.

Chem. Assoc, 53_, 669, (1970).

115. Funke, W. and Haagen, H. , "Empirical or Scientific Approach to Evaluate

the Corrosion Protective Performance of Organic Coatings," Ind. Eng. Chem.

Prod. Res. Div. , 17_, No. 1, 80 (1978).

116. Murray, J. D. , "Permeability Properties of Surface Coatings Toward

Chloride Ion," J. Oil Col. Chem. Assoc, 56_, 210 (1973).

117. Glass, A. L. and Smith, J. , "A Radio Chemical Investigation Into the

Diffusion of Ions Through Protective Films," Paint Technology, 38 , No.

495, 203 (1966).

118. Glass, A. L. and Smith, J., "A Radiochemical Investigation on the Effect

of Pigment on Diffusion of Ions Through Protective Films," J. of Paint

Technology, 39, No. 511, 491 (1967).

119. Suoboda, M. , Knapek, b. and Klicover, H. , "The Effect of Pigments on the

Penetration of Sulphur Dioxide into Coatings," J. Oil Col. Chem. Assoc,

56_, 172 (1973).

120. Malik, W. U. and Aggarwal, L. , "Ion Exchange Behavior of Some AntiCorrosive Paints and Paint Media," J. Oil Col. Chem. Assoc, 57_, 131

(1974).

121. Ruggeri, R. T. and Beck, T. R. , "The Transport Properties of Polyurethane

Paint," Corrosion Control by Organic Coatings Conference, Lehigh, PA,

August, 1980, p. 196.

32
122. Ulfuarson, U. and Khullar, M. , "The Relationship Between the Ion Exchange

Capacity and the Corrosion Protection Efficiency of Alkyd Based AntiCorrosive Paints," J. Oil Col. Chem. Assoc, 54, 604 (1971).

123. Wang, W. and Leidheiser, Jr. , "Delamination of Epoxy-Amino Coatings During

Cathodic Treatment," Corrosion Control by Organic Coatings Conference,

Lehigh, PA, August 1980.

124. Corcoran, E. M. , "Determining Stresses in Organic Coatings Using Plate

Beam Deflection," J. Paint Technology, 41_, No. 538, 635 (1969).

125. Farrar, N. R. and Ashbee, D. H. G. , "Destruction of Epoxy Resins and of

Glass-Fiber-Reinforced Epoxy Resins by Diffused Water," J. Phys. D:

Appl. Phys., II, 1009 (1978).

126. Artamonova, R. V., Vinogradova, L. M. , Garanina, S. D. , Zherdev, Y. U. V.,

Korolev. A. Ya. , "Internal Stresses, and Water Diffusion in Polymers."

Vysokomol. so ed. A12 ; No. 2, 336 (1970) (Translated in Polymer Sci.

U.S.S.R. 12_, No. 2, 385 (1970)).

127. Saarnak, A. , Nilsson, E. , and Kornum, L. 0. , "Usefulness of the

Measurements of Internal Stresses in Paint Film," J. Oil Col. Chem. Assoc,

59, 427 (1976).

128. Croll, S. G. , "Internal Stress in a Solvent-Cast Thermoplastic Coating,"

J. Coatings Technology, 50_, No. 638, 33 (1978).

129. Croll, S. G. , "Adhesion Loss Due to Internal Strain," J. Coatings

Technology, 52_, No. 665, 35 (1980).

130. Hill, L. W. , "Stress Analysis - A Tool for Understanding Coatings

Performance," Prog, in Org. Coatings, 5_, 277 (1977).

131. Whiteley, P. , Rothwell, G. W. and Kennedy, J. , "Determination of Paint

Films by Water," J. Oil Col. Chem. Assoc, 58, 200 (1975).

132. Talen, H. W. , "The Mechanical Properties of Paint and Varnish Films,"

Paint and Varnish Production, 2_3, February 1961.

133. Kruger, J., "Use of Ellipsometry in the Study of Corrosion," Corrosion,

22_, 88 (1966).

134. Ritter, J. J. and Kruger, J., "Studies on the Sub-Coating Environment

of Coated Iron Using Qualitative Ellipsometric and Electrochemical Techniques," Corrosion Control by Organic Coatings Conference, Lehigh, PA,

August 1980.

135. Gettings, M. , Baker, F. S. and Kinlosch, A. J., Use of Auger and X-Ray

Photo-Electron Spectroscopy to Study the Locus of Failure in Structural

Adhesive Joints," J. Applied Polymer Sci., 21_, 2375 (1977).

33
136. Hammond, J. S., Holubka, J. W. and Dickie, R. A., "On the Locus of Paint

Adhesion in Failure in Corrosion," A. C. S. Org. Coat. Plas. Chem. 39 , 506 (1978).

137. Hearst, P. J., "Effect of Atmospheric and Laboratory Irradiation on Attenuated Total Reflectance Spectra of Organic Coatings," Naval Civil

Engineering Laboratory, Technical Note N-1411, 1975.

34
Appendix A. Reliability-Based Accelerated Aging Tests

The objective of accelerated aging tests is to make service life predictions

at normal operating conditions from tests in which one or more of the stresses

typically encountered in-service are operated at levels higher than normal.

In operating at these high stress levels, it is presumed that the degradation

process is accelerated but that the predominant failure mechanism is the same

as that occurring in-service. Methods are available to verify these

presumptions.

Mathematically, the accelerated aging process is stated as follows. Let E denote a vector of stresses to which the material is expected to be subjected

in-service. Let S be any one of these stresses such that

S < S1 < ... < S ± < Si +1 <...<...< Sm

. (1)

That is, the i*1 " level of this stress is less severe than the (i + l) tn level

of stress; Sq symbolizes the level of stress at normal conditions, and Sm

symbolizes the most severe level of applied stress yet still obeying the imposed

assumptions.

If now n specimens are randomly selected from a nominal population and

subjected to stress level S^, then each specimen will fail at some time T.

Assuming all n specimens are exposed simultaneously, then the observed times

to failure are ordered; that is

t]_ < t2 < « • • < t2 < • • • < t r where r < n.

These ordered times to failure form an empirical cumulative distribution

function. To this empirical cumulative distribution function a theoretical

cumulative distribution is fit such that the probability that a specimen will

fail at or before time t is denoted by

fs (t; e ± ) = p s (t < t) (2)

i i

where

Fg (t) is the cumulative distribution function of the random variable T

i

for specimens subjected to stress S^;

P (•) is the probability of the event in parentheses;

6^ is a parameter vector for Fg (t); and t is a time.

i

Assuming that the form of S-(t) does not change from stress level to stress

level (the parameter vector 8 is allowed to change) and assuming knowledge

of S.(t) is available at two different stress levels, e.g., the i and the

(i + l) tn stress levels, then an acceleration of the degradation process has

occurred if for all t >

35
fs <t; e i) < F s U; 8 i + 1) (3)

1 1 + 1

where i = 1, 2, ...; m-1. Given Eq. 3, a function, p(t; S,9), exists relating

the cumulative distribution function at any elevated stress level I to that at

normal conditions.

Fs.(t; e ) F sJp(t; s,e>) (4)

i

where p(t; S,0) is the acceleration function relating the parameter vector 9 to

applied stress S. Commonly, the acceleration function is assumed to be independent of exposure time; that is,

p(t; S,9) = T(S,9) • t (5)

Examples of ¥(S,9) include the Arrhenius model, the Eyring model, the inverse

power law; and the William, Landell, and Ferry type superpositions. Assuming

the inverse function F~ (t;9) exists, then p(t; S,9 T )) can be determined by

p(t; S,9) = F'Vo (t;9 T )). (6)

It is the mission of accelerated aging programs to determine the acceleration

function p(t; S,9). Knowledge of this function, along with any cumulative distribution function at an elevated stress level Sy(t; 9-r) permits estimates

of the cumulative distribution function at the normal stress level, S Q

(t; 9),

(see eq. 4).

The obvious advantages of this procedure are that once p(t; S,9) is known, then

drastic reductions in the accelerated aging program can be made by conducting

accelerated aging tests at the maximum allowable stress Sm obeying the assumptions. Large savings in time are also realized since estimates of S^Ct; 9) can

be made without observing all of the failure times. Perhaps the most important

advantage of using this procedure is that coatings having like performance

under similar conditions can now be grouped into performance classes, hence,

any improvement on one coating system from the class may apply to all coatings

in the class, thus effecting great savings in research.

36 MJ.S. GOVERNMENT PRINTING OFFICE: 1981—340-997/1811—1-3
NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS TN 1149

2. Performing Organ. Report No. 3. Publication Date

September 1981

4. TITLE AND SUBTITLE

Short-Term Evaluation Procedures for Coatings on Structural Steel

5. AUTHOR(S)

Paul G. Campbell, Jonathan W. Martin, and Mary E. McKnight

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

. Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

U.S. Department of Transportation

Federal Highway Administration

Washington, DC 20590

10. SUPPLEMENTARY NOTES

J Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

This report presents the findings of the first of a two-phased study to aid the

Federal Highway Administration in evaluating and selecting protective coatings for

steel. The objectives of the study are to review existing short-term test procedures for selecting coatings, and to discuss analytical measurement techniques for

characterizing coating systems and for monitoring coating degradation.

In assessing current accelerated aging testing procedures^ several deficiencies became

apparent. These included the reported lack of reproducibility in the rankings for

different iterations of the same short-term test and the lack of correlatability

between the rankings of short-term laboratory and long-term outdoor exposure tests.

It was concluded that, at the present time, coating manufacturers and users do not

depend heavily on accelerated test results for making durability assessments. Instead

durability assessments are based on outdoor exposure performance. Recommendations

are made to design future short-term test procedures using reliability theory.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

accelerated aging tests; coatings; corrosion; time to failure;

predictability; reproducibility.

13. AVAILABILITY

|x_J Unlimited

| | For Official Distribution. Do Not Release to NTIS

Ix^l Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

Order From Sup. of Doc, U.S. Government Printing Office, Washington, DC

20402.

14. NO. OF

PRINTED PAGES

43

15. Price

$3.00

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technology, with primary emphasis on work at NBS. The magazine

highlights and reviews such issues as energy research, fire protection, building technology, metric conversion, pollution abatement,

health and safety, and consumer product performance. In addition, it reports the results of Bureau programs in measurement

standards and techniques, properties of matter and materials, engineering standards and services, instrumentation, and

automatic data processing. Annual subscription: domestic $11;

foreign $13.75. NONPERIODICALS

Monographs—Major contributions to the technical literature on

various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial

practice (including safety codes) developed in cooperation with in- terested industries, professional organizations, and regulatory

bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special publications

appropriate to this grouping such as wall charts, pocket cards, and

bibliographies. Applied Mathematics Series—Mathematical tables, manuals, and

studies of special interest to physicists, engineers, chemists,

biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program coordinated by NBS under

the authority of the National Standard Data Act (Public Law

90-396).

NOTE: The principal publication outlet for the foregoing data is the Journal of Physical and Chemical Reference Data (JPCRD)

published quarterly for NBS by the American Chemical Society

(ACS) and the American Institute of Physics (AIP). Subscriptions,

reprints, and supplements available from ACS, 1 155 Sixteenth St., NW, Washington, DC 20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research results,

test methods, and performance criteria related to the structural and

environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of

the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

organizations. Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest to the con- sumer. Easily understandable language and illustrations provide

useful background knowledge for shopping in today's technological marketplace. Order the above NBS publications from: Superintendent of Documents, Government Printing Office, Washington, DC 20402. Order the following NBS publications—FIPS and NBSIR's—from

the National Technical Information Services, Springfield, VA 22161

.

Federal Information Processing Standards Publications (FIPS

PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register

serves as the official source of information in the Federal Govern- ment regarding standards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended.

Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717(38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or

final reports on work performed by NBS for outside sponsors

(both government and non-government). In general, initial dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Services, Springfield, VA 22161,

in paper copy or microfiche form.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. DC 20234

OFFICIAL BUSINESS

Penalty for Private Use, $300

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