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Progress in Organic Coatings 46 (2003) 148157

EIS studies of coated metals in accelerated exposure

Gordon Bierwagen a,, Dennis Tallman b, Junping Li a, Lingyun He a, Carol Jeffcoate c

a Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105, USAb Department of Chemistry, North Dakota State University, Fargo, ND 58105, USA

c Honeywell, 55 Federal Road, Danbury, CT 06810, USA

Abstract

One of the most popular uses of electrochemical impedance spectroscopy (EIS) is the characterization of the protective properties of

coatings on corrodible metals. From early studies up to the present time, many EIS studies have been devoted to the study of the changes

in the impedance of coated metals as they undergo either natural or artificial exposure to conditions that cause corrosive failure of such

systems. With the current improvements in instrumentation and software for EIS studies of coated metals, one no longer needs to be an

expert electrochemist to utilize EIS in ones studies of protection by coatings. In this paper, the use of EIS from the point of view of the

coatings scientist will be presented, with an emphasis on its application simultaneous with accelerated exposure. EIS is used by coating

scientists for several purposes, among them the detection of changes due to exposure, prediction of the lifetime of corrosion protection,

identification of the corrosion processes that lead to failure, ranking of coatings systems, measurement of water uptake by coatings, and

the development of models for coating/metal system performance. This paper will discuss several specific examples of the use of EIS in

the study of coatings in accelerated exposure and the analysis of EIS data from such studies. The importance of cyclic vs. steady state

exposure of samples will be shown by EIS results, and some of the problems in the use of standard continuous salt fog exposure as

exemplified by ASTM B117 for a coating specification will be discussed. Considering Tg effects on EIS data will show the importance of

considering thermal effects in the testing of coatings. The extremely important role of water uptake in coatings during exposure will also

be discussed using EIS results to analyze changes in both the coating resistance (low frequency |Z| data) and capacitance (higher frequency

Zdata). During exposure to cyclic changes in temperature and electrolyte solution concentration, a coating over a metal substrate appears

to undergo both physical aging and chemical degradation. The coating appears to have a memory of past exposure events such thateach subsequent exposure to water and temperature creates and enlarges transport pathways within the coating for water and electrolyte.

As cyclic exposure continues, damage to the bulk-coating layer above the coating/metal interface accumulates until there begins to be a

permanent accumulation of electrolyte at this interface and local small-scale corrosion begins. This is the initiation of corrosion failure of

the system, but it only occurs following the decrease of bulk-coating layer barrier properties caused by cyclic temperature and humidity

processes characteristic of exterior exposure. This whole process can be accelerated by immersion in a flowing electrolyte, emphasizing

the role of transport processes in coating degradation processes. If there is simultaneous UV exposure, as Skerry has so well described, one

must also account for photodegradation of the outermost layer of the coating system. The role of the coating scientist is now to assimilate

the data that EIS now provides us during the exposure process and develop meaningful models for the molecular level changes that occur

in the coating film in order to enable use of the EIS results for true coating performance ranking and lifetime prediction.

2003 Published by Elsevier Science B.V.

Keywords: EIS; Photodegradation; Electrolyte; Accelerated testing; Lifetime prediction

1. Introduction

1.1. Brief history of EIS use in coatings

Review of the use of electrochemical impedance spec-

troscopy (EIS) in the study of corrosion protection by coat-

ings has been considered by several authors [15]. Early

measurements of EIS values on coated metals were very

difficult due to the lack of computer controlled data acqui-

Corresponding author. Tel.: +1-701-231-8294; fax: +1-701-231-8439.

sition systems. Much of this has been reviewed before, and

will not be repeated here. From the point of view of coating

scientists, when relatively easy to use, control and standard-

ized EIS equipment became available, the domination of

this area of research by pure measurement electrochemists

ceased. At this time, coating users began to see EIS as a

valuable tool to characterize coating systems. The use of

equivalent circuit methods to model the physical behavior

of coatings as they aged and failed in immersion was intro-

duced, with software that allowed the easy fitting of coating

data [1].

0300-9440/03/$ see front matter 2003 Published by Elsevier Science B.V.

doi:10.1016/S0300-9440(02)00222-9

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G. Bierwagen et al. / Progress in Organic Coatings 46 (2003) 148157 149

1.2. EIS usage by electrochemists and

metallurgists in coatings

The use of EIS by electrochemists to study the corrosion

protective properties of organic coatings over metals has

been dominated by an emphasis on the methodologies and

instrumentation of EIS. The primary consideration is the cor-rosion occurring on the metal itself, and on the metal/coating

interface and what happened when this occurred [6]. Very

often the period of time during which a coating completely

protected a metal was given minimal consideration, for the

interest was in what happened to a metal surface as it cor-

roded, and how the coating changed the mechanisms of cor-

rosion [3]. Attention was not directly paid to the coating and

the measurement of the properties of the system before fail-

ure by corrosion began at the metal/coating interface. Much

attention was paid to the metal substrate and its composi-

tion. Classes of coatings were compared by EIS, and the EIS

modeling of failing systems was very popular. Equivalent

circuits with many elements were used to provide physical

models for failed systems, but not much modeling of the

changes occurring in coatings systems leading to failure was

performed.

1.3. EIS measurements by coatings scientists

The coatings scientist views EIS as a tool to exam-

ine coatings and the way they protect metals. The metal

substrate was viewed by the coatings scientist as a fea-

ture of the system controlled by the user of the coating,

and as something beyond their control [7]. They view the

pre-treatment of the substrate as very important to the pro-tective performance of the coating, much like it is for all

other coating/substrate systems. They tend to be very con-

cerned with the variability seen in EIS characterization of

coated metals [4,8]. Wet adhesion, the ability to maintain

adhesion to a metal substrate in the presence of water or

electrolyte, was identified as being a very important coating

parameter in the performance of coated metals [9].

2. Accelerated life testing in coatings

2.1. Goals of accelerated life testing

Accelerated life testing is used in many areas of science

and technology to determine the effective performance life-

time of various types of systems. In general, one seeks a

physical or chemical acceleration of the failure of a specific

system by placing the system under stress larger than it

would receive in its normal lifetime, and monitor its per-

formance to failure from that stress. Ideally the stress only

causes the system to fail faster than it normally would, hence

the term accelerated, but the mechanism of failure remains

the same as in the non-accelerated conditions. A good intro-

duction to accelerated testing along with some introductory

references to this area of science is given in the recent book

chapter by Meeker et al. [10]. The goal of accelerated life

testing of corrosion protection of coatings is to impose a

repeatable, measurable stress, in excess of that it normally

undergoes, to a coating/metal system. One then determines

the time to failure under this stress as well as the changes in

system variables under the stress conditions chosen. Ideally,as mentioned above, these imposed stress conditions will

not cause the mechanism of corrosion protection failure to

change from that seen in normal use of the coating. The goal

of accelerated testing is most often to predict the lifetime

of the system in question under normal use conditions.

Accelerated testing when properly performed enables the

user of a system to obtain good estimates of when to replace

that system as well as allowing the developer of such sys-

tems to study, rank and predict performance lifetimes of

new systems with no prior field use history without com-

plete field use testing. The latter use is especially important

for good systems because the increasing lifetimes of per-

formance make the acquiring of true field performance life-time data so time-consuming that it is too impractical and

expensive to acquire. A proper accelerated test also requires

a very clear definition of what constitutes failure so that

proper measurement methods can be chosen and utilized to

characterize the properties involved in failure [11].

2.2. Accelerated testing of the lifetimes of coatings

corrosion protection

The goal of accelerated testing for coatings corrosion

protective lifetimes is twofold: first, to screen and test

newly developed coatings systems and second, to qualifynew coatings for field use. This type of testing usually

involves two components: the imposed stress environment

that drives the system to failure and the measurement of

system quality during stress imposition. Examples of the

stress environments used for accelerating coatings system

failure are immersion in electrolyte, continuous salt fog at

35 C, SO2 salt fog, and cyclic salt fog [12]. These methods

were developed around the idea that electrolyte and oxygen

are needed for corrosion at a metal surface while increasing

temperature increases the transport of oxygen and elec-

trolyte through paint films, and also increases corrosion

reaction rates. Skerry and co-workers [1315] provided evi-

dence that true emulation of the effects of exterior exposure

on corrosion protective coatings systems requires inclusion

of UV effects. The proper measurement of coating quality

during the imposed stress environment is not totally re-

solved. The ASTM B117 continuous salt fog test in its most

used form involves only qualitative examination of panels

after exposure by a trained observer, with some guidance

given on blister density, appearance judgments, etc. This

makes this protocol very weak and almost unusable due to

the almost entirely objective nature of the characterization

performed, irregardless of the fact that the stress used in the

test is unrealistic and takes many coatings above their Tgs,

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150 G. Bierwagen et al. / Progress in Organic Coatings 46 (2003) 148157

rendering them non-protective and poor in film properties.

A discussion of quantitative coatings measurements which

may be used in a corrosion protection lifetime test protocol

is given in a recent paper from this laboratory [16].

2.3. Requirements and recommendations for accelerated

test protocol for corrosion protection by coatings

A proper test protocol has a stress environment specified,

that accelerates the failure of a system by the same mech-

anism that is observed in field use of the system without

altering the failure mechanism. It also provides quantitative

measurement(s) which can be performed on the system

which will allow clear identification of the failure time

as well as the changes in the system with time under the

imposed stress. For coatings systems that must provide cor-

rosion protection under exterior exposure conditions, the

test protocol must provide the primary stresses that coat-

ings systems endure in exterior exposure. Most generally,these are UV exposure as from the sun along with (cyclic

temperature + dilute electrolyte) exposure. This has been

most carefully examined, prior to our studies on aircraft

coatings, in industrial maintenance coatings and automotive

coatings. A more complete discussion of these issues is

given in Refs. [1215]. Based on these references, for our

work on exterior coatings, the ASTM D 5894-96 exposure

protocol supplemented by various physical measurements

was chosen. The details of this are given in Fig. 1.

This protocol has been successfully used in our laborato-

ries for many coatings systems, especially aircraft coatings,

and it has proven quite useful in examining and ranking

coatings [17]. Other test cycles including thermal cyclinghave also been examined in our laboratory [18], but the

one that is generally most useful for ranking and compar-

ing systems in exterior exposure is the protocol shown in

Fig. 1.

Fig. 1. Test protocol used with exterior coatings.

2.4. Problems in present use of accelerated

testing of coatings

Many problems exist in the present use of accelerated

testing of the corrosion protective properties and lifetimes of

coatings. One problem that has arisen constantly is that users

are unwilling to stop use of a specification test protocol thathas been in place for any extended amount of time. Such a

specification test protocol is the ASTM B117 test protocol

and the accompanying qualitative observations are used for

the measurements for this protocol. The measurements are

all qualitative and subjective, and the test method does not

emulate the conditions of use and failure in use conditions.

The continuous high temperature (35 C) and continuous

high salt concentration (5% NaCl fog) do not fit any common

use condition. One must make sure that the temperature does

not exceed the Tg of the coating under test otherwise false

failures will result [18,19].

3. Lifetime prediction from EIS data

3.1. Previous use of |Z(t)| data

Impedance data on coatings have been in use extensively

in a semi-quantitative way to measure and predict the corro-

sion protective lifetimes of organic coatings on metals. As

Mansfeld et al. [20] have observed, the most efficient way to

analyze EIS data when examining coatings is in Bode plot

format. The EIS data from newly applied coatings on metals

is often purely capacitive in nature with only one time con-

stant, and only levels off to a low frequency limit at quite ahigh values of |Z|. As the coating performance decays, the

signal begins to show non-linear behavior at intermediate

frequencies and displays more than one time constant. Many

former workers have examined time changes in EIS data

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from coated metals in exposure [6,7,21,22]. However, very

few authors have considered examining the low frequency

|Z| values as a function of exposure time for extended ex-

posure times. This type of data analysis has been done in

our laboratories for several types of coatings, and the initial

results have already been published [17].

3.2. Observed trends in EIS vs. exposure time

As mentioned above, there have been an extensive num-

ber of EIS measurements of coating performance in actual

or accelerated exposure. However, much of the focus of

these studies has been on the period of time after the metal

surface begins corroding, and not on the time over which

the coating performance is degrading but the metal surface

is essentially intact. However, work in our laboratory indi-

cates that for accurate evaluation of the protective lifetimes

of organic coatings over metals, the period of time one

should consider in detail is that, before the onset of sig-nificant corrosion of the metal substrate during which the

coating somehow degrades and loses its corrosion protec-

tive properties. When the coating is still largely intact (no

physical damage like scratches or stone dings) the metal is

not undergoing significant local corrosion. The most impor-

tant property of the coating is its ability to impede the flow

of current between anodic and cathodic areas of the metal

substrate. This property is the resistance of the coating and

is best characterized from EIS measurements by examining

the low frequency limit of |Z()|. In a practical measure-

ment sense, this is the value of |Z()| at the lowest value of

for which there is still no significant noise as 0, and

which does not require an unreasonable measurement time.If one considers the EIS literature on coatings, this value of

is somewhere between 103 and 5102 Hz.

In data from our laboratory for |Z| at the lowest frequency

that can be accurately measured as a function of exposure

time for coatings that are physically intact and provides cor-

Fig. 2. |Z|0.012Hz vs. exposure time for various aircraft coatingsSet A.

rosion protection, and only for systems which are in steady

state over the measurement time necessary to acquire the

data, an exponential decay of the low frequency modulus

with exposure time has been observed. This has been more

completely described in Ref. [17]. Other authors have seen

this type of data also and commented likewise on this trend

in data of |Z|lowfreq when plotted vs. exposure time [23].Examples of these types of data are shown in Figs.2 and3.

These data are from aircraft coatings as identified in the

figure over Al alloy 2024 T-3 panels exposed in the exposure

cycle described in Fig. 1. The topcoats are DoD Specification

MIL-C -85285, the extended lifetime topcoat (ELT), and

primer MIL P23377 materials from Deft Coatings Inc. In

Fig. 4 are shown photographs of the panels from which the

data in Fig. 3 was measured after the indicated exposure

time.

The data of Figs. 2 and 3 can be analyzed from the fol-

lowing equations:

|Z|(t) = |Z|m + (|Z|0 |Z|m) expt

(1)

ln

|Z|(t) |Z|m

|Z|0 |Z|m

=

t

(2)

where |Z|m is the limiting bare metal value |Z|, |Z|0 is the

film resistance values at t= 0, and the constants, , has the

dimension of time and can be considered the characteristic

decay times for the coating under consideration. The decay

constant (the inverse of the slope of the exponential fit

line in the graph of ln(|Z|(t) |Z|m) vs. exposure time) is

presented for different samples.

If one takes Eq. (2), and solves for the time it takes todecay to a specific failure value, |Z|fail, one has an expression

for the failure time of the coating as a function of the chosen

failure modulus:

tfail =

ln

|Z|0 |Z|m

|Z|fail |Z|m

(3)

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Fig. 3. |Z|0.012Hz vs. exposure time for various aircraft coatingsSet B.

This analysis holds true for other types of coatings as well,

such as alkyd marine coatings and epoxy marine coatings

[17], and should be considered for other types of coatings

as well. Eq. (3) implies that the failure time is a function

of the chosen failure modulus value. This is illustrated

in Fig. 5, and the corresponding values of the failure rate

constant are given in Fig. 6. The failure rate constant is

most useful for comparative ranking of substrate lifetimes

when a complete prediction of failure time is not necessary,

such as in rapid screening testing.All coating systems examined under the exposure protocol

ofFig. 1 have shown the first-order decay behavior described

above that allow the use of this lifetime prediction protocol.

This is not always true for other exposure protocols, or for

shorter time periods of exposure, and this will be discussed

in a later publication.

Fig. 4. Photographs of panels used in testing of Fig. 3.

4. Thermal effects in coatings examined

electrochemically

4.1. General comments

The general effects of temperature are often used to

accelerate exposure tests for coating lifetime such as the

high temperature periods of the Prohesion test cycle. The

reasoning behind the use of increased temperature to

accelerate failure is that there is an increase in rates ofchemical reactions, transport properties, molecular mobil-

ity, etc. with increasing temperature. An Arrhenius-type of

effect is often invoked in the use of increasing temperature

to accelerate the failure of materials [10]. As discussed in

Ref. [10], caution must be exercised in extrapolation of

results from thermally accelerating failure. The ideal acce-

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Fig. 5. Failure time dependence on failure modulus.

lerated test increases the rate of failure without changing the

mechanism of failure. We have examined the effects of tem-

perature on coatings performance by EIS as an attempt to

identify the validity of thermal acceleration of coatings fail-

ure and to provide numerical evaluation of coatings proper-

ties vs. exposure at elevated temperatures [19].

4.2. Early studies: examination of pipe-line coatings

Our earliest studies of thermal effects were performed

in conjunction with our examination of pipe-line coatings

[24]. We showed that thermal acceleration study had to

Fig. 6. |Z|(t) decay constant () for different aircraft coating systems.

consider the effects of the glass transition temperature, Tg,

in the coatings under study as well as the effects of the wa-

ter plasticization of the coating during the measurements.

This was more fully documented in Ref. [19], it gives a full

discussion of how examination of EIS data as a function of

temperature can be used to characterize the Tg in coatings

in immersion, as well as the plasticization effect of water on

many coating polymers. The barrier and electrical resistance

properties of coatings are significantly lowered above theirTgs, and care must be taken not to exceed the Tg of the

coating under study during accelerated testing if one wants

an accurate evaluation of the coating in question. Similarly,

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use of coatings above their Tg will drastically shorten their

lifetime of protection.

4.3. Thermal cycling effects

What all of these studies have shown is that EIS mea-

surements track very effectively the effects of temperature,water ingress and coating plasticization, and Tg in coatings.

Ref. [18] discusses the effect of reversible and irreversible

changes due temperature cycling on Tg and other coating

properties, and suggests that irreversibility in coatings prop-

erties is due to permanent degradation of the coatings prop-

erties. Observation of irreversible degradation of coating

properties in thermal cycling is an early indicator of coating

failures.

4.4. Tg effects in dry and wet systems

Below the Tg, the activation energies for diffusion andconductivity are both quite high and the magnitudes of the

diffusion coefficient and the conductivity are quite small (re-

sistance is large). Above the Tg, or any other orderdisorder

transition, the energies of activation for transport processes

undergo threshold behavior, and diffusion sharply increases

while electrical resistance drops drastically. Plotting the

properties of polymer coating films above and below the

Tg has shown a distinct change in slope around the Tg. The

extent of irreversibility in cycling above and below the Tgis dependent on the coating composition. For coatings in

immersion, the plasticization effect of water must be con-

sidered, and the reversibility of this plasticization is very

important [18,19]. For ionomer types of coating polymers,

above the Tg, there is considerable water ingress due to

solubility and plasticization effects [25,26].

4.5. Thermal effects on water ingress and uptake as

assessed by EIS

We have earlier examined water ingress and uptake by EIS

for an epoxy powder coating over steel and a ionomer coat-

ing over steel [25,27]. It was noted that the ionomer coat-

ing, once taken over its Tg in immersion, had irreversible

changes in its low frequency |Z| values and its capacitance

as estimated from EIS. Measurement of the electrochemical

properties of the film by impedance spectroscopy enables us

to calculate the capacitance which incorporates the dielectric

constant of the film. Both these properties change with in-

creased water content and plasticization of the polymer film.

Caution is required over the choice of frequency since

one needs to choose a frequency at which the dielectric

properties rather than the electrochemical are measured.

Too low a frequency, (below the break point frequency) and

the electrochemical properties of the film and any corrosion

processes occurring will be measured. The dielectric con-

stants for most polymers are in the range of = 34 [28].

Table 1

DSC data on powder coating and ionomer coating

Estimated Tg (C) Wf (%)

Epoxy powder coating detached film sample

Dry 100 Nor detected

In 3% NaCl 8 months at

25

C+ 1h at 90

C

82 1.4

Ionomer powder coating detached film sample

Dry 81 Not detected

In 3% NaCl 24h at25 C 75 0.3

In 3% NaCl 1 h at100 C 74 1.6

Table 2

Weight measurements on coating films

Estimated Tg (C) Weight (%)

Epoxy powder coating detached film sample

In 3% NaCl for 8 months

at 25 C+ 1h at 90 C

73 1.35

Ionomer powder coating detached film sampleIn 3% NaCl for 1 h at100 C 74 1.6

Entry of water which has a much higher dielectric constant

(w = 80 is the dielectric constant of water at room temper-

ature) into the coating will increase the coating capacitance

(Cc). From the Brasher and Kingsbury (BK) [29] empirical

formula, measuring Cc should be a measurement of water

permeation into the coating as given by

=log(Cc/C0)

log w(4)

where is the volume fraction, Cc the coating capacitance,

C0 the coating capacitance at the beginning of exposure,

and w is the dielectric constant of water.

There is good agreement between the thermal and weight

measurements for water uptake. A slight modification of the

BK empirical equation to

=(Cc/C0)

w

1

w(5)

gives better agreement of these impedance calculations to

the thermal and weight measurements for these samples.

This was not true for all of our impedance data.

As expected, good agreement is possible from weight andthermal measurements of the water uptake by a film. DSC,

weight measurement, TGA data are shown in Tables 13.

The water uptake data for our samples as measured by

TGA was the most reliable and the most reproducible results

of the methods applied. This method provided us with a very

Table 3

TGA measurements of relative water uptake

Wet film boiled

in 3% NaCl

FBE Sample 1 FBE Sample 2 Ionomer

Water uptake (%) 1.15 1.14 3.27

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Fig. 7. Water uptake estimated by EIS using the BK equation: ionomer powder coating.

accurate, almost absolute method for determining water con-

tent when the EIS data based on the BK estimating proce-

dure for water content gives very misleadingly high results.

The data calculated by the BK and modified BK equations

are given in Figs. 7 and 8 for the ionomer and the epoxy

coatings, respectively. As can be seen from these figures, the

estimates of water content when using the BK equation are

quite large as compared to the weight measurements. The

estimate for water content calculated with the modified BK

are much closer to the experimentally determined values.The disparities between the TGA and DSC results and the

Fig. 8. Water uptake estimated by EIS using the modified BK equation: epoxy powder coating.

BK estimates seem to be significant when some of the as-

sumptions in the BK estimate, especially the assumption that

there is a separate water phase with no solvency interactions

between water and coating, are no longer true. The ionomer

coating, as one might expect, shows the greatest disparities

between the electrochemical and weight-based estimates of

water content. In our measurements, the failures of the as-

sumptions seem to occur especially when water enters the

coating above the Tg and/or the water is a strong plasticizer

for the coating, or enters it as a neutralizing electrolyte as itdoes with the ionomer.

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This is documented in a recent study in our laboratory.

The techniques that we developed in our thermal cycling

studies [18] have shown that when panels are exposed above

their Tg to aqueous electrolyte and then cooled, the apparent

water content as measured by EIS via a BK type of rela-

tionship does not equal the amount of water as determined

gravimetrically or by TGA. This implies that the water im-bibed above the Tg is not phase-separated, as required by

the BK relationship, but probably acts as a partial solvent

for the polymer phase which apparently alters the dielec-

tric constant significantly yielding a higher apparent water

content than predicted by BK. Others have seen related

results [30].

5. Summary and conclusions

In summary, EIS techniques provide an excellent method

to examine the corrosion protective properties of organic

coatings as well as other properties of the coatings suchas thermal properties, water solubility, and perhaps even

physical aging processes. In conjunction with accelerated

weathering protocols developed by corrosion scientists to

qualitatively estimate the corrosion protective lifetimes of

coatings, the numerical evaluation of metal substrate/organic

coatings systems by EIS measurements provides an objective

assessment of system performance that tracks performance

changes in such systems quite well. For relatively undam-

aged systems, a simple parallel Randles circuit (RC) is suffi-

cient to model systems performance, but as systems become

significantly damaged due to exposure and corrosion at the

metal/coating interface, more complex circuits are required.These may offer insight into final failure mechanisms.

From the work we have presented and the work of others

cited in this work, it can be definitely concluded that EIS

and other electrochemical measurements of metal/organic

coating systems offer significant advantages to those eval-

uating such systems, especially when used with accelerated

exposure protocols. The numerical, objective nature of the

results from EIS enable quantitative assessment of changes

in metal/coating systems as they are artificially or naturally

weathered, and allow objective evaluation of these sys-

tems. EIS is an important measurement tool that should be

used routinely by all those examining corrosion control by

organic coatings.

Acknowledgements

This work was performed with the support of the

following: (a) Office of Naval Research under grant no.

N00014-95-10507, Dr. A.J. Sedriks, program manager;

(b) Air Force Office of Scientific Research under grant

F49620-96-1-0284, Lt. Col. P. Trulove and Maj. H. De-

Long, program managers; (c) a sub-contract from Boeing,

Dr. J. Osborne, PI, the prime contractor for DARPA under

contract no. F33615-96-C-5078; (d) a subcontract with U.

Missouri-Columbia, Prof. H. Yasuda, PI, the prime con-

tractor for DARPA under contract no. F33615-96-C-5055.

All of the military coatings samples studied here were gra-

ciously provided by Deft Coatings Inc., 17451 Von Karman

Ave, Irvine, CA 92614.

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