United States
Environmental Protection
Agency
Atmospheric Research and
Exposure Assessment Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-90/032 July 1990
&EPA Project Summary
Preliminary Exposure Study to
Determine the Effects of Acid
Deposition on Coated Steel
Substrates
Patrick J. Moran, Theresa C. Simpson, Howard Hampel, Guy D. Davis,
Barbara A. Shaw, Chike O. Arah, Tammy L. Fritz, and Ken Zankel
This report describes the progress
that has been made within the
Coatings Effect Research Program
that the Environmental Protection
Agency conducts for Task Group VII
within the National Acid Precipitation
Assessment Program. This project
involves the evaluation of the effects
of acidic pollutants on painted metal
substrates. The project examined a
commercially available alkyd paint/
primer system applied to a low car-
bon steel substrate exposed under a
variety of simulated acidic conditions
to determine the micro/ macro effects
of such exposure. This report details
the results which include the screen-
ing and development of sensitive
analytical techniques and the use of
these techniques to investigate
effects of SO2 on alkyd painted steel
coupons during laboratory exposure.
The techniques that were identified
as those most sensitive and
applicable during the study include
tensile adhesion testing, electro-
chemical impedance spectroscopy
(EIS), and x-ray photoelectron spec-
troscopy (XPS). A novel electrochem-
ical monitor was developed during
this program that allowed the
continual monitoring of coating
degradation during chamber expo-
sure. It was further found that a good
correlation existed between tensile
adhesion strength measurements
and electrochemical impedance par-
ameters. The program determined
that the rate of degradation of the
alkyd painted steel coupons was
accelerated in the presence of SO2.
This effect was most pronounced on
samples that contained defects
(scribes), that were allowed to form
condensed dew during the exposure
period, and that had a horizontal
orientation during exposure.
This Project Summary was
developed by EPA's Atmospheric
Research and Exposure Assessment
Laboratory, Research Triangle Park,
NC, to announce key findings of the
research project that is fully
documented in a separate report of
the same title. (See Project Report
ordering information at back.)
Introduction
The Atmospheric Research and
Exposure Assessment Laboratory of the
Environmental Protection Agency (EPA)
has conducted a program entitled, Effects
of Acid Deposition on .Coated Steel
Substrates for Task Group VII, Effects on
Materials and Cultural Resources. This
Task Group is one of several groups that
conducted research within the National
Acid Precipitation Assessment Program.
This report reviews the progress that
was made to develop and select
analytical techniques that are appropriate
to identify early micro/macro failure pro-
cesses associated with acidic deposition
on coated steel substrate systems and to
use these techniques to evaluate such
damage. The candidate techniques were
those that would evaluate changes in the
painted steel coupons that would lead to
such macroscopic failures. Those deter-
mined to be the most appropriate for our
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painted steel system include tensile
adhesion testing, electrochemical impe-
dance spectroscopy, and surface
analytical techniques. The recent focus of
the program involved refined types of
exposures, i.e., controlled atmosphere in
an environmental chamber and the use of
those analytical techniques deemed most
appropriate in year one to examine the
incremental effects of acid deposition on
painted metal substrates.
Technical Approach
The first year of this program involved
the completion of four primary tasks: the
design and construction of an environ-
mental test chamber, the selection of an
initial coating system for study and the
development of a method for accelerated
cure of the system, preliminary sample
exposures (primarily via immersion), and
technique screening. The second year of
the program involved the use of the most
promising analytical techniques in refined
types of exposures to evaluate the dam-
age to painted steel substrates exposed
to acidic environments.
Environmental Exposure
Chamber
The environmental test chamber is
composed of a 4" diameter, 2 ft. long
sample compartment which can
accommodate up to 78 1" x 1" x 1/16"
painted steel coupons mounted flush to
the chamber wall. It is capable of
controlled introduction of pollutants and
regulation of conditions of temperature
and relative humidity. Since flow rates in
the chamber are not sufficient to ensure
turbulent flow, baffles have been
incorporated between sample sections to
promote mixing and minimize the
development of a boundary layer. The
inside walls of the chamber are quartz,
and the remainder of the system that is
exposed to the gas is teflon. The
environmental exposure system is moni-
tored and controlled using a computer, to
collect the data and to control the 60
1/min. mass flow controllers (wet stream)
and the sample heater/chiller unit, which
regulates the sample temperature.
Sample Preparation
All test panels made from steel (ASTM
A 569 CQ). Coupons of two different
sizes (4" x 6" x 1/16" and 1" x 1" x
1/16") were used in these studies. The
coupons were solvent degreased and
grit-blasted to a white metal finish. A
commercially available alkyd primer/
topcoat paint system, typically used for
steel storage tanks, was selected for
these studies. The primer/top coat
system, which does not contain sulfur,
was spray-applied onto the clean
coupons. The front side of the coupons
was coated with a layer of primer
followed by a single or double layer of
topcoat. The back side of the coupons
was coated with one layer of primer and
two layers of topcoat. The edges were
coated with primer and brush coated
twice with topcoat. Free-standing paint
films (200-250nm) were prepared by
pouring the paint onto a teflon plate,
spreading it out using a doctor blade, and
allowing to dry on the plate overnight at
room temperature under standard
laboratory conditions. All the coupons
and films were then oven cured in air at
100°C for 1 hr, after a gradual heat up
rate (taking approx. 70 min.) from room
temperature to 100°C. Specimens had
dry film thicknesses of either 120-170nm
(later studies) or 250-300nm (immersion
studies and early chamber studies).
Some specimens contained a scribe ex-
tending diagonally across the specimen
according to ASTM D-1654. Detailed
description of the techniques that were
used to provide reproducible specimens
for the exposure studies are provided in
the report.
Promising Analytical Techniques
A number of analytical techniques,
including tensile adhesion, XPS, SEM,
TGA, etc., have been applied to charac-
terize degradation of chamber-exposed
and aqueous-immersed painted steel
coupons. EIS was also used to examine
the electrochemical properties of the
paint/substrate system. An atmospheric
electrochemical monitor (ATMEIS) was
developed for conducting these measure-
ments during atmospheric exposures.
EIS was the only analytical method, used
within this program, that was appropriate
for in-situ testing in the atmospheric
exposure chamber. The fact that samples
remain in the chamber throughout such
studies makes EIS invaluable in deter-
mination of the onset and extent of
degradation and establishing the relative
contributions from each pollutant/
condition.
Results and Discussion
Painted steel coupons were immersed
in one of four different environments:
aerated solutions of either nitric acid,
sulfuric acid, or distilled water, or a
deaerated solution of sulfurous acid. The
acidic solutions were maintained at pH 3
throughout the exposure time. Free
standing paint films were also exposed to
each solution. It should be emphasized
that all immersion studies were
conducted on samples containing
single layer of topcoat on the front side oi
the sample and a double layer of topcoa|
on the back and edges of the sample.
After two weeks of exposure, the
edges of the panels in the nitric anc
sulfuric acid baths began to show verO
slight rust stains. The edges of the panelq
exposed to the sulfurous acid solution die
not appear to have been attacked)
however, small blisters began to form or
the surface. At later exposure times each
of the exposed sample coupons exhibitec
blisters which appeared to increase ir
number with time. The blisters were fille
wilh an aqueous solution whose pH is
approximately 8.
ED-XRM line profiles were made
across primer/paint cross-sections for as-j
prepared samples and for samples
exposed for 1, 4, and 8 weeks. Samples
exposed to water, sulfuric and nitric acids
exhibited no indiffusion or leaching ot|
material (elements with Z > Na). Ir
contrast, samples exposed to sulfurous
acid exhibited an indiffusion of S through
the paint and primer and a leaching of|
primer constituents starting after 1 week
of exposure. The comparison of the ED-|
XRM line profiles and the XP!
measurements suggests that the S froml
the sulfurous acid penetrates the paintl
and primer films readily and does notl
accumulate at the intact paint-electrolyte]
interface.
EIS was also used extensively toj
characterize immersed samplesl
throughout the exposure time in each ofl
the three acids. From initial impedance!
values, 5 hours after exposure, evidence!
of pinholes was apparent. EIS data!
throughout the exposure period were!
dominated by the presence of preexisting!
pinholes, predominately located at thel
edges of the samples. A cell which does!
nol: expose the edges of the sample to|
solution was subsequently used for EISI
analyses of immersed specimens tol
avoid the contribution of pinhole defects.!
Samples exposed in this cell exhibited]
very high impedance value (1010 ohms!
cm2) a few hours after exposure, and did!
not show evidence for appreciable!
defects until 1-2 weeks after exposure.!
These data confirmed that defects are!
largely localized on the edges of the|
samples.
A primary goal of the initial immersion
studies was the screening and develop-
memt of techniques that would be appro-
priate for chamber and field exposure
studies. EIS is a desirable technique to
monitor paint degradation since it is quite
sensitive and essentially nondestructive.
Our efforts involved the design and
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testing of an atmospheric electrochemical
monitor (ATMEIS) to be used to generate
EIS data during atmospheric exposure.
The ATMEIS monitor consists of a
painted steel coupon upon which a gold
electrode (covering less than 10% of the
front surface of the sample coupon) is
electron-beam deposited. The impedance
of the substrate/coating system is deter-
mined in a two electrode measurement
using the steel substrate as one electrode
and the electron-beam-deposited gold as
the second electrode. The monitor can
be used, therefore, in the absence of a
remote reference or counter electrode.
The monitor was developed to assess the
degradation of organic coatings in atmo-
spheric exposures simulating acid depo-
sition. A description for the preparation of
the monitor is presented in the report.
The monitor relies on the assumption
that the impedance of the interface of the
deposited electrode lying on the surface
of the coating is low relative to the
impedance of the coating and the steel-
coating interface; an assumption which
has been verified. This enables a two
electrode measurement rather than a
conventional three electrode measure-
ment to be completed. The two electrode
approach is valid, because placement of
the deposited electrode at the surface of
the coating enable its entire interface with
the coating to be wetted and used,
resulting in a low interfacial impedance
relative to the coating or the steel-coating
interface. The primary value of this
monitor is that it has the potential of
being- used for completely in-situ
monitoring of atmospheric or vapor phase
coating deterioration on a real-time basis
and/or on real structures. Such a method
is currently unavailable.
Although the ATMEIS monitor was
designed to be used for atmospheric
monitoring of coating degradation,
preliminary evaluation and validation of
the monitor was conducted using
immersion exposures. Results from
exposures for time periods up to several
months demonstrate that the EIS data are
essentially identical regardless of whether
1) remote reference and counter elec-
trodes are used, 2) the gold electrode is
used as the reference/counter combina-
tion with the sample in the aqueous acid
electrolyte, or 3) the gold electrode is
used as the reference/counter combina-
tion with the sample removed from the
electrolyte in air.
In addition, the ATMEIS monitor has
shown behavior which is nearly identical
to that of an as-prepared sample
(conventionally coated without a sputter
coated gold electrode) for equivalent
exposure times. Within the error
associated with sample to sample varia-
tion and measurement constraints, there
is no significant difference in the behavior
of samples with or without the monitor.
Thus, the presence of the gold electrode
on the sample surface does not appear to
affect the normal chemistry/degradation
process of the coated metal sample.
Tensile adhesion tests were used to
measure the cohesive/adhesive strength
of the paint system as a function of time
in each exposure environment. Studs
were bonded to the painted steel prior to
and/qr following exposure. The force
required to separate the steel and the
paint was measured. The failure type and
locus of failure in the unexposed coupons
was cohesive in the primer. After one
week exposure, no change in the tensile
strength was noted for the water, nitric,
and sulfuric acid-exposed samples, and
failure continued to be cohesive in the
primer. Some decrease in strength
(approx. 30%) was seen for the sulfurous
acid-exposed samples, and failure was
partially cohesive in the primer and
partially cohesive in the paint with the
strength being inversely correlated with
fraction of paint failure. The nitric and
sulfuric acid-exposed specimens retained
their strength at 4 weeks immersion, but
lost most of it by 8 weeks. The water and
sulfurous acid-exposed specimens lost
their .tensile strength by 4 weeks. In each
case,; strength was dependent on the
time'-. from removal of specimens from
immersion to time of measurement, the
lowest strength being seen if the
measurement was completed shortly
after removal from exposure. Tests
performed a few days after removal from
immersion indicated a partial regaining of
strength. Presumably this time
dependence was a result of the
paint/primer system losing some of its
absorbed water once it was removed
from the immersion environment. The
locus of failure of the degraded samples
generally changes to the metal-primer
interface. XPS analysis of spots
commonly found on failure surfaces
following long exposures revealed Fe,
indicating the onset of corrosion of the
substrate.
Chamber Exposures
Chamber exposures of alkyd painted
steel coupons to 1-ppm SO2 with an air
flow,of 60 1/min. at a relative humidity of
95% were conducted. Control
experiments without SO2 were also
performed. Samples were either kept at a
constant temperature (20-23°C) for dry
deposition or were thermally cycled
(alternating 12hr periods at 25-28° C and
15-18°C) for both wet and dry deposition
during the exposures. The samples were
removed at various time intervals (in the
middle of the dry cycle). Samples
designed for in-situ EIS monitoring were
also included in the latter experiments. It
should be noted that all chamber-
exposed samples except those for in-situ
EIS monitoring during the chamber run
contain two layers of topcoat on both
sides of the sample. The in-situ EIS
samples contain a single layer of topcoat
on the front side of the specimen.
As the samples were removed from
the environmental chamber, some blister-
ing occurred on the samples used in the
final chamber studies (these were the
samples with the dry film thicknesses of
120-170nm). Since these samples were,
on average, the thinnest of all the
samples examined, the occurrence of
blistering was likely related to coating
thickness. In addition, after ambient
laboratory storage for a few days, brown
spots appeared on all (independent of
thickness) the SO2, dew-exposed
samples that were mounted horizontally
(with the top surface exposed). The
discoloration is likely an example of a
synergistic effect between condensed
moisture and SO2- The brown spots
appeared after removal from the chamber
only on specimens with condensed dew
that were mounted horizontally. The
discoloration may be attributed to a
moisture-induced reaction with the
adsorbed S whose local concentration
has been increased as a result of the
evaporation of condensed dew. On
specimens that are mounted vertically,
the dew drips off and removes much of
the S from the paint surface. However, on
horizontal surfaces, not only does the S
remain on the surface, but as the dew
evaporates during the dry cycle, parts of
the surface dry first with the S
concentrating in the remaining wet areas.
The discoloration does not occur until the
absorbed moisture, as indicated by the
weight gain of each specimen, is lost
after a few days in the desiccator.
XPS measurements show that all the
nominally dry samples adsorbed SO2
from the test atmosphere. As one would
expect, no S was detected on paint
surfaces exposed only to high humidity
(control run). The rate -.of adsorption was
greatest initially with 1-2 at.% S present
on the surfaces after the first few days;
the S concentration subsequently takes
the remainder of the 28-day period to
approximately double. There is no
orientation dependence, but samples that
were pre-exposed to UV exhibit ~70%
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greater uptake of S than samples that
were not pre-exposed. High-resolution
XPS spectra indicate that the S is in the
form of a sulfate.
The results from the samples exposed
to dew are very different. First, there is a
strong orientation dependence - samples
that were mounted vertically exhibit very
little S adsorption (<0.5 at.%) whereas
samples that were mounted horizontally
exhibit up to 4 at.% S adsorption.
Second, the amount of S adsorption on
the horizontal samples varies greatly
across the surface - the brown areas on
horizontally mounted samples have high
S concentrations (comparable to that of
the dry samples), white areas have
significantly less than brown areas, but
have higher S uptake than the samples
that were mounted vertically. Again, the S
was in the form of a sulfate.
The tensile adhesion tests indicate a
degradation of the mechanical properties
of the paint system during the chamber
exposures. Initially, the tensile strength is
high (5-6 MPa) with the failure occurring
cohesively in either the primer or the top
coat. However, upon exposure to SO2,
the strength decreases, until the average
value is approximately one-third of the
initial value. Concurrent with this weak-
ening of the paint system is a shift of the
locus of failure from within the paint
system to the primer metal interface.
The mechanical properties of the paint
system also degrade during the control
run, but at a slower rate. SO2-exposed
samples exhibited a significant loss in
strength even after exposures as short as
one day. Even though there was more
variation in control samples, strength
losses occurred much more gradually
over the 28-day period and final strength
values were higher than those witnessed
for SO2 exposure.
The relationship between locus of
failure and strength remains constant
regardless of variations in test method or
exposure condition. As the specimens
are exposed in the chamber, the tensile
strength decreases significantly and the
locus of failure shifts from within the
polymer system to the metal/primer
interface. The decrease in tensile
strength appears to be dominated by a
loss or weakening of adhesion between
the substrate and the primer and that any
reduction in the cohesive strength of the
paint system plays a secondary role. This
mechanism of failure appears to be the
same with and without the presence of
SO2, but SO2 increases the rate at which
the de-adhesion occurs.
Decreases in near DC impedance
correspond well to changes in the
mechanical properties. ATMEIS data are
given for two different samples and
tensile adhesion values are averages of
at least 3 specimens. Locus of failure in
the tensile specimens occurs initially
within the paint system but shifts to the
metal primer interface at later exposure
times. Visual examination of the failure
surface indicates that this failure is
associated with corrosion at the
metal/primer interface. Thus, EIS is
detecting corrosion at the interface.
The relationship was also observed
between EIS data obtained with the
ATMEIS monitor and tensile adhesion
strength for samples exposed to the
same environment. This provides
encouragement that EIS data may be
used as an indicator of mechanical
properties and the changes in these
properties due to degradation. Although
one other study reports similar effects,
we are not aware of any studies which
have evaluated the extent of this
correlation for the same samples.
Conclusions
Major findings for this study are
summarized as follows:
• Specific analytical techniques capable
of detection of coating degradation at
early exposure times have been either
identified or developed. A good
correlation exists between non-
destructive electrochemical (EIS) and
mechanical test methods indicating
new possibilities for evaluation of
coating degradation.
• Under aqueous conditions S
penetrates the paint film upon
exposure to sulfurous acid but does
not penetrate the film upon exposure
to sulfuric acid. In addition, the
presence of water, in immersion
exposures, overrides the effects of the
individual pollutants.
• Exposure to 1 ppm SO2 at 95% RH
causes significantly more deterioration
of coated metal substrates over expo-
sure to 95% RH in the absence of
SO2. This deterioration is manifested
as localized discoloration, damage at
defects, and changes in tensile
adhesive strength and locus of failure
in chamber-exposed samples as a
function of exposure time. These
discolored regions which occur only
on samples on which dew condensed
and was allowed to dry are areas of
high S concentration. Significant
blistering and rusting occurred on
samples containing intentional defects.
This damage was most severe for
samples that were dew cycled and
mounted horizontally during chambet|
exposure.
Data from chamber exposed samples
incorporating the ATMEIS electro-]
chemical monitor also indicates
coating deterioration as a function oil
exposure time to 1ppm S02 at 95%|
RH with diurnal cycling.
Pre-exposure of samples to UV and/or]
diurnal cycling (during exposure) can
affect the severity of this degradation!
as indicated by the discoloration and|
•the amount of S detected on the
surface.
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Patrick J. Mora/?, Theresa C. Simpson, and Howard Hampel are with The Johns
Hopkins University, Baltimore, MD 21218. Guy D. Davis, Barbara A. Shaw,
Chike O. Arah, and Tammy L. Fritz are with Martin-Marietta Laboratories,
Baltimore, MD 21227. Ken Zankel is with Versar, Inc., Columbia, MD
21045.
John W. Spence is the EPA Project Officer (see below).
The complete report, entitled "Preliminary Exposure Study to Determine the
Effects of Acid Deposition on Coated Steel Substrates," (Order No. PB
90-201 799/AS; Cost: $15.00, subject to change) will be available only
from:
National Technical Information Service,
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park NC27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-90/032
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