United States
Environmental Protection
Agency
Atmospheric Research and v
Exposure Assessment Laboratory ^
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-88/045 Feb. 1989
&EPA Project Summary
Analytical Techniques for
Assessing the Effects of Acid
Deposition on Painted Steel
Substrates
P. Moran, T. Simpson, G. Davis, and C. Arab
This report summarizes the out-
come of studies performed for the
period of October 1, 1987 - May 15,
1988 of the first year of this program
at The Johns Hopkins University and
Martin-Marietta Laboratories. To
date, initial exposure studies of
painted steel (ASTM A569 CO) cou-
pons coated with a commercial alkyd
primer/top coat system for steel
structures (not bridges) and prelim-
inary characterization of the
freestanding paint films have been
completed. Surface analytical tech-
niques including x-ray photo-
electron spectroscopy (XPS), energy
dispersive spectroscopy (EDS), and
scanning electron microscopy (SEM)
have been used in the charac-
terization process. In addition,
electrochemical impedance spec-
troscopy (EIS), Fourier-transform
infrared spectroscopy (FT-IR),
weight loss/gain measurements, and
coating/steel adhesion-strength
studies have been performed to
evaluate performance of sample
coupons and paint films in aqueous
acid environments.
This Project Summary was devel-
oped by EPA's Atmospheric Research
and Exposure Assessment Laboratory,
Research Triangle Park, NC, to
announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report
ordering information at back).
Introduction
The overall goal of this program is to
determine the most sensitive techniques
for signaling the oncoming degradation of
paint coatings and the subsequent
corrosion of the steel substrate, and to
correlate this information with the
deterioration of actual paint films. This
program is designed to answer the
following two questions: (1) What are the
mechanisms by which the coating on a
painted steel structure fails when
exposed to the environment and (2) what
incremental role do pollutants and acid
deposition play in accelerating the failure
of the paint/steel system?
The primary goals of the first year of
the project are the construction of an
environmental test chamber and the
preliminary screening of suitable
analytical techniques for the early
characterization of corrosion of painted
metal substrates exposed to acid
deposition environments.
This report is divided into five primary
sections which summarize the current
state of this project. The first section
provides information regarding the
exposure chamber and outlines the
exposure conditions of the initial tests.
The second section of the report explains
the techniques used for sample
preparation including the coatings used,
the methods for coating and curing of the
paint films, and the pre-exposure
chemical information regarding the paint
films. The third section specifies the
conditions of the immersion studies on
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four different solutions (sulfurous, nitric,
or sulfuric acids at pH3, and distilled
water) and the analytical methods
chosen for preliminary evaluation of the
mechanism of degradation of both free
standing and coated steel samples. The
fourth section describes the effects of
exposing the paint film to the four
solutions. The mechanism of paint failure
under each condition is discussed. The
results of this effort have provided insight
into the potential benefits and drawbacks
of each analytical technique. The final
section of the report is a discussion of
the program thus far and the conclusions
reached to date.
Progress to Date
Environmental Test Chamber-
Construction and Preliminary
Studies
The basic design and operation of the
environmental test chamber is described
as follows: Air from a laboratory supply is
pumped through a dryer, and charcoal
and HEPA (0.2 jim) filters to remove
inorganic and organic contaminants
(SOa, pump oil, HaO, hydrocarbons).
The purified air is broken into two
streams and routed to a set of
computer-controlled flow controllers.
One controller is used to regulate the
flow of air into a humidifier, where the air
is saturated with water at an elevated
temperature. The moist air then
immediately passes into a condensor to
reduce its temperature to the chamber
temperature and ensure that it is
saturated with water. The two air streams
are then remixed at a known ratio to
provide a controlled and reproducible air
mix with a well defined relative humidity.
The system has been designed to add
pollutants from bottled gas through mass
flow controllers and is capable of flow
rates as high as 100 l/min. Because this
flow rate will not provide sufficiently high
velocities in the chamber to ensure
turbulent flow (Re < 4000), baffles have
been incorporated between sample
sections to promote mixing and minimize
the development of a boundary layer.
The air containing the pollutants is mixed
with the moist air from the condensor in
an adiabatic mixing chamber to obtain a
uniform gas composition. The inside
walls of the chamber are quartz, and the
remainder of the system that is exposed
to the gas is teflon.
The sample chamber has a 4"
diameter, is 2-ft long, and is able to
hold up to 78 samples at any one time.
The samples are square 1x1x1/16 in.
coupons and are held in place by
magnets which are mounted flush to the
chamber wall. Although the temperature
of the sample chamber is constant (20-
30°C), it will be possible to vary
temperature of individual sections of the
chamber to permit condensation and
evaporation on specific samples. The
environmental exposure system will be
monitored and controlled using a
computer, and software has been written
to collect the data and to control the 100
l/min mass flow controllers (wet stream)
and the sample heater/chiller unit, which
will regulate the sample temperature.
Sample Preparation and Pre-
Exposure Characterization
Samples for atmospheric exposure,
immersion studies, and chamber studies
have been prepared. All test panels have
been made from steel (ASTM A569 CQ)
in two different thicknesses (1/16" and
1/8"). Coupons of three different sizes
(27/8 x 47/8 x 1/8 in., 4x6x1/16 in. and
1 x 1 x 1/16 in.) have been used in the
initial studies.
All sample coupons were prepared via
the following procedures. The coupons
were solvent degreased and grit-blasted
to a white metal. A primer/top coat
system that is low in sulfur and does not
contain chromates or lead was then
applied to the clean coupons. After
discussions with ASRL, a commercial
alkyd primer and top coating were
selected for these studies. The front side
of the coupons was coated with a layer
of primer followed by a layer of the top
coat. The back surface of each coupon
was coated with one layer of primer and
two layers of the top coat. The edges
were coated with the top coat twice using
a brush. All of the coupons were then
oven cured in air at 100"C for 1 hr., with
a heat up rate of 1 °C/min. Some concern
resulted regarding the ability of a single
layer of topcoat to prevent corrosion
propagation at preexisting pinholes from
being the primary mode of degradation.
Preliminary testing via EIS and other
analytical methods had indicated that this
may be a serious concern. For this
reason, samples have been prepared
containing two layers of topcoat on both
the front and back surfaces and are
currently being evaluated.
Bulk paint films have also been
prepared in the following manner. The
formulation of adequate viscosity is
poured onto a teflon plate and spread out
to a thickness of about 10 mils using a
doctor blade. The films are then allowed
to dry on the plate overnight at room
temperature under standard laboratory
conditions. Samples are then oven cured
using the same procedure described
earlier.
Sample preparation for in-situ elec-
trochemical impedance spectroscopy
(EIS) studies has begun. Two designs are
being evaluated for their feasibility in
such studies. The first incorporated
platinum mesh into the coating to
function as reference and counter
electrodes. Small sections of 52 or 100
mesh platinum gauze, covering less than
10% of the sample area, have been
incorporated into the paint coating. The
reference electrode is in the first layer of
top coat of a 1 x 1 x 1/16 in. sample
coupon. A second layer of coating is then
applied, containing the counter electrode.
The second design involves sputter
depositing gold onto different layers of
the coating for the same purpose.
Exposures and Analytical
Methods
Coupons for initial exposure tests were
sent to EPA in December, 1987 and
January 1988. In addition, freestanding
paint films have also been recently
prepared for EPA. Twenty 1 x 1 x 1/16 in.
samples have been sent to Sam Williams
(USDA Forest Service) to be pre-
exposed to UV radiation in a weather-
ometer prior to exposure in the
atmospheric chamber at MML. Coupons
for laboratory tests have also been
prepared and immersion studies have
been ongoing at MML and JHU since
January 1988. Coupons have been
immersed in one of four different
environments: aerated solution of either
nitric acid, sulfuric acid or distilled water,
and a deaerated solution of sulfurous
acid. The acidic solutions have been
maintained at pH3 throughout the
exposure time. Free standing paint films
have also been exposed to each solution.
Examination of a variety of analytical
techniques to determine those most
appropriate for early detection of
corrosion of painted metal substrates and
identification of failure mechanisms has
also been completed.
Surface Techniques - Sample surfaces
have been examined with XPS using a
Surface Science Instruments SSX-
100-03. In this spectrometer, MgK alpha
x-rays are focused to a 600nm spot on
the sample; the emitted photoelectrons
are analyzed using a hemispheric energy
analyzer with multichannel detections to
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determine the concentration and chem-
ical state of elements (Z > Li) in the near
surface region (depth < 5nm).
The cross sections of the paint films
were analyzed using high resolution
scanning electron microscopy (HSEM)
on a JEOL 100 CX scanning trans-
mission electron microscope (STEM),
which has a point to point resolution of
20 angstroms. Elemental analysis of the
top coat and primer were evaluated using
EDS with electrons at an accelerating
voltage of 40 KeV. The use of EDS
permitted detection of elements as low
as F on the periodic table, and the
detection volume is several micrometers
into the bulk of the paint.
Fourier Transform Infrared Spectros-
copy - The Nicolet 7199 and the 5S XC
spectrometers each equipped with a
Hg-Cd-Telleride (MCT) detector were
used for analyses. A Spectratech IR Plan
microscope capable of providing spatial
resolution necessary to produce high
quality spectra is attached to the 7199
spectrometer. The viable angle attenu-
ated total reflectance (ATR) manifold
enables analyses to depths of 40-50nm
below the surface of the film.
The system for measuring perme-
ability, solubility, and diffusivity of the
pollutants in the paint has been designed
and constructed, and is undergoing
calibration tests. The gas detector for the
transport properties experiment has been
selected. It consists of a permanently
aligned long-path gas cell (#6 PA,
Infrared Analysis, Inc.) mounted in an
FT-IR spectrometer (Nicolet 55XC
equipped with a MCT-A detector). This
system will continuously monitor the
gases permeating through the paint film.
Preliminary experiments using sulfur
dioxide indicate that the system has a
minimum detectable concentration (mdc)
of 30 ppb.
Electrochemical Impedance Spec-
troscopy - Coupons exposed via
complete immersion in each of the three
acids for 65 days have been charac-
terized by electrochemical impedance
spectroscopy (EIS) at regular intervals
throughout the exposure time. EIS
studies were conducted using an EG&G
PAR Potentiostat/Galvanostat Model 273
in combination with a 5208 Two Phase
Lock-In Analyzer. Data was collected
over a frequency range of 0.005hz-
100kz. Impedance data was analyzed
using the Nyquist and Bode modes of
presentation. An equivalent circuit model
for the coated metal system has been
developed which appears to describe the
experimentally determined behavior of
the system very well.
Results of Exposure Tests
Surface Analyses
XPS measurements on exposed sam-
ples indicated a small adsorption of S
(0.2-> 0.6 at. %) as a sulfate and N
(2-4 at. %) as an amine on all samples,
including those immersed in distilled
water. The source of the S and N is
being currently investigated. For samples
exposed to water, sulfurous and nitric
acids, a small amount of Zn (approx. 0.2
at. %) was also observed, most likely
originating from the primer. The
comparison of the EDS line profiles and
the XPS measurements suggests that
the S from the sulfurous acid penetrated
the paint and primer films readily and
does not accumulate at the electrolyte-
paint interface.
EDS line profiles were made across
primer/paint cross-sections for as-
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
of material (elements with Z > Na). In
contrast, samples exposed to sulfurous
acid exhibited an indiffusion of S through
the paint and the primer starting after 1
week of exposure.
Fourier Transform Infrared
Spectroscopy
Preliminary results indicate that
conventional FT-IR spectroscopy is not
an adequate technique for the charac-
terization of the bulk films. Sufficient
transmission through films incorporated
into KBr pellets or inserted between NaCI
plates does not occur. For this reason, IR
studies are restricted to those employing
the variable angle ATR manifold. Current
variable angle ATR spectra (< =45) of
bulk paint films after immersion in
different aqueous acid solutions do not
reveal any noticeable deterioration.
Electrochemical Impedance
Spectroscopy
EIS has also been used extensively to
characterize immersed samples
throughout the exposure time in each of
the three acids. From initial impedance
value, 5 hours after exposure, evidence
for pinholes is apparent. A single coating
of the paint does not appear to be
sufficient to prevent behavior char-
acteristic of pinhole defects from being
observed even at the shortest exposure
times. More aggressive conditions,
aerated nitric or sulfuric acids, simply
enhance this situation. In the case of
sulfurous acid behavior characteristic of a
defected coating is not well defined until
exposure times nearing 30 days. This
may indicate better sample preparation of
the sulfurous sample or that sulfurous is
inherently less aggressive to the metal. In
addition, the absence of oxygen may
play a role in determining the observed
behavior. This phenomenon has not been
completely explained and requires further
evaluation.
Using our equivalent circuit model
certain parameters corresponding to the
relevant electrochemical processes were
calculated as a function of exposure time
to identify trends. The coating
capacitance has been calculated to be or
the order of 2 x 10'9 farads in each ol
the three acids. This is consistent witr
predicted values for coatings of this type
and thickness.
We postulate that EIS data throughou
the exposure period for these single coat
specimens is dominated by the presence
of preexisting pinholes, most probably
localized at the edges of the sample
which simply increase as a function o
time. At later times build-up of corrosior
products could serve to slow th<
corrosion rate and is most likely th<
reason for the fluctuations in corrosior
resistance observed in sulfuric acid. It h
possible that new sites of attack an
forming during the exposure period
however, confirmation of such <
phenomenon requires testing via othe
methods. This is somewhat supported b;
the increasing number of blister;
observed to develop during the exposure
One method suitable for such studie:
which should give preliminary informatioi
regarding the nature of attack am
propagation of corrosion is SEM
Identification of pinhole sites on tes
coupons prior to and throughout th<
exposure process could indicate the typi
of attack which predominates. Sucl
studies are anticipated to begin in th
upcoming months. Current studies t
determine the role of pinholes localize'
at the edges of samples are using
defined area exposure cell for impedanc
measurements. The sample is mounte<
on the cell via an o-ring seale<
polymethyl methacrylate holder whic
results in an exposed area of 2.5 cm2
The cell is designed such that th
counter electrode resides in the larg
compartment of the cell and th
reference electrode in the fritted glas
tube. Preliminary results using th
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defined exposure cell are consistent with
our postulates. Impedance data at early
times for doubly coated specimens in
this cell which does not expose the
edges or back surface to the solution
indicate purely capacitive behavior. Thus,
pin hole defects are most likely localized
at the edges of these samples. Further
testing of these samples to verify these
results and look at behavior at longer
exposure times is underway.
In addition, in-situ EIS samples to be
exposed in the atmospheric chamber
have been designed and are in the
process of being constructed. The
behavior of these samples will be
evaluated in the defined exposure cell to
adequately best design and the
feasibility of use of such a cell to
adequately model the behavior of
specimens without implanted electrodes.
Visual Observations
After two weeks of exposure, the
edges of the panels in the nitric and
sulfuric acid baths began to show very
slight rust stains. The edges of the
panels exposed to the sulfurous acid
solution did not appear to have been
attacked, although small blisters began to
form on the surface. At later exposure
times each of the exposed sample
coupons was covered with blisters which
appeared to increase in number with
time. The blisters were filled with an
aqueous solution whose pH is approx-
imately 8. The curing process did not
seem to effect the formation of blisters
which occurred on both as-coated and
oven-cured samples. The primary
cause of the blistering is felt to be the
uptake of water through the coating.
Adhesion Studies - Tensile
Pulls
Stubs were bonded to the painted steel
prior to and or following exposure and
the force required to separate the steel
and the paint was measured. The locus
of failure in the unexposed coupons
visually occurs at the primer-top coat
interface. After one week exposure, no
change in the tensile strength was noted
for the water, nitric, and sulfuric acid-
exposed samples and failure was
generally cohesive in the primer. Some
decrease in strength (~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, the
lowest strength was measured shortly
after exposure; tests performed a few
days after immersion indicated a partial
regaining of strength. Presumably this is
a result of the paint/primer system losing
some of its absorbed water. The locus
of failure of the degraded samples
generally changed 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.
Weight Gain
Each specimen gained weight upon
exposure: 1% gain after 8 hours and
2.0-2.5% after 5 days. For longer
exposures (up to 4 weeks), samples in
the acidic solutions exhibited smaller
weight gains (-1.5%) while the samples
immersed in distilled water continued to
slowly gain weight (to approx. 3%).
Recent tests on the samples exposed to
distilled water have shown that the
samples begin to lose weight after 8
weeks of exposure.
Conclusions
To date, construction of the envi-
ronmental test chamber; preparation of
samples for atmospheric, immersion, and
chamber studies; initial atmospheric and
immersion exposures; and preliminary
screening of analytical techniques have
been completed. The environmental test
chamber is fully operational and initial
exposures are anticipated to begin in the
next few weeks. Samples for in-situ
electrochemical testing in the exposure
chamber are being prepared. Immersion
studies will be conducted on these
samples using a defined area exposure
cell to evaluate and refine the design for
chamber studies. General methods of
sample preparation for all types of
exposure have been refined. The curing
process, 1 hour at 100°C does not seem
to degrade the paint film to any
appreciable extent and results in near full
cure. In preliminary immersion studies
the effects of water overwhelmed the
contribution to degradation due to
pollutants present under most conditions.
In the case of sulfurous acid, however,
some indiffusion of sulfur and rapid loss
of the cohesive strength was witnessed. It
is expected that chamber exposures will
be more useful in evaluating the effects
of the pollutants on coating degradation.
Several analytical tech- niques have
been used in preliminary studies to begin
the screening process for those which
will be the most sensitive in signaling
degradation. At this point in the program
it is too early to reject any of these
techniques and continued evaluation of
each of the methods is ongoing.
Proposed studies, expected to begin
within the next few months, include the
complete evaluation and testing of the
atmospheric chamber and the onset of
chamber exposures. The preparation and
evaluation of samples for in-situ
electrochemical testing has already
begun. Initial evaluation of samples in the
chamber are expected to begin within the
next few months. The effects of UV-
radiation via pre-exposure of films will
also be evaluated. Refinement and
further testing of the system for
measuring permeability, solubility, and
diffusivity of the pollutants in the paint is
in progress.
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P. Moran and T. Simpson are with The Johns Hopkins University.Baltimi
Davis and C. Arah are with Martin Marietta Laboratories, Baltimore, Mu.
J. W. Spence is the EPA Project Officer (see below).
The complete report, entitled "Analytical Techniques for Assessing the Effects of
Acid Deposition on Painted Steel Substrates" (Order No. PB 89-127 5001 AS;
Cost: $15.95, 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, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-88/045
0GQQ329
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