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Rapid
Electrochemical Assessment of Paint (REAP)
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Overview
The Rapid Electrochemical Assessment
of Paint (REAP) is a procedure designed to estimate the long-term corrosion resistance of
coated metals using short term electrochemical tests. It involves running a series of
experiments over a 24-hour period on two or more samples. The REAP approach
was developed by researchers at Rockwell (Reference 1)
in 1998.
You can use the REAP parameters to
estimate the relative time-to-failure (TTF) of coatings and rank coatings in terms of
their expected long term performance. Another application of REAP testing is the quality
control of coated products. You can follow the variation of the REAP parameters over time,
as a measure of coating quality.
Overview of REAP Testing.
Measurement of the REAP
parameters involves tests on at least two painted metal samples. One of the samples is
scribed with a 2x2 cm right angle cross.3 Both samples are
exposed to a 0.5M aqueous NaCl test solution at room temperature for 24± 2 hours. An
overview of the testing process with two samples is illustrated above.
Immediately after filling the cells,
the corrosion potential of the unscribed painted sample is measured. The measured
potential determines the DC potential used in the
electrochemical impedance spectroscopy
(EIS) experiments on the
sample. An EIS experiment is run immediately after the corrosion potential experiment. A
second EIS experiment is run after soaking the unscribed sample in the NaCl test solution
for 24 hours.
In between the two EIS tests, a 24-hour
potentiostatic experiment is run on the scribed sample. A cathodic potential (-1.05V vs.
SCE) is used to accelerate disbonding of the coating. This test measures one of the REAP
parameters, the disbond rate of the coating (dx/dt), in units of mm per hour. Typical
rates measured via cathodic disbonding are 0.1mm/hr or lower.
Comparison of the results of the two
EIS experiments allows you to evaluate the second REAP parameter, water uptake by the
coating. Water uptake is expressed as an apparent volume fraction, %v. %v is calculated
from the change in the measured capacitance of the coating (CC) over the
24-hour period. These capacitance values are obtained via analysis of the EIS data using
an equivalent circuit model (shown in simplified form below).
Equivalent circuit
(simplified) used in EIS data analysis.
The third REAP parameter is obtained
from the EIS data recorded after 24 hours. It is the corrosion resistance (Rcor)
of the underlying steel and results directly from the equivalent circuit analysis of the
EIS data.
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Background
The procedure described in this application note is a
condensed summary of a proposed method being considered by the American Society for
Testing Materials (ASTM). The ASTM G01.11
subcommittee on Electrochemical Measurements in Corrosion Testing used an article by
Kendig et. al.1 as the basis of the proposed method for testing painted
steel.
Kendig’s paper studied the
correlation of various electrochemically-measured parameters with the long-term time to
failure (TTF) of painted carbon steel samples. TTF was evaluated by exposure to a salt
fog.2 The three parameters mentioned above were found to
correlate well with the TTF. The rapid electrochemical assessment method does not attempt
to measure the time-to-failure directly. Instead it relies on the correlation of the TTF
with REAP parameters as demonstrated in the Kendig paper.
This Application Note is a description
of the REAP testing process using Gamry’s products rather than a detailed review of
the original paper or the proposed ASTM method. More information can be obtained from the
paper or the ASTM proposal. You might consider joining the ASTM committee considering this
proposal. For information concerning ASTM, see the
endnotes.
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Cell Design & Setup
Each REAP measurement requires two
electrochemical cells.
The proposed REAP method calls for five
replicate tests requiring the use of 10 samples. In most cases, you will want to run the
replicate tests simultaneously, so ten cells are needed.
If you wish to build your own cells,
the proposed REAP method describes a cell that holds a flat painted metal sample. This
cell exposes 56 cm2 of the specimen to the electrolyte. The reference electrode
is a saturated calomel electrode (SCE), while the counter electrode can be platinum, gold,
graphite, nickel or passivated stainless steel. Graphite rods are probably the most
economical choice. As mentioned above the test solution used is a 0.5 M NaCl solution in
water and the experiments are performed at room temperature.
If you wish to make use of a ready made
cell kit, you can consider commercial cell kits from a number of vendors. Depending on
your budget, the need for multiple cells can limit your choices. Gamry’s
PTC1 Paint Test Cell is designed to be
inexpensive and easy to use.
Gamry’s PTC1 Cell Kit.
The PTC1 is shown above. It consists of
a simple glass tube that clamps onto a flat metal specimen. A rubber O-ring is used to
create a watertight seal. The top is a rubber stopper with holes suitably sized for the
reference & counter electrodes. The base is polypropylene. The PTC1 comes with an SCE
reference electrode & a graphite rod counter electrode. The sample exposed in the PTC1
is 4.3 cm in diameter for an area of 14.6 cm2.
Although neither the REAP paper or
proposed method mentions it, you will also want to run your EIS tests in a Faraday
Cage. A
Faraday Cage is a metal enclosure that shields the cell from noise. Shielding is required
for accurate EIS tests on very good coatings. A Faraday Cage can be as simple as a
cardboard box lined with aluminum foil. Make sure that the Faraday Cage does not come in
contact with any of the cell electrodes. You must also connect the black ground lead from
the cell cable to the Cage.
Click here for information on Gamry's VistaShield Faraday Cage.
Needed Files
The experiments are standard
techniques, which can be carried out using Gamry’s EIS300
software and any of Gamry's Potentiostats. Because of it's
sensitivity, the Gamry
Reference 600
Potentiostat is the best choice for EIS on high impedance coatings. The
Series G Potentiostat,
while it is less sensitive, is also a good choice.
The EIS300 Model Editor in
the Gamry Echem Analyst includes the reap.mdl and reap2cpe.mdl, model files used in the analysis
of the EIS data.
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Experiments
1. Run Corrosion Potential Experiment on the Unscribed Sample.
The first experiment to be performed is
just a corrosion potential or open circuit vs. time experiment. Look for the
Corrosion Potential experiment on the DC Corrosion submenu on the Experiment pull-down
menu and select it.
You will be presented with a setup
dialog box like the one shown below. These
are the default values except for the Output File and the Stability parameter (shown set
to 0).
Connect the cell to the unscribed
painted sample and then click on the OK button to start the experiment. The whole purpose
of this step is to determine the DC Voltage setting to be used in the EIS experiments. If
the open circuit readings are stable, the EIS experiments will be run at 0 V vs. Eoc.
If your metal sample has a very good
coating , the coating will have a very high resistance and may approach the behavior of an
ideal insulator. If this happens, the open circuit potential will drift to very high
potentials with time. In this case, you cannot use the open circuit potential as the DC
Voltage setting in the EIS experiments. The proposed REAP method uses a potential of -600
mV vs. SCE when the open circuit potential is not stable.
When the experiment is finished, click
on the F2-Skip button to close the experiment window.
Settings
for Corrosion Potential Experiment.
2. Run EIS on Unscribed Sample Before 24-Hour Soak Test.
Next you will run a Potentiostatic EIS
experiment on the same cell used for the open circuit test. Look for Potentiostatic EIS
experiment on the EIS submenu on the Experiment pull-down menu and select it. You will be
presented with a setup dialog box like the one shown below.
Settings for EIS Experiment Before
24 Hour Soak Test.
If you have a copy of REAP.SET,
use the Restore button to recall EIS_OHRS from REAP.SET. Otherwise use the settings shown
above. If an unstable open circuit potential was determined in Step 1, change the DC
Voltage setting to -600 mV vs. Eref.
Remember to update the Area
setting to the correct value for the cell you are using — 14.6 cm2 for the
PTC1.
The proposed method recommends allowing
the painted sample to equilibrate for 10 minutes. For this reason the Initial Delay
setting is On and the Time is set to 600 s. Do not use the F2-Skip key to bypass the
Initial Delay.
Simply hook up the cell with the
unscribed painted sample and click on OK to start the experiment. After the experiment has
finished, use F2-Skip to close the experiment window.
3. Run Potentiostatic Test on Scribed Sample for 24 Hours.
Disconnect the cell with the unscribed
painted sample, but leave the cell filled with the solution for the 24-hour soak test.
Next hook up the cell with the scribed painted sample. Use the Experiment pull-down menu
to bring up a Potentiostatic experiment setup dialog box.
Settings for Potentiostatic Cathodic Disbonding
Test.
A Potentiostatic experiment
setup dialog box will appear. Change the experiment parameters to the values shown above. When you are done, click on
OK to start the experiment. The data actually collected during this 24 hours of cathodic
disbonding is not used for data analysis, but in general higher currents seen in this
experiment will lead to higher disbond rates. At the end of the 24 hours, click on F2-Skip
to close the experiment window.
4. Run EIS on Unscribed Sample After 24-Hour Soak Test.
After the 24-hour cathodic disbonding
is over, disconnect the cell with the scribed sample and hook up the cell with unscribed
sample again. Start up another Potentiostatic EIS experiment and use Restore to recall the
EIS_24HRS setup or change the setting to the values shown below. Make sure the DC Voltage
setting is the one used in Step 2.
Settings for EIS Experiment After 24-Hour Soak Test.
Click on OK to start the
experiment. After the experiment has finished, use F2-Skip to close the experiment window.
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Data Analysis
5. Perform Tape Pull on Scribed Sample. Measure
dx/dt.
After the potentiostatic test has run
for 24 hours on the scribed painted sample, you need to measure the disbond rate,
dx/dt.
Remove the scribed sample from the cell. Immediately rinse the sample with deionized
water. Remove excess water with a lint-free cloth or paper towel. A piece of adhesive tape
is applied to the region around the scribe and pulled to remove any loose coating around
the scribe. Probe the remaining coating with a knife edge to make sure that the loose
material has been completely removed by the tape pull.
Use a ruler to measure the width of the
tape pullback area. You should to measure this width in several places and average the
results. The disbond rate is the average width of the disbonded stripe divided by two
times the elapsed time. Kendig’s paper use a default value of 10-4 mm/hr
whenever no appreciable pullback was observed.
6. Analyze EIS Data Using REAP.MDL. Extract CC,0, CC,24
& Rcor.
As mentioned before, analysis of the
REAP impedance data is accomplished using an equivalent circuit suitable for a coated
sample. The circuit shown here differs from the one shown in the introduction in that the
capacitors have been replaced by constant phase elements. This circuit is contained in the
model file, REAP.MDL.
Using constant phase elements
(imperfect capacitors) can give a better fit to experimental data.
Launch the Echem Analyst and load the first EIS data file, EIS0HRS.DTA. Now use the Named Model menu
option on the EIS pull-down menu to load and run REAP.MDL.
REAP.MDL used in
Analysis of EIS data.
You will be presented with a dialog box
shown below which requests starting values for the model parameters. You need to enter
realistic estimates for these values, but they don’t have to be exact, just in the
same ballpark. Some suggested values are shown below.
Note that n and m are shown as locked
parameters. You will probably get better results if you run the calculation first with n
and m locked and then run the calculation a second time letting them vary. The software
will prompt you to use the latest results as the starting point for the second analysis.
REAP.MDL
Parameter Setup Dialog Box.
If the fit does not converge to
reasonable values and the theoretical curve does not closely fit the experimental data,
you need to vary the starting values. Resistance values are entered in ohms and the
capacitances in farads.
Note that use of a constant phase
element instead of a capacitor inverts the parameter. For example, if you think a typical
capacitance for your coatings is 100nF (1.0E-7 F), you would actually need to enter 1.0E+7
(=1/100nF). Likewise the calculated results have to be inverted to get the resulting Cc.
For example, the screen may show 1/CC as 1.0E+9. In this case CC is
actually 1.0E-9 which corresponds to a 1 nF capacitor.
You may want to run the model analysis
a third time using the results of the calculation with n and m varying as the starting
point for the third calculation. Anytime the numbers change by a factor of 10 or more, you
should suspect a bad fit and might want to try again with different starting parameters or
a different model.
Record the value of 1/CC,0
for the EIS data at 0 hours. Alternatively, you can use the Report option on the EisCurve
pull-down menu to print out a copy of the results. Next use New Graph on the EisCurve
pull-down menu to load and display the EIS24HRS.DTA file. Again analyze it using the
REAP.MDL file.
You might want to use the numbers that
result from the EIS data at 0 hours as the starting point for the fit for the EIS data
taken after 24 hours. Print out a report of the results or record 1/CC,24 and Rcor.
By fitting both the EIS data taken
before and after the 24 hour soak test of the painted unscribed sample, you now have the
coating capacitance at 0 and 24 hours (CC,0 & CC,24) and the
corrosion resistance, Rcor. For Rcor, use the value obtained from
the EIS data after the 24 hour soak test.
7. Calculate Coating Water Uptake, %v.
Once you have extracted the coating
capacitance from the EIS data, you can use the formula above to estimate the water uptake
of the coating as a volume percent. Note that the capacitance values are expressed as 1/Cc
rather than Cc. This accounts for the parameter inversion caused by the
use of the constant phase elements. The formula used in the proposed REAP method is
equivalent (shown below):
8. Predict Relative TTF
The last step in REAP testing is to
interpret the numbers. Ideally, you would like to simply take the three REAP parameters
measured and combine them into a single value that can predict time-to-failure
(TTF) values.
Kendig et. al. did that by analyzing
the long term tests on carbon steel samples exposed to a salt fog. They had two different
criteria for determining TTF. They found the REAP parameters correlated best with the TTF
determined from the pullback scribe tests. They came up with the following formula to
predict the pullback TTF from the measured values of the three REAP parameters:
These numbers result from linear
regression of the pullback TTF values with the REAP parameters for the specific mild steel
samples used. These numbers may not apply to other metal samples and probably depend on
other factors such as the test solution and cell used. Determination of the true value of
these predictive constants for another system would require long term testing to determine
the TTF values for that metal system. In general, though, the formula shows that higher
values of dx/dt and %v lead to shorter time to failure, while higher Rcor
values lead to longer time to failure.
If the REAP method is applied to a
painted metal sample and then a single component of the system is varied, you can predict
the effect of the change on the TTF for the system. For example, comparison of the REAP
parameters for several different coating materials for a given alloy and test solution
could lead to noticeable differences in the disbond rate. Assuming the water uptake and
corrosion resistance did not alter appreciably, the best coating would be the one with the
lowest disbond rate.
If you need more detailed background
information after reading through this application note, it is suggested that you obtain
copies of the proposed ASTM method and/or the original Kendig paper. As always, if you
have questions concerning the use of Gamry’s products, feel free to give us a call at
215-682-9330.
Contacting ASTM
If you wish to contact ASTM directly to
obtain more information about any of their standards or current proposals, they can be
reached at:
ASTM
100 Bar Harbor Drive
West Conshohocken, PA 19428-2959
Tel: 610-832-9585
FAX: 610-832-9555
Internet: www.astm.org
References
1 Kendig, M.,
Jeanjaquet, S.,
Brown, R., Thomas, F., J. Coatings Tech., 1996, 68, 39-47.
2 ASTM B117 describes salt
fog testing.
3 ASTM D1654 describes a
procedure for scribing a sample.
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