Electrochemical impedance spectroscopy (EIS) (sometimes also called AC
impedance or dielectric spectroscopy) is an electrochemical technique that
surfaced in the late 1960's but did not become extensively studied until the
late 1970's and early 1980's when computer controlled laboratory
equipment became the norm.
Electrochemical Impedance Spectroscopy (EIS) is a non-destructive
technique used to track the condition of a coated metal sample as it
changes. In EIS, an AC voltage of varying frequency is applied to the
sample and a plot of frequency vs. impedance change is traced for analysis.
EIS generates quantitative data that relates to the quality of a coating on a
metal substrate by studying the nature of change of impedance of the
EIS also provides a technique to optimize coatings while reducing the time
of coating evaluation and gives insight into the chemical properties of the
Dielectric spectroscopy (sometimes called impedance spectroscopy), and
also known as electrochemical impedance spectroscopy (EIS), measures
the dielectric properties of a medium as a function of frequency.
The pathway to coatings failure is monitored by making periodic EIS
measurements on the sample as it is stressed. Changes in the Coatings
Capacitance, Pore Resistance, Polarization Resistance, and Double Layer
Capacitance can be measured and related to the rate of failure of the
Polarization resistance of paints is experimentally observed to be inversely
proportional to probability of corrosion to set in on the metal substrate
Active corrosion at the metal surface is often accompanied by delamination
of the coating and formation of blisters. This is signalled by a decrease in
the Double Layer Capacitance and a slight increase in the Coating
WHAT CAN EIS TELL US ABOUT PAINTS?
It is useful because using EIS to characterize a painted
metal substrate simultaneously measures three
The deterioration of the organic coating caused by
exposure to an electrolyte
The increase in corrosion rate of the underlying
substrate due to the deterioration of the coating and
subsequent attack by the electrolyte
The loss of adhesion between the coating and the
substrate, i.e. Delamination problems
ELECTROCHEMICAL INSTRUMENTATION FOR
EIS EVALUATION OF COATINGS
An electrochemical impedance instrument consists of a
potentiostat to apply the AC voltage and measure the
current(key component to measure current in the system)
Associated electronics to measure the impedance
An electrochemical cell to contain the sample.
Electrolyte(resembling service conditions in which the
coating will be used)
Frequency response analyzer (Computer system)
Alternate Representations of electrochemical cell
Real life EIS machine for coating evaluation
NDSU has developed this
parallel dip coating
apparatus that can coat up to
four different paints on a
single substrate. The
apparatus is design to use the
minimal amount of paint and
can be adjusted for rate of
withdrawal. The substrate
used is compatible with our
spectroscopy tool as well as
the other combination tools
in the lab.
A Case Study
Substitution of zinc chromate or zinc yellow, traditionally used as anticorrosive pigment,
for other phosphate-based pigments that are not hazardous to health and have the
same anticorrosive behavior or even better, is studied in this paper. Four alkyd paints
were specially prepared; two of them contained calcium acid phosphate or micronized
zinc phosphate as anticorrosive pigments respectively. A paint containing zinc chromate
was used as reference and a paint without anticorrosive pigments was used as a blank,
in which the other ingredients were increased proportionally to attain the desired PVC
relationship. It was observed after EIS testing, Results of all tests showed that the paint
with calcium acid phosphate and especially that with micronized zinc phosphate
exhibited better behavior than paint with zinc chromate. Analysis of impedance
parameters (ionic resistance and capacitance of the paint film) against immersion time
allowed the paints to be ranked in the same order as that obtained with salt spray and
humidity chamber tests.
Citation: L.S. Hernandez, B. del Amo, R. Romagnoli, (2008) "Accelerated and EIS tests
for anticorrosive paints pigmented with ecological pigments"
Electrochemical impedance spectroscopy (EIS) vs
salt spray testing
EIS methods can detect deterioration at metal/coating interfaces well before defects
become visible, and with greater accuracy than artificial testing methods such as salt
spray. More importantly, changes in electrical properties determined by EIS experiments
have been found to closely resemble long-term performance of coatings.
For the improved test, outdoor specimens are placed in a typical EIS test cell and
immersed in a 3.55% NaCl solution. Impedance measurements are taken ranging in
frequency from 0.01 Hz to 100 kHz. The results are processed, establishing parameters
for galvanic and barrier mechanisms in an equivalent-circuit model Early changes in
electrical properties of coatings will predict deterioration of the coating system.
The EIS method is also preferable to salt spray and salt fog testing, both techniques that
artificially increase exposure to corrosion. This is because the results of artificial
experiments tend to be uncorrelated with those performed under natural
conditions. Results are often distorted, as well.
Advantages of EIS technique over other
traditional techniques for coating evaluation
EIS is a very sensitive detector of the condition of a coated metal, so
the EIS response can indicate changes in the coating long before any
visible damage occurs.
Non- destructive technique, hence less loss of material while testing.
Quantitative data available to back up the claims made by the
Takes lesser time to evaluate coatings in comparison to other tests like
salt spray evaluation or PAC test.
Time dependent data is available
Shortcomings of EIS technique for
Requirement of complex data analysis for
Heavy Requirement of complex lab computers
and peripherels,compilers etc for the process
Requires skilled operator
It can be easily concluded that EIS is a superior modern
technique to study and test surface coatings .What used to
take between 1-1/2 and 5 years of exposure to marine
conditions to gain definitive information on coatings can
now be accomplished in about 8 weeks or less with EIS.
It has been proposed that better quantitative approach to
surface coating testing is the need of the hour and bodies
like the American standards for testing of
materials(ASTM) have held discussions on the possibility
of including EIS techniques as a “pass-fail” test for
evaluation of common paints
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