Contents Fundamental of AC theory Review of EIS Equivalent Circuit - - PowerPoint PPT Presentation

Contents

• Fundamental of AC theory
• Review of EIS
• Equivalent Circuit Models for Coatings
• Change in EIS during Deterioration of a

Coating

• Combination of assessment of coating
• Few coating examples and case studies

• A metal is held at its OCP, E corr, in an electrolyte
• A small sinusoidal AC voltage is applied to the metal, e.g., 10-100 mV
• The response, a small sinusoidal current, is measured
• The ratio of the two is not a resistance, but an impedance
• The frequency of the AC voltage is varied from 105to 10–2Hz (typically)
• The Impedance value is plotted vs. the frequency
• The curve is modeled by an Equivalent Circuit that gives the same

frequency dependence

• The EC consists of resistors, capacitors and other elements, whose

values can be calculated if the model fits the data

• These elements represent the Polarization Resistance Rp and Double

Layer Capacitance Cdl of the metal in the electrolyte

• As it is based on AC, EIS can also be applied to metals with a dielectric

coating, such as a paint, immersed in an electrolyte

What is EIS and what can do?

EIS is not only a corrosion measurement technique

• In that case we measure the properties of the coating, such as

thickness, dielectric constant, porosity, water uptake, etc.

• The equipment we use in EIS (potentiostat, corrosion cell, reference

electrode, counter electrode), are the same as in the DC methods

• In addition, a frequency response analyzer and a lock-in amplifier

are used

• EIS has become very popular in recent years and is now a routine tool
• An ASTM standard has been written
• The areas that are considered appropriate for using EIS are:

1.Rapid estimation of corrosion rates, within 30 min. 2.Estimation of very low corrosion rates (<0.01 mpy) 3.Estimation of corrosion rates in low-conductivity media 4.Rapid assessment of corrosion inhibitor performance 5.Rapid evaluation of coatings 6.Evaluation of metal pretreatments, e.g., chromates, phosphates

What is EIS can do?

SYSTEM Linear Input/output Systems Input Output

Light Current Temperature Voltage Sound Voltage Ohm’s law: R = V / I Impedance (Z) = across function (VO) / through function (IO)

EIS – Theory - Simple

Input and output as function of time

Magnitude and Phase shift are frequency dependant, Impedance is vector quantity IZI = Z’(real) – j Z’’(imaginary)

EIS – Frequency response analysis

Sinusoidal response in a linear system

How does EIS work?

Sinusoidal response in a linear system

EIS

How is shown on a oscilloscope?

E (t) plotted vs. I (t)

• Frequency response

analyzers and lock-in amplifiers are required to convert these figures to EIS spectra.

• During the recording of

an EIS spectrum this figure is displayed

Electrochemical flat cell Reference electrode sample in contact with the electrolyte through a hole

• f 1cm2

Counter electrode

DC/AC-Experimental set up

Electrochemical Impedance

Data Acquisition

• Measure Eocp and allow it to stabilize
• Apply DC voltage equal to the measured value of Eocp
• In addition to the DC voltage, apply a small sinusoidal voltage (10

mV) perturbation of fixed frequency and measure the current response

• Calculate the impedance and the phase shift.
• Repeat the measurement at a wide range of frequencies.

Data Analysis

• Model the electrochemical process with electrical circuit elements

such as resistors, capacitors, and inductors. Adjust the values of the circuit elements to fit the model to the EIS data.

• Coatings performance prediction

(e.g pre-treatment/primer or any polymer system)

• Film evaluation (free standing or CCVD)
• Corrosion prevention/control
• Inhibitor studies
• Electroplating, Electro deposition
• Conducting polymers
• Battery and fuel cells, Membrane/separators
• Metal processing/recovery
• Corrosion (Pitting / SCC etc.)
• Metal oxide formation

EIS applications

A Bode Plot: Magnitude & Phase Shift

EIS data can be presented as a Bode Plot or a Nyquist Plot

Modelling

• Electrochemical cells can be modeled as a network of

passive electrical circuit elements.

• The network is called an “equivalent circuit”.
• The EIS response of an equivalent circuit can be

calculated and compared to the actual EIS response of the electrochemical cell.

• The values of the circuit elements in the model are

calculated by varying them in an iterative fashion until acceptable agreement is reached (non-linear least squares) with the actual EIS curve of the sample.

Frequency Response of Electrical Circuit Elements

• Resistor (ohms)

Z = R 0° Phase Shift

• Capacitor (Farads)

Z = -1/jC

• 90° Phase Shift
• Inductor (Henrys)

Z = jL +90° Phase Shift

• A real response is in-phase (0°) with the excitation. An

imaginary response is 90° out-of-phase. j = -1, ω= 2f radians/s, f = frequency (Hz or cycles/s)

EIS of a Resistor

Time Magnitude Applied Voltage Measured Current Phase Shift of 0º

Imag Emag Zmag = Emag / Imag = R

EIS of a Capacitor

Time Magnitude Measured Current Applied Voltage Phase Shift of 90º

Zmag = Emag / Imag = 1/(2fC) Imag Emag

Rs Rp

Wmax =1/RpCdl

Rs + Rp Rs + Rp Rs Cdl frequency decreases

Metal Electrolyte

Rs Rp Cdl Interface Metal/electrolyte

Equivalent circuit for a simple corroding system with an electrolyte resistance. Nyquist impedance plot

Bode IZI and phase angle plots

EIS- Non destructive technique

Rpo and Cc

Z” Z’

Rct and Cdl

• Water uptake
• Evaluate coating resistance/capacitance
• Corrosion underneath the coating
• Blistering-delamination
• Quantification of substrate area non protected ?

EIS- Non destructive – Organic coated products with pore/defect

Electrolyte

Rpo Cdl Rct

pore or defect

Rs Cc

Coating Metal

Coating Impedance vs. water permeation

Two “Special Problems” with the Measurement of the EIS of Coatings

• 1. Because of the barrier nature of coatings,

currents will always be small, so use a sensitive potentiostat and a Faraday Cage.

• 2. The initial Open-Circuit Potential of an intact

coating is subject to Capacitive Drift. For the first few measurements, use the Eocp of the bare metal.

EIS Seems More Complex, than It Is!

• Discussed the theory of EIS/modeling

and it’s not particularly simple.

• You will be pleased to know that the

real-world applications of EIS for coating evaluation are relatively simple!

EIS of Coatings on Metallic Substrates

• For coatings on a metal substrate, EIS acts

as a very sensitive quantitative detector of changes in both the coating and the metal substrate during long-term exposure to an electrolyte.

• Changes in the coating will be apparent in

EIS long before any visible damage occurs.

• Apply stress to the sample to cause it to fail.

The stress should simulate the service environment, which could be weathering or a specific chemical attack, e.g., seawater.

• Measure an EIS Curve immediately upon exposure

and periodically thereafter until the test is complete. Changes in the EIS Curve with time reflect changes in the coating or the metal substrate. These changes are accelerated by the artificial stress.

• Fit an equivalent circuit to the data to determine the

value of the circuit elements.

• Evaluate the data to select an “indicator” of coating
• deterioration. The indicator may be Ztotal,

capacitance, pore resistance, etc.

• In many cases, Z at low frequency is satisfactory.

EIS of Coatings on Metallic Substrates

What Do We Mean by “Stress”?

• To test a coating, we must cause it to

fail by applying an “artificial” stress

• The stress must resemble the service

environment and it must not change the failure mechanism

• The stress may be a solution under

various conditions, a temperature, climate or a voltage.

Electrified interface structure for a corroding coated metal

Equivalent Circuit and Schematic of an Organic Coating on a Metal Substrate

Typical Model of a Coated Metal

Ru = Uncompensated Resistance Ccoating = Coating Capacitance Rpore = Pore Resistance Rp = Polarization Resistance (Corrosion of the Metal Substrate) Cdl = Double Layer Capacitance at the Metal Substrate

Alternative Model of a Coated Metal to Account for Blistering*

Ru = Uncompensated Resistance Ccoating = Coating Capacitance C blister = Coating Capacitance Above the Blister Cdl,blister = Double Layer Capacitance of the Metal Under the Blister Rp,blister = Polarization Resistance (Corrosion of the Metal) Under the Blister Rpore = Coating Pore Resistance Cdl = Double Layer Capacitance of the Metal Under the Pore Rp = Polarization Resistance (Corrosion of the Metal) Under the Pore

*Kern et al, Journal of Coatings Technology, 71, 67 (1999)

Six Steps of Coating Degradation*

• 1. Initial Immersion, Purely Capacitive
• 2. Absorption of Water
• 3. Development of a Pore Resistance
• 4. Diffusion-Controlled Corrosion Through Pores
• 5. Free Corrosion During Blistering
• 6. Major Coating Damage

*J. N. Murray, Progress in Organic Coatings, 31, 375-391 (1997)

• 1. Initial Immersion, Purely Capacitive

Note:

• 90o Phase Angle
• -1 Slope in Impedance
• High Impedance at Low Freq
• Impedance @ 0.16 Hz ~ 1GW

• 1. Initial Immersion, Purely Capacitive
• Barrier coating

intact.

• No intrusion of

water or electrolyte.

• C = e eo A / d

About 1 nF/cm2.

• 2. Absorption of Water

Note:

• Still 90o Phase Angle
• -1 Slope in Impedance Shifted
• Lowered Impedance at Low

Frequency

• Impedance @ 0.16Hz ~100MW

• 2. Absorption of Water
• Impedance remains purely

capacitive.

• Water dielectric constant

( 80 ) is higher than coating dielectric constant (2-5)

• C = e e0 A /d
• Capacitance will increase.
• Impedance will decrease.

Uptake of Water by the Coating

• The Brasher-Kingsbury Equation allows

a convenient estimate of water uptake.

• Volume Fraction H2O = (log Ct/C0)/log

εw

– Ct: Coating capacitance at time t – C0: Coating capacitance at time zero – εw = 80 (dielectric constant of water)

Brasher and Kingsbury, J. Applied Chemistry, 4, 62 (1954)

Swelling of the Coating

• Swelling of the coating

will change thickness, d

• C = e e0 A / d
• Swelling of the coating

will decrease the capacitance and increase the impedance!

• 3. Development of a Pore Resistance

Note:

• Phase Angle No Longer

Constant

• Impedance Magnitude

Flattens Off at Low Frequency

• 3. Development of a Pore Resistance
• Early stage of

attack.

• Diffusion, migration

through coating begins.

• Rate limiting step.
• Double-layer and

corrosion not significant.

• 4. Diffusion-Controlled Corrosion

Through Pores

1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 11.00

• 2.00
• 1.00

0.00 1.00 2.00 3.00 4.00 5.00 6.00 Log Freq (Hz) Log Modulus (Ohm)

• 100.00
• 90.00
• 80.00
• 70.00
• 60.00
• 50.00
• 40.00
• 30.00
• 20.00
• 10.00

0.00 Phase (Degree)

Note:

• Impedance and Phase

Angle Begins to show two time constants

• Impedance Magnitude

Continues to Decrease at Low Frequency

• 5. Free Corrosion During Blistering

Note:

• Impedance and Phase Angle

Clearly Shows Two Time Constants

• Impedance Magnitude

Continues to Decrease at Low Frequency

• 6. Major Coating Damage

Note:

• Phase Angle no longer

–90º at High Frequency

• Impedance Magnitude Has

Decreased Six Orders of Magnitude During Experiment

• 6. Major Coating Damage

EIS of a Coated Metal

• Y. T. Al-Janabi et al, NACE Corrosion 2000, Paper 759, NACE, Houston, TX (2000).

At long times, Z increases which was attributed to the clogging of the pores with corrosion products.

Adhesion and EIS

• Adhesion is a complex phenomena typically

measured with qualitative tests

– ASTM D 3359: Tape pull-off test – ASTM D 6677: Knife Adhesion Test

• Rp and Cdl may be related to the adhesion of the

coating film, but the relationship is very complex

• A high value of Rpore may complicate the

measurement of Rp and Cdl

• EIS is not the best measurement test for

adhesion.

Immersion Test: The Simplest Case

• Gray and Applemen, J. Prot. Coat.

Linings, p. 66 (2003).

• Immerse the sample in 5% NaCl and

periodically measure the EIS curve.

• Heat the samples to 65º C to accelerate

attack.

• Measure EIS after 1, 4, 7, 14, and 28

days.

Immersion Test: The Simplest Case

• Coating quality is indicated by the limiting

impedance at low frequency

• Zlow freq = Rpore + Rp +Runc
• Zlow freq >109 Ω Excellent corrosion protection
• Zlow freq >107 Ω Adequate corrosion protection
• Zlow freq <107 Ω Poor corrosion protection

EIS Response of a Pipeline Coating

EIS of Free standing coating Films

• No substrate – study film degradation only!
• Simpler analysis.
• Requires 4-electrode potentiostat, four point

probe.

• Two reference

electrodes

• Plastisol film in 3% NaCl
• Note changes in Rpore, Ccoating

Castella & Simoes, Prog. Org. Coat., 46, 130 (2003).

EIS of Free standing coating Films

Atmospheric Corrosion Tests

• For a test to predict time-to-failure,

atmospheric tests are the Gold Standard.

• Atmospheric tests generally continue for

several years!

• Deterioration of the coating can be detected

with EIS long before visual defects appear.

Atmospheric Tests

• Expose the samples
• Run an EIS curve on each sample periodically

(e.g., every six months)

• In the EIS measurement, use an electrolyte that

simulates the environment, e.g., artificial sea water

• Use the same area for the EIS measurement
• Make sure the Open-Circuit Potential has

stabilized before starting the EIS Test!

EIS and Cabinet Tests

• Cabinet tests are commonly used to test coatings
• The best cabinet tests can reproduce the failure

mechanism and correlate with exposure tests

• Standardized cabinet tests specify the exposure

conditions and times

• The standard methods do not provide test

methods or pass-fail criteria

• Evaluation of test specimens is given in ASTM D
• 1654. All techniques are qualitative.

EIS and Cabinet Tests

• EIS can provide a quantitative measure of coating quality

earlier in the test cycle!

• ASTM B 117 Salt Spray
• ASTM D 5894 Cyclic Salt Fog/UV Exposure
• SAE J 2334 Laboratory Cyclic Corrosion Test
• For EIS testing of barrier properties, panels are not scribed

ASTM B 117 Salt Spray

• Uses a salt fog of 5% NaCl
• B 117 Salt Spray has been used for many

years for a wide variety of samples, so there is large database of history for the test.

• Several researchers have stated that B117

is a good quality control test

• B 117 is not recommended for testing of

paints on metallic substrates.

ASTM D 5894 Cyclic Salt Fog/UV Exposure (Prohesion)

• Developed specifically for coatings and generally

agreed to give reasonable agreement with exposure tests.

• One-week exposure cycles of 4-hr UV (340 nm) at

60º C, then 4-hr condensation @ 50º C

• One-week fog/dry cycle of 1-hr fog (0.05% NaCl,

0.35% NH4SO4) at ambient temp, then 1-hr dry-off at 35º C

Test Protocol for Aircraft Coatings*

* Bierwagen et al, Prog. Org. Coatings, 46, 148 (2003)

Z vs. Exposure Time for Airc. Coatings

Accelerated Tests

• Cabinet tests, while faster than atmospheric

tests, still takes weeks or months.

• To accelerate the evaluation, induce failure in

a shorter time with more aggressive stress conditions!

• Careful! Make sure that the stress does not

change the mechanism of failure!

• Careful again! The stress must resemble the

service conditions to be relevant.

Thermal Cycling

• Increasing the temperature accelerates the

rate of chemical reactions.

• In the case of coatings, a temperature

increase will increase the rate of diffusion through the coating.

• Expect pore resistance to decrease with

increasing temperature.

Thermal Cycling

• Immerse sample in 0.05% NaCl, 0.35% NH4SO4

(ASTM D 5894).

• Measure EIS after 20 min immersion.
• Ramp temperature up, repeat EIS after 20 min.
• Repeat @ 35°, 55°, 75°, 85°.
• Ramp temperature down in same sequence.
• Good coating should return to same impedance level.
• Complete protocol:

– Three temperature cycles. – Three day immersion, then EIS. Bierwagen, et. al., Prog. Org. Coatings, 39, 67 (2000).

Thermal Cycling - Good

Bierwagen, et. al., Prog. Org. Coatings, 39, 67 (2000).

Thermal Cycling - Failed

Bierwagen, et. al., Prog. Org. Coatings, 39, 67 (2000).

Thermal Cycling

• Complete protocol:
• Three temperature cycles
• Three day immersion, then EIS
• Thermal cycling test – <1 week
• Prohesion test – 4-12 weeks

Rapid Electrochemical Assessment

• f Paint (REAP)
• The REAP protocol seeks to determine

the time-to-failure in 24 hours.

• It is unique in that it combines two tests:

– EIS to determine the barrier properties – Cathodic disbonding to determine adhesion

• The test was developed for automotive

coatings on mild steel.

Kendig, et al, J Coatings Tech, 68, 39 (1996).

REAP: EIS Test

• EIS measured in 0.5 M NaCl
• EIS was measured immediately after immersion

and again after 24 hours immersion

• Frequency range: 10,000 Hz to 10 mHz
• Fit to an equivalent circuit (next slide) to

determine element values.

Equivalent Circuit Used in REAP

• Identical to the “standard’ coatings except for the Constant

Phase Element. A CPE models an “imperfect” capacitor.

• Zcpe = (1/Y0) / (jω)α
• Note that Y0 = capacitance when α = 1.

Cathodic Disbonding in REAP

• Samples are scribed (“X”) to expose the substrate-

paint interface

• Polarized at –1050 mV in 0.5 M NaCl for 24 hours
• ½ O2 + 2 e- + H2O  2OH-
• Dry the sample, apply tape, and pull to remove

disbonded coating

• Measure the average pull-back from the scribe

REAP Results

• Time-to-failure criteria was based on a corrosion

rating and a pull-back measurement from a 1200 hour ASTM B117 Salt Fog Test

• TTF = Overall Time-to-Failure based on average of

pull-back and corrosion

• TTF(PB) = Time-to-Failure based on >3 mm pull-

back

REAP Comments

• Assessing both barrier properties and adhesion in

different tests is an interesting and rational step!

• The REAP “approach” is probably the most likely to

yield believable results for most coatings

• A quantitative time-to-failure probably is not

possible

• For an in-house test, a REAP technique with pass-

fail criteria is relatively simple to establish.

AC-DC-AC Technique

• Cathodic (reducing) potential is applied to

substrate (-1  -2 V for an intact coating).

• Water that permeates the coating is reduced at

the metallic substrate.

• H2O + e-  H2 + OH-
• H2 exerts pressure to disbond, OH-

compromises adhesion.

• EIS is run before and after cathodic

delamination step.

• Simulates cathodic regions of the substrate.

AC-DC-AC Technique

Suay, et. al., J Coatings Tech, 75, 103 (2003).

Cr3+ - Pre-treatment coating degradation evaluation (Exam 1)

(EZ Cr coating dried at 60, 110 and 210oC )

 1 1 C

• 

2 1 C

• 

6 C

• 

E G

60oC 110oC 210oC EZ

EIS vs LPR: Chromate coating degradation

(EZ Cr coating dried at 60, 110 and 210oC )

• X. Zhang, S. Bohm et al., Materials and Corrosion, 55 (7), pp 501-510, 2004

Polarisation results shows that Cr-treatment dried at 110oC Lowest corrosion current.

Polarization in 3.5% NaCl, pH: 5.8 Potential (VSCE)

• 1.4
• 1.2
• 1.0
• 0.8
• 0.6
• 0.4
• 0.2

0.0

Current Density i (A/cm

2)

1e-10 1e-9 1e-8 1e-7 1e-6 1e-5 1e-4 1e-3 1e-2 1e-1 (a) EGCr10s60 (b) EGCr10s110 (c) EGCr10s210 (d) EG (a) (b) (c) (d)

(1)

Chromate Coating influence of drying temperature using EIS

Chromate coating dried at 60oC demonstrate best performance. Fitting results @ 210oC, Charge transfer resistance (Rct & RP) are smaller and the capacitance (Cdl and Cc) are higher.

• X. Zhang, S. Bohm et al., Materials and Corrosion, 55 (7), pp 501-510, 2004

Cr- VI replacement for Al substrate- Case study

• The objective of this project was to develop alternative metal pre-

treatment, that could replace chromate

• The chromate films give added corrosion protection and excellent paint

adhesion

• Note that in this study the paint and the metal were constant, the only
• Variable was the type of metal pre-treatment
• The tests included EIS & 1500 hrs salt spray exposure,

EIS in 3.5 % aerated NaCl

EIS in 3.5 % aerated NaCl

The Model used for fitting

After 17 days using the Model for fitting

Time intervals EIS

Rpo vs. time of immersion showing that the pre-treatment has an effect on the water uptake or degradation of the coating

Time intervals EIS

Cc vs. time of immersion showing that the pre-treatment has an effect on the water uptake of the coating

Time intervals EIS

Rct (Rp) vs. time of immersion showing that the Rp decreases, i.e., the corrosion rate increases, as the coating degrades

Mechanism

Cr- VI replacement for Al substrate- Case study - outcome

• The pre-treatment has a strong effect on the performance of a

painted metal Thus, rankings between systems can be obtained in a few hours in EIS instead

• f 1500 hours in the salt spray test
• Capacitances typically increase with aging, all resistances

typically decrease with aging (except Rs)

• The Warburg impedance also decreases with time, but more

slowly than Rp and Rpo

• The water uptake of a coating as well as the coating degradation

(Rpo) depend on the strength of the interface

• The same ranking in performance of pre-treatment's was obtained

in corrosion tests without the paint coating

• One of the alternative systems matches the performance of the

chromate pre-treatment

Barrier properties of organic coatings and the additives impact – Paint suppl

• Corrosion processes depend on the availability of

– Water, oxygen, aggressive ions e.g. chloride

• The penetration of water, oxygen and ions into an
• rganic coating depends on the structure (micro

porosity) and composition of the coating

• The EIS technique provides a convenient,

relatively straightforward method for assessment

• f “water uptake” by coatings

– uptake of oxygen and ions cannot be quantified directly

Assessment of “water uptake”

• Organic coatings possess a measurable

capacitance, typically between 10-10 and 10-9 F cm-2

• The dielectric constant of an organic coating, εr,

increases as the coating absorbs water

• Thus, “water uptake” can be assessed by

measuring the capacitance of a coating as a function of time

Ccoating = A εr εo d

A = area εr = dielectric constant of the coating εo = physical constant, 8.85 x 10-14 F cm-1 d = coating thickness

EIS capacitance

• Step 1: immerse the coated specimen in an aqueous

solution and measure Ccoating as a function of time

• Step 2: determine the initial capacitance of the dry

coating by extrapolating the measured capacitance values back to t=0

Typically, Ccoating rises rapidly during the first day of exposure to an aqueous solution, and then remains roughly constant once the coating has become saturated with water

0.00 0.05 0.10 0.15 0.20 24 48 72 96 Time / hours Coating capacitance / nF cm

• 2

P Silver P Silver

Water uptake using EIS

• Step 3: the “Brasher-Kingsbury equation”

provides a convenient estimate of water uptake

• Vol. fraction H20 =

log (Ct/Co) log εwater

Ct = capacitance of coating at time t Co = initial capacitance of coating εwater = dielectric constant of water

2 4 6 8 24 48 72 96 Time / hours Estimated water uptake / volume %

P Silver PSilver

“Matt system” (panels with matted topcoats only – no primer)

Water uptake correlates with quantity of matting agent in coating

3 6 9 2 4 6 8 10 (Time / hours)

1/2

Estimated water uptake / volume %

level 1 gloss level 2 level 3 level 4 matt

level 1 (no matting agent) level 4 (fully matted system) level 2 level 3

“Matt system” (panels with matted topcoats only – no primer)

Matting agent reduces overall impedance of coating, indicating reduced corrosion resistance

1.00E+07 1.00E+08 1.00E+09 1.00E+10 1.00E+11 12 24 36 48 Time / hours Impedance |Z|

f = 1 mHz / Ω cm 2

level 1 gloss level 2 level 3 level 4 matt

level 4 (fully matted system) level 1 (no matting agent) level 2 level 3

Impedance magnitude |Z| at frequency 1 mHz

Do we completely understand the product performance and interfacial coatings failure mechanisms from ASTM accelerated tests? NO! What are other alternative solution for coatings evaluation / performance prediction? Electrochemistry linked with natural/accelerated!!

Electrochemistry EIS, SKP, SVET, and Surface Eng. etc.

Natural Accelerated Fundamental

Summary & Conclusions

• Interfacial understanding of coating can be only

predictable, if we link the results from accelerated, natural and fundamental applied electrochemistry.

• DC/AC electrochemistry can be used for coating

evaluation and improve industrial product development and process parameters. Results are relatively rapid, providing “real time” information of the corrosion rates.

• Electrochemistry combined with surface technology-

A powerful tool for predicting the in-service corrosion performance and durability of coatings

EIS along Long-Term Exposure Tests

• Degradation of low frequency impedance correlates to time-to-

failure for some coatings (as evaluated by ASTM D 610 and 714). J.

• R. Scully, J. Electrochem. Soc., 36, 979-989 (1989).
• Erik and Hans de Wit found correlations between coating

capacitance (Cc) and time-to-failure. E.P.M. van Westing et al, Corrosion Science, 36, 979-994, (1994).

• Review by J. N. Murray lists EIS studies by paint category.

Progress in Organic Coatings, 31, 375-391 (1997), Critical measurement indicators are dependent on paint system being studied.

References for EIS

• Electrochemical Impedance and Noise, R. Cottis and S.

Turgoose, NACE International, 1999. ISBN 1-57590-093-9. An excellent tutorial that is highly recommended.

• Fundamentals of Electrochemical Impedance
• Spectroscopy. JCT CoatingsTech, pp. 46-52, August 2004.

Application of EIS to Coatings. JCT CoatingsTech, pp. 88-93, October 2004. Protocols for Testing Coatings with EIS. JCT CoatingsTech, 22- 27, February 2005. Electrochemical Impedance: Analysis and Interpretation, STP 1188, Edited by Scully, Silverman, and Kendig, ASTM, ISBN 0- 8031-1861-9. ASTM Standard G 106 –89 Verification of Algorithm and Equipment for Electrochemical Impedance Spectroscopy. Volume 3.02.

Additional Source Materials for EIS of Coatings

• ASTM (D01.27.32) and ISO (TC 35/SC 9/WG

29) are preparing standards on EIS evaluation of paints and coatings.

• One of the more active academic research

groups using electrochemical techniques for the evaluation of coatings is Dr. Gordon Bierwagen, Dept. of Polymers and Coatings, North Dakota State University.

Thank you Acknowledged my PhD Supervisor Prof Laurie Peter