Paints and Coatings

The use of Electrochemical Impedance Spectroscopy (EIS) to evaluate paints for marine applications is discussed in the October 2007 edition of the JOURNAL OF PROTECTIVE COATINGS AND LININGS.  A paper presented at PACE 2006 by Korean researchers from Hyundai Industrial Research Institute compares EIS to conventional exposure tests such as salt spray.

 

Gamry has published a series of three papers on the "Evaluation of Organic Coatings with Electrochemical Impedance Spectroscopy" in JCT CoatingsTech.  We start with a definition of EIS and follow through to specific protocols for testing coatings with EIS.  EIS is a complex technique and we labored mightily to civilize it!  We hope you like it.

Part 3 of the series on EIS and Paints was published in the February 2005 issue of JCT CoatingsTech.  The  cover of this issue is a very nice photo of a Boeing C-17 in a hangar.  Two Boeing scientists are taking an EIS measurement on the aircraft coating with a Gamry FAS2 Femtostat.  

The photo came about as a result of a serendipitous interaction between the FSCT (publishers of JCT CoatingsTech), Boeing, and Gamry.  The photo was taken by Boeing photographers while the Boeing scientists were making periodic EIS measurements on the aircraft.

Gamry scientists have published a series of three papers on the use of Electrochemical Impedance Spectroscopy (EIS) to characterize paints and coatings.  The articles discuss the principles of EIS, the instrumentation, the interpretation of the EIS response, and the use of the EIS in a testing protocol for organic coatings.  The articles are directed at the coatings scientist who has no previous experience with EIS.

JCT CoatingsTech is published monthly by the Federation of Societies for Coatings Technology.

The authors are David Loveday, Pete Peterson, and Bob Rodgers, all members of the Sales and Marketing Group at Gamry.  Contact any of these gentlemen for a scintillating discussion of EIS and coatings!

Part 1: Fundamental of Electrochemical Impedance Spectroscopy.  JCT CoatingsTech, 46-52, August 2004.
This paper discusses the theory of EIS and the reasons it is such a powerful electrochemical technique. Special attention is paid to the capacitance of an electrochemical cell, because this is an important characteristic of a coated metal substrate. The instrumentation and the cell are introduced and some brief comments are noted regarding the interpretation of EIS results.

Click here to download a PDF file of "Fundamentals of Electrochemical Impedance Spectroscopy, JCT CoatingsTech, p. 46, August 2004.

Part 2. Application of EIS to Coatings.  JCT CoatingsTech, 88-93, October 2004.
The second installment of the Series discusses the unique characteristics of a coated metal substrate and the reasons that the EIS response is useful for understand the integrity of the coating.  The instrumental challenges are described in detail and the EIS response is presented and explained for a coating in various degrees of degradation.

Click here to download a PDF file of "Application of EIS to Coatings". JCT CoatingsTech, p. 88, October 2004.

Part 3. Protocols for Testing Coatings with EIS. JCT CoatingsTech, 22-27, February 2005.
The final paper of the series brings it all together and explains how to use EIS in an experiment designed to evaluate coating quality.  The principal focus of this paper is the mechanism by which the coating is stressed and caused to fail.  EIS is used to detect the quality of the coating during the stress process. The paper discusses several cabinet tests such as ASTM B117 and D5894.  It also highlights some very, very rapid tests to evaluate coatings in 1-7 days.  A distinction is drawn between a failure of the barrier properties of the coating and the loss of adhesion. Several ASTM tests for adhesion are discussed.

Click here to download a PDF file of "Protocols for Testing Coatings with EIS", JCT CoatingsTech, p. 22, February 2005.

Paints and organic coatings are often used to protect metals that are used in aggressive environments.  These environments are typified by industrial facilities and seawater.  The protective mechanism of the paint may be physical (i.e., the paint acts as a barrier) or chemical in which the paint incorporates specific reagents for the prevention of corrosion.

Electrochemical Impedance Spectroscopy (EIS) is the electrochemical technique that is most commonly used to test coatings.  Over twenty years of research has demonstrated that EIS is a viable technique for coatings testing.  The best place to gather information on EIS and its application to coatings is the three JCT CoatingsTech ariticles above.

An excellent article entitled "Electrochemical Impedance Spectroscopy: A Tool to Predict Remaining Coating Life?" by Linda Gray and Bernard Appleman of KTA-Tator can be found in the Journal of Protective Coatings & Linings, Vol. 20, p. 66 (2003).  That's the February 2003 issue.  A Gamry PC4/750 Potentiostat running EIS300 Software was used in this coatings study.

This website is a good place to familiarize yourself with Electrochemical Impedance Spectroscopy and coatings.  There's an excellent general introduction in our EIS Primer.  (Be sure to look at the Literature section at the end of this Note.)  Paints and coatings are somewhat unique samples for EIS because they typically exhibit a very high impedance (10⁶ to 10¹² ohms).  That's a very different impedance from others samples (corroding metals or batteries, for example) and can be a problem unless you understand the implications.  We tell you all about it in our Application Note on EIS on Coatings.  Finally, for tips on improving your sample throughput with our ECM8 Multiplexer, take a look at EIS on Coatings Using the ECM8 Electrochemical Multiplexer and FAS2 Potentiostat.

EIS can only be used to evaluate coatings on metallic substrates, which excludes some systems, e.g., architectural coatings.  It does, however, include marine and industrial coatings, where coating performance is critical.  Its appeal lies in its ability to (1) detect very small changes in the coated substrate, to (2) distinguish changes in the coating from changes in the substrate, and to (3) do it very quickly compared to tests which depend on visual observations of the coating.  EIS will reveal changes in a coating long before any visual damage to the coating.

The reason that EIS is the method-of-choice for coatings evaluation is relatively simple.  Most electrochemical techniques are designed to study an electron-transfer reaction.  For example, corrosion (another name for “oxidation”) is an electron-transfer reaction that takes place with a metal.  An organic coating, however, is definitely not going to participate in an electron-transfer reaction.  So how does it work?

In EIS, an AC voltage of varying frequency is applied to the sample, and this allows the measurement of the capacitance of the sample.  An organic coating will definitely affect the capacitance of the sample.  As the organic coating deteriorates with time, EIS can track changes in the capacitance of the coating as well as changes in the porosity of the coating.  EIS can also monitor the rate of corrosion of the metallic substrate that generally increases as the protective coating fails.

EIS is an excellent tool to measure the performance of a series of coatings in a given environment.  The EIS response may differ by coating classification (epoxies, alkyds, etc.).  For a good discussion on these issues, see “Electrochemical Test Methods for Evaluating Organic Coatings on Metals: An Update”, John N. Murray, Progress in Organic Coatings, 31, 375 (1997).  As with cabinet tests, however, it is difficult to to predict a time-to-failure in service.

Several researchers have also used electrochemical noise measurements to evaluate organic coatings.  Electrochemical noise measurements apply no stimulating signals to the sample, so they are absolutely non-intrusive.  Electrochemical noise has not reached the level of acceptance of EIS for the evaluation of paints and coatings. 

Electrochemical impedance spectroscopy is employed in virtually every academic coating laboratory and in many industrial labs.  There have been a number of recent publications on the use of EIS to study coatings and we've listed them below:

1.   "Electrochemical Impedance Spectroscopy: A Tool to Predict Remaining Coating Life?" Linda Gray and Bernard Appleman, Journal of Protective Coatings & Linings, Vol. 20, p. 66 (2003).  A Gamry PC4/750 Potentiostat running EIS300 Software was used in this coatings study.

2.  "Rapid Assessmengt of Automotive Epoxy Primers by Electrochemical Techniques", J. J. Suay et al, Journal of Coatings Technology, Vol. 75, 103 (2003).  Researchers at the Universitat Jaume I in Spain have described a number of electrochemical tests to evaluate automotive primers in a relatively short period of time.  EIS, AC/DC/AC, and cathodic disbonding tests were employed to test the epoxy primer on both steel and phosphatized steel.  

3.  "Electrochemical Impedance Spectroscopy: Testing Coatings for Rapid Immersion Service", M. O'Donoghue et al, MATERIALS PERFORMANCE, September 2003, page 36. Workers at Powertech Laboratories (Surrey, BC, Canada) and ICI Devoe Coatings Canada (Vancouver, BC) report the EIS evaluation of two epoxy coatings that are designed for immersion within 24-48 hours of application. The Gamry PC4/300 Potentiostat with EIS300 Software at Powertech was used for the test. EIS showed significant nonvisual differences in the two coatings. 

4.  "Electrochemical Impedance Spectroscopy of Epoxy-Vinyl Coating in Aqueous Medium Analyzed by Dipolar Relaxation of Polymer", S. Duval, M. Keddam, M. Sfaira, A. Srhiri, and H. Takenouti, Journal of the Electrochemical Society, 149, B520 (2002).  Researchers from the very respected electrochemical laboratory at the Pierre et Marie Curie University used a Gamry Femtostat to examine several coatings with excellent barrier properties. Because of the virtual absence of flaws in the coatings, the authors argue that dipolar relaxation can be used to characterize these coatings.
5.  "Two-Electrode Electrochemical Impedance Sensor: Part 1--Response to Coating Degradation on Conductive Substrates", Qi, Hinderliter, and Gelling, CORROSION, 65, 343 (2009). North Dakota State University sets the standard for the use of EIS for coatings evaluation.  In this paper, a Gamry Femtostat is used with an NDSU-designed cell to study coatings in the field.

 

Two Short Courses on Corrosion and Coatings are available from North Dakota State University
The Department of Polymer science at North Dakota State University is presenting a couple of interesting Short Courses this summer.  Contact Dr. Vicki Gelling at 701-231-8027 for more information.

Fundamentals of Coatings Science 
The Fundamentals of Coatings Science Short Course is designed for all levels of scientists and technologists working in the field of coatings. For those relatively new to the field, the short course provides a comprehensive discussion of the basic principles of coatings science. For more experienced chemists, the short course may provide a broader perspective and more fundamental understanding of coatings science. Participants should have had some background in college level chemistry, including organic chemistry.

Corrosion: Coatings and Electrochemistry 
The Corrosion Science Short Course will be a comprehensive survey of all aspects of the corrosion process, characterization of corrosion, corrosion protection by coatings, testing of coating and lifetime prediction.

Using electrochemistry to evaluate polymeric coatings is not an obvious leap of scientific reasoning.  So how did EIS become such a powerful technique for the evaluation of coatings?  Well, the way these things normally get done is that someone, generally in academia, has the foresight to see the possibilities and the fortitude to make it happen.  Dr. Gordon Bierwagen of the Dept. of Polymer Science and Coatings at North Dakota State University in Fargo, North Dakota, was one of the earliest users of electrochemistry for coatings evaluation and his laboratory continues to find new ways to put these techniques to work.

Dr. Bierwagen was kind enough to give us permission to present a listing of his papers that are related to the electrochemical evaluation of coatings.  The information in these papers will give you a significant head start in using EIS, and perhaps even Electrochemical Noise, to design better coatings for your application. 

Electrochemical experiments on coatings always exhibit low, low currents because of the barrier effect of the coating.  Measuring a low current reliably is much more challenging than measuring a high current.  To our knowledge, there is nothing better for coatings that the Reference 600 Potentiostat running the EIS300 Electrochemical Impedance Spectroscopy Software.  You can measure impedances as high as 10¹² ohms with this system!

The PTC1 Paint Test Cell is an electrochemical cell designed for testing coatings with EIS. The PTC1 contains all the electrodes, a convenient platform for mounting the coated sample, and a large electrode area (15 cm²), which is preferred for coated samples.  Also, it's reasonably inexpensive, so you can afford to have several in your lab for testing multiple samples.

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