Electrochemical test methods for evaluating organic coatings on metals: an update. Part III: Multiple test parameter measurements1
Introduction
The status of evaluating organic coated metals utilizing electrochemical means was reviewed for the period of 1988–1994. The general improvements in the overall technology are presented in three parts, each as a separate publication. Part I covers the test cell configurations, changes in testing approaches and a brief survey of measurement equipment. Part II covered the characterizations of coatings using the single data point test methods such as monitoring the panel or `coating' potential, the current flow under constant applied potentials, electrochemical noise method (ENM), the dc resistance as well as the coating dielectric response measured using a single frequency, small magnitude ac input signal. In Part III, the focus is on test methods where the sample potential is deliberately changed. In terms of monitoring coating performance periodically over long periods of exposure, the usual intention is to minimize affecting the coating system by whatever level and duration of applied input signal one selects. The use of electrochemical noise measurements (discussed in Part II) is truly the least intrusive of all the electrochemical evaluation techniques for coatings. Of the five techniques to be discussed in Part III, electrochemical impedance spectroscopy (EIS) with an normal input signal of from 5 to 10 mV ac (RMS) is usually considered to be the least intrusive. However, the determination of the polarization resistance (Rp) by dc means using ASTM-G-59 or some modification thereto would be somewhat less intrusive simply by perturbation duration considerations. Small signal chronopotentiometry (constant current voltametry) and small signal chronoamperometry (constant voltage, current–time) studies can also be minimally disturbing in the signal ranges selected and the duration of the applied signal. The determination of the full range polarization curves can be the most damaging technique and one might deliberately accelerate the destruction of a coating system by applying cyclic voltametry over a sufficiently wide applied potential range. With the newer and more resistive coatings these techniques require either very large samples or the use of extremely sensitive ammeters. In any event, these five approaches are not truly non-destructive techniques (NDT), but rather they are minimally destructive techniques (MDT).
Section snippets
Polarization resistance
In 1973, Wolstenholme [3]provided a review of the use of polarization resistance (Rp) measurements for painted specimens. There have been very few recent publications regarding the use of ASTM-G-59 (or modifications thereto) with metals coated with organic films. The ASTM method requires a potential change rate of 0.6 V/h and an applied, smoothly ramped, signal range of ±30 mV around Ecorr is indicated although ±10 or ±5 mV might be preferred from a damage control viewpoint. One can therefore
Chronopotentiometry
The application of a small constant dc current to a sample and monitoring the change in sample potential with continued elapsed time is one of the earliest techniques employed in electroanalytical testing. This approach has been shown to be useful in determining, for example, the amounts of oxide films on metallic surfaces, the potential ranges for given electrochemical reactions, as well diffusion constants of species dissolved in solutions. If one has a range of approximately 100 mV where no
Chronoamperometry
The use of chronoamperometry has been revived by Granata 89, 91for determining the same basic model parameters as in the polarization decay approach. Success has been noted in the analysis of data from a simple RC circuit [91]and the requirement of a more refined analysis technique was indicated to determine the parameters from the nested model (Fig. 1).
There have been only a handful of recent papers presenting the monitoring of currents resulting from longer term applications of potentials in
Polarization curves
Obtaining meaningful polarization curves is not an easy task as was discussed by Wolstenholme in the 1973 review [3]. First there is the problem of correcting the test piece voltage data for the bulk electrolyte resistance and more importantly, the ionic resistance in the coating pores to obtain `true' or `IR-free' polarization curves. The overall equation describing the electrochemical behavior of excellent coatings which, on at least steel, combine to act as air (O2) reduction electrodes in
Electrochemical impedance spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) normally involves applying a small magnitude (usually ≤10 mV) ac voltage signal to a sample contained in a conventional three electrode (working or sample, reference and counter) electrochemical cell. With coated metals, the sample area is usually quite large relative to cells for studying bare metal surfaces and a sample size of approximately 1 cm2 per 2.5 μm (0.0001 inch) of organic coating thickness was suggested by Kendig and Scully [42]. The
Acknowledgements
The author is quite pleased to acknowledge the review efforts and encouragements by his NSWCCD Code 613 colleague, Dr. Harvey P. Hack. The NSWCCD program on electrochemical methods for evaluating organic coatings was initiated by Dr. John R. Scully, (now at the University of Virginia) and has been funded and sponsored by Mr. Ivan C. Caplan (NSWCCD, Code 0115) and by Dr. A. John Sedriks (ONR, Code 332). Mr. E.R. Wilkinson's (NSWCCD Code 3412) talents converted the author's crude sketches into
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Note: References 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106were initiated within Parts I and II of this three part article. References also cited within this Part III are repeated below as a convenience to the reader