Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2001-07-10
2003-08-26
Metjahic, Safet (Department: 2171)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
Reexamination Certificate
active
06611151
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to methods and apparatuses for evaluating conditions of coatings on metallic substrates, more particularly to such methods and apparatuses involving electrochemical noise.
Electrochemical impedance spectroscopy (EIS) is an electrochemical methodology in which an ac signal (typically, a small voltage signal) is applied to an electrode (e.g., a corroding metal) and the response is measured. The current-time and the voltage-time measurements are processed to provide a representation of the ac impedance at different frequencies, known as the “impedance spectrum.”
The term “impedance” is the ac analogue of dc resistance. The relationship for dc is given by Ohm's law, V=IR, wherein V (e.g., in volts) is the voltage across a resistor R (e.g., in ohms) and I (e.g., in amps) is the current. Similarly, the relationship for ac is given by V=IZ, wherein Z is the impedance of the circuit. Unlike resistance R, impedance Z may depend on the frequency f (e.g., in hertz, which is the number of cycles per second) of the applied ac signal.
Two parameters which relate the output current to the input voltage define the impedance of a system at a given frequency. The first parameter is the amplitude of the ac current divided by the amplitude of the ac voltage. The second parameter is the phase angle, which is proportional to the shift in time between peak current and peak voltage. The impedance spectrum comprises an accumulation of values of these parameters for various frequencies.
In the past teen to twenty years, electrochemical impedance spectroscopy (EIS) has become widely accepted as a nondestructive technique for evaluating the electrochemical properties of non-conductive coatings applied to metallic substrates. EIS data (such as maximum impedance, Z
max
) have been successfully equated to given coating conditions. For example, a Z
max
value of 10
9
−10
11
ohms-cm
2
indicates a ‘good’ coating while a Z
max
value less than 10
6
ohms-cm
2
indicates a ‘bad’ coating.
U.S. Navy researchers have used EIS in the laboratory to characterize and evaluate many organic coating systems on metallic substrates for long periods (up to 10 years) of exposure to saltwater. Due to the logistics of the test method, however, it is not practical to perform EIS testing in the field to evaluate the condition of coatings on ships and vehicles. While EIS can be performed in the field, it requires a relatively large amount of time to perform each test, and the data are somewhat complicated. Efforts to make in-field EIS testing more logistically feasible have shown varying degrees of success, but the lack of straightforward, easy-to-interpret output from EIS tests remains a hindrance to the widespread use of EIS testing as a monitoring technique in the field.
Electrochemical noise (ECN), also referred to as electrochemical noise analysis (ENA) or electrochemical noise measurement/method(s) (ENM), is a nondestructive analysis technique in which the direct, current “noise” and voltage “noise” associated with electrochemical reactions on a metallic surface are each measured and recorded. The meaning of the word “noise” in the context of ECN is distinguishable from its commonly understood meaning, wherein the word “noise” refers to unwanted sound. Electrochemical noise does not involve audible sounds (i.e., fluctuations in air pressure or acoustic noise), but rather is concerned with fluctuations in electrochemical potential and electrochemical current. Electrochemical potential noise is the fluctuation in the electrochemical potential of an electrode relative to a reference electrode. Electrochemical current noise is the fluctuation in an electrochemical current.
Generally, measurement of ECN involves the utilization of three test electrodes. For instance, two steel electrodes are connected to an ampmeter, and current therebetween is recorded; one of the two steel electrodes and a reference electrode are connected to a voltmeter, and voltage therebetween is recorded. Although only one of the two steel electrodes is connected to the voltage meter, the two steel electrodes effectively behave as a single electrode of twice the area, since the ammeter used to measure current is assumed to behave ideally (i.e., measuring current with no voltage drop). While the three test electrodes are immersed in a salt solution, two kinds of “time records” are effectuated, viz., current-against-time (variation of current with time) and potential-against-time (variation of electode potential with time).
ECN testing has been used in the past ten years to evaluate the kinetics of localized electrochemical reactions and processes, such as pitting reactions on passive alloys. EIS has been much more commonly effectuated than has ECN for evaluating coating conditions. More recently, ECN has been gaining interest as a technique for evaluating coatings, albeit that EIS testing remains the more “tried-and-true,” traditional approach for such purposes.
For instructive discussion regarding EIS and ECS in relation to the electrochemistry of corroding metal samples, see Robert Cottis and Stephen Turgoose, Electrochemical Impedance and Noise (Corrosion Testing Made Easy), NACE International, 1440 South Creek Drive, Houston, Tex. 77084, 1999, incorporated herein by reference; see, especially, Chapter 1, pages 1-7.
Also incorporated herein by reference are the following articles: John N. Murray, “Evaluation of Electrochemical Noise to Monitor Corrosion for Double Hull Applications,” Technical Report, Naval Surface Warfare Center, Carderock Division, CARDIVNSWC-TR-61-94/29, August 1994; Gordon P. Bierwagen, Carol S. Jeffcoate, Junping Li, Seva Balbyshev, Dennis E. Tallman, Dougals J. Mills, “The Use of Electrochemical Noise Methods (ENM) to Study Thick, High Impedance Coatings,”
Progress in Organic Coatings
29, 1996, pp 21-29; Colin J. Sandwith and Robert L. Ruedisueli, “Corrosion and Aging Tests—Via Measurements of Insulation Resistance, Impedance, and Electrochemical Noise—on Jackets of Small-Diameter, Armored, Fiber-Optic Cables with and without Simulated Biofouling Damage,”.
Proceedings of the Ocean Community Conference
1998, Marine Technology Society, Baltimore, Md., Nov. 16-19, 1998, pp393-397; Gretchen A. Jacobson; Managing Editor, “Corrosion Control,” Materials Performance, January 2000, pp 22-27; Jeffery R. Kearns, John R. Scully, Pierre R. Roberge; David L. Reichert, John L. Dawson, Eds., “Overview,” pp ix-xvii, Electrochemical Noise Measurement for Corrosion Applications, ASTM, 100 Barr Harbor Drive, West Conshohocken, Pa., ASTM Publication Code No. 04-012770-27, First International Symposium on Electrochemical Noise Measurement for Corrosion Applications, Montreal, Quebec, Canada, May 15-16, 1994; David L. Reichert, “Electrochemical Noise Measurement for Determining Corrosion Rates,”
Electrochemical Noise Measurement for Corrosion Applications
, Jeffery R. Kearns, John R. Scully, Pierre R. Roberge, David L. Reichert, John L. Dawson, Eds., ASTM, 100 Barr Harbor Drive, West Conshohocken, Pa., ASTM Publication Code No. 04-012770-27, First International Symposium on Electrochemical Noise Measurement for Corrosion Applications, Montreal, Quebec, Canada, May 15-16, 1994, pp 79-89; Gordon P. Bierwagen, Douglas J. Mills, Dennis E. Tallman, Brian S. Skerry, “Reproducibility of Electrochemical Noise Data from Coated Metal Systems,”
Electrochemical Noise Measurement for Corrosion Applications
, Jeffery R. Kearns, John R. Scully, Pierre R. Roberge, David L. Reichert, John L. Dawson, Eds., ASTM, 100 Barr Harbor Drive, West Conshohocken, Pa., ASTM Publication Code No. 04-012770-27, First International Symposium on Electrochemical Noise Measurement for Corrosion Applications, Montreal, Quebec, Canada, May 15-16, 1994, pp 427-445; John N. Murray, “Electrochemical Test Methods for Evaluating Organic Coatings on Metals: An update. Part I. Introduction and Generalities Regarding Electrochemical Testing of Organic Coatings,” Reprinted from
Progress in Organic Coatings
30
Bowles Christine A.
Layer Brian D.
Ruedisueli Robert L.
Kaiser Howard
LeRoux Etienne P
Metjahic Safet
The United States of America as represented by the Secretary of
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