Abstract:
Apparatus for using electrochemical noise analysis to assess the coating upon a conductive substrate comprises an electrolyte-containing cell, a witness electrode, a reference electrode and a working conductor. The electrolyte-containing cell is securely coupled with the coated conductive substrate so that the electrolyte is contiguous with a coated area of the metal substrate, which is demarcated by an electrolytic contact-permitting aperture in the electrolyte-containing cell. The witness electrode contacts the electrolyte and connects to an ammeter. The reference electrode contacts the electrolyte and connects to a voltmeter. Via the working conductor, the ammeter and the voltmeter each connect to a noncoated region of the metal substrate. The conductive substrate&#39;s coated area which contacts the electrolyte effectively represents a working electrode. Practice “in the field” is possible relative to coated conductive substrates associated with diverse entities.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/234,214 filed Sep. 21, 2000, entitled “In Situ Coating Assessment System Based on Electrochemical Noise,” joint inventors Robert L. Ruedisueli, Christine A. Bowles and Brian D. Layer, incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     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&#39;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, 1997, pp 225-233; John N. Murray, “Electrochemical Test Methods for Evaluating Organic Coatings on Metals: An update. Part III. Multiple Test Parameter Measurements,” Reprinted from  Progress in Organic Coatings  31, 1997, pp 375-391; Gordon Bierwagen, Douglas J. Mills, “Characterization of Corrosion under Marine Coatings by Electrochemical Noise Methods,” Final Report for the period Sep. 1, 1992-Aug. 30, 1994, Grant Number N 00014-93-1-0013, The Office of Naval Research, 800 N. Quincy Street, Arlington, Va. 22217-5660 (61 pp plus cover page, errata page); F. Mansfeld, L. T. Han, C. C. Lee, “Analysis of Electrochemical Noise Data for Polymer Coated Steel in the Time and Frequency Domains,”  J. Electrochem. Soc ., Vol. 143, No. 12, December 1996, pp L286-L289; Gordon Bierwaen, Junping Li, Seva Balbyshev, Jason Lindquist, “Electrochemical Noise Methods Applied to the Study of Organic Coatings,” Final Office of Naval Research Report, Grant No. N00014-95-1-0507, July 2000, Department of Polymers &amp; Coatings and Department of Chemistry, North Dakota State University, Fargo, N.Dak. 58105 (191 pages). 
     Further incorporated herein by reference are the following United States patents: Pope et al. U.S. Pat. No. 5,888,374 issued Mar. 30, 1999; Murray U.S. Pat. No. 5,746,905 issued May 5, 1998; Shih et al. U.S. Pat. No. 5,373,734 issued Dec. 20, 1994; Berg et al. U.S. Pat. No. 5,236,564 issued Aug. 17, 1993; Baer et al. U.S. Pat. No. 5,093,626 issued Mar. 3, 1992; Kihira et al. U.S. Pat. No. 4,806,849 issued Feb. 21, 1989; Hladky U.S. Pat. No. 4,575,678 issued Mar. 11, 1986; Mansfeld et al. U.S. Pat. No. 4,221,651 issued Sep. 9, 1980. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide method and apparatus for performing on-site (e.g., in-field or in-service) evaluations of coatings on metallic substrates. 
     The present invention provides a device, system and method for evaluating coating condition using electrochemical noise (ECN) and a “witness” (e.g., steel or platinum) specimen. According to this invention, the witness specimen is a bare (uncoated) electrode and contacts (e.g., is immersed in) the same (identically contained) electrolyte solution as do a standard “reference” electrode and the coated metallic substrate (such as a coated test panel, a tank wall area or a ship&#39;hull area), thus obviating the need for a conventional “salt bridge” between plural discretely contained electrolyte quantities. 
     In accordance with many embodiments of the present invention, coating assessment apparatus is provided which is suitable for on-site use in association with a coated metal substrate and with electrochemical noise instrumentation including an ammeter (which measures current) and a voltmeter (which measures voltage). The inventive apparatus comprises a receptacle, a witness electrode, a reference electrode and lead means. The receptacle is for containing electrolyte and for being coupled with the coated metal substrate whereby the electrolyte contacts the coated metal substrate. The witness electrode is for contacting the electrolyte and for connecting to the ammeter. The reference electrode is for contacting the electrolyte and for connecting to the voltmeter. The lead means is for connecting a metal portion of the coated metal substrate to the ammeter and the voltmeter. 
     Further provided by many embodiments of the present invention is a system, based on electrochemical noise, for evaluating the condition of a coating on a metallic substrate. The inventive system comprises an ammeter, a voltmeter, an electrolyte reservoir, a witness electrode, a reference electrode, a first conductor, a second conductor and a third conductor. The electrolyte reservoir is attachable to the metallic substrate whereby the electrolyte communicates with the metallic substrate. The witness electrode is contactable with the electrolyte (e.g., communicable with a liquid electrolyte-containing sponge-like member placed inside the reservoir; or, immersible in the reservoir which contains free-standing liquid electrolyte). The reference electrode is contactable with the electrolyte (e.g., communicable with a liquid electrolyte-containing sponge-like member placed inside the reservoir; or, immersible in the reservoir which contains free-standing liquid electrolyte). The first conductor is for connecting the metallic substrate with the ammeter and with to the voltmeter. The second conductor is for connecting the witness electrode with the ammeter. The third conductor is for connecting the reference electrode with the voltmeter. 
     Also provided by many embodiments of the present invention is a method, based on electrochemical noise, for evaluating the condition of a coating on a metallic substrate. The inventive method comprises the steps of: attaching an electrolyte reservoir to the metallic substrate whereby the electrolyte communicates with the metallic substrate; causing a witness electrode to contact the electrolyte (e.g., immersing the witness electrode in the electrolyte reservoir; or, causing the witness electrode to touch a sponge-like member placed inside the reservoir); causing a reference electrode to contact the electrolyte (e.g., immersing the reference electrode in the electrolyte reservoir; or, causing the reference electrode to touch a sponge-like member placed inside the reservoir); electrically connecting the metallic substrate with an ammeter and with a voltmeter; electrically connecting the witness electrode with the ammeter; and, electrically connecting the reference electrode with the voltmeter. 
     In accordance with typical embodiments of the present invention, a device for using electrochemical noise analysis for purposes of assessing the coating upon a conductive (e.g., metal) substrate comprises an electrolyte-containing vessel, a witness electrode,sa reference electrode and a working conductor. The electrolyte-containing vessel is adaptable to being firmly coupled with the coated conductive substrate so that the electrolyte is adjacent to a coated region of the metal substrate located inside the electrolyte-containing vessel. The witness electrode is contactable with (e.g., immersible in) the electrolyte and is adaptable to being connected to an ammeter. The reference electrode is contactable with (e.g., immersible in) the electrolyte and is adaptable to being connected to a voltmeter. The working conductor is adaptable to connecting the ammeter and the voltmeter to a noncoated region of the metal substrate located outside the electrolyte-containing vessel. 
     In inventive principle, the region of the coated conductive substrate which is contacting the electrolyte and is bounded by an electrolytic contact-permitting opening in the electrolyte-containing vessel is effectively rendered a working electrode. In situ application can be propitiously afforded by fixing and sealing the bottom of the device with respect to a coated conductive substrate such as would be part of a marine or nonmarine transportation vehicle or a tank wall. If the vessel is infused with free-standing liquid electrolyte, a top seal is preferably provided. If an electrolyte-soaked sponge or foam member is disposed within the vessel, a top seal may be optional. 
     Inventive practice will normally prescribe that the same electrolytic entity (e.g., a liquid electrolyte-saturated sponge disposed inside a cell; or, a body of electrolyte liquid contained by a cell) will be in contact with the witness electrode, the reference electrode and the coated metallic substrate test region (e.g., the portion of the metallic substrate which is bordered by the perimeter of the base opening of the cell and is adjacent to the electrolytic entity). 
     According to typical inventive embodiments, a generally cylindrical electrolyte reservoir (such as includes a vessel, container, chamber, receptacle, cell, etc.) is capable of being coupled with the coated metallic substrate so that the electrolyte is contiguous with the coated metallic substrate through the open lower end of the reservoir. A first conductor means is situated outside the reservoir and connects an uncoated portion of the metallic substrate (e.g., a bare spot, a projection or a stud) to an ammeter and a voltmeter; that is, the first conductor means is in electrical contact with the uncoated metallic substrate portion, and with the ammeter, and with the voltmeter. A second conductor means passes through the upper end of the reservoir and connects the witness electrode to the ammeter; that is, the second conductor means is in electrical contact with the witness electrode and with the ammeter. A third conductor means passes through the upper end of the reservoir and connects the reference electrode to the voltmeter; that is, the third conductor means is in electrical contact with the reference electrode and with the voltmeter. The coated metallic substrate portion which is in electrical contact with the electrolyte and is inside the periphery of the lower open end of the reservoir functionally represents the working electrode; in other words, this electrically connected (coated) metallic substrate portion effectively acts as a working electrode. 
     According to many inventive embodiments, the chamber will be provided with a first opening means (including at least one opening) for allowing the leads to enter the chamber, and a second opening means (including at least one opening) for allowing the electrodes to contact the substrate. Typically, the first opening means (for permitting lead entry) and the second opening means (for permitting electrode contact) will be located at generally or approximately opposite extremes of the chamber. As an example, if a cap-seal-type sealing device is implemented for sealing the chamber at its lead-entry end, the cap-seal should be appropriately apertured with one or more openings for accommodating the passing therethrough of the leads; for instance, according to some embodiments a cap-seal has two small holes for correspondingly accommodating the passing therethrough of two leads. 
     This invention can be practiced in the field (e.g., on ships) to evaluate coating condition. Furthermore, the data obtained using the inventive sensor and method have been demonstrated by the inventors to correlate well with data obtained unsing electrochemical impedance spectroscopy (EIS). It is noted that the U.S. Navy bases its databases pertaining to long-term coating performance on, inter alia, EIS methodology. The U.S. Navy also bases such databases on salt fog spray chamber tests, field exposure tests, visual evaluations, scratch and prohesion. Based on inventive testing, in situ (e.g., in-the-field, or in-service) coating assessments effectuated using inventive ECN sensors are demonstrably relatable to EIS laboratory databases, and may be relatable to other databases, as well. 
     The present invention uniquely features the performance of ECN tests between a coated steel specimen and a bare steel specimen, which serves as a probe or “witness” specimen. This inventive ECN approach offers numerous advantages not only over conventional ECN approaches but also over conventional EIS approaches. ECN offers a logistically feasible means of evaluating coating condition in the field, with shorter test times than EIS and with test output that is more straightforward and easier to interpret than EIS data. However, long-term ECN databases on Naval and Department of Defense (DoD) coatings do not exist. By correlating EIS and ECN data, ECN offers a practical means of bridging the gap between laboratory-based data bases and in-situ, condition-based monitoring in the field. The correlation of data from the two techniques (EIS and ECN) offers practical and useful benefits for the evaluation of coating condition and prediction of coating life. 
     As described herein (including in Appendix A), the inventors investigated a practical methodology for correlating test results of the inventive ECN sensor and procedure with electrochemical impedance spectroscopy (EIS) data. This investigation was conducted at the Naval Surface Warfare Center, Carderock Division (NSWCCD) located in West Bethesda, Md. 
     ECN measurements obtained using the inventive method can serve as a practical link between existing coatings data bases (which are based on samples tested using EIS throughout long periods of exposure) and inventively acquired data pertaining to in-service coatings performance on ships and vehicles. ECN data from the apparatus and method according to this invention can be correlated with existing EIS data in order to demonstrate that ECN can quantitatively distinguish between “good” coatings and “bad” coatings. The inventors have demonstrated the ability of the inventive ECN system to quantitatively differentiate between ‘good’ and ‘bad’ coatings, and thus have as demonstrated the feasibility of using the inventive ECN system to quantitatively probe the condition of ships and vehicles in the field and predict coating life based on existing data bases. 
     Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE APPENDICES 
     The following appendices are hereby made a part of this disclosure: 
     Attached hereto marked APPENDIX A and incorporated herein by reference is the following 19-page high school science project report, authored by joint inventor Brian D. Layer, which discloses various aspects of the present invention: Brian D. Layer, “The Uses of Electrochemical Noise in the Evaluation of Coating Condition,” displayed at the science fair which took place at the Poolesville High School science fair on Jan. 15, 2000 in Poolesville Md. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein like numbers indicate the same or similar components, and wherein: 
     FIG. 1 is a diagrammatic perspective view of the prototypical embodiment, used in testing conducted by the U.S. Navy, of an ECN sensor in accordance with the present invention. 
     FIG. 2 is a diagrammatic perspective view, similar to the view shown in FIG. 1, of an embodiment of an ECN sensor in accordance with the present invention, particularly illustrating a cap seal. The inventive embodiments depicted in FIG.  2  through FIG. 5 are similar to the inventive prototype shown in FIG. 1, but are especially suitable for in situ utilization in association with coatings on ship hulls or tank walls. 
     FIG. 3 is a diagrammatic perspective view of an embodiment of an ECN sensor in accordance with the present invention, particularly illustrating a sponge-containing electrolyte reservoir. 
     FIG. 4 is a diagrammatic perspective view of another embodiment of an ECN sensor in accordance with the present invention, particularly illustrating an injectable electrolyte reservoir. 
     FIG. 5 is a schematic and block diagrammatic representation of an embodiment of an ECN system in accordance with the present invention. 
     FIG. 6 is a diagrammatic bottom plan view of a magnetic hold-down ring/seal assembly for sealing and securing the electrolyte reservoir with respect to the coated metal substrate. 
     FIG. 7 is a flow diagram illustrative of ECN testing, including utilization of ECN instrumentation and data processing, which was performed by the U.S. Navy on some occasions while implementing conventional ECN apparatus and on other occasions while implementing the inventive prototype apparatus shown in FIG.  1 . 
     FIG. 8 is a graphical representation of correlation of an inventively obtained ECN current-deviation product (an example of a single-value figure of merit) with EIS Z max  values for two different witness electrodes, viz., steel and platinum (Pt). As illustrated in this figure, in inventive practice, platinum is generally superior to steel as a witness material. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to establish correlativeness of data obtained from EIS and ECN tests, two pairs of coated steel test specimens, both of which were included in the long-term EIS coatings database, were evaluated. The first pair consisted of two, “bad” specimens which had consistently shown low EIS impedance values and shown visual evidence of degradation in the form of heavy rust. The second pair consisted of “good” samples with high impedance values and no visual evidence of coating failure. 
     ECN tests were performed in a conventional way, implementing an electrolyte bridge (“salt bridge”), on each pair of samples (“good” and “bad” pairs), and the resultant data were compared with EIS data previously obtained for each individual sample; for an instructive representation of a classical laboratory experimental ECN set-up such as used in this conventional ECN testing, see “FIG.  1 ” in the aforementioned U.S. Navy technical report 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. 
     In addition to performing conventional ECN tests on pairs of coated steel specimens, the inventors performed inventive ECN tests. Referring now to the figures herein, in accordance with the present invention, testing was performed between bare steel “witness” specimens and coated specimens representing the “good” and “bad” conditions. According to the present invention&#39;s prototypical test methodology, the steel “witness” specimens served as ECN probes for evaluating the condition of the coated specimens; a separate set of similar inventive experiments was conducted using bare platinum “witness” specimens. 
     ECN tests were inventively performed using the prototype inventive ECN device illustrated in FIG.  1 . Since the bare carbon steel (or platinum) witness specimen (witness electrode  16 ) was immersed in the same test solution as the coated panel (32-coated metallic substrate  30 ), this eliminated the need for a salt bridge. The inventive ECN device  10  embodiments depicted in FIG.  2  through FIG. 5 are similar to that shown in FIG.  1 . Among the inventive features shown in FIG.  2  through FIG. 6 which foster suitability for in situ implementation in association with coatings such as on ship hulls or tank walls are: top sealability means; bottom sealability means; and, bottom attachability/detachability means (such as via magnetic attraction for magnetic substrates, or suction or adhesive for non-magnetic substrates) with respect to a metallic substrate. 
     The feasibility of the present invention was demonstrated by the experimental analysis which was conducted utilizing the inventive prototype portrayed in FIG.  1 . Implementing an inventive ECN sensor  10  such as shown in FIG. 1, ECN measurements were taken between “witness” specimens (witness electrodes  16 ) consisting of bare steel or bare platinum (representing the substrate material of a ship hull) and the “good” and “bad” coated steel specimens (metal substrates  30  coated with coating  32 ) representing good and bad areas of a coating on a ship&#39;s hull or tank wall. 
     Thus, by using an inventive monitoring system such as shown in FIG. 5, wherein a “witness” specimen (witness electrode  16 ) represents the substrate material of interest, the condition of a coating  32  in service can be inventively evaluated quantitatively using ECN. Furthermore, by inventively correlating the ECN data obtained in this manner with existing EIS databases, meaningful and reliable predictions of coating life will be possible. 
     An inventive ECN test system comprising a “witness” probe and appropriate instrumentation can be used in the field to monitor the condition of a coating such as paint on the hull of a ship, the wall of a tank, or the body of a vehicle. As shown in FIG. 3, a saturated sponge  34  (made of sponge, foam or other porous or open cell material) can contain the necessary electrolyte  14 . As shown in FIG. 4, liquid electrolyte can be introduced through the top of cell  12 , which would previously or subsequently be capped. FIG. 5 shows an inventive ECN setup for in situ monitoring a ship&#39;s hull or other coated entity. As shown in FIG.  5  and FIG. 6, the inventive ECN cell  12  is held to the surface of the 32-coated metal substrate (e.g., hull)  30  via a magnetic ring/seal assembly  25 . 
     With reference to FIG.  1  through FIG. 6, inventive ECN device  10  includes electrolyte reservoir  12 , liquid electrolyte  14 , witness electrode  16 , reference electrode  18 , working lead  20 , witness lead  22  and reference lead  24 . Top cap-seal  26 , sponge  34  and/or bottom magnetic hold-down ring/seal assembly  28  are also included in many inventive embodiments. As illustrated in FIG.  1  through FIG. 5, ECN testing is being inventively performed in relation to a conductive substrate  30 , typically a conductive metal/metallic substrate  30 , which is provided with a coating  32 . 
     Electrolyte reservoir  12  is a clamp-on, seal-to-surface, cylindrical cell structure which is partially or substantially filled with liquid electrolyte  14 . Reservoir  12  is fixedly mounted upon 32-coated metallic substrate  30  using a magnetic clamping base sealing and coupling device such as hold-down ring/seal assembly  28 , shown in FIG.  5  and FIG. 6, having clamp fasteners/tighteners  23 , a flanged (e.g., split) magnetic clamp ring  25 , an o-ring-type seal  27  and a central hole  29  compatible with the circumference of reservoir  12 . The magnetic attraction of ring  25  of hold-down ring/seal assembly  28  to metal substrate  30 -helps to fix reservoir  12  in relation to 32-coated metal substrate  30 . 
     Many substrates of large structures are magnetic (typically, made of steel). However, in accordance with this invention, nonmagnetic adherence means (e.g., adhesive, suction or another mounting or “clamping” technique) can additionally or alternatively be used in order to stably-affix reservoir  12  to non-magnetic 32-coated metallic substrate  30 . Stainless steel, aluminum, copper, etc. are examples of non-magnetic conductive substrate materials. 
     Reservoir  12  as shown in FIG. 1 is representative of the inventive cell which was used in inventive laboratory tests on single coated panels. FIG.  2  through FIG. 6 portray an inventive arrangement  10  including reservoir  12  which is suitable not only for in-laboratory ECN single panel  30  testing of out-of-service coatings  32  but also for on-site, ECN single panel  30  field testing of in-service coatings  32 . Inventive device  10  is endowed with certain features e.g., top cap-seal  26 , sponge  34  and/or bottom magnetic hold-down ring/seal assembly  28 —which promote on-site adaptability such as in relation to a ship&#39;s hull or a tank wall. 
     Once reservoir  12  is appropriately securely situated atop coating  32  of metallic substrate  30 , electrolyte  14  can be caused to be contained by reservoir  12  according to various inventive techniques, such as elaborated upon hereinbelow. For instance, electrolyte  14  liquid can be injected into a sealed reservoir  12  after being placed on 32-coated metal substrate  30  (e.g., hull or wall). Or, electrolyte  14  liquid can be contained within a sponge or foam structure  34  situated inside an open reservoir  12  cell before or after placement on 32-coated metal substrate  30 . 
     As shown in FIG. 3, sponge  34  is a porous absorbent member (such as characterized by a sponge-like or foam-like structure) which is at least substantially saturated with electrolyte  14  and is inserted in reservoir  12 . A cap-seal  26  can be removable and securely replaceable (e.g., like a medicine bottle cap) for purposes of introducing and withdrawing contents (such as electrolyte  14 -soaked sponge  34 ) therein and therefrom. According to some inventive embodiments utilizing an electrolyte  14 -soaked sponge  34 , the upper end of reservoir  12  can be left open during operation of inventive ECN device  10 , thus obviating telexed altogether for sealing means such as cap-seal  26  at the top end of reservoir  14 . 
     As shown by vertical arrow e in FIG. 4, electrolyte  14  is fluidly inserted through the top end of reservoir  12 . For instance, electrolyte  14  can be injected in liquid form through an opening in the cap-seal  26  of reservoir  12 . Alternatively, cap-seal  26  can be removed, electrolyte  14  poured into reservoir  12 , and cap-seal  26  replaced. Alternatively, cap-seal  26  can be made of a membranous (e.g., rubber) material which is impermeable to electrolyte  14  but which is permeable to a hypodermic syringe-type device usable for injecting electrolyte  14  therethrough. 
     Witness electrode  16 , reference electrode  18  and 32-coated metallic substrate  30  each must contact electrolyte  14  during inventive operation of ECN device  10 . For this purpose, reservoir  12  is at least partially open at the bottom end. A round opening  13 , approximately concentric with the circumference of reservior  12 , is provided at the bottom end of reservoir  12  for promoting communication between electrolyte  14  and 32-coated metallic substrate  30 . 
     As illustrated in FIG. 3, approximately cylindrical sponge  14 , diametrically slightly or somewhat smaller than the circular bottom opening  13  of reservoir  12 , sits atop coating  32  of metal substrate  30  at the bottom of reservoir  12  approximately coaxially therewith. Witness electrode  16  and reference electrode  18  are each touching the top flat surface of electrolyte 34-soaked sponge  34 , and coating  32  is touching the bottom flat surface of sponge  34 . Alternatively, to ensure good contact, witness electrode  16  and reference electrode  18  can be “sleeved” snugly into electrolyte 34-soaked sponge  34 . 
     Neither opening  13  nor sponge  34  need be circularly or cylindrically shaped in inventive practice; preferably, however, opening  13  accommodates sponge  34 , and sponge  34  includes a flat lower surface portion adaptable to contiguity with  32 -coated metallic substrate  30 . In fact, it is readily apparent to the ordinarily skilled artisan who reads this disclosure that, in inventive practice, reservoir  12  can be any of diverse kinds of containers, vessels, receptacles, etc., and need not have a cylindrical or near cylindrical shape. Any reservoir  12  shape will due which advances the inventive principles of (i) electrolyte containment, (ii) electrolyte contiguity (in relation to the subject coated metal substrate), (iii) base fixability (in relation to the subject coated metal substrate) and (iv) base sealability (in relation to the subject coated metal substrate). Glass, plastic or other nonconductive (preferably transparent) materials are suitable for the material composition of reservoir  12 . 
     In accordance with the inventive electronic arrangement, the metallic substrate  30  itself constitutes, in effect, the “working electrode” of inventive ECN device  10 . That is, the working electrode is the portion of conductive substrate  30  (e.g., metal panel, ship&#39;s hull or tank wall) which lies beneath the coating  32  area being tested—more specifically, the portion of the coating which contacts the electrolyte  14  and is circumscribed by the electrolytically vehicular opening  13  of reservoir  12 . 
     Witness electrode  16 , made of steel in inventive testing but preferably made of platinum or other noble metal, is supported within reservoir  12  on witness lead  22 , which serves as the electrical connection means from witness electrode  16  to ECN instrumentation  36 . Reference electrode  18  is made of a standard electrochemical (half-cell) reference material (such as silver-silver chloride) and is similarly supported by reference lead  24 . Reference lead  24  similarly serves as the electrical connection means from reference electrode  18  to ECN instrumentation  36 . 
     According to typical inventive practice, the lower end sections of witness lead  22  and reference lead  24  (which are connected to witness electrode  16  and reference electrode  18 , respectively) are each a rigid, insulated, conductive shaft or a heavy gauge wire. Referring to FIG.  1  and FIG. 2, these lower extreme portions, witness lead portion  22   a  and reference lead portion  24   a  (which are connected to witness electrode  16  and reference electrode  18 , respectively) should each be of sufficient length to clear the top of reservoir  12 , at which point conventional insulated (e.g., copper) wire lead  22   b  and lead  24   b  portions correspondingly connected thereto can serve to complete the corresponding connections to ECN instrumentation  36  (in particular, to ammeter  38  and voltmeter  40 , respectively). 
     Reference is now made particularly to FIG. 1, FIG.  2  and FIG.  5 . Witness electrode  16  is connected, via witness lead  22 , to ammeter (e.g., a zero resistance ammeter, or ZRA)  38 . Reference electrode  18  is connected, via reference lead  24 , to voltmeter  40  (e.g., an electrometer). Metallic substrate  30  is connected, via working lead  20 , to both ammeter  38  and voltmeter  40 . Witness lead  20  is the insulated wire conductor which effects connection between ammeter  38  and the witness electrode  16  metal (e.g., platinum) in reservoir (e.g., cell)  12 . Reference lead  24  is the conductor which effects connection between voltmeter  40  and reference electrode  18 . Working lead  20  is the conductor which effects connection between the test subject coating&#39;s metal substrate  30  (electrically conductive ship&#39;s hull, tank wall or other structural member) and ECN instrumentation  36  (which includes ammeter  38  and voltmeter  40 ). 
     It is notable that, according to many embodiments of the inventive configuration: Witness electrode  16  is directly connected to ammeter  38 ; reference electrode  18  is directly connected to voltmeter  40 ; and, metallic substrate  30  is directly connected to ammeter  38  and voltmeter  40 . That is, conspicuously absent in the inventive circuitry are any electronic elements (such as series resistors) between witness electrode  16  and ammeter  36 , as well as between reference electrode  18  and voltmeter  38 , as well as between metallic substrate  30  and either ammeter  36  or voltmeter  38 . 
     Connection of working lead  20  to metallic substrate  30  at working connection  42  will normally be more easily accomplished in the laboratory than in the field. In a laboratory, the inventive practitioner can grind off a small section of the coating (e.g., paint)  32  on the test panel substrate  30 , and then clip working lead  20  to the working connection  42  location in the test panel substrate  30  using an alligator clip or similar means. In the field (e.g., on a ship), a connection between working lead  20  and metallic substrate  30  can be made at an uncoated surface working connection  42  location in metallic substrate  30 , such as a bare (uncoated) spot, a projection (e.g., lug) or a fastener (e.g, stud  44  shown in FIG.  5 ). Typically, lugs (e.g., for hatch covers) or studs or other types of hardware are imbedded in a ship&#39;s hull or tank wall. In a worst case scenario, the inventive practitioner would have to grind through the coating  32  (e.g., paint) in a small area (or plural small areas if the test domain is very large), effecting a connection at the “de-coated” area through a conductive adhesive or gel analogous to what may be used when electrocardiogram (ECG) electrodes are connected to a person. 
     As shown in FIG. 5, the working connection  42  point is somewhat distanced or removed (typically a few or several inches, if not several feet or more, in inventive in situ applications) from the inventive ECN sensor  10  test site. Working lead  20 , when connected at working connection  42 , effectuates an actual electrical connection between ECN instrumentation  36  and the metal substrate  30  which lies under coating  32 . Working connection  42  serves to render the portion of metal substrate  30  which is in the vicinity of working connection  42  at the same electrical potential as the portion of metal substrate  30  which lies beneath coating  32  within the test cell perimeter (e.g., the circumference of circular opening  13  at the bottom end of generally cylindrical reservoir  12  shown in FIG.  1  through FIG.  5 ). 
     According to frequent inventive practice, working lead  20 , witness lead  22  and reference lead  24  are preferably shielded to eliminate “noise” other than ECN in reservoir  12 . Moreover, working lead  20  and reference lead  24  can preferably each be provided with “dual connections” or “sensing” capability in order to eliminate IR from voltage measurements. For instance, working lead  20  will actually comprise two “sub-leads” (not separately shown) running together, viz., a working electrode sublead (the function of which is to conduct electronic signals from metal substrate  30  to ECN instrumentation  36 ) and a “working sense” sublead (the.function of which is to measure and cancel the voltage drop along the working electrode sublead). In typical inventive practice, both the working electrode sublead and the working sense sublead should be shielded to minimize unwanted electrical noise pick-up (such as may derive from overhead lights). 
     FIG. 5 illustrates that a computer system  50  comprising data processing, signal processing, data acquisition and/or information storage can be connected to ECN instrumentation  36 . Aforementioned U.S. Pat. No. 5,888,374 to Pope et al. is instructive regarding principles of computer monitoring such as would be inventively practicable in association with inventive ECN device  10 . In the light of this disclosure, the ordinarily skilled artisan will understand how known data processing and data acquisition methodologies can be effectively practiced in relation to the present invention. 
     Reference now being made to FIG. 7, the ECN tests conducted in this demonstration were performed as follows. Tests were run on pairs of samples for six minutes using a commercial ECN test setup. Voltage and current noise were measured using a zero-resistance ammeter (ZRA) which effectively shorts the two samples. The test solution contained within the specimens&#39; test cells was linked with a salt bridge to allow ionic exchange. Noise data was acquired during four-second intervals every six seconds throughout the test period. Thus, the data acquisition cycle was 4 seconds “on” (data collected for 4 seconds) followed by 2 seconds “off” (no data collection for 2 seconds), repeated continuously for 6 minutes. During the 4-second acquisition period, voltage and current were measured every 10 ms. ECN tests were performed inside of a Faraday cage. Output data consisted of the average noise values for each 4-second interval plotted as a function of time. 
     The ECN data inventively obtained for the witness specimens and coated panels were compared to the aforementioned conventional ECN data taken on the pairs of panels. Moreover, witness specimens with freshly sanded and rusted surfaces were inventively tested to evaluate the effect of witness specimen surface condition on the ECN output. Results of initial experiments validated that ECN could indeed differentiate, as desired, between “good” and “bad” coating conditions. In general, nominal current noise values were found to vary by an order of magnitude for the two coating conditions examined. Furthermore, the initial testing demonstrated that the novel sensor setup and procedure according to this invention, in which a “witness” specimen is utilized as a probe, is feasible as a method of quantitatively assessing coating condition. Test results also showed that the presence of rust on the “witness” specimens affected the ECN data, and that nominal ECN values obtained using witness specimens were not the same as those obtained between pairs of coated panels. Overall, the prototype inventive ECN sensor was shown to function as desired, and further development and characterization of the inventive sensing method is warranted. 
     During some inventive testing, steel was used as the material for witness electrode  16 . Instead of steel, the bare “witness” specimen, witness electrode  16 , can be made of another material such as graphite or platinum or another noble metal. The term “noble metal” is generally considered to describe any of several metallic chemical elements characterized by outstanding resistance to oxidation, even at high temperatures. The “noble metal” grouping, though not strictly defined, is usually understand to include rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold; these elements are the metals of groups VIIb, VIII and Ib of the second and third transition series of the Periodic Table. 
     With reference to FIG. 8, platinum (Pt) was also used during some inventive testing, and was found to compare favorably to steel as the witness electrode  16  material. The graph in FIG. 8 illustrates the EIS Z max  versus the ECN current-times-deviation product for two different witness electrodes  16 , viz., a witness electrode  16  comprising steel and a witness electrode  16  comprising platinum. The “ECN current-deviation product” is an example of a “single-value figure of merit” which may be obtained in accordance with the present invention. As the graph in FIG. 8 suggests, for most inventive embodiments, platinum (vice steel) is the superior “witness.” 
     An alternative method of obtaining ECN data in service was investigated by joint inventor Ruedisueli and Dr. J. N. Murray at the Naval Surface Warfare Center, Carderock Division (NSWCCD). This sub-film ECN sensor-based method involved the development and fabrication of thin-film, interdigitated steel sensors to serve as under-film ECN sensors for painted surfaces. The sensors were applied to selected locations on the bare steel interior walls of a chemical storage tank prior to painting. Upon painting the tank walls, the sensors themselves were covered with paint. By performing ECN tests on the sensors throughout the service life of the tank, differences in nominal current noise could be monitored at the sensor locations. According to this sub-film ECN sensor approach, however, the noise data is indicative of the electrochemical activity of the sensor itself rather than of the tank wall itself. Moreover, according to this sub-film ECN sensor technique, ECN testing is limited to the pre-selected areas where sensors have been installed prior to painting. Thus, this sub-film ECN sensor methodology is limited in applicability and flexibility in comparison with the inventive “witness” specimen-based ECN sensor methodology. 
     Another method currently under development at NSWCCD is a variation on the EIS laboratory method. According to this limited-frequency EIS approach, only the high frequency component of the measurement spectrum is utilized, thus allowing for shorter, more practical measurement times. A field-portable prototype of this limited-frequency EIS system, including a special hand-held sensor head that reportedly does not require hull (coating substrate) grounding, was nearing completion for testing as of about May 1999. 
     Other embodiments of this invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. Various omissions, modifications and changes to the principles described may be made by one skilled in the art without departing from the true scope and spirit of the invention which is indicated by the following claims.