Other Electrochemical Test Techniques

Use and Uimitations of Electrochemical Techniques A major caution must be noted as to the general, indiscriminate use of all electrochemical tests, especially the use of AC and EIS test techniques, for the study of corrosion systems. AC and EIS techniques are apphcable for the evaluation of very thin films or deposits that are uniform, constant, and stable—for example, thin-film protective coatings. Sometimes, researchers do not recognize the dynamic nature of some passive films, corrosion produc ts, or deposits from other sources nor do they even consider the possibility of a change in the surface conditions during the course of their experiment. As an example, it is note-... 

Two other aspects of electrochemical phenomenology associated with localized corrosion should be appreciated before we discuss individual test techniques common observations during potentiostatic testing and common observations during open circuit testing. Careful interpretation of these tests can provide useful information on the processes that control localized corrosion. 

Several researchers experimented with electrochemically driven techniques. Budd and Booth used a potentiostatic approach [23]. Others have tried impressed current tests. Although both appeared promising, these procedures did not lend themselves to the multiple testing required for production control of heat treatment. Neither type of test has been standardized to date. 

Electrochemical tests are rapid techniques to determine mechanisms, determine the effect of various parameters on corrosion rate, and screen out a large number of materials [43]. They usually involve measurement of corrosion potentials, corrosion currents, polarization curves, and electrochemical impedance. They are used to evaluate metals and alloys and the behavior of metallic, inorganic, and oiganic coatings. The simplest test involves the measurement of the corrosion potential and its use in conjunction with other measurements. A zero resistance ammeter (ZRA) is commonly used to measure corrosion currents between dissimilar metals and alloys. Controlled potentitd tests and anodic and cathodic polarization curves using potentiostats are the most commonly used electrochemical tests. These are powerful tools for investigating the effect of various parameters on corrosion behavior. These incorporate the use of cycUc polarization and polarization resistance for localized corrosion and corrosion rate measurements. Table 4 lists electrochemical tests that can be used for corrosion tests in the automobile industry. 

Other plant test specimen designs and evaluation techniques are available. Some fit in restricted spaces and/or monitor electrochemical parameters. Examples of such fixtures, electrode polarization probes, appear in Fig. 4. These allow monitoring of the corrosion process and can detect changes in plant operation that cause excessive attack. 

Potentiodynamic anodic polarization techniques were used by Bennett [85] in laboratory testing to study Type 316L stainless steel in CIO2 bleaching environments. Other researchers used electrochemical testing via an anodic scan with reversal to locate the pitting potential and the repassivation potential [59]. 

Where experiments are conducted which simulate process conditions, the effectiveness of electrochemical testing is more controversial. Postreaction EIS studies of alloys exposed to autoclave corrosion were not able to illuminate differences in the performance of the alloys. By contrast, imaging SIMS provided u.seful insight into the structural and compositional differences, leading to a characterization that might possibly be used to qualify other alloy specimens. Alternatively, one could consider the surface techniques to be at their best when performing a postfailure analysis. 

The electrometer has very high input impedances with respect to voltage measurement and zero input impedance with respect to current sensing along with high precision and resolution. Here we focus on the aspects of this potentiodynamic technique. Aspects of instrumentation are beyond the scope of this chapter. There are several other pseudo-steady state methods that are often used in other electrochemical systems. Some I-V tests include ... 

For electroactive thin films, the electrochemical impedance technique alone is not able to discriminate among the various models which have been proposed in the literature. So the measurement of an electrogravimetric transfer function, Am/AE, by using a fast QCM, in parallel to measurement of the impedance has been proposed to test these various models and to obtain a complete description of the ionic and solvent exchanges between the film and the solution. So far, two groups, one in Korea and the other one in France, have used this technique. 

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