Local electrochemical methods

Background Global Electrochemical Methods Local Electrochemical Methods Surface Spectroscopy and Imaging Methods... 

That is, to determine the correct corrosion rates in pitting corrosion, as shown in Fig. 37, it is necessary to know the local corrosion currents on the electrode surface. The corrosion current observed is, however, obtained as the total current, which is collected by the lead wire of the electrode. From the usual electrochemical measurement, we can thus determine only an average corrosion current (i.e., the corrosion rate). Hence if we can find some way to relate such an average rate to each local corrosion rate, the local corrosion state can be determined even with the usual electrochemical method. 

MIC depends on the complex structure of corrosion products and passive films on metal surfaces as well as on the structure of the biofilm. Unfortunately, electrochemical methods have sometimes been used in complex electrolytes, such as microbiological culture media, where the characteristics and properties of passive films and MIC deposits are quite active and not fully understood. It must be kept in mind that microbial colonization of passive metals can drastically change their resistance to film breakdown by causing localized changes in the type, concentration, and thickness of anions, pH, oxygen gradients, and inhibitor levels at the metal surface during the course of a... 

More recent work has utilized advanced electrochemical methods to create coatings that can provide drug to the local environment as well as provide for more biocompatible and corrosion-resistant surfaces (5). In this study, the final processing of the device surface included an electrochemical method to create voids on the surface. Pharmaceuticals were loaded into the voids and released over a long period of time. The coating also enhanced corrosion resistance. There is no doubt that an understanding of surface electrochemistry, specifically at the metal-blood interface, will allow for further biomimetic enhancement. 

Subcell Approach Stumper et al.135 presented the subcell approach to measure localized currents and localized electrochemical activity in a fuel cell. In this method a number of subcells were situated in different locations along the cell s active area and each subcell was electrically isolated from each other and from the main cell. Separate load banks controlled each subcell. Figure 8 shows the subcells in both the cathode and anode flow field plates (the MEA also had such subcells). The current-voltage characteristics for the... 

When one is dealing with localized corrosion processes, the tendency is experimentally to determine or model whether a particular process can occur in a specific environment i.e., to determine the susceptibility. Such procedures are invaluable in materials selection, and the use of electrochemical methods is an integral part of these efforts. However, in some environments it is injudicious to assume that localized corrosion will not occur. One example would be SCC in nuclear reactor heat exchangers and other components. In other applications, the need to minimize materials costs leads to the selection of materials for which there is no guarantee of immunity to localized corrosion. For such applications there is a strong need for models that will predict how fast such processes will propagate once they are initiated and what kind and extent of damage will accumulate. 

A general scheme for the development of corrosion models based on electrochemical principles has been described, and a number of examples for active, passive, and localized corrosion has been given. This chapter is by no means comprehensive, and a search of the scientific and technical literature will unearth many additional examples. The value in using electrochemical methods both to develop understanding of the corrosion process and to measure the values of specific modeling parameters is obvious. However, their application alone would not provide all the elements and parameter values required for the development of corrosion models, so the use of supplementary techniques is necessary. It is necessary also to keep in mind that electrochemical techniques inevitably accelerate the corrosion process one is interested in. Consequently, the scaling of electrochemi-cally determined parameter values to the rates and time periods of interest in the corrosion process to be modeled should be undertaken carefully and with a full knowledge of the limitations involved. 

Electrochemical Studies of Localized Corrosion. There are several electrochemical methods to determine the electrochemical conditions of pitting and crevice corrosion. However, emphasis is placed here on pitting as an example ... 

Measixrement techniques Bridge mechanical generator Bridge electronic generator Impulse method, oscillograph, Laplace transform Analogue impedance measurement, potentiostat (AC + DC) Digital impedance measurement, cormection with computer Local electrochemical impedance spectroscopy (LEIS)... 

R. S. Lillard, R J. Moran, and H. S. Isaacs, "A Novel Method for Generating Quantitative Local Electrochemical Impedance Spectroscopy," Journal of The Electrochemical Society, 139 (1992) 1007-1012. 

The results obtained in the system k x hkl)/Cv , where 2D Me UPD phenomena occur followed by a Stranski-Krastanov growth mechanism in the OPD range, show that electrochemical 3D Me phase formation processes can be used for structuring and modification of metal single crystal surfaces in the nanometer range. Local electrochemical processes are initiated by in situ local probe methods using appropriate polarization routines. 

Recently, two new electrochemical mapping techniques have become available the scanning vibrating electrode technique (SVET) and the localized electrochemical impedance spectroscopy (LEIS) technique. These techniques provide the capability to identify and monitor electrochemical behavior down to the micron level. These represent significant advances over traditional electrochemical methods (cyclic voltammetry, EIS, and even EQCM), which provide data that reflect only an average over the entire sample surface. Although such data are very useful, a major drawback is that no local or spatial information is obtained. 

An interesting electrochemical method for the determination of bound sialic acid has been developed, making use of a potentiometric four-channel thick-film sensor [236]. The sialidase sensor consists of a bilayer of a membrane containing Clostridium perfringens sialidase immobilized in a poly(vinyl acetate)-polyethylene copolymer, which is placed on top of an fT -selective poly(vinyl chloride)-poly(vinyl acetate) indicator membrane. The enzyme-induced release of bound sialic acid leads to a concomitant decrease in pA a of the carboxyl function of sialic acid. This decrease affords a local pH change inside the sialidase-containing sensor membrane, which is monitored by the H -selective indicator membrane. The pH optimum of the sialidase sensor was pH 4 for sialyllactose, mucin and colominic acid. 

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