Localized corrosion electrochemical phenomenology

The phenomenology of localized corrosion helps to define certain requirements for localized corrosion that can be expressed in terms of the concepts already discussed in Chapter 2. In order for localized corrosion to occur, there must be a spatial variation in the electrochemical or metallurgical conditions. The occurrence of discrete sites of attack demonstrates that passivity must be able to coexist on the same surface with active regions. In fact, this is one of the scientifically interesting aspects of localized corrosion. Under normal circumstances, one would expect that a surface would either be completely passive or completely active, not a mixture of the two. Finally, there is a physical separation of the anodic and cathodic reaction sites during localized corrosion. In order to understand localized corrosion and thus how to test for resistance to localized corrosion, we must understand each of these aspects and their interrelations. 

This section provides a basic explanation of the underlying physical processes that control localized corrosion in order to lay the foundation for an understanding of the tests that are discussed in the next section. The manifestations of these physical processes through electrochemically measurable quantities are then discussed. Some generalized phenomenology is presented through illustrative examples from the literature. Full mechanistic understanding of localized corrosion has not yet been achieved. Information on the various models proposed can be found in review articles (11,12) and corrosion texts (13,14). 

Armed with an understanding of the underlying physical processes, the electrochemical phenomenology of localized corrosion can be better understood. Figure 23 shows three schematic polarization curves for a metal in an environment in which it spontaneously passivates and (1) can be anodized, (2) transpassively dissolves at higher potentials, and (3) pits upon further anodic polarization. We have discussed cases 1 and 2 in the section on passivity. For case 3, the region of passivity extends from to a potential labeled EM at which point the current increases dramatically at higher potentials. 

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. 

The most common electrochemical test for localized corrosion susceptibility is cyclic potentiodynamic polarization. As was discussed briefly in the section on the electrochemical phenomenology of localized corrosion, this test involves polarizing the material from its open circuit potential (or slightly below) anodically until a predetermined current density (known as the vertex current density) is achieved, at which point the potential is scanned back until the current reverses polarity, as shown in Fig. 42. The curve is generally analyzed in terms of the breakdown (Ebi) and repassivation potentials (Elf). Very often, metastable pits are apparent by transient bursts of anodic current. The peaks in current shown in Fig. 42 for a potentiodynamic scan are due to the same processes as those shown in Fig. 25 for a potentiostatic hold. 

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