1156Pitting Corrosion

Pitting corrosion is often considered to be identical to that of crevice corrosion because the nature of the corrosion processes that occur are almost identical. The difference is, however, the nature by which each type of corrosion initiates. In the case of pitting local surface film damage or material heterogeneities lead to the formation of a pit, as opposed to differences in oxygen concentration, as is the case for crevice corrosion. Localized pitting may result from the preferential dissolution at the site of a nonmetallic inclusion or where the surface oxide film is broken, for example, by scratching the surface (see Fig. 11.10) [23].

Pitting is often associated with materials that show a high tendency to passi-vate, for example, stainless steels. Localized metal loss via pitting corrosion is governed by the following:

1. Spatial separation of anodic and cathodic sites

2. Presence of an oxidant, for example, dissolved oxygen and negative ions (e.g., SO42-, Cl-, but not OH-)

3. Concentration of salts within the pit, that is, solubility of the corrosion product

Passive

Film breakdown

Repassivation

^ Metastable pitting and repassivation

^ Stable pitting and pit growth

^ Stable pitting and pit growth

FIGURE 11.10 Pit development and growth following surface film breakdown (after [23]).

FIGURE 11.10 Pit development and growth following surface film breakdown (after [23]).

Stable pit propagation is the result of the inability of the pit surface to repassivate. This is due to the aggressivity of the pit solution. As can be seen from Fig. 11.10, metal dissolution and cation hydrolysis lead to the formation of a local low pH environment; see Eqs. 11.13 and 11.14:

The aggressiveness of the environment will be determined by the pit depth and local metal dissolution current [24]. Oxygen depletion occurs inside the pit further reducing the lack of repassivation. The pitting mechanism is said to be autocatalytic with anodic dissolution, hydrolysis of metal ions, outward transport of metal ions, and inward transport of anions such as chloride. The processes by which this occurs include diffusion, convection, and ionic migration due to the potential gradient between pit and bulk solution. Where sulfur is present, as, for example, in the form of sulfide and oxysulfate oxysulfide inclusions, the pitting resistance of the metal is reduced [25], hence the steel-making trend toward that of low sulfur steels. In addition it has been reported that treatments that result in a redistribution of the sulfur at the surface of the metal, lead to an improvement in pitting resistance [26].

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