Pitting Corrosion

In this chapter, the subject is pitting corrosion as a dangerous process causing deep localized attack in metal surfaces, including perforations of thin-walled structures. In this context, we do not consider trivial cases of localized corrosion caused by accidental defects in coatings. Instead, the matter of serious practical concern is unexpected pitting of metals passivated by oxide films. Attending to this type of pitting only, two other types of attack also resulting in pitting are disregarded: One is pitting of iron after prolonged exposure to acid solutions containing an adsorption inhibitor [2a]. The other is pitting of Gs [3a], which is interesting, as apparently a space charge layer in the semiconductor surface is now in the role of the oxide film on passive metals. The reason to concentrate on pitting of passive metals is its outstanding practical importance for safety considerations. In contrast to uniform corrosion, whose rate will normally be predictable from vast experience, pitting corrosion tends to proceed at an unexpected and high rate of localize metal dissolution. This major aspect of the matter is, at least in principle, easily understood considering that passive metals usually are at a corrosion potential, £_corr which by the presence of the oxide film is shifted far beyond the equilibrium potential of most of the metals alloys components, E_Me/Mez+. Whenever in pore of the film bare metal is exposed to the electrolytic solution, the overpotential, n_Me = E_corr − E_Me/MeZ+, available for the acceleration of anodic metal dissolution thus tends to be very high.