Pit growth process

Stable pit growth processes are not presented in this chapter. The key point is that conditions are established in which repassi-... 

Owing to the hydrolysis reaction, pit development is an autocatalytic process and often there is an induction period before pit growth attains... 

Figure 45. Semilog plot of the pit-growth current, J vs. V = 50 mV, [NiCU = 5 mol in 3, (NaCIJ = 1000 mol m 3. T=300 K, J0 is the current component shown in Eq. (Ill), which becomes unstable at the minimum state and Iq is the growth factor of the pits expressed by Eq. (112). (Reprinted from M. Asanuma and R. Aogaki, Nonequilibrium fluctuation theory on pitting dissolution. III. Experimental examinations on critical fluctuation and its growth process in nickel dissolution, J. Chem. Phys. 106,9944, 1997, Fig. 14. Copyright 1997, American Institute of Physics.)...

The resistance of many metals and alloys to corrosion depends critically upon the presence of a thin (10-1000 A [168]) passive surface film [169]. In "aggressive environments, this film may become damaged locally via several processes, e.g. surface stress effects (either flow-induced [170,171] or as a result of anion adsorption [168]), the impingement of small particles on the surface [169], spontaneous depassivation [169]. Retention of the protective film by the metal only results if repassivation of the unprotected area is feasible compared with pit growth. 

Localized corrosion usually involves change in pH and chloride ion concentration around corrosion pits. Therefore, potentiometric SECM tips can be used to study corrosion processes. When a pH microsensor was used as the SECM tip for the in situ measurements during localized corrosion of stainless steel, it was found that pH decreases at the pit initiation stage and increases during pit growth and repassivation (113). 

This model assumes the transport of both anions (oxygen ions) and cations (metal) ions or then-respective vacancies. The anions diffuse to the metal surface with a resultant thickening of the film. Cation diffusion cause film dissolution and metal vacancies are created at the metal/film interface. When the rate of production of metal vacancies in the metal exceeds their rate of migration, the vacancies pile up at the metal/film interface and result in void craters. This is the process of pit incubation. At a certain critical size, the passive film suffers local collapse and the incubation period ends. The collapsed site dissolves much faster than at any other place and leads to pit growth. 

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