Localized Corrosion: Passive Film Breakdown vs. Pit Growth Stability: Part III. A Unifying Set of Principal Parameters and Criteria for Pit Stabilization and Salt Film Formation

Abstract

The framework for pit stability established in Part II of this series is elaborated upon and expanded. The maximum pit dissolution current density, idiff,max, which depends on temperature and maximum potential at the pit surface in a given pit environment, is proposed and compared with the diffusion current density to determine the conditions for pit stability and salt film formation. For an open pit, the criterion for pit stability is that idiss,max must be greater than or equal to the critical diffusion current density required for maintaining a critical local environment for active dissolution, idiff,crit. The critical condition of idiss,max = idiff,crit is associated with three parameters that must be exceeded for stabilization of an open pit: critical temperature Tcrit, critical potential Ecrit and critical pit depth rcrit. Analogously, the criterion for salt film formation is idiss,max must further exceed the diffusion-limited current density ilim, and three parameters associated with salt film formation are: saturation temperature Tsat, saturation potential Esat and saturation pit depth rsat. Unifying criteria for control of pit growth by charge transfer or diffusion are proposed, which provide interrelationships for all principle parameters. The pit stability criteria for covered pits, such as metastable pits, are also described.