Considerations of the Stability of Pit Repassivation during Pitting Corrosion of Passive Metals

The repassivation of pits, by which pitting corrosion initiated in an aggressive environment ceases, is studied from the kinetic point of view. Consideration of irreversible thermodynamic stability suggests that the repassivation process starts as soon as the monolayer formation of the metal oxide becomes stable at the bottom of the pit. The critical pitting potential is defined so that the rate of the monolayer formation of the metal oxide overcomes the rate where the monolayer oxide changes to the halide. It is shown that the repassivation potential is very close to the critical pitting potential, whereas the pit initiation potential is intrinsically different and coincides only when the passive film is very thin. The present study presents the theoretical significance of the repassivation potential as a characteristic value of pitting.