The role of oxygen on the stability of crevice corrosion

Crevice corrosion can initiate in a spontaneously active metal/electrolyte system even with some appreciable oxygen concentration in the crevice electrolyte, as indicated by the measured instantaneous milliampere current and active E-x=L potential values at the bottom of the crevice. However, for crevice corrosion to occur, the oxidant concentration has to be below a certain level, set by the metal/electrolyte system and the experimental conditions. Similarly, the oxidant concentration needs to be increased to some higher level to terminate an on-going crevice corrosion process. Reinitiation of crevice corrosion (on a passive crevice wall) is much more difficult than the original initiation. This is because the passive crevice wall provides a small ionic current which generates only a modest IR voltage that cannot shift the potential far enough on the crevice wall to reach the active region. Oxidant reduction at the anodic sites on the crevice wall resulted in a significant reduction in the ionic current, I, and hence in IR. This decrease in IR caused E-x=L to increase to a value in the passive region which resulted in passivation of the entire crevice wall and termination of the crevice corrosion process. Conversely, removal of the oxidant caused I to increase and E-x=L to shift negatively back to the active region resulting in reactivation of crevice corrosion. (C) 2002 The Electrochemical Society.