The Stability of Pitting Dissolution of Metals in Aqueous Solution

The stability criteria for pitting dissolution of metals are examined on the basis of recent results. Pitting dissolution is stable if the local ion concentration buildup at pit sites exceeds a certain critical value, which for pits at noble potentials appears to correspond to the lowest ion concentration required for the transition from etching dissolution to brightening dissolution. The smallest pit size for stable pitting decreases with anodic potential, and hence the larger defect on the metal surface gives rise to the less noble pit initiation potential. The local p H at pit sites seems to contribute to the initiation of etching pits at less noble potentials, whereas the stability of brightening pits at noble potentials is determined by the local concentration of the aggressive anion. The metal dissolution rate less than a certain value (∼0.1 A · cm −2 ) does not produce sufficient ion buildup for the initiation of stable pits on the flat metal surface.