Random Instability Induced by Active Dissolution

With the nonequilibrium fluctuation theory, pitting process in the active state was examined: in the active state, anodic dissolution yields random irregular surface inside a pit. This type of morphology is quite different from that of the polishing-state pit, which is composed of hemispherical round hollows. Consequently, it was concluded that pit diameter is, as well as the polishing-state pit, determined by the asymmetrical fluctuations which result from strong specific adsorption of anions. On the other hand, the irregularity inside the pit is created by the symmetrical fluctuations arising from the variation of the amount of adsorbed ions. That is, the aggressive anions form a complex with metal, which accelerates irregular dissolution. In the polishing-state pit, passive film hinders the formation of the complex, so that the symmetrical fluctuations do not occur. As a result, only hemispherical pits are yielded by the asymmetrical fluctuations.