DISSOLUTION AND REPASSIVATION KINETICS OF FE-CR ALLOYS IN PIT SOLUTIONS .1. EFFECT OF THE SURFACE SALT LAYER

A study of the dissolution and repassivation of an Fe-17.4 a/o Cr alloy in a pencil-type artificial pit simulating one-dimensional diffusion, has been carried out in a chloride-containing bulk solution. The attainment of saturation of dissolution products within the artificial pit was characterized by the formation of a salt layer on the alloy surface. A transport model has been used to calculate the decrease in concentration at the metal interface for currents below that required to maintain the salt layer. The concentration at the metal interface was calculated from the balance of electrochemical dissolution and diffusion in the localized environment. The dissolution rate of the alloy showed a maximum at a particular concentration and, both this concentration, and the rate, increased with potential. Passivation took place below a critical concentration. The potential and critical current for passivation increased with potential. The dissolution kinetics were affected by a remnant film that was present on the metal surf ace after the salt layer had dissolved. Its effects during active anodic dissolution slowly disappeared with time.