ON PASSIVITY OF IRON-CHROMIUM ALLOYS .3. EFFECT OF POTENTIAL

The kinetics of growth of the primary passivating film and of the secondary film on an Fe-24% Cr alloy in H//2SO//4 have been studied over a potential range of several hundred millivolts above the activation potential. The data have permitted a direct evaluation of the effectiveness of an adsorbed film and of an oxide film in giving rise to passivity. It is concluded that an adsorbed film is responsible for passivity at the activation potential. In the potential region Ea less than E less than Ea plus 200 mv this same film is responsible for intiating passivity. However, the steady- state film is an oxide. Aboveea plus 200 mv the adsorbed film is not an effective passivator, and an oxide film is required to initiate and maintain the passive state. In the low-potential region (e less than Ea plus 200 mv) anodic dissolution during the passivation process occurs by the direct, activated transfer of the ions from the metal ino solution. The data also indicate that the rate of growth of both the primary and the secondary films is potential dependent.