THE DISSOLUTION AND REPASSIVATION KINETICS OF FE-CR ALLOYS IN PIT SOLUTIONS

A study of the dissolution and repassivation of Fc-11.6 at% Cr, Fe-17. 4 at% Cr and Fe-24.3 at% Cr alloys in a pencil type artificial pit sim ulating one-dimensional diffusion has been carried out in chloride-con taining bulk solution. The attainment of saturation of dissolution pro ducts within the pit was characterized by the formation of a salt laye r on the alloy surface. A transport model has been used to calculate t he decrease in concentration at the metal interface for currents below that required to maintain the salt layer. The concentration at the me tal surface was calculated from the balance of electrochemical dissolu tion and diffusion in the localized environment. It was found that the dissolution rate of the alloy showed a maximum at a particular concen tration and that the rate increased with increasing chromium concentra tion in the alloy. Passivation took place when the concentration dropp ed below a critical concentration. The concentration at which the repa ssivation took place and hence, the tendency for repassivation was fou nd to increase with increasing chromium content in the alloy. It was a lso found that the dissolution kinetics were affected by a film that w as present on the metal surface after the salt layer had dissolved. It could be slowly removed or undermined by maintaining high anodic curr ents in the absence of a salt layer.