Surface Composition of Stainless Steel during Active Dissolution and Passivation

Anodic segregation of selected alloy constituents is found by ESCA analysis to occur as a result of active dissolution and during the passivation of stainless steel in deaerated acid. The resulting surface composition of the steel is radically different from the bulk and conditions seem to favour the formation of surface phases of intermetallic compounds. The resulting surface phases appear to control the kinetics of active dissolution and the composition of the passive film. In this respect we propose that in the more complex stainless steels passivation may be due to the formation of a duplex kinetic barrier consisting of an outer oxide based passive film and an inner intermetallic surface phase having a higher lattice binding energy than the bulk alloy. This paper will focus on the influence of anodically segregated Ni, Mo and N in a range of austenitic stainless steels exposed to deaerated HCl solutions.