Assessment of the Processes leading to Pit Nucleation

Abstract

The literature relating to the stages prior to and during pit initiation on iron and aluminium in aggressive environments, such as halides, has been reviewed by members of two groups which have approached this problem from differing viewpoints. There are certain similarities, and certain apparent differences, in the breakdown of the air-formed or passive films, which tend towards an insulating nature on aluminium and are semi-conducting on iron. Both systems show so-called induction periods and breakdown potentials, with characteristic potential or current surges before breakdown is achieved. Evidence is presented on the controversial issue regarding possible aggressive anion entry into passive films and its relation to passive film thinning and salt accumulation. Consideration is given to the current disagreements regarding the relative importance of local salt or anion accumulation, and pH reduction, as a means of initiating and stabilising pitting. The growing evidence of pre-existing flaw involvement in pit intiation is emphasised and related to the salt accumulation and pH reduction theories. In particular, further understanding is required regarding the various possible, initially highly localised, processes occurring at flaw bases, where pitting is initiated, and of transport processes in flaws. These include salt accumulation, complex formation, pH reduction, crack/heal processes and local heating. The need to use model flaws is emphasised and thefuture role of techniques such as scanning AES, XPS and STEM/EDAX analysis is indicated.