The use of X-ray photoelectron spectroscopy in corrosion science

Abstract

Most metals in commercial use are protected from reaction with an environment at ambient temperature by an oxide layer of ~5 nm in thickness. This layer may increase in thickness, by solid state diffusion, at high (>300°C) temperature or may dissolve and be replaced by soluble species by electrochemical reactions in aqueous environments. These are the twin processes of corrosion and the problem of preventing such destructive transformations had led to a longstanding interest by corrosion scientists in the thin film: and indeed in sub-monolayers. Prior to the introduction of the techniques of surface derived ion and electron spectroscopy the chemical analyses of films, which had known physical and electronic characteristics, were unavailable. This review attempts to specify the requirements of an analytical technique for thin films and examines the present role of XPS in this context. The areas covered will include elemental analysis, evaluation of valence state, non-destructive information, elemental distribution and finally the use of ion-etch depth profiling. Examples are drawn from corrosion problems in both aqueous and gaseous environments.