Surface science approach of corrosion phenomena

Abstract

This article presents a brief review of data on adsorption, anodic dissolution and passivation of well-defined single crystal surfaces of metals (Ni, Cr) and alloys (stainless alloys), which exemplify the concept of surface science approach of corrosion phenomena. This approach aims at a better understanding of corrosion phenomena at the atomic or molecular level. In the first part, the effects of sulphur, adsorbed or segregated on the surface, on corrosion or passivation are described, including the sulphur-induced enhancement of dissolution and the blocking of passivation. It is shown how the conditions of stability of adsorbed sulphur monolayers can be predicted on thermodynamics grounds and this is illustrated by a potential-pH diagram for adsorbed sulphur on nickel in water at 25 °C. The next part of this paper deals with the structure and chemistry of thin oxide overlayers (passive films), with special emphasis on recent data on the atomic structure of passive films obtained by scanning tunneling microscopy (combined with surface chemical analysis by X-ray photoelectron spectroscopy). The major problems which are addressed are the crystallinity, the epitaxy and the nature of defects of passive films, which are key factors in the resistance to both general and localized corrosion. The recently observed structural modifications of passive films on stainless steels provided by aging in aqueous solution are indicated (crystallization and coalescence of Cr 2O 3 islands). Finally, an attempt to rationalize the role of alloyed elements in terms of passivity promoters (elements that enhance passivity) or dissolution moderators or blockers (elements that slow down the anodic dissolution rate) is described. The proposed model is based on a comparison, for different metals, of the tendency of the metal for oxygen adsorption (reflected in the heat of adsorption of oxygen) and the relative facility in disrupting metal-metal bonds (a necessary step in the transition betwen two-dimensional and three-dimensional oxide, leading to passivity.