Surface and interface segregation in the oxidation of metals

Abstract

For the protection of high-temperature alloys against corrosion, a slowly growing, dense, well-adherent scale must be formed. At elevated to high temperatures either chromia (Cr2O3) or alumina (Al2O3) can act as such a protective scale. Surface and interface phenomena in the nucleation, growth and adherence of oxide scales have been studied, mainly by AES; these phenomena are described and their mechanisms are discussed. Various positive effects on nucleation, growth and adherence are exerted by alloying with reactive elements (i.e. Ce, Y, La, Ti, Zr ...) in small concentrations. They are acting in their oxidized state, segregated on the metal surface in oxide nucleation and segregated in oxide grain boundaries in the oxide growth mechanism. Non-metal elements such as nitrogen and carbon can enhance Cr2O3 nucleation by intermediate co-segregation with chromium, in contrast sulphur impedes chromia formation. Segregation of sulphur to the surface of voids and cracks forming at the metal/oxide interface deteriorates the scale adherence. The detrimental sulphur can be scavenged in precipitates formed by the reactive elements (oxides and oxysulphides) and at their interfaces.