The formation of subscales of varying compositions

Abstract

An earlier paper (Bastow, Whittle and Wood 1977) presented an analysis of the diffusion processes taking place in the surface scale when a binary alloy was oxidized and the resulting scale consisted of a solid solution of the oxides of the individual alloy components. However, it is shown here that if the diffusion of the oxidant into the alloy is significant then, in addition to the formation of a surface scale, a subscale consisting of precipitate particles of the solid-solution scale can form within the alloy. Unlike the classical cases of internal oxidation, the precipitated oxide is the same phase as the surface scale, although its composition is different and varies with depth below the surface. In addition, the fraction of precipitated subscale also varies with depth below the surface. Diffusion equations are established to describe these systems. The importance of the thermodynamic parameters, namely the relative stabilities of the two scale components, and the kinetic parameters, oxidant diffusivity in the alloy and overall growth rate of the scale, are demonstrated. The variation of the ratio of subscale/surface scale thickness with bulk alloy composition goes through a maximum at a value of the latter which depends on both the thermodynamics and kinetic parameters of the system, and which falls commonly around 0·1–0·2.