Chapter 6 Oxidation of Alloys II: Internal Oxidation

When an alloy component is selectively oxidized but cannot reach the surface quickly enough to develop a scale, then internal oxidation results. Furthermore, an alloy that initially contains sufficient of the reacting metal to form a scale can become depleted in that component to the extent that internal oxidation commences. Under some circumstances, internal oxidation and external scaling can occur simultaneously. It is emphasized that “oxidation” means forming a compound of the reactive alloy solute metal, and the description given in the chapter applies to internal oxidation, carburization, nitridation, etc. Internal oxidation is the process in which a gas phase oxidant dissolves in an alloy and diffuses inward, reacting with a dilute solute metal to form dispersed precipitates of metal oxide or metal carbide, etc. This class of precipitation reactions is distinguished by its dependence on gas–solid interaction, and the formation of a reaction product zone adjacent to the alloy surface. A large percentage of high-temperature corrosion failures involve internal oxidation. The internally precipitated reaction products cause embrittlement and dilation of the alloy subsurface region, which can cause the affected zone to flake off. Both chromia and alumina formers can be attacked by internal oxidation, even when the alloys contain sufficient chromium or aluminium to sustain external scale growth according to Wagner's criterion. The chapter considers some experimental results with the aim of relating reaction morphologies to the phase diagrams that describe the phase assemblages encountered. The conditions under which these morphologies develop are then established, and the kinetics described using Wagner's diffusion analysis.