The Influence of Fine Structure, Morphology and Composition of Alloy and Oxide on the Growth of Cr₂O₃ Scales

Abstract

The kinetics of morphological and structural development of Cr₂O₃ scales on Fe-Cr alloys, with and without yttrium additions, under isothermal conditions at 1000°C have been studied in an attempt to elucidate the growth processes. The steady state scale on the yttrium-free alloy is uniformly continuous and highly convoluted, with a porous, equiaxed grain structure. It is considered that such a configuration results from transport processes in the scale and the development of high compressive stresses, leading to lateral growth of the oxide. In particular, both inward movement of oxygen and outward movement of chromium occur, enabling new oxide to form within the bulk scale with accompanying oxide dilatation.

The presence of active elements, such as yttrium, in the alloy has a marked effect on the oxidation process. In particular, there are considerable changes in the microstructure and the adherence of the scale. This results in a large decrease of the amount of scale spalling upon cooling or thermal cycling. The influence of the active element is accounted for in terms of its effect on transport processes in the scale, i.e. the suppression of chromium diffusion. This enables oxygen to penetrate to the scale/metal interface where new oxide is formed without significant stress development.