The Influence of Fine Structure, Morphology and Composition of Alloy and Oxide on the Growth of Cr2O3 Scales
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 continuous and highly convoluted, with a porous, equiaxed grain structure. It is suggested that such a configuration results from the transport processes in the scale and from 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 dilation. 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 resulting in a considerable decrease in 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 the transport processes in thescale, i.e. the supression of chromium diffusion.
This enables oxygen to penetrate to the scale/metal interface where new oxide is formed without significant stress development.
KeywordsOxide Scale Alloy Surface Growth Stress High Compressive Stress Fracture Section
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