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Abstract

It is generally agreed that differences in oxidant transport behaviour explain the development of single, duplex, and breakaway oxide scales. In view of the low lattice mobility of oxygen, it is necessary to postulate the transport of oxidant in molecular form through micropores in the scale. It is proposed that the oxidant has some degree of access to the oxide/metal interface in all types of scale and that volume constraints on the gaseous reactions at this site determine which type of scale forms. Single layers form when dislocation climb eliminates space created at this interface by outward metal diffusion. Factors which inhibit this volume collapse, such as alloying or sample corners, lead to duplex scales. However, the rate of reaction is still limited by the rate at which cation effusion creates the volume. Breakaway occurs when factors such as non-sealing interfaces caused by oxide carburisation overcome this constraint. The breakaway rate is then controlled by the rate the oxide can grow while deforming by creep. The breakaway oxidation rate is found to be given by (k_pk_c/2)^1/2, where k_p is the underlying parabolic oxidation rate and k_c is the oxide creep rate at its yield stress. The rates given by this expression are found to be in good agreement with experimental rates in high pressure CO₂.

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Criteria for formation of single layer,duplex,and breakaway scales on steels

Abstract

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