An approximate treatment of the transient state in the oxidation of binary alloys forming the most-stable oxide. Part II: Single-phase alloys in systems with a solubility gap
- Universita de Genova (Italy); and others
The transient state in the oxidation of solid solutions of a most-noble component A in a most-reactive component B (beta phase) for binary A-B alloys presenting a limited reciprocal solubility of the two metals has been examined assuming that their B content is sufficient for the exclusive growth of external BO scales. Above a critical B content of the alloy the BO scales may grow directly on the surface of the beta phase, while below this limit they can only form over a layer of B-depleted solid solution of B in A (alpha phase), which appears after a critical time. An approximate analytical expression for the profile of concentration of B in the alloy has been adopted to calculate the flux of B in the metal. Moreover, the effect of the reaction between the gas and the external oxide surface has been taken into account using a procedure developed by Wagner for the oxidation of pure metals. The corrosion kinetics as well as all the parameters involved, such as the concentration of B at the alloy-scale interface, have been calculated as functions of the corrosion time. The results show that, as time proceeds, the instantaneous parabolic rate constant increases and the concentration of B at the alloy-scale interface decreases, both tending gradually to their corresponding steady-state values. It is also predicted that if the rate of transformation of the beta phase in the alpha phase after the alloy reaches the composition of equilibrium at the interface with the oxide is lower than the corresponding rate of displacement of the alloy surface due to scale growth, the corrosion of beta-phase alloys may eventually lead to the formation of two-phase mixtures of alpha phase and BO which are expected to grow at much larger rates than in the presence of a uniform continuous alpha layer.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 539371
- Journal Information:
- Oxidation of Metals, Journal Name: Oxidation of Metals Journal Issue: 5-6 Vol. 47; ISSN OXMEAF; ISSN 0030-770X
- Country of Publication:
- United States
- Language:
- English
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