An effect of chemisorbing surface reaction poisons on the transition from internal to external oxidation
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I.
INTRODUCTION
B O T H steam and trace quantities of sulfur are effective additives for mitigating corrosion of iron and nickel based alloys in petrochemical environments. Past research has identified mechanisms for the beneficial effects of each of these additives in the absence of the other. Steam raises the P02 which promotes the thermodynamic stability of protective oxide scales on the alloy surfaces. 1'2'3 On the other hand, sulfur, in the absence of oxygen, chemisorbs on the alloy surfaces and blocks the surface sites required for the decomposition of the gas phase molecules responsible for corrosion. 4 When sulfur is considered in the presence of steam, however, it is not clear that the above mechanisms can apply simultaneously. In particular, if oxide scales separate the alloy from the environment, we might ask what benefits, if any, would result from sulfur or any other chemisorbing surface reaction poison. One possible effect which has not been considered is that sulfur promotes protective oxide scale formation by slowing down the decomposition of reactive molecules and allowing oxide scale forming elements in the alloy sufficient time to diffuse to the surface to react externally rather than internally. Prior to exploring the potential role of sulfur in promoting protective oxide scale formation in complex environments, we need to consider its potential role in promoting oxide scale formation in simpler model gas environments for which we can assume plausible surface reactions with straightforward surface reaction rate laws. The purpose of this paper is to establish a theoretical framework for treating the effect of a chemisorbing surface reaction poison on the transition from internal to external oxidation in just such a model gas phase environment. Our approach is to reconsider C. Wagner's analysis of the transition from internal to external oxidation, s modifying only his assumption that the oxygen concentration at the surface of the alloy immediately reaches equilibrium with the environment. GREGORY LUCKMAN, formerly with Exxon Research and Engineering Company, currently is with Olin Corporation, 91 Shelton Avenue, New Haven, CT 06511. RICHARD S. POLIZZOTrI (LF370) is with Exxon Research and Engineering Company, Corporate Research Science Labs, Annandale, NJ 08801. Manuscript submitted October 27, 1983.
METALLURGICALTRANSACTIONS A
II.
THE MATHEMATICAL MODEL
A. Assumptions of the Model We consider a binary alloy of reactive metal A and noble metal B in a gas with an oxygen partial pressure high enough to oxidize A but not B. In addition to oxygen, we assume that the gas contains a strongly chemisorbing species which can poison the surface reaction required for the uptake of oxygen into the alloy. That species is present at a chemical potential high enough for significant chemisorption but too low for bulk compound formation with A, B, or oxygen. We assume that surface coverage by oxygen chemisorption is insignificant compared with chemisorption of the poisoning element. We treat the oxidation p
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