Corrosion by naphthenic acids and sulfur compounds has long plagued the refining industry, generating a large body of research and field literature. However, the corrosion mechanism is not well understood and the industry relies on empirical models. In the present study, a mechanistic model has been developed on the basis of extensive experimentation. The model incorporates surface chemical reactions, solid-state diffusion through growing corrosion product scales, and flow dynamics. It is built on evidence that demonstrates that both sulfidation and naphthenic acid corrosion proceed on the outer surface of an inner corrosion product scale. The controlling step in corrosion involving naphthenic acids and sulfur compounds is either the solid-state diffusion of iron through the inner scale or the rate of chemical reactions on it. The model has been validated by the corrosion by model compounds and crude oil fractions. Key issues on the modeling and corrosion mechanism will be discussed on the basis of experimental results.

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