In crude oil refineries, high temperature corrosion in nonaqueous phase, predominantly by carboxylic acids (also known as naphthenic acids in crude oil) and sulfidation, is an old problem and has been studied for almost a century. Despite the large body of laboratory study and field experience, the mechanism of corrosion by naphthenic acids and sulfidation is not fully understood so that models in the public domain are empirical rather than mechanistic. Previously, a protective inner iron oxide scale was found when both naphthenic acids and sulfur compounds were present in our prior corrosion study. In the current study, it is shown that the high-temperature corrosion by naphthenic acids and sulfidation depends on the solid state diffusion of iron through the inner scale. It is postulated that corrosion rates are governed by either chemical kinetics on the surface of the inner scale or self-diffusion of iron through the inner scale. A model was built to simulate this mechanism for corrosion and validated with experimental data from a flow through corrosion test.