Abstract
The potential corrosivity of crude oils is a major concern for refineries. Plant experience has shown that current methods based on crude sulfur content and total acid number (TAN) do not reliably predict corrosion rates. In particular, a better understanding of the relative importance of sulfidic and naphthenic acid corrosion mechanisms, when both are present, is needed to better predict crude corrosivity. Previous work focused on the influence of organic acid structure on corrosivity. In this paper, the relative corrosivities of different types of sulfur species are explored. Four model sulfur compounds were chosen based on relative thermal stability of carbon-sulfur bonds, which increased in the order of 1-octanethiol < dioctyl sulfide < diphenyl sulfide < dibenzothiophene. Corrosion rates for these compounds in white oil were measured for 1018 carbon steel (UNS(1) G10180) coupons in a test unit that simulated a vacuum distillation tower. Test conditions were varied, including temperature, total sulfur content, and whether or not naphthenic acids were present. The results were consistent with a sulfidic corrosion mechanism that depends on the release of hydrogen sulfide (H2S) by thermolysis of the carbon-sulfur bonds. When naphthenic acids were present, there was clearly competition between H2S and naphthenic acids for metal surfaces. Corrosion rates of three types of stainless steel (UNS S41000, S30400, and S31600) were then compared to that of the 1018 carbon steel.
