Characterization of sulfide layers formed in a refinery crude oil at 300°C on carbon steel, alloy steel and stainless steel

摘要

High temperature sulfidic corrosion has caused safety and reliability issues in refinery units for decades. Whereas sulfidation of steels in pure H_2S environment is well documented, it is not the case for more complex mixtures as those encountered in the refinery industry. This is all the more important that sulfided steels can also react with naphthenic acids during refinery processes. The long-term objective of this work is to understand the chemical mechanisms of the attack of iron sulfide layers by naphthenic acids. The first step reported here was to investigate the kinetics of sulfidation under refinery conditions, and to characterize, at the nanometer scale, the sulfide layers formed on different steels. The experiments were performed in an autoclave filled with a refinery high sulfur crude oil at 300°C on XC18 carbon steel (Fe-0.18%C), P5 alloy steel (Fe-5%Cr-0.5%Mo) and 304L stainless steel (Fe-18%Cr-8%Ni), which are commonly used in refinery units. Pure chromium was also studied as a reference material. The duration of the test ranged from a few hours to a few days. The kinetics of sulfidation was followed by mass loss and by X-ray Photoelectron Spectroscopy (XPS), after sample cleaning in order to reduce the surface contamination by crude oil. Mass loss results showed that a parabolic law can fit the kinetics of sulfidation of XC18 and P5 steels. The chemical composition of the outermost sulfide layers formed on the steels was characterized by XPS and the structure of the crystallized phases was investigated by X-Ray Diffraction (XRD) and Transmission Electronic Microscopy (TEM). XRD patterns showed the presence of pyrrhotite (Fe_(1-x)S) and iron oxide (Fe_3O_4) on XC18 and P5 steels. On 304L stainless steel, only the pyrrhotite phase was identified. By TEM, it was possible to estimate the size of iron sulfide crystals, of the order of a few hundreds of nanometers. For all alloys, the S 2p core level spectra (XPS) showed the presence of five sulfur species on the surface: sulfide, disulfide or mercaptan, elemental sulfur or thiophene, sulfite and sulfate, the last four species resulting from an oxidation in air during the sample transfer to the spectrometer. The Fe 2p core level examination (XPS) confirmed the presence of iron sulfide (Fe_(1-x)S) on the surface. On stainless steel, the presence of Ni_xS_y was revealed by XPS, while chromium sulfide was hardly detected, as with pure chromium samples. Iron and chromium oxides were also detected at the external surface and their presence is discussed.