Sulfidation kinetics of industrial steels in a refinery crude oil at 300 °C: reactivity at the nanometer scale

High temperature sulfidic corrosion has caused safety and reliability issues in refinery units for decades. The objective of the work 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). The duration of the test ranged from a few hours to a few days. The combination of X-ray diffraction and transmission electronic microscopy showed the presence of pyrrhotite (Fe 1-x S) on XC18, PS and 304L samples. For all alloys, the S 2p core level decomposition (X-ray photoelectron spectroscopy) proved the presence offive sulfur species on the surface: sulfide (main species), disulfide, elemental sulfur, sulfite and sulfate. On 304L, the presence of chromium oxide and nickel sulfide was revealed by XPS, whereas chromium sulfide was hardly detected. The macroscopic kinetics of sulfidation was followed by mass loss. The results showed that a parabolic law can fit the kinetics of sulfidation of XC18 and P5 steels. The kinetics of sulfidation of the adherent sulfide layer formed on 304L steel, determined from XPS measurements, follows a logarithmic law.