The mechanisms of corrosion of mild steel, and associated corrosion product formation, in high temperature sour environments are still largely unknown although they directly relate to pressing operating issues in the oil and gas industry. Previous studies have shown that, from 80°C to 200°C in an H2S only environment, magnetite forms as an inner layer while iron sulfides are found in the outer layer. Although magnetite is thermodynamically less stable than iron sulfide, it was always observed as a defined inner layer. In this work, experiments were conducted to investigate the formation mechanisms of magnetite and iron sulfide in a H2S environment at high temperature. The corrosion behavior of mild steel was first investigated in environments with and without H2S at 120°C, showing that magnetite is the dominant corrosion product layer in the initial stages of corrosion, due to a much faster kinetics of formation than iron sulfide (mackinawite). Magnetite is assumed to be responsible for the initial rapid decrease of the corrosion rate in this environment. In another experiment, the conversion of magnetite into mackinawite was investigated by exposing a preformed magnetite layer on an inert steel substrate (nickel) to an H2S environment. Consequently, it is postulated that Fe3O4 experiences a simultaneous and continuous process of formation at the steel/magnetite interface and conversion to mackinawite at the magnetite/mackinawite interface. A descriptive model for the formation mechanisms of magnetite and iron sulfide at high temperature is presented.
Persistent energy demand moves the exploration and production of hydrocarbons towards ever deeper and harsher reservoirs, both onshore and offshore. These wells are frequently operated under high temperature and high pressure conditions in the presence of H2S.1-4 As a result, these operating environments present a constant challenge for new developments in materials selection, design technology, corrosion management, and corrosion modeling in the oil and gas industry.5-8