Electrochemical growth of iron sulfide films in H₂S-saturated chloride media

The chemical degradation of carbon steel components in sulfur-containing environments is a major concern in oil and gas production. In a previous study, a direct current (DC) polarization technique was used to infer the corrosion mechanism of steel in aqueous chloride solutions containing H₂S at a temperature of 220°C. A mechanism involving the growth of a pyrrhotite scale on carbon steel by the outward migration of iron atoms and subsequent dissolution of the sulfide at the interface with the aqueous solution was proposed. The present study focuses on confirming whether this mechanism applies at lower temperatures, for example, in the 22 to 95°C temperature regime. These investigations have been carried out using alternating current (AC) impedance spectroscopy. The impedance results have clearly identified two processes in corrosion: a charge transfer process that occurs at the interface between the iron sulfide corrosion product and the metal and a transport process through the sulfide film. An analysis of the impedance curves indicates that the latter process is rate-limiting in agreement with earlier observations at high temperatures.