Iron sulphide formation and interaction with corrosion inhibitor in H2S-containing environments

Corrosion of carbon steel in H2S-containing environments poses serious challenges to operations in oilfield and geothermal applications. Carbon steels exposed to H2S-containing environments are often susceptible to pitting and uniform corrosion, and linked to the properties of iron sulphide (FeS) layers that forms at the corroding interface. This paper investigates the corrosion characteristics of carbon steel in H2S-containing environments focussing on the early stage kinetics of FeS formation and its interaction with corrosion inhibitors (CI). Experiments were conducted in 3.5 and 5.8 wt% NaCl solution at 80 °C in 10% H2S-90% CO2-containing environments. In-situ electrochemical responses were measured using the linear polarisation resistance (LPR) technique. This was combined with post-experiment surface analysis; scanning electron microscopy (SEM) and X-ray diffraction (XRD) to investigate the nature and morphology of formed FeS. 3D surface profilometry was used to investigate the pitting corrosion characteristics of carbon steel caused by FeS interaction with CI. The results show a fast kinetics of FeS formation by direct reduction of H2S(aq) with a distinct and easily identifiable morphology, and evidence of minimum level of ferrite dissolution. The early stages of FeS formation and its interaction with CI have a strong influence on the overall corrosion characteristics of carbon steel. A dose of 30 ppm of a commercial corrosion inhibitor (CI); salts of nitrogenous molecules with a fatty acid group, at the corrosion interface induced a competition for surface absorption between CI and emerging FeS. Over the 48 h duration of this test, the FeS formation process was delayed and the mechanisms of its formation quickly became the main driver of pitting corrosion. The amount of CI dosed in this study was not able to prevent FeS-induced pitting corrosion attack on the surface.