Characterization of Surface Films Formed During Corrosion of a Pipeline Steel in H2S Environments

Abstract

In this work, the surface films formed on an X52 pipeline steel in H2S-containing environments with various pH values and H2S concentrations were characterized by surface analysis techniques and electrochemical impedance spectroscopy. A stoichiometric FeS film is formed during H2S corrosion of the steel. At low pH (e.g., 3.5) and low H2S concentration (e.g., 0.2 mmol/L), the film is primarily crystalline FeS. When the H2S concentration increases to 2 and 20 mmol/L, mackinawite is also formed. At high pH of 5.5 and low H2S concentration of 0.2 mmol/L, the film is amorphous FeS. With the increase in the H2S concentration to 2 and 20 mmol/L, the film changes to crystalline FeS and the mixture of crystalline FeS and mackinawite, respectively. In low-pH solution (pH 3.5), the compact, crystalline FeS is more protective for steel corrosion compared to mackinawite. As the H2S concentration increases, the corrosion is increased. At high pH of 5.5, when the H2S concentration is 0.2 mmol/L, the low corrosivity of the environment causes production of amorphous FeS only. As the H2S concentration is increased, a thick film is generated, reducing somewhat the steel corrosion.