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

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.