Electrochemical Behavior of A516 Carbon Steel in Solutions Containing Hydrogen Sulfide

The electrochemical behavior of A516 grade 70 (UNS K02700) carbon steel with a ferrite-pearlite structure in acidic chloride (Cl^–) solutions containing hydrogen sulfide (H₂S) at 25°C was investigated by electrochemical impedance spectroscopy (EIS). Results showed the impedance of A516 steel increased in an acidic Cl^– solution with H2S (compared to a solution without H₂S) at the open-circuit potential (OCP). The corrosion process for A516 steel in the H₂S-containing solution was in two stages. Polarization resistance (R_p) initially increased with time for immersion up to ~ 10 h but then decreased gradually. In the early stage, the impedance was characterized by one large semicircle in the Nyquist plot that increased with time for immersion up to 3 h. In the later stage, precipitation of a sulfide film modified the surface properties and led to variation in EIS data through a capacitive effect in the low-frequency region.