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 degrees 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 similar 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.