Study of the catalytic behavior of H₂S on the corrosion of high strength carbon steel in deaerated marine environments

Electrochemical investigations were performed for corrosion of high strength low alloy carbon steel in neutral and basic brine solutions at temperatures around 30 °C. Thermodynamic modeling at these conditions predicted that HS^- would be the dominant species for dissolved H₂S at basic pH and would increase in concentration at constant H₂S partial pressure with increasing pH. Undissociated H₂S was predicted to increase in concentration with increasing solution temperature, but was not expected to vary greatly with solution pH. Initial measurements using impedance spectroscopy after 60 hours of exposure found that while the impedance magnitude increased with decreasing sulfide concentration and increasing solution pH, the phase angle behavior was relatively unchanged. There was a significant shift in the high frequency phase angle when sulfide-free solution was tested. When the rate of stirring was decreased, LPR slopes for the cathodic polarization remained unchanged for sulfide-containing solutions but increased in slope for sulfide-free solution. Initial results suggest that the presence of dissolved H₂S may affect the cathodic reaction mechanism beyond simply reducing the solution pH.