Effects of Hydrogen Sulfide on the Corrosion Behavior of High Strength Steel in Alkaline Solutions

The effects of hydrogen sulfide (H₂S) on the corrosion of high strength carbon steel have been studied due to their relevance to the oil and natural gas industries. Since H₂S behaves as an acid in aqueous solutions, the presence of dissolved H₂S results in a decrease in pH through dissociation into H⁺(aq) and HS^-(aq) ions. It is known that the effects of sulfide on iron oxidation dependent on the solution pH. For example, at very low pH the metal surface is free of corrosion products and resulting in high rates of active corrosion. In a pH region from 3 to 5, the addition of H₂S can result in an initial increase in corrosion rate that decreases over time as a protective iron sulfide layer forms and passivates the metal. Our goal was address the corrosion processes on high strength carbon steel due to H₂S at high pH values.     

In-situ electrochemical corrosion measurements were performed for high strength low alloy steel in 5 %wt. NaCl solution at 85 °C. The solution pH was tested from 7 and 12 to simulate conditions that may be present for drill pipes operating in deep-well environments. Exposure for each sample was at least 60 hours to allow for the observation of steady-state behavior. It was found  that the calculated corrosion rate dramatically increased from around 3 mm y^-1 or less to over 10 mm y^-1 as the solution pH fell below 9, which is an event that could occur if pH control is temporarily lost during gas or oil extraction operations.

The change in corrosion rate corresponded to an observed change in the electrochemical impedance spectroscopy (EIS) behavior, with a shift from two well-defined time constants at more alkaline conditions to what appeared to be a single low-frequency time constant at more neutral pH. Varying the rate of stirring in the test vessel did not appear to change the corrosion behavior across the pH range tested, suggesting that the corrosion process was not diffusion limited.

The results were correlated to previous work that found the mechanical failure mechanism to shift from stress corrosion cracking at alkaline conditions to hydrogen induced cracking over a similar pH range. This would suggest that changing the pH alters the nature of the corrosion process, possibly by changing the mechanism or the nature of the corrosion product film. Thermodynamic modeling has been performed to investigate the stable corrosion products and aqueous species at each condition. Pourbaix diagrams suggest that corrosion products shift from iron sulfides to iron oxides as the solution pH is increased.