Hydrogen Permeation and Corrosion Fatigue Crack Growth Rates of X65 Pipeline Steel Exposed to Acid Brines Containing Thiosulfate or Hydrogen Sulfide

Corrosion fatigue crack growth rates were obtained for X65 pipeline steel in acid brines containing thiosulfate (S₂O₃²⁻) or hydrogen sulfide (H₂S). Samples were exposed for 72 h at the open-circuit potential to allow bulk hydrogen charging. The corrosion fatigue crack growth rate increased with partial pressure of H₂S and correlated with the steady-state flux of hydrogen permeation during corrosion in the same solutions. The rate of hydrogen absorption increased with increasing S₂O₃²⁻ concentration to a maximum at 10⁻³ M S₂O₃²⁻, owing to a competition between increased surface concentration of H₂S from S₂O₃²⁻ reduction and increased rate of iron sulfide film formation. Corrosion fatigue behavior in S₂O₃²⁻-containing acidified brines is the same as in solutions with low partial pressures of H₂S, in accordance with predictions from the hydrogen permeation results. This suggests that, for corrosion fatigue studies, S₂O₃²⁻ solutions are possible candidates for replacement of H₂S gas, as long as the H₂S partial pressure of interest is low. Also, the expected concentration of hydrogen inside the metal specimen at the end of the exposure period correlates with the rate of corrosion fatigue crack growth, suggesting that hydrogen charged during the pre-charging period controls corrosion fatigue crack growth.