Analysis of Pit Stability in 316L Stainless Steel Exposed to H2S Saturated Dilute Chloride Solutions Above the Critical Pitting Temperature

The effect of H₂S in the transition from metastable to stable propagating pits is studied for 316L steel using solutions with chloride contents between 10^-3 M to 10^-2 M. By means of potentiostatic tests done at discrete over-potentials above the open circuit potential (OCP) at 50 mV intervals up to 500 mV. The experiments were performed at 75ºC and 150ºC with H₂S and N₂ saturation. Results indicate that the transition to stable pit growth is hindered by the H₂S saturation at both temperatures. Preliminary assessment of the results based on the analysis of the chrono amperometric transients suggests that this effect can be related to the availability of H₂S within the pit cavity and increase the net anodic current by the formation of metallic sulfides providing an alternate source of H⁺ ions for the acidification of the pit anolyte. However, since sulfide formation is irreversible, the H₂S reduces the availability of metal cations within the pit anolyte dissolving the equilibrium layer of metal chloride salt critical for pit stability. In this context, pit repassivation can be associated with the reduction in the concentration of H₂S at the pit bottom due to diffusion control.