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

This study examines the effect of hydrogen sulfide (H2S) on the transition from metastable to stable propagating pits using Type 316L (UNS S31603) steel in brines with Cl-concentrations of 10(-3) M to 2 x 10(-2) M at 75 degrees C and 150 degrees C. The potentiostatic tests were performed at discrete intervals of 50 mV from the recorded open-circuit potential (OCP) to 300 mV above the OCP. The results based on analysis of the chrono-amperometric transients suggests that the transition to stable pit growth is hindered by H2S saturation at both temperatures. The comparison of the experimental results with similar tests in an acidified control brine (without H2S saturation) indicates that the effect of H2S in the promotion of pit metastability is associated with the diffusion of H2S to the bottom of the cavity. This increases the net anodic current by irreversibly forming metallic sulfides, which, in turn, affect the pit's anolyte by reducing the metal salt concentration. The consequence of this effect is that small pits get activated but seldom reach stability, except when there is a high Cl-concentration within the brine.