Analysis of pit stability in 316l stainless steel exposed to H2S saturated dilute chloride solutions above the critical pitting temperature
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abstract
This study examines the effect of H2S in the transition from metatstable to stable propagting pits, using 316L steel in brines with Cl- concentrations of 10-3 M to 210-2 M at 75C and 150C. The potentio-static tests were performed at discrete intervals of 50 mV each from the recorded open circuit potential up to 300 mV. Preliminary assessment of the results, based on the analysis of the chrono-amperometric transients, suggest that the transition to stable pit growth is hindered by the H2S saturation at both temperatures. The comparison of the experimental results with similar test in an acidified control brine (with no H2S saturation) indicate that the effect of the H2S in the promotion of pit meta stability is associated to the diffusion of the H 2S species to the pit cavity bottom, which increases the net anodic current by forming irreversible 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 higher Cl concentration within the brine. 2011 by NACE International.
