ANALYSIS OF PIT STABILITY IN 316L STAINLESS STEEL EXPOSED TO H_2S SATURATED DILUTE CHLORIDE SOLUTIONS ABOVE THE CRITICAL PITTING TEMPERATURE

摘要

This study examines the effect of H_2S in the transition from metatstable to stable propagting pits, using 316L steelin brines with Cl- concentrations of 10-3 M to 2x10-2 M at 75℃ and 150℃. The potentio-static tests wereperformed at discrete intervals of 50 mV each from the recorded open circuit potential up to 300 mV. Preliminaryassessment of the results, based on the analysis of the chrono-amperometric transients, suggest that thetransition to stable pit growth is hindered by the H_2S saturation at both temperatures. The comparison of theexperimental results with similar test in an acidified control brine (with no H_2S saturation) indicate that the effect ofthe H_2S in the promotion of pit meta stability is associated to the diffusion of the H_2S species to the pit cavitybottom, which increases the net anodic current by forming irreversible metallic sulfides, which in turn affect thepit’s anolyte by reducing the metal salt concentration. The consequence of this effect is that small pits getactivated but seldom reach stability except when there is a higher Cl- concentration within the brine.