Pitting mechanism of mild steel in marginally sour environments - Part Ⅱ: Pit initiation based on the oxidation of the chemisorbed iron sulfide layers

摘要

The role of trace concentrations of O2 (ca. 20 ppb(w)) relating to pitting in marginally sour environments was investigated in this work. Laboratory experiments were conducted with X65 mild steel specimens at pH 5.00 ? 0.01, 30 ?C, 0.97 bar CO2 and 0.04 mbar H2S with/without O2 for up to 7 days. Acquired corrosion rate, the composition of the corrosion product layer, and surface profile after the layer was removed were compared. As a result, only H2S chemisorption layer was formed without oxygen; this layer could be partially compromised when [O2]aq 3 ppb(w), leading to pitting initiation.