Localized Corrosion of Mild Steel in Marginally Sour Environments

Localized corrosion has been a challenge for the integrity of mild steel pipelines, specifically at operating conditions where a trace amount of H2S is present alongside CO2 at lower temperatures. The presence of H2S will lead to the formation of a protective iron sulfide layer that will decrease the general corrosion rate; however, a trace amount of H2S may only form a partially protective mackinawite layer that could result in localized corrosion. In the current study, mild steel specimens (API 5L X65) were exposed to a 1 wt% NaCl solution sparged at 0.096 MPa pCO2 and 15 ×10−6 MPa or less pH2S (≤150 ppm H2S/CO2). At pH 5.0 and 30°C the bulk solution was undersaturated with respect to iron sulfide—mackinawite and iron carbonate. At these marginally sour conditions, a H2S/CO2 threshold of approximately 100 ppm was deduced, below which localized corrosion happened. No localized corrosion occurred for the same environmental conditions when H2S/CO2 ratio was above 100 ppm or when there was no H2S present.