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.