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 leads to formation of a protective iron sulfide layer that decreases the general corrosion rate; however, a trace amount of H2S may only lead to 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 under-saturated 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. At the tested conditions no localized corrosion occurred for the same environmental conditions when H2S/CO2 ratio was above 100 ppm or when there was no H2S present.
Localized corrosion presents a threat to the integrity of mild steel pipelines and equipment in the oil and gas industry, given that it often proceeds at a much faster rate than uniform corrosion at the same conditions. It frequently results in failure, when the penetration depth exceeds far beyond the built in corrosion allowance, which is based on the predicted uniform corrosion rates. The lack of ability to predict localized corrosion and to detect it by using conventional corrosion monitoring methods makes a difficult situation even worse.
Since the presence of H2S in upstream oil and gas pipelines has been associated with an increased risk of localized corrosion1, a significant research effort has been focused on understanding H2S corrosion mechanisms. Mild steel corrosion in such environments has been investigated since the 1940s, and was mostly concentrated on uniform corrosion2-19. Rather few studies available in the open literature have addressed localized corrosion in H2S containing environments. There, most of the effort was on aqueous solutions that contain moderate to high amounts of H2S (pH2S = 0.01 MPa to 0.5 MPa) with temperatures ranging from 50°C to 150°C.20-24