Galvanic Corrosion between Mild Steel and Iron Sulfides in Acidic Solutions: Part I. Experimental Factors

With the increase in production of sour oil and gas fields, mitigation of production-related failures due to H 2 S corrosion of mild steel is a key challenge. In H 2 S environments, most failures occur due to localized corrosion originating from the galvanic coupling between mild steel and conductive iron sulfide corrosion products. However, the mechanism of the galvanic coupling between mild steel and iron sulfides and the effect of influential parameters, have not been studied yet. Here, we provide a systematic experimental investigation on the galvanic corrosion between mild steel and iron sulfides by examining the effect of the critical factors: iron sulfide type, cathode to anode surface area ratio, and salt concentration. Specifically, we focus on pyrite and pyrrhotite as the main corrosion products found in localized corrosion of mild steel in H 2 S environments. Our results show that the cathodic current obtained on pyrrhotite was an order of magnitude higher than that obtained on pyrite, leading to a higher galvanic current for coupled mild steel-pyrrhotite compared to coupled mild steel-pyrite. Moreover, our study reveals that the increase of cathode to anode surface area ratio and, to some extent, the increase of salt concentration, enhance the galvanic current for the coupled materials.