With the increase in producing sour oil and gas fields in the world, mitigation of production related failures due to H2S corrosion is a key challenge. In H2S environments, localized corrosion is the type of attack which contributes to the most failures in oilfields. The main cause of localized attack is the galvanic coupling between steel and iron sulfide corrosion products due to their electrical conductivity. However, the mechanism of the galvanic coupling between steel and iron sulfides and the effect of experimental parameters on it, have not been unraveled yet. The present study investigates the effect of three different experimental parameters: iron sulfide type, cathode to anode surface ratio, and salt concentration, on the galvanic coupling between steel and iron sulfides in acidic solutions. Pyrite and pyrrhotite were selected as iron sulfide specimens since these corrosion products have been mostly found when localized corrosion of mild steel was observed in sour environments. The results show that the cathodic current of pyrrhotite was an order of magnitude higher than the cathodic current of pyrite, leading to a higher galvanic current as well as a higher galvanic potential for coupled steel-pyrrhotite compared to coupled steel-pyrite. In addition, it was found that the increase of cathode to anode surface area ratio as well as the increase of salt concentration to some extent, increased the galvanic current for the coupled materials.
H2S corrosion, also known as sour corrosion, is one of the most researched types of metal degradation in oil and gas transmission pipelines requiring a wide range of environmental conditions and detailed surface analysis techniques. This is because localized or pitting corrosion is known to be the main type of corrosion failure in sour environments which caused 12% of all oilfield corrosion incidents according to a report from 1996.1 Therefore, control and reduction of this type of corrosion could prevent such failures in oil and gas industries, and significantly enhance asset integrity while reducing maintenance costs as well as eliminating environmental damage. The unpredictability of pitting corrosion in sour media is a complicated challenge in this area as several factors, such as the nature of the corrosion products and the contribution of galvanic coupling, play a role in this type of corrosion.2,3