Abstract
Localized corrosion has been reported to happen in sour environments via a number of mechanisms. Some of these mechanisms are well understood and described (elemental sulfur, oxygen ingress) while considerable uncertainties still remain on the influence of other environmental parameters (FeS polymorphs, organic acid and etc). This paper addresses the occurrence of localized corrosion at the top of the line in a condition that is typically described as “marginally sour”: CO2 environments with low H2S concentrations. Little has been published on this topic so far and the mechanisms involved are not well defined. This research presents a systematic study of the effect of low H2S concentrations on corrosion of an API 5L X65 carbon steel exposed to top of the line conditions. Special emphasis is given to the transition between sweet and sour corrosion and how it relates to localized corrosion. Experiments were performed for 7 days at 1 bar total pressure in a CO2/H2S environment. The H2S partial pressure was varied from 0 to 0.15 mbar at two different gas temperatures (40°C and 60°C). Corrosion rate measurements and surface analyses revealed extensive localized corrosion at H2S partial pressure below 0.08 mbar, being more severe at a gas temperature of 40°C than at 60°C. No localized corrosion was observed without H2S or at H2S partial pressure above 0.08 bar in the conditions tested. The occurrence of localized corrosion is speculated to be due to the formation of a non-homogenous corrosion product layer and to the unfavorable balance between the rates of protective layer formation and undermining by corrosion.
