Carbon steel is mostly used as the material for oil and gas pipelines because of its economic advantage over other steel alloys. Due to the structural demands of oil and gas exploration on pipeline materials and oil country tubular goods (OCTG), the microstructure of carbon steel is often adjusted through carefully designed heat treatments to achieve more desirable mechanical properties. While it is important to optimize the structural properties of carbon steel, it is also necessary to understand how the resulting microstructure affects corrosion and formation of corrosion product layers. The effect of ferritic/pearlitic microstructure on formation of iron sulfide was investigated in this study. This microstructure was chosen based on the relatively wide use of UNS G10180 steel in different oil and gas production applications.

More specifically, the impact of cementite, present in the pearlite microconstituent of the ferritic/pearlitic microstructure, on the corrosion behavior in sour environment was investigated by using two substrates: UNS G10180 and 99.9% pure Fe. These substrates were exposed to brine solutions saturated with 10% by volume of H2S in a mixture with nitrogen, at temperature of 30°C and pH of 5 and 6 for a period of 4 days. The morphologies of the FeS layers that developed after 1 day, 3 days and 4 days exposure were compared, and the changes in corrosion of each substrate were established from the monitored corrosion rates during these exposure periods. The corrosion product layers were analyzed using EDS and XRD in order to determine the composition of the corrosion product layers. Furthermore, the surface of the substrates was examined after removal of the corrosion product layers for possible localized attacks. SEM analysis of the corrosion product layers showed that the presence of cementite affected the morphology of the FeS layers formed on UNS G10180 with higher precipitation occurring in the pearlite region. The sharper decrease in the corrosion rates observed with UNS G10180 suggests that these layers became protective quicker than those that formed on the pure Fe substrate. The findings from this study were compared to the observation of other researchers when ferritic/pearlitic UNSG 10180 steel was exposed to CO2 environment.

Subject
Materials, Carbides, Precipitation (solids), Substrates, Corrosion rate, Microstructure, Iron sulfides, Iron, Corrosion products, Sodium chloride solution, Pearlite, Surface layers, Steel
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2020
Association for Materials Protection and Performance (AMPP)
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