The effect of microstructure and Cr content in steels on CO2 corrosion was investigated by using steels containing Cr content from0 to 13 mass% melted in laboratory and Steels J55, N80 and L80(AP1 Grade) melted in the mill. Temperatures and H2S contamination were considered as environmental factor. In CO2 environments, the temperature giving a maximum corrosion rate, Tmax. existed in carbon and Cr steels. Tmax. increased together with Cr content, and Tmax. of 0, 1, 2 and 13% Cr steels was about 80, 100, 120 and 225 °C, respect ively. Because of this behavior, the relationship between Cr content and corrosion rate was linear at 60°C, but the corrosion rate was highest on the steel with around 1mass70 Cr at 100°C. H2S contamination for CO2 corrosion suppressed the corrosion rate and localized-corrosion in the temperature region whose corrosion rate showed a maximum value. It was clarified that this was related to the formation of Fe-sulfides from EPMA analysis and the volubility of the corrosion products. Concerning microstructure, Steel J55 with ferritic-pearlitic microstructure showed good corrosion resistance for localized-corrosion compared with Steel N80 and L80 with martensitic microstructure.
Keywords: Martcnsitic steel, Ferritic-pcarlitic steel, Mesa corrosion, Ringworm corrosion, Carbon dioxide(CO2), Hydrogen sulfide(H2S), Oil country tubular goods(OCTG)