Corrosion protective film is important to prevent CO2 corrosion. The formation behavior of corrosion protective film on C-Mn steel and low Cr bearing steels with martensitic microstructure was investigated in a CO2 environment at 60°C. Immersion tests were performed with test periods of 24, 48, 96 and 720 hours. At each of these periods the corrosion protective film was examined using a scanning electron microscope (SEM) and an X-ray diffraction analyzer. Quantitative analysis of the Cr element in the corrosion protective film was carried out by using energy dispersive X-ray analysis (EDX). Three measures were used to estimate the C-Mn and low Cr bearing steels' resistance to CO2 corrosion: (1)
weight loss in test specimens; (2) thickness of the corrosion protective film on the test specimens; and (3) Cr enrichment of the corrosion protective film. In C-Mn steels, it was found that J55 steel, with ferritic-
pearlitic microstructure, lost weight at higher rate than N80 steel, with martensitic microstructure, in the first 48 hours of immersion (the initial stage of corrosion). In contrast to N80 steel the rapid weight loss in
J55 steel resulted in the test solution reaching FeCO3's solubility earlier, which in turn led to FeCO3 film forming on J55 earlier than N80 steel. Therefore, J55 steel has better resistance to localized corrosion.
The low Cr bearing steels, 3CR and 5CR steels had better resistance to CO2 corrosion than 1CR steel. Weight loss in both 3CR and 5CR steels remained almost constant after 96 hours of immersion. Cr enrichment in the corrosion protective film on 5CR steel was greater than the other low Cr steels, with approximately 50 mass %. The corroded surface of the 5CR steel test specimens was smoother than that of 3CR steel test specimens.
Key words: CO2 corrosion, Localized corrosion, General corrosion, Cr bearing steel, Oil country tubular goods (OCTG), Ferritic-pearlitic steel, Martensitic steel