THE FORMATION BEHAVIOR OF CORROSION PROTECTIVE FILMS OF LOW CR BEARING STEELS IN CO_2 ENVIRONMENTS

摘要

Corrosion protective film is important to prevent CO_2 corrosion. The formation behavior of corrosion protective film on C-Mn steel and low Cr bearing steels with martensitic microstructure was investigated in a CO_2 environment at 60 deg 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 CO_2 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 FeCO_3's solubility earlier, which in turn led to FeCO_3 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 CO_2 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 percent. The corroded surface of the 5CR steel test specimens was smoother than that of 3CR steel test specimens.