Autoclave is employed to simulate the corrosion for J55 steel in CO_2 environments and to study its corrosion process. The
corrosion samples are wore in self-made wearing equipment to gain various
corrosion scales, and they are used to observe their surface and profiles with Scanning Electron Microscope (SEM), and their phase structures and chemical composition are analyzed by X-ray diffraction (XRD) and X-ray Energy Dispersive Spectrometry (EDS). The effect of
temperature or CO_2 pressure on the average thickness and grain size of the scales is studied in this paper. The results show that, the puce scales are double-layers in the cross-section, which is more bulky and more regular grains on the outer scale but finer grain scale alike dried slurry in inner scale. The major chemical composition of the scale is ferrous carbonate, and other iron oxides are not observed in the scale even gained at 180℃. And also a small quantity of cementite is found in inner scale besides the ferrous carbonate. The variation of average thickness and surface grain size of the scale for J55 steels with respect to the temperature reaches all the maximum values at 120℃, and the minimum at 160℃, but there is a little difference between twocurves. The variation with respect to the CO_2 pressure at 90℃ reaches all one maximum value at 6.89MPa, the other one at 3.45MPa for grain size and at 2.41MPa for scale thickness. There is a minimum value between two maximum, at 4.13MPa for surface grain size and at 3.45MPa for scale thickness, respectively. When CO_2 pressure is higher than 6.89MPa, grain size and scale thickness decrease rapidly.