CHARACTERISTICS AND FORMATION MECHANISM OF CORROSION SCALES ON LOW--CHROMIUM X65 STEELS IN CO2 ENVIRONMENT

The corrosion rates of X65 steels with different Cr contents were measured in CO₂ environment under high temperature and high pressure condition. ESEM, EDS, XPS and SEM were employed to analyze the morphologies and characteristics of corrosion scales on the steels. The results show that the corrosion rate significantly decreased with increasing Cr content in the steels. An increase in Cr content< leads to a lower susceptibility to local corrosion. When the Cr content reaches up to 3%, the local corrosion can be eliminated. The competitive deposition of FeCO₃ and Cr(OH)₃ on low-chromium steels results in a multilayer structure of the scales. The scales on 1Cr-X65 and 3Cr-X65 steels have three-layer structure and scale on 5Cr-X65 steel has two-layer structure. Cr exists mainly as amorphous compound Cr(OH)₃ in specific layer of the corrosion scales on low-chromium X65 steels. The Cr(OH)₃ content of the scale increases remarkably with increasing Cr content in the steels. The high Cr(OH)₃ content improves the protection performance of the scales and the corrosion resistance of low-chromium X65 steels. The rate of general corrosion and susceptibility to local corrosion are mainly dependent on the formation of codeposition layer of FeCO₃ with Cr(OH)₃ on low-chromium steels.