Corrosion Resistance of Modified Heat-Treated 16Cr-4Ni Steel for Geothermal Steam Turbine Blades
Immersion corrosion tests and electrochemical corrosion tests were carried out in the simulated geothermal water on a modified heat-treated material of 16Cr-4Ni steel, which is used for geothermal steam turbine blades. Incidentally, the purpose of the modified heat treatment is for reducing the susceptibility of stress corrosion cracking (SCC) of the steel in the geothermal fluids. For comparison, a conventional heat-treated material of the 16Cr-4Ni steel was also used in the corrosion tests. In the immersion corrosion tests up to 4,000 hours, the modified heat-treated material showed lower corrosion rates than those of the conventional heat-treated one in the test water. In the electrochemical corrosion tests, the modified heat-treated material exhibited noble and stable corrosion potential behavior. The XPS analysis results exhibited that the passive films formed on the materials were consisted mainly of Cr-oxides (CrOOH, CrO3) and Fe-oxides (FeO, Fe2O3). Furthermore, the composition ratio of Cr-oxides in the outermost surface of the passive film formed on the modified heat-treated material was higher than that on the conventional heat-treated one. It was suggested that the better corrosion resistance of the modified heat-treated 16Cr-4Ni steel was contributed to the formation of the passive film with higher compositions of Cr-oxides.