Corrosion Resistance of Chromium-Coated Ferritic-Martensitic Steel in Supercritical Water

CNS-I (9Cr) and modified CNS-II (12Cr) were specifically designed for supercritical water reactors. Chromium (Cr) coatings with thicknesses of about 1 mu m were deposited on the CNS-I and modified CNS-II substrates by magnetron sputtering to improve their corrosion resistances. Corrosion behavior was investigated in supercritical water at 823 K and 25 MPa with an oxygen concentration of 200 ppb for 200, 400, 600, 800, and 1,000 h. The coated CNS-I and coated modified CNS-II showed negligible weight gains of 0.542 mg/dm(2) and -13.351 mg/dm(2), respectively, considerably lower than those of the corresponding bare samples (602.17 mg/dm(2) and 459.42 mg/dm(2), respectively) after 1,000 h of exposure. The remarkable improvement in corrosion resistance could be attributed to the formation of dense chromium oxide (Cr2O3) layers on the Cr coatings. Interestingly, the Cr-coated CNS-I consisting of 9 wt% Cr displayed an equally excellent corrosion resistance to the coated modified CNS-II consisting of 12 wt% Cr due to the better mechanical properties and machinability of CNS-I.