Electrochemical Crevice Corrosion Behaviors of Low-Pressure Steam Turbine Materials in the Simulated Boiler Water Added Chloride and Sulfate Ions

Abstract

Using a rotor material, 3.5NiCrMoV steel, and a blade material, 13Cr steel, for low-pressure (LP) steam turbines of thermal power plants, electrochemical crevice corrosion tests were conducted in the simulated AVT (All Volatile Treatment) boiler water added chloride and sulfate ions. The crevice corrosion behaviors as well as the films formed on the specimen surfaces inside crevices were investigated. The 3.5NiCrMoV steel and the 13Cr steel in the test water showed crevice corrosions in a general type and a pitting type, respectively. For both the two steels, however, passive films formed on the specimen surfaces inside crevices. It was found that the passive film formed on 3.5NiCrMoV steel was composed mainly of Fe₃O₄, while that on 13Cr steel was composed mainly of Cr-oxides and partly of inward Fe-oxides. Especially, it was confirmed that CrOOH and CrO₃ were concentrated in the outermost surface of the passive film formed on 13Cr steel.