Effect of W on Corrosion Behavior of Low Alloy Steel in Wet/dry Environment

摘要

The chemical state of the tungsten (W) in the rust of low alloy steel was analyzed by EPMA and XPS, and the electrochemical behavior of rust was investigated by alternating current (AC) impedance method after wet/dry cyclic corrosion test with chlorides. The 1.0 mass percent W- bearing steel showed higher corrosion resistance than carbon steel in the corrosion test. EPMA and XPS showed that the tungsten existed as WO_4 compounds in the rust of W-bearing steel. A Fe-W binary potential-pH diagram was made to investigate the thermodynamic stability of Fe-W oxides, and this showed that WO_4 was stable in the rust, which confirmed the physical analysis. An AC impedance measurement was made to obtain the corrosion reaction resistance (R_t) of the rust, and it was found that R_t of W-bearing steel was much larger than that of carbon steel. The corrosion of the W-bearing steel is suppressed by (1) the insoluble FeWO_4 formed near the base metal suppressing the anodic reaction, (2) the WO_4~(2-) ions in the rust changing the selective permeability to prevent the transport of Cl~- ions.