An Electrochemical Investigation of the Pitting Corrosion of TZM Alloy in Chloride Solution

In this study, cyclic polarization (CP) measurements were conducted on the molybdenum-based titanium–zirconium–molybdenum (TZM) alloy in 3.5% NaCl solutions under varying pH conditions, and the results were compared with those of pure molybdenum. No passivity breakdown was observed during cyclic polarization in acidic and neutral chloride solutions. The surface film formed on the TZM, and pure Mo samples displayed a dual-layered structure, comprising an inner layer of p-type semiconductivity and an outer layer of n-type semiconductivity. The defect density of the n-type layer ranged from 7.5 × 1017 to 7.5 × 1019 cm−3, while the p-type layer had a carrier density ranging from 2 × 1018 to 9 × 1019 cm−3. The pure molybdenum samples demonstrated lower passive current densities, lower charge carrier densities, and higher impedance than the TZM alloy. The lower corrosion resistance of TZM alloy could be attributed to the higher dislocation density, which acted as short-circuit paths for Mo diffusion, and the presence of carbides that exhibited a microgalvanic effect. Overall, this study clarified that the localized corrosion reported in the literature was not due to the breakdown of the passive layer but may be linked to the heterogeneous microstructure.