Fretting Corrosion Behavior of Multilayer Structure on Nitrided 2.25Cr-1Mo Steel in 723 K Liquid Sodium

In this study, multilayer modified structures were fabricated on the surface of nuclear-grade 2.25Cr–1Mo steel via salt bath nitriding at different temperatures. Fretting corrosion tests were subsequently conducted in liquid sodium at 723 K. The results indicate that the multilayer structures formed by salt bath nitriding effectively enhance the cross-sectional hardness and improve the wear resistance of the substrate. However, after prolonged exposure to liquid sodium at 723 K, these multilayer structures undergo failure, primarily manifesting as cracking, spalling, and corrosion micropores. Material degradation of the nitrided steel is governed by the synergistic effects of tribological removal, chemical corrosion, and thermal acceleration. Notably, the QPQ 550 treatment, featuring a thinner compound layer, exhibited superior tribological performance during extended testing. This is attributed to the fact that while a higher salt bath nitriding temperature (QPQ 590) yields a thicker multilayer structure, it simultaneously induces premature failure—characterized by microcracking and the formation of corrosive channels—which ultimately compromises the wear performance.