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In this study, the influence of nanocrystalline structure on the plasma nitriding (PN) behavior of 17-4PH steel and corresponding corrosion mechanisms under different treatments were systematically investigated. The results showed that nanocrystalline structures introduced by ultrasonic surface rolling process (USRP) would affect the nitrides morphology, composition and distribution via accelerating element diffusion. The fabricated nanocrystalline structures could enhance the corrosion resistance of original and plasma-nitrided 17-4PH steel, which was attributed to its various microstructures. Homogeneous microstructures suppressed or alleviated the formation of micro-galvanic couple in USRP-treated 17-4PH steel and USRP-15 + PN specimen.