Gradient-nanostructured Alloy 600MA was fabricated via surface mechanical attrition treatment (SMAT) for 0–180 min. Pre-corrosion XRD confirms that SMAT with severe plastic deformation does not induce phase transformation of the γ-FCC Ni-Cr-Fe matrix, while progressive peak broadening verifies grain refinement and accumulated lattice strain, consistent with TEM-observed gradient nanolayers. Quantitative grain-boundary misorientation and mechanical-twinning information cannot be obtained solely from TEM-SAED. Pre-corrosion SEM-EDX reveals no obvious elemental segregation after SMAT. Corrosion behavior was evaluated in oxygen-saturated deionized water at 290 °C and 8.7 MPa for a single duration of 720 h. Raman spectroscopy detects crystalline corrosion products of NiCr₂O₄, though amorphous or disordered nanocrystalline oxides cannot be excluded. Under 720 h exposure, all SMAT-treated samples exhibit higher mass loss than the pristine alloy; the 120 min-SMAT sample shows moderate mass-loss among treated groups. Since defect-driven electrochemical activity and Cr diffusion were not directly measured, the competitive mechanism between defect-accelerated corrosion and Cr-enriched passivation is proposed based on experimental results and published literature. This work provides fundamental data for gradient-nanostructured nickel-based alloys under high-temperature pressurized-water conditions.