<p>This study examines the cavitation erosion (CE) behavior and cavitation erosion–corrosion interactions of high nitrogen austenitic steel (HNS) in distilled water (DW) and 3.5&#xa0;wt.% NaCl solution. The alloy was evaluated in the as-received (AR) hot-rolled state and after solution annealing (SA) at 1100&#xa0;°C for 2&#xa0;h. Microstructural characterization (SEM/EBSD) showed that the AR condition possessed finer grains and deformation twins, resulting in higher hardness, whereas the SA condition exhibited grain coarsening and reduced residual strain, leading to a lower hardness. EBSD analysis further revealed that solution annealing increased the fraction of coincidence site lattice (CSL) boundaries, particularly Σ3 twin boundaries, which are known to exhibit lower interfacial energy and enhanced resistance to localized electrochemical attack. Cavitation testing demonstrated that hardness played a decisive role in cavitation resistance: The higher hardness AR steel consistently showed lower mass loss and slower pit-to-crack evolution compared to the softer SA steel. In DW, damage progressed predominantly through mechanically driven impact fatigue, whereas in NaCl solution, chloride-induced passive film breakdown promoted localized dissolution and accelerated crack propagation. Mechanical stresses break the passive film, while Cl⁻ ions hinder its regeneration, accelerating localized damage, confirming a strong cavitation–corrosion synergistic effect. Although solution annealing improved electrochemical corrosion resistance due to microstructural homogenization and increased CSL boundary fraction, the associated reduction in hardness compromised cavitation resistance, making the AR condition more effective against cavitation-dominated degradation. Overall, the results show that mechanical hardness is the primary factor controlling cavitation erosion resistance, while the presence of chloride ions substantially intensifies material loss through synergistic corrosion-enhanced mechanical damage.</p>