Effect of ultrasonic impact treatment on microstructural evolution and stress corrosion cracking behavior of AISI 300 M steel in chloride environments

This study investigates the influence of ultrasonic impact treatment (UIT) on the microstructure evolution and corrosion behavior of 300 M steel in sodium chloride (NaCl) solutions with varying chloride ion (Cl - ) concentrations. UIT induced severe plastic deformation, forming a gradient microstructure in the near-surface layers approximately 60 μm thick with refined grains, increased dislocation density, and residual compressive stress. Surface roughness decreased by 43.5%, while microhardness increased by 9.8%. Electrochemical tests showed that UIT enhanced corrosion resistance in 3.5 wt% NaCl, as evidenced by a more positive corrosion potential, lower corrosion current density, and higher charge transfer resistance. However, this protective effect diminished at higher Cl - concentrations. Slow strain rate tensile tests revealed a transition from ductile to brittle fracture with increasing Cl - concentration, indicating increased stress corrosion cracking susceptibility. Finite element simulations demonstrated that Cl - accumulates at stress concentration zones, creating a “corrosion-stress” feedback loop. Molecular dynamics simulations confirmed that UIT induces extensive lattice distortion and high-density dislocations. In conclusion, UIT effectively improves the surface properties and corrosion resistance of 300 M steel in mild chloride environments but shows limitations under highly aggressive conditions. Combining UIT with complementary surface protection strategies is recommended for long-term service in severe environments.