Altered microstructure characteristics and enhanced corrosion resistance of UNS S32750 duplex stainless steel via ultrasonic surface rolling process

The UNS S32750 duplex stainless steel was treated by ultrasonic surface rolling process (USRP), and effects of the plastic deformation on microstructure evolution and corrosion behavior were investigated. Results demonstrated that grain refinement, a considerable increase of the fraction of low-angle grain boundaries (LAGBs, <10°), high density of dislocations and high surface compressive residual stress were produced by the USRP. In particular, the intensity of the evolution was inhomogeneous and it was higher for austenite than ferrite phase. What’s more, an evolution toward <001> of crystallographic orientation, a decrease in intensity of texture in α phase while increase in γ phase were found for the surface deformation layer. In addition, a small amount of deformation twins in γ phase were induced on account of the USRP treatment. Taking advantages of the microstructure evolution, the corrosion potential and critical pitting potential increased, while the passive current density and metastable pitting susceptibility decreased significantly for the USRP sample. The refined grains, high-density of dislocations and high surface compressive residual stress synergistically promoted the enrichment of Cr2O3 in the passive film on the deformation layer. The Cr/Fe ratio increased from 0.73 to 0.94 with the help of USRP, revealing a less defective and more stable passive film. Therefore, USRP is a promising and effective surface strengthening technique for DSS to improve the corrosion resistance in aggressive environments.