Achieving excellent corrosion resistance properties of 7075 Al alloy via ultrasonic surface rolling treatment

The corrosion resistance of 7075 aluminum (Al) alloy treated by ultrasonic surface rolling process (USRP) in a chloride environment was studied in this work. The effects of different USRP passes on the surface state (surface roughness and residual stress), surface microstructure, and corrosion resistance of this alloy were investigated through microstructural characterization, stress relaxation, immersion testing and electrochemical measurement. The results revealed that all USRP-treated samples demonstrated significantly improved corrosion resistance. However, the main factors that led to this performance enhancement were not the same. After 3 USRP treatment passes, the reduction in surface roughness and the increase in surface compressive residual stress improved the corrosion resistance of the 7075 Al alloy. After 7 USRP passes, the larger number of grain boundaries caused by surface grain nanocrystallization led to the rapid enrichment of passive elements, which formed a dense passive film on the alloy surface. At the same time, the dissolution of precipitates and the disappearance of precipitation-free zones (PFZ) reduced the corrosion caused by anodic dissolution. Meanwhile, the primary mechanism that controlled the corrosion rate was transformed from the surface state to the surface microstructure, which further improved the corrosion resistance of the 7075 Al alloy.