Optimization of Surface Layer Properties of Mg–9Li–1Zn Alloy by Ultrasonic Surface Rolling Process and its Impact on Corrosion Behavior

The Mg–9Li–1Zn (LZ91) alloy was subjected to an ultrasonic surface rolling process (USRP) with varying passes for the purpose of modifying its surface state. The USRP transformed surface residual stress from initial tensile stress to compressive stress, decreasing the surface roughness and increasing the ratio of the β-Li phase. The USRPed LZ91 sample (3 passes) showed superior corrosion resistance, with the corrosion current density changing from 57.11 to 24.70 μA cm⁻², and the polarization resistance increasing from 576.3 to 1146.1 Ω cm². According to the corrosion procedure evaluations, in situ observation revealed that the LZ91 alloy initially experiences pitting, which subsequently develops into cracking. The substantial area coverage of the β-Li phase facilitates the formation of a protective film on the surface, effectively delaying localized corrosion.