Grain-Refined Microstructure and Hard Surface Layer Produced by SMRGT Process for Improved Corrosion Behavior of Mg-3Al-1Zn Alloy

To improve the surface properties and performance, a grain-refined surface layer was produced on AZ31B Mg alloy by a newly developed surface nano-crystallization and hardening process called surface mechanical rolling grinding treatment (SMRGT). The grain size refinement and minimal surface hardening were confirmed by the microstructure observations, XRD results and microhardness tests and were attributed to the multipass small strain-induced plastic deformation of the material during the SMRGT process. A nano-grain surface layer (average grain size of ~ 100 nm) and a graded microhardness variation (average ~ 113 HV adjacent to surface) along the thickness direction were generated. The E_corr values of the as-SMRGTed samples were − 1.43 ± 0.03 and − 1.42 ± 0.02 V, increasing by ~ 30-40 mV compared with the values of the as-received (AR) sample (− 1.46 ± 0.02 V), corresponding to i_corr ranging from 3.0 × 10⁻⁵ to 1.0 × 10⁻⁵ and 5.0 × 10⁻⁶ A/cm², respectively. Charge transfer resistance (R_ct) increased from 93.4 to 292.4 and 578.3 Ω (~ 3-6 times) with the increasing number of SMRGT passes. The corrosion resistance improvement was attributed mainly to the dramatic strain-induced surface grain refinement and minimal surface hardening that give rise to a more smooth and densely packed surface state.