Influence of bimodal grain size distribution on the corrosion behavior of friction stir processed biodegradable AZ31 magnesium alloy

In the present study, AZ31 magnesium alloy sheets were processed by friction stir processing (FSP) to investigate the effect of the grain refinement and grain size distribution on the corrosion behavior. Grain refinement from a starting size of 16.4 ± 6.8 µm to 3.2 ± 1.2 µm was attained after FSP. Remarkably, bimodal grain size distribution was observed in the nugget zone with a combination of coarse (11.62 ± 8.4 µm) and fine grains (3.2 ± 1.2 µm). Due to the grain refinement, a slight improvement in the hardness was found in the nugget zone of FSPed AZ31. The bimodal grain size distribution in the stir zone showed pronounced influence on the corrosion rate of FSPed AZ31 as observed from the immersion and electrochemical tests. From the X-ray diffraction analysis, more amount of Mg(OH)2 was observed on FSPed AZ31 compared with the unprocessed AZ31. Polarization measurements demonstrated the higher corrosion current density for FSPed AZ31 (8.92 × 10−5A/cm2) compared with the unprocessed condition (2.90 × 10−5A/cm2) that can be attributed to the texture effect and large variations in the grain size which led to non-uniform galvanic intensities