In many cases degradation of a material initiates at its surface, including wear, corrosion, fretting, etc. Such deterioration / failure modes are hence surface properties sensitive. This study is one discrete effort towards the optimization of the surface microstructure for specific properties by understanding the fundamentally unknown ‘corrosion – grain size relationship’ for magnesium. There is a special need to understand this relationship as we outline in some detail within this study. Results showed that there was a significant variation in corrosion resistance with grain size, which is a key finding; however these trends were also strongly dependent upon the specific thermomechanical processing used to prepare the specimens.