The Role of Intermetallics on the Corrosion Initiation of Twin Roll Cast AZ31 Mg Alloy

The micro-galvanic coupling between the microconstituent phases and the α-Mg matrix in the twin roll cast AZ31 Mg alloy sheet of 6 mm thickness has been investigated using scanning Kelvin probe force microscopy (SKPFM) and electron microscopy. The β-Mg 17 (Al,Zn) 12 phase at the interdendritic spaces, along with the Al 8 Mn 5 particles with rosette/flower-shaped morphologies and Fe-particles have been observed in the surface/near-surface region of the alloy sheet. Volta potential differences exhibited by the microconstituents relative to the α-Mg matrix followed the order of Fe-particles > Al 8 Mn 5 intermetallics > β-Mg 17 (Al,Zn) 12 phase. The susceptibility to localized corrosion initiation by the intermetallics predicted from the SKPFM was confirmed by immersion testing conducted in naturally aerated 3.5% NaCl solution at ambient temperature.