Evaluation of microstructural effects on corrosion behaviour of AZ91D magnesium alloy

Abstract

The effect of microconstituents on the corrosion and electrochemical behaviour of AZ91D alloy prepared by die-casting and ingot casting route has been investigated in 3.5% NaCl solution at pH 7.25. The experimental techniques used include constant immersion technique, in-situ corrosion monitoring, and potentiodynamic polarisation experiments. Surface examination and analytical studies were carried out using optical and scanning electron microscopy, EDX and XRD. The corrosion behaviour of microconstituents namely primary α, eutectic α and β phases was significantly different. Coring of aluminum showed influence on corrosion behaviour more significantly in ingot material. Areas with aluminium concentration less than about 8% were found to be prone to corrosion attack compared with either those with higher amount of aluminium or β phase. Die-cast material with smaller grain size and fine β phase offered marginally lower corrosion rate and better passivation compared with the ingot. In die-cast and ingot, hydrogen evolution took place preferentially on β phase. XRD pattern of non-corroded and corroded surface revealed the removal of β phase from alloy surface during corrosion. The corrosion products for ingot consisted of Mg(OH) 2 with small amounts β phase, magnesium-aluminium oxide and MgH 2 while for die-cast, the product showed a highly amorphous structure.