An AZ91D ingot in the as-cast condition was homogenized by solution treatment and then aged for various periods of time. The microstructures produced were studied in detail and the beta phase volume fraction was measured quantitatively. The Corrosion resistance of all the different microstructures was studied in 3.5% NaCl solution through weight loss measurement in constant immersion conditions and potentiodynamic polarization experiments. The corroded surfaces were analysed using SEM and XRD. The volume fraction of the beta phase was found to have a significant influence on the corrosion behaviour. The T4 condition improved the corrosion resistance of AZ91D alloy compared to the T6 heat treatment. The results support the idea of microgalvanic coupling between cathodic beta phase and anodic alpha matrix.