β-Mg17Al12, η-Al8Mn5 and an α-magnesium phase have been synthesized from pure components by controlled solidification procedures. These phases have been studied using different electrochemical techniques including the scanning Kelvin probe (SKP). From the results, it was possible to determine the nobility and the rate of the cathodic reaction of the different phases. Measurements have also been made on an AZ91D Mg-alloy using scanning Kelvin probe force microscopy (SKPFM) and field emission gun scanning electron microscopy (FEG-SEM). The results show that the Volta potentials measured with the scanning Kelvin probe (SKP) on bulk intermetallics are comparable with those recorded with the SKPFM on the AZ91D alloy. It is shown that SKPFM provides information on the local nobility of the different intermetallic particles and phases on the submicron scale. Both the η-Al8Mn5 phase and the β-Mg17Al12 phase in AZ91D showed a more noble potential than the α-magnesium phase. It is also shown that the aluminium-rich coring along the grain boundaries results in measurable changes in the Volta potential. Finally, the role of the β-Mg17Al12 phase and the η-Al8Mn5 phase in the corrosion behaviour of AZ91D is discussed in term of local nobility, surface coverage of the cathode and the cathodic activity of the different phases.