Microstructure and corrosion behaviour of a Mg-9Al alloy with minor alloying additions (Mn, Y, Nd, Ca and Sn) are evaluated. All of the additions form Al-rich intermetallics with varying amounts of Fe, while Ca and Sn are also incorporated into the eutectic β-phase (β-Mg₁₇Al₁₂). Special attention is given to the surface potential values of the microconstituents, which are related to the formation of microgalvanic couples. Scanning Kelvin Probe Force Microscopy (SKPFM) measurements revealed that all the alloying elements, except Sn, reduced the potential difference between the secondary phases and the matrix, which is the main reason for the improvement of the corrosion resistance of Mn-, Y- and Nd-modified alloys. The beneficial effect of Ca is additionally related to the microstructure refinement and an increased area fraction of the β-phase.