The mutual effect of Al and Mg on the corrosion of model binary Mg-Al alloys (Mg from 0 to 100 wt%) in 3 wt% sodium chloride (NaCl) aqueous solutions was studied by atomic emission spectroelectrochemistry (AESEC). Mg2+ ions retard the formation of passive oxide on Al and alloys during spontaneous reaction but inhibit the cathodic dissolution of Al from Mg-Al alloys. The ratio between cathodically generated OH- and dissolved Al increases significantly with Mg2+ in the solution, suggesting the formation of an insoluble product. Al3+ in the solution does not influence the anodic current evolution on Mg but decreases the Mg dissolution rate, indicating that oxidized Mg stays in the insoluble form. The titration experiments and the x-ray diffraction (XRD) analysis confirm the co-precipitation of Al3+ and Mg2+. The rapidly formed semiconducting spinel is supposed to be responsible for the reduced concentrations of soluble Al and Mg and delayed in comparison to Mg-free solution passivation of Al surface. The interpretation is made that the key factor for the reactivity of Al-Mg binary alloys is the modified solution composition near the surface, which is controlled by the chemical composition of the alloy via the selective dissolution processes.

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