The aim of this work is to contribute to the understanding of the effects of crystalline structures on the hardness and corrosion characteristics of aluminium–magnesium alloys. So, a series of nominal Al–4, 16, 40 and 50 wt% Mg compositions was rapidly cooled at ambient temperature by a vacuum high-frequency induction magnetic melting process and characterized by means of x-ray diffraction and optical observation analyses as well as Vickers indentation testing and electrochemical measures. Measured hardness and corrosion characteristics were compared with that of pure aluminium and industrial steel sheets. It was found that the crystalline microstructures consist of the intermetallic compounds fcc β-Al3Mg2 and sc γ-Al12Mg17 phases in equilibrium with the solid solution fcc α-Al of the matrix. Surface morphologies exhibit fine textures for the lower Mg contents and dendritic–eutectic microstructures for the higher contents. Observed combined high microhardness strengthening and corrosion resistance of aluminium matrix by magnesium contents are essentially due to the hardening cfc β-Al3Mg2 phase precipitation in the binary Al–Mg alloys.