Four binary Mg-Ni ribbons with Ni content up to 18.3 atm% were rapidly solidified by melt-spinning. The microstructure changed from alpha -Mg plus intermetallic Mg2Ni to a fully amorphous structure with increasing Ni content. Both the hydrogen evolution test and anodic potentiodynamic polarization revealed that the corrosion resistance of the amorphous ribbons was superior to that of ribbons with alpha -Mg plus intermetallic structure in 0.01 M NaCl solution (adjusted to pH 12 by NaOH), The reduced dissolution rate is attributed to the formation of a fully amorphous structure so that no intermetallic Mg2Ni is present and all Ni atoms are embedded in the Mg matrix. The enhanced passivity is due to the incorporation of NiO in the surface of the passive film as a consequence of its p-type semiconducting properties, thus preventing the entry of aggressive CO32- and Cl- ions into the passive film.