The electrochemical behaviour of amorphous Mg65Y10Cu25 ribbons in electrolytes with pH=8.4 and pH=13 is compared to that of the multiphase crystalline alloy and to pure magnesium. Alloying with Cu and Y leads to a remarkable reduction of the corrosion rates. The most protective anodic surface layers form on the single-phase amorphous alloy. The electrolyte-dependent layer formation mechanisms are discussed in detail. Upon cathodic polarisation, up to 34 wt.-% hydrogen is absorbed in the as-quenched amorphous alloy and transformation to a nanocrystalline structure is observed.