Stability of rapidly quenched and hydrogenated Mg–Ni–Y and Mg–Cu–Y alloys in extreme alkaline medium

Amorphous-nanocrystalline Mg50Ni30Y20 and Mg63Ni30Y7 and amorphous Mg65Cu25Y10 alloys were produced by melt-spinning and characterized regarding their microstructure and thermal behaviour using XRD, TEM and DSC. Their electrochemical behaviour in the as-quenched state and after hydrogen charging at −25 mA/cm2 for up to 20 h was studied in electrolytes with pH 5–7 and 13, but mainly in a battery electrolyte: 6 M KOH with pH 14.8 by means of anodic and cathodic polarization measurements. In the as-quenched state, the highest alloys stability was observed at pH 13. At pH 14.8, gradual oxidation and dissolution of copper or nickel governs the anodic behaviour before a passive state is attained. The dissolution of nickel is much more inhibited than that of copper due to its lower tendency to form soluble oxidized ions and to a stabilizing effect of higher fractions of yttrium in the alloy on the passivation. By galvanostatic charging, the Mg65Cu25Y10 alloy shows the highest hydrogen absorption capacity followed by Mg50Ni30Y20 and Mg63Ni30Y7. During the charging process, the alloys exhibit a change in the surface state chemistry, i.e. an enrichment of nickel- or copper-rich species, causing preferential oxidation and dissolution during subsequent exposure under free corrosion and anodic conditions. Mg–Ni–Y alloys demonstrate a higher stability during this treatment in extreme alkaline medium. The reasons for this and consequences regarding the use as electrode materials are discussed in detail.