Mg-alloys in water – hydrophilic ionic liquid mixtures: Is there a negative difference effect?

Commercial purity Mg and Mg-alloy AZ61 were exposed at open circuit potential and under anodic polarization in two ionic liquids containing 10 wt% of water, formed by acetate anion and methylpyrrolidinium cation, substituted with either aliphatic (ILA) or etheric (ILE) chain. Hydrogen evolution strongly increased under anodic polarization (negative difference effect, NDE) in ILA, but not ILE. In both electrolytes Mg(OH)2-film formed on Mg but not on AZ61. The NDE suppression did not correlate neither with the presence of carboxylate, nor with the absence of Mg(OH)2, indicating strong differences between magnesium-water interactions in IL-water mixtures and in inorganic aqueous solutions. •Negative difference effect (NDE) was studied in new H2O-ionic liquid (IL) mixtures.•NDE was detected on both pure Mg and AZ61alloy in aliphatic substituted MPyrr+OAc-.•NDE was suppressed on both alloys in etheric substituted MPyrr+OAc-.•Mg(OH)2 formed only on pure Mg in both etheric and aliphatic substituted MPyrr+OAc-.•NDE suppression in OAc- containing H2O-ionic liquid mixtures is governed by IL cation.