Electrochemical reactivity of trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate ionic liquid on glassy carbon and AZ31 magnesium alloy

The electrochemical behaviour of the ionic liquid trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate ([P6,6,6,14][(iC8)2PO2]) over different potential ranges, on both an inert substrate (glassy carbon), and AZ31 magnesium alloy has been investigated. On the glassy carbon electrode the ionic liquid exhibits an electrochemical window of 4 V in an argon environment. Anodic cycling of AZ31 in the ionic liquid resulted in a reduction in current density of over two orders of magnitude after one cycle and also stifled ionic liquid oxidation at approximately 2.0 V, indicating a passive response in the ionic liquid as a result of the deposition of a surface film. In contrast, cathodic cycling of AZ31 led to an increasing current density response with progressive cycles. The understanding of this electrochemical behaviour will allow for future optimisation of electrochemical parameters to obtain a robust and corrosion protective film on AZ31 alloy.

► AZ31 passivation in the IL occurs when the substrate is polarised anodically. ► Anodic passivation is believed to be a result of film formation. ► Film formation kinetics are slow. ► IL electrolyte oxidation (V > 2 V vs. Pt) is suppressed with water levels of 0.1 wt%, however in wet IL (6 wt%) severe oxidation occurs. ► [P6,6,6,14][(iC8)2PO2] IL may be suitable as an electrolyte for anodising AZ31.