Characterization of the Magnesium Alloy AZ31 Surface in the Ionic Liquid Trihexyl(tetradecyl)phosphonium Bis(trifluoromethanesulfonyl)amide

Commercially available magnesium alloy AZ31 is extensively used in structural engineering components although, like many magnesium-based materials, it suffers from poor corrosion resistance, particularly in marine environments, which limit wider application. Previously, the ionic liquid (IL) trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)amide was shown to improve the corrosion resistance of magnesium alloy AZ31 in humid environments and in the presence of chloride-containing aqueous environments. Here, we investigate the morphology and composition of the protective surface film that forms upon immersion of the Mg alloy in the IL, using grazing angle X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), time of flight–secondary-ion mass spectrometry (TOF-SIMS), solid-state NMR, and transmission electron microscopy (TEM). XRD indicates that an amorphous film is present on the surface subsequent to exposure to the IL, whereas XPS etching experiments indicate that the film is multilayered. The innermost layer is predominantly inorganic fluoride salts as well as native oxide/hydroxide surface species. TOF-SIMS spectra support these observations and indicate an outermost, thin, adherent layer of IL species. Multinuclear NMR spectroscopy confirms the presence of a multiphase composition as well as the presence of metal fluorides and complex organic species. The surface film appears to be of the order of 100 nm according to the TEM/energy-dispersive X-ray spectroscopy observations.