Characterization of the magnesium alloy AZ31 surface in the ionic liquid trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)amide

Patrick Howlett, Jim Efthimiadis, Penny Hale, G Van Riessen, Douglas Macfarlane, Maria Forsyth

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Abstract

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 ([P66614][NTf2]) 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  ([P66614][NTf2]) 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.
Original languageEnglish
Pages (from-to)C392 - C398
Number of pages7
JournalJournal of the Electrochemical Society
Volume157
Issue number11
DOIs
Publication statusPublished - 2010

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