Novel ionic liquid corrosion-resistant coatings for magnesium

Magnesium and its alloys exhibit poor corrosion resistance thereby not allowing the full potential of these materials to be realized. To date the most common protection scheme for magnesium is a chromate conversion coating, and recent work has sought to replace chromate coatings with other less toxic materials and processes. Ionic liquids (IL) have been proposed as a novel method of forming a corrosion protective surface film. The work presented here reports an investigation of how a number of different ILs interact with the surface of pure magnesium (as opposed to an alloy). The ILs under investigation are a phosphonium (tris (hexyl) tetradecyl phosphonium) based organophosphinate (P 6,6,6,14 M 3PPh) and a phosphonium bistriflamide (P 6,6,6,14 (Tf) 2N). The resulting films were investigated by electrochemical techniques, namely D.C. Potentiodynamic Polarisation (PP) using a Micro-Capillary Electrochemical Cell, and A.C. Electrochemical Impedance Spectroscopy (EIS), where it was found that ILs can shift the corrosion potential (E corr) to more noble values and decrease the current density (i corr) by an order of magnitude. The coated magnesium specimens were also subjected to immersion corrosion tests and metallographic examination. The results of these measurements show that both ILs being investigated offer some degree of corrosion protection. The results of these tests are compared with the results obtained for magnesium alloy AZ31.