Anodic treatments in an ionic liquid for the corrosion protection of magnesium alloy AZ31
thesisposted on 17.02.2017, 04:03 by Latham, Julie-Anne
Recent work in our group found that applying a potential bias to magnesium alloys during immersion in ionic liquids (ILs) could improve the homogeneity and corrosion protection properties of the film (when immersed in 0.01 M NaCl). To gain an understanding of the effect of potential bias cyclic voltammetry was used to evaluate the behaviour of magnesium alloy AZ31 in the IL trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl) phosphinate ([P6,6,6,14][(iC8)2PO2]). The behaviour of the system was investigated using scan rates of 1, 10 and 100 mV/s, conditions of wet (6 wt% water) and dry (0.1 wt% water) IL, access to and absence of air and dilution of the IL in a solvent. These were all replicated on an inert glassy carbon (GC) substrate for comparison to AZ31. It was found that scanning the potential anodically resulted in anodic film growth under the “high field” mechanism of film growth. During film growth the IL was adsorbed into the oxide/hydroxide film to form a resistive film on AZ31. The presence of excess water in the system results in breakdown of the anion of the IL through hydrolysis. Ionic liquid films were formed on magnesium alloy AZ31 in ([P6,6,6,14][(iC8)2PO2]) ionic liquid using potentiostatic and galvanostatic anodising methods. We have found that galvanostatic and potetiostatic anodising in the IL is effective and can be used to control the dimensions of the film by varying the time of treatment and magnitude of potential. The films are comprised of both the anion and cation but the growth of the film is likely dominated by the anion whereas the cation is adsorbed. Film formation is still possible when the IL is diluted in a non-reactive solvent, but the morphology of the film is altered. In all cases, deposition under anodic bias was only effective over the α matrix-phase. Performing an acid pickling pre-treatment (mixture of nitric and phosphoric acid) coupled with galvanostatic anodising in the IL were effective in reducing the corrosion rate of AZ31 by a factor of 3 and reduced the anodic dissolution reaction by 1.5 orders of magnitude (vs. a blank ‘as-polished’ AZ31 surface). The principal outcomes of this research provide an improved understanding of the formation of IL films on AZ31 for corrosion protection.