Corrosion properties of mg alloy ze41 and its mitigation using ionic liquids

Magnesium (Mg) alloy ZE41 (Mg-Zn-RE-Zr), which is used extensively in the aerospace industry, possesses excellent mechanical properties albeit poor corrosion resistance. The first part of this work investigates the mechanism of corrosion, and the interaction between the grain boundary intermetallic phases, the zirconium (Zr)-rich regions within the grains and the bulk Mg rich matrix in both the as-cast and heat-treated conditions. The results of optical and scanning electron microscopy (SEM) together with potentiodynamic polarisation show the importance of the microstructure in the initiation and propagation of corrosion in an aqueous environment. The Zr-rich regions play a distinct role in the early stages of corrosion in this alloy. It is also shown by altering the Zr distribution by means of heat treatment, that the corrosion mechanism can be altered significantly. The second part of this work investigates the interaction of two different ionic liquids (ILs) with the surface of the ZE41 alloy. ILs based on trihexyltetradecylphosphonium (P6,6,6,14) coupled with either diphenylphosphate (DPP) or bis(trifluoromethanesulfonyl) amide (Tf2N) have been shown to react with Mg alloy surfaces, leading to the formation a surface film that can improve the corrosion resistance of the alloy. The interaction of the ILs with the ZE41 surface has been investigated by optical microscopy and SEM. Surface characterization has been performed using a variety of techniques, namely Raman Spectroscopy, Solid-state Nuclear Magnetic Resonance (NMR), Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS). The surface characterization and microscopy revealed the preferential interaction with the grain boundaries and grain boundary phases. Thus the morphology and microstructure of the Mg surface seems critical in determining the nature of the interaction with the IL. The effect of altering the IL interaction through activation of the surface by pre-treatment of the surface with sodium hydroxide (NaOH) or by the application of a potential bias during the iii treatment was explored. The biased treatment proved to alter the IL film deposition in order to produce a more homogeneous film. Characterization of the ZE41 alloy surface treated with the Tf2N IL indicates that this anion reacts to form a metal fluoride rich surface in addition to an organic component. The DPP IL reaction with the ZE41 appears to be via a combination of chemical adsorption as well as a chemical process on the metal surface. The corrosion resistance of the IL films formed on the ZE41 surface was investigated. It was observed that the regular DPP IL treatment produced a film which provided protection to the grain boundaries as well as the fine pitting within the grains, but failed to stop the progression associated with the Zr-rich regions in the centre of the grains. The biased DPP IL treatment produced a film which limited the corrosion associated with the Zr-rich regions, but had corrosion present along the grain boundaries. The Tf2N IL treatment produced a film which suppressed some of the grain boundary corrosion as well as some of the corrosion associated with the Zr-rich regions, however there was fine pitting on the surface of the grains. The work thus far shows that the use of IL treatments to form corrosion protective surface films is promising, however, much work is still needed to produce a truly uniform protective surface. [Selected publications are not attached to pdf]