Doctor of Philosophy
Shoesmith, David W.
Owing to their excellent strength-to-weight ratio and low density, magnesium alloys have the potential to significantly reduce the weight of automobiles leading to decreased emissions and greater range for electrical vehicles. However, the practicality of magnesium alloys for automotive and aerospace applications is severely hindered by their poor corrosion resistance in aqueous environments. Despite intensive research effort, the underlying mechanism(s) responsible for this poor corrosion resistance remains elusive. Further complicating the situation is the presence of secondary microstructures which are necessary for desirable physical properties but lead to microgalvanic coupling which exacerbates the poor corrosion resistance of magnesium alloys. This work utilizes a combination of electrochemical and surface analytical techniques to investigate the role of microstructure in magnesium alloy corrosion. The role of magnesium as an alloying addition to another lightweight material, aluminum, has been investigated. A combination of bulk electrochemistry (ECORR and PDP), localized electrochemistry (SECM) and surface analysis (SEM) are utilized. Bulk electrochemistry reveals that greater amounts of magnesium, which is insoluble in aluminum, leads to activation of the matrix towards oxidation. Time dependent SECM tracking local cathodes over several days reveals very little difference on the cathodic kinetics as a function magnesium content. The effect of microstructure size and distribution of secondary microstructures present in an Mg AM50 alloy are investigated as a function of Clconcentration, electrolyte solvent, time and microstructure size and distribution using EIS. By fitting data with a physically justified equivalent electrical circuit values of RP are extracted. This shows that as conductivity of solution decreases the effect of microstructure size and distribution diminishes, trending toward unity in highly resistive electrolytes. Cathodic galvanostatic polarization of a Mg ZEK100 alloy is shown to exhibit cathodic current densities at anodic applied potentials. Surface analysis (XRD and SIMS) reveals that the pretreated sample is enriched with magnesium hydrides. Furthermore, the hydrides are ii shown to exist within and leading the common “filiform-like” corrosion morphology that is typical of magnesium alloys. Wedge casting is utilized to examine the effect of cooling rate on the corrosion performance of Mg ZEK100 alloy. A combination of bulk electrochemistry and surface analysis are utilized to show that a faster cooling rate leads to the initiation and propagation of filiform-like corrosion more quickly than slower cooling rates.
Binns, Wilfred J., "Corrosion Studies on Lightweight Automotive Alloys: The Effect of Microstructure and Fundamental Mechanisms" (2019). Electronic Thesis and Dissertation Repository. 6054.