On the development of magnesium alloys with improved corrosion resistance

2017-02-02T02:37:34Z (GMT) by Sudholz, Aaron
The work presented herein represents a research endeavour undertaken by Aaron Südholz at Monash University for the degree of Doctor of Philosophy. The research program investigated the technological challenges of Mg alloys in regards to the inherent susceptibility of Mg metal to corrosion, examined in context of the improvement of the corrosion resistance of Mg. A broad range of atypical alloying additions were initially considered in conjunction with the commercial AZ91 alloy. Additions were found to influence the formation of the incumbent β-phase intermetallic when added in small concentrations and lead to the introduction of an additional intermetallic phase when added in excess of the respective solubility limit. This work aided the identification of elements which could be beneficial to the reduction of corrosion kinetics in AZ91 alloys. The investigation proceeded to model Mg alloys to identify individual effects of elements commonly applied in commercial Mg alloys such as; Al, Zn, Ce, La, Nd and Y. It was found that each of these elements enters the solid solution matrix below their respective solubility limit and each element forms an intermetallic phase rich in the alloying element when in excess of the solubility limit for that element. The respective intermetallic phases were found to be more noble than the parent Mg alloy, in all cases support enhanced levels of cathodic kinetics and supported reduced rates of dissolution than α-Mg when anodically polarised. This work concluded with the integration of the AZ91 Mg-based alloy with small amounts of rare earth elements in order to exploit the beneficial properties of RE elements in the solid solution of the α-matrix and its subsequent influence on the formation of the Mg17Al12 β-phase. As a result, this work prompts discussion of strategies in regards to improving the corrosion resistance of Mg alloys and these strategies are demonstrated in a number of new cast Mg alloys designed by the author. This work also presents and tests hypotheses which are generally applicable to Mg alloys with respect to corrosion. The information contained in this thesis is intended to fill the current knowledge gap of the corrosion behaviour of Mg alloys with previously unreported findings and introduce, through the analysis of these findings, strategies for improving the corrosion resistance of Mg alloys for future applications.