Corrosion of magnesium alloys: the role of alloying

The demand for light-weighting in transport and consumer electronics has seen rapid growth in the commercial usage of magnesium (Mg). The major use of Mg is now in cast Mg products, as opposed to the use of Mg as an alloying element in other alloy systems and there is an emerging market of wrought Mg products and biomedical Mg components – such that the past two decades have seen a significant number of new Mg-alloys reported. None-the-less, the corrosion of Mg alloys continues to be a challenge facing engineers seeking weight reductions by deployment of Mg. Herein, authors review the influence of alloying on the corrosion of Mg-alloys, with particular emphasis on the underlying electrochemical kinetics that dictate the ultimate corrosion rate. Such a review focusing on the chemistry–corrosion link, both in depth and in a holistic approach, is lacking. As such the authors do not describe aspects such as high-temperature oxidation or cracking, but focus on delivering the state-of-the-art with regards to alloying influences on corrosion kinetics. It has been demonstrated that Mg itself will not be thermodynamically passive in environments of pH<11, regardless of the extent and type of alloying and hence corrosion kinetics require unique attention. Authors consolidate the presentation to include essentially all commercially available alloys and in excess of 350 custom alloys with wide variations in composition; in addition to reviewing the range of intermetallic compounds and impurities that form in such alloys systems. An update is also given regarding mechanistic advances and the role of grain size on corrosion of Mg. A wider understanding of the role of chemical effects upon corrosion of Mg is both timely and serves to highlight metallurgical approaches towards kinetically retarding the corrosion problem. The latter is of key relevance to next generation lightweight alloys and rational design of wrought Mg and bio-Mg.