Corrosion of Mg Alloys

Abstract

We review the state of the art of Mg alloy corrosion performance and progress towards a Mg alloy with a corrosion rate much less than the intrinsic Mg corrosion rate of 0.3 mm/y in a concentrated chloride solution, and summarise recent developments in the understanding of Mg corrosion. A critical examination of recent claims of Mg alloys with corrosion rates much less than that of pure Mg indicates that these claims were not consistent with the literature existing at that time. The only successful Mg alloy development strategy relates to Mg alloys that have produced more protective surface films on immersion in solution. Corrosion of Mg occurs at breaks in a thin (2–7 nm) MgO surface layer on top of which is a much thicker, porous Mg(OH)2 layer. Atmospheric corrosion has produced corrosion rates of Mg-Al alloys an order of magnitude lower than the intrinsic corrosion rate of Mg in a concentrated chloride solution of 0.3 mm/y. Corrosion rates for Mg alloys measured by electrochemical methods are typically lower than those measured by weight loss, often by orders of magnitude. Thus, weight loss measurements (or were appropriate hydrogen evolution measurement) should always be included to substantiate corrosion behaviour, and reliance should never be placed on only electrochemical measurements. Corrosion morphologies provide evidence that the steady state corrosion mechanism may be significantly different to the corrosion mechanism soon after specimen immersion in solution. Evaluation of the recent literature indicate that the only viable Mg corrosion model is that based on the uni-positive Mg+ ion. The formation energy of the uni-positive Mg+ has been recently evaluated. Thus, it is possible to provide the free energies for the critical reactions of the uni-positive Mg+ ion model. The Fe tolerance limit is very sensitive to small quantities of Si in the Mg alloy. The Fe impurity content can confound studies aimed to study other aspects of the Mg corrosion behaviour.