Improving the Corrosion Resistance of Magnesium-Aluminum Alloys Using Microstructural Design

Abstract

Magnesium-aluminum (Mg-Al) alloys are crucial lightweight materials with exceptional properties. However, unfortunately, they exhibit rather poor corrosion resistance, and, thus, they have attracted enormous research efforts and triggered a great deal of technological developments during the last decade. Despite the impressive progress in understanding the corrosion mechanism of Mg and Mg alloys, several shortcomings and bottlenecks still exist, preventing more widespread use of Mg alloys in various applications. Recent studies have shown that the corrosion properties of commercial Mg-Al alloys can be greatly improved by applying a semi-solid casting procedure. This promising method involves the rheocasting (RC) technique, in which the required slurry is prepared by the RheoMetal process, enabling the manufacturers to design the alloys’ microstructure with a rather high degree of freedom. A series of comprehensive accelerated corrosion tests in constant/cyclic relative humidity and temperature show that the alloys produced by the RC technique exhibit up to four times better corrosion resistance than their high pressure die-cast (HPDC) counterparts. In this study, the influence of alloy microstructure, in terms of the effect of all the microstructural constituents (α, β, η and casting pores), on the mechanisms of degradation of the RC and HPDC Mg-Al alloys in ambient atmosphere is discussed in detail. The results imply that it is (in principle) possible to use the component fabrication step (i.e., casting) to modify alloy microstructure so as to mitigate corrosion properties. The idea is generic and may also be applicable to other alloy systems.