Influence of the β-phase morphology on the corrosion of the Mg alloy AZ91

The influence of the microstructure, particularly the morphology of the beta-phase, on the corrosion of Mg alloys has been studied using AZ91 as a model Mg alloy. The corrosion behaviour was characterized for five different types of microstructure produced by heat treatment of as-cast AZ91. The influence of microstructure can be understood from the interaction of the following three factors: (i) the surface films can be more or less effective in hindering corrosion and more or less effective in controlling the form of corrosion as uniform corrosion or localised corrosion, (ii) the second phase (the beta-phase in AZ911) can cause micro-galvanic acceleration of corrosion and (iii) the second phase can act as a corrosion barrier and hinder corrosion propagation in the matrix, if the second phase is in the form of a continuous network. It is expected that these factors are important for all multi-phase Mg alloys because all known second phases have corrosion potentials more positive than that of the a-phase. A particular example of the corrosion barrier effect is provided by the fine (alpha + beta) lamellar micro-constituent; when a p-phase plate nucleates this micro-constituent, the p-phase plate acts as a corrosion barrier. In contrast, nano-sized 0 precipitates, produced by aging, caused micro-galvanic corrosion acceleration of the adjacent a-phase. However, it is an important finding that the corrosion rate of the a-phase was decreased by the aging treatments that caused the precipitation of the nano-sized p particles. (c) 2008 Elsevier Ltd. All rights reserved.