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9/27/04

9:55 PM

Page 2525

The Role of Microstructure in Localized Corrosion of Magnesium Alloys R.K. SINGH RAMAN The article presents new findings on the influence of microstructural changes on corrosion behavior of magnesium alloy AZ91 in chloride solution, with a particular attention to the role of the  phase (Mg17Al12) and the surrounding Al-rich- area. The as-cast alloy was subjected to solutionizing and aging heat treatments, in order to incorporate variation in morphology and distribution of the intermetallic  phase and the surrounding Al-rich- (also known as eutectic ). Although previous workers have ascribed the higher corrosion resistance of a fine-grained alloy to the formation of the finely distributed  phase, the present work suggests that it is the ratio of the  phase to the Al-rich- that governs the localized corrosion of the aged alloy. Corrosion characteristics were investigated by immersion and electrochemical tests. Surface microtopography, optical microscopy, and scanning electron microscopy (SEM) were employed to characterize the localized corrosion.

I. INTRODUCTION

THE high strength-to-weight ratio and low density of magnesium promise great potential of magnesium alloys as light construction materials for automotive applications,[1,2] which will benefit the large magnesium-mineral resources in Australia. A major stumbling block that restricts the commercial application of magnesium alloys is their poor corrosion resistance. New magnesium alloys have been investigated over the past decade for structural applications.[3] The present investigation of the role of alloy microstructure on the localized corrosion has been carried out using one of the common Mg alloys, AZ91 (i.e., Mg-9 wt pct Al-1 wt pct Zn), for which microstructural development is well characterized. The AZ91 has high melt fluidity, making it ideal for casting components of intricate shapes, and thus eliminating the requirement of welding or bolting. Early Mg-Al-Zn castings suffered severe corrosion in wet or moist conditions. Magnesium, like aluminum, quickly develops an oxide film on its surface, and should be quite resistant to corrosion. However, unlike the impervious barrier-type oxide that develops on aluminum, the oxide layer on magnesium is porous and non-protective. This “quasi-passivity” of Mg causes poor pitting resistance of both pure magnesium and its alloys.[4] Oxide films formed on magnesium also tend to dissolve in water or break down chemically in the environments of chloride, bromide, sulfate, nitrate, and chromate. Hence, although magnesium alloys have been tried in dry and chloride-free environments, their poor corrosion resistance in chloride environments has grossly limited their engineering application. The corrosion resistance of AZ91 is influenced by the microstructural features of the alloy.[5,6,7] The Mg-Al-Zn

R.K. SINGH RAMAN, Senior Research Fellow, is with the School of Physics and Materials Engineering, Monash University (Melbourne), Victoria, 3800, Australia. Contact e-mail: raman.singh@s