(186i) Comparing the Corrosion Effects of Two Environments On as-Cast AM60 and AZ91

Martin, H. J., Center for Advanced Vehicular Systems, Mississippi State University

A desire to decrease the weight of automobiles has increased the appeal of lightweight metallic alloys, such as magnesium [1,2]. The low density, high mechanical stiffness, excellent castability, and easy machinability of magnesium make it desirable for use in intricate structural components [2]. Unfortunately, magnesium corrodes easily in the presence of saltwater, such as puddles containing de-icing salt, that relegate its usage to unexposed locations within the automobile [3-5]. It has been shown that the microstructure of the magnesium alloy has considerable influence over the corrosion rate [6-8]. The addition of various elements, including aluminum, manganese, and zinc, has been undertaken in an effort to decrease the corrosion rate of magnesium. The addition of up to 10% aluminum improves the corrosion resistance of magnesium through the formation of a finely divided but continuous β-phase, which acts as an anodic barrier [5,9-10]. Also, the presence of an as-cast skin, which consists of very small grains formed during alloy cooling, improves the corrosion resistance of the magnesium alloy 10 fold [7,9]. While the corrosion resistance is improved by the addition of aluminum, manganese, and zinc, an understanding of the corrosion mechanisms associated with these magnesium alloys is not available.

In the Center for Advanced Vehicular Systems (CAVS) at Mississippi State University, the reaction of chloride ions at the surface of the magnesium alloy to produce a pit surface is currently being investigated in an effort to determine the corrosion mechanisms. Two environments, an immersion bath and a cyclical salt spray chamber, are being examined to determine the effects of exposure to chloride ions on the corrosion of as-cast magnesium. Continuous exposure, in the form of the immersion bath, and cyclical exposure, where the coupons are exposed to salt spray, 100% humidity, and drying, have demonstrated differences in the number of pits present as well as the size of the pit based on the environment [5,11-12]. In addition to the two environments, two as-cast magnesium alloys, AM60, which contains approximately 6% aluminum and 0.25% manganese, and AZ91, which contains approximately 9% aluminum and 1% zinc, are being examined to determine the effect of various elements on the corrosion rate of magnesium. While the corrosion mechanisms has previously been examined using AE44 magnesium, there has been no examination of the corrosion mechanisms on as-cast AM60 or AZ91. The research presented here will cover the differences in corrosion mechanisms on as-cast AM60 and AZ91 when exposed to two different environments, immersion and cyclical salt spray.

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