(164f) Comparing the Corrosion Effects of Environment and Form On Various Magnesium Alloys
Improving gas mileage by reducing the weight of vehicles is currently a goal of the U.S. government. Lightweight metallic alloys, such as magnesium, are a favorable alternative to heavy steel parts, due to their low density, castability, machinability, and high mechanical stiffness [1-2]. However, these magnesium alloys must be able to withstand environmental exposure, which is a problem due to its high corrosion rate. The form of magnesium, as-cast versus extruded, plays a role in corrosion resistance. The as-cast material, which possesses a skin created by very small grains formed during alloy cooling, has a much higher corrosion resistance that the extruded material [3-4]. In addition, the addition of elements, such as aluminum, manganese, rare earth elements, and zinc, can reduce the corrosion rate [4-6]. While the addition of elements and the method of magnesium alloy formation both affect the corrosion rate, there currently is no understanding of how the corrosion mechanisms are affected by the various elemental additions.
In the Center for Advanced Vehicular Systems (CAVS) at Mississippi State University, investigating the relationship between environment, form, and additive elements on the corrosion characteristics is currently ongoing. Two environments, an immersion bath and a cyclical salt spray chamber, are used to examine the effects of exposure to chloride ions on the corrosion of as-cast and extruded magnesium. Three extruded alloys (AZ31, AZ61, AM30) and three as-cast alloys (AM60, AE44, AZ91) were exposed to the two environments using a 3.5% sodium chloride aqueous solution over 60 hrs. The differences in form and additive elements are being elucidated by examining the bulk surface characteristics, pit number density, pit area, and nearest neighbor distance, and the individual pit characteristics, pit depth, pit surface area, and pit volume. While the two environments have noticeable differences in pit number and pit size, the major differences in pit and surface characteristics are seen based on the additive elements and form [7-12]. The research presented here will cover the differences of the individual and bulk characteristics seen between the six magnesium alloys based on environment, form, and additive elements.
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