(692a) Comparing Corrosion Mechanisms On Extruded AZ61 and AZ31 Exposed to Immersion and Salt Spray Environments

Martin, H. J., Center for Advanced Vehicular Systems, Mississippi State University
Horstemeyer, M. F., Mississippi State University
Wang, P. T., Mississippi State University

Lightweight metallic alloys, such as magnesium, are currently being investigated for use as structural parts within the automotive industry because of excellent castability and easy machinability [1,2]. However, magnesium corrodes easily in the presence of seawater, as compared to aluminum or steel, relegating its use to areas that are not exposed to the environment [3-5]. In an effort to decrease the corrosion rate of magnesium, various alloys have been developed, utilizing varying weight percentages of aluminum and zinc, among others. It has been shown that the addition of aluminum, up to 10%, improves the corrosion resistance by creating a continuous, finely divided β-phase which acts as an anodic barrier [5-7]. While the corrosion resistance is improved with the addition of aluminum, there is currently little understanding of the corrosion mechanisms of these magnesium alloys containing various amounts of aluminum.

In the Center for Advanced Vehicular Systems (CAVS) at Mississippi State University, a conductor is being exposed to an electrolyte in an effort to determine the associated corrosion mechanisms. Two extruded magnesium alloys, AZ61, which contains approximately 6% aluminum and 1% zinc, and AZ31, which contains approximately 3% aluminum and 1% zinc, are being examined to determine the effect of the amounts of aluminum on the corrosion resistance of magnesium. In addition, two exposure environments, an immersion bath and a cyclical salt spray chamber, are being utilized to determine the environmental effects on the corrosion mechanisms. An immersion bath is used for continuous exposure to the aqueous sodium chloride solution, while a cyclical salt spray chamber is utilized for periodic exposure to the aqueous sodium chloride solution, 100% humidity, and drying. Previous work on an as-cast AE44 magnesium has demonstrated that pitting is initially the dominant corrosion mechanism for both environments, but that pit characteristics, such as pit number density, pit area, and nearest neighbor distance were more numerous following exposure to the immersion environment [5,8-9]. However, the data elucidating the corrosion effects has not been collected on extruded magnesium, nor magnesium containing zinc instead of rare earth metals. The research presented will cover the effects of an immersion environment and a cyclical salt spray environment on extruded AZ61 and AZ31 magnesium.

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[8] Alvarez, R.B.; Martin, H.J.; Horstemeyer, M.F.; Chandler, M.Q.; Williams, N.; Wang, P.T.; Ruiz, A.; Corros. Sci., 52 (2010) 1635-1648.

[9] Martin, H.J.; Horstemeyer, M.F.; Wang, P.T.; Corros. Sci. In Review.