(179e) Corrosion Mechanisms Occurring On AM60 Mg Exposed to a 3.5% NaCl Aqueous Solution: Experimental Effects

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

The use of lightweight metallic alloys, such as cast magnesium, has recently gained traction in the automobile industry [1, 2]. The low density (1.74 g/cm3), high mechanical stiffness, excellent castability, and easy machinability make cast magnesium a strong candidate for structural components that are intricate and lightweight [2]. However, these benefits are greatly affected by a high corrosion rate when compared to aluminum or steel. In addition to a relatively high corrosion rate, magnesium has a high electrochemical potential, causing magnesium to corrode easily in the presence of seawater [3]. The high electrochemical potential of magnesium allows for coupon dissolution when magnesium is placed in a NaCl aqueous solution.

The high corrosion rate has relegated the structural use of magnesium to areas that are not exposed to the environment, including car seats and internal electronic boxes [4, 5]. Therefore, the use of cast magnesium as a structural material requires a detailed understanding of the metal's response to corrosion conditions. Corrosion mechanisms in cast magnesium have been linked to the microstructure, suggesting that the performance of magnesium in corrosive environments could be significantly improved [6-8]. In order to improve corrosion resistance, additional elements have been introduced in an effort to develop new magnesium alloys [4]. These new magnesium alloys must be rigorously evaluated to determine the change in galvanic corrosion within the alloy due to the introduction of additional elements [4].

In the Center for Advanced Vehicular Systems (CAVS) at Mississippi State University, the electrochemistry between a conductor, AM60 magnesium (Mg), and an electrolyte, sodium chloride, is being investigated. Three different mechanisms are responsible for the corrosion of magnesium, including pitting, intergranular corrosion, and general corrosion. The influence of aluminum (A: 5.5-6.5%) and manganese (M: 0.25% min) on the corrosion mechanisms is also being studied. Previous work using polished AM60 Mg demonstrated that pitting is initially the dominant corrosion mechanism [5, 9]. Following the initial exposure to the NaCl aqueous solution, changes in the surface resulted in changes to pit characteristics, including pit depth, pit area, and pit volume, likely the result of the formation of a protective Mg(OH)2 film [9]. This film was eventually degraded as the dominant corrosion mechanism changed from pitting to general [9]. Intergranular corrosion also contributed to changes in the pit characteristics. While the corrosion mechanisms of AM60 Mg have been previously studied, these results were gathered using highly polished AM60 Mg coupons. The corrosion effects of 3.5% NaCl on cast AM60 have not previously been studied. Two different methods of salt corrosion were investigated, including constant immersion and salt-spraying. The research presented will cover the effects of immersion and salt-spray on the corrosion mechanisms of AM60 Mg.

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