(414f) Corrosion Mechanisms Occurring On AE44 Mg Exposed to a 3.5% NaCl Aqueous Solution: Experimental Effects

Among the lightweight metallic alloys, those of magnesium present a great interest in the aerospace and automobile industry due to their low density (1.74 g/cm3) and high mechanical stiffness. These benefits are contrasted by a high relative corrosion rate when compared to aluminum or steel. As such, corrosion has limited magnesium's use as a structural component in industry, restricting it to nonexposed environmental locations, such as seats of cars and internal electronic boxes [1, 2]. Furthermore, magnesium alloys have been recognized as metals that corrode easily in the presence of seawater, due to their electrochemical potential as illustrated in the galvanic series [3]. The high electrochemical potential of magnesium allows for coupon dissolution when magnesium is placed in a NaCl aqueous solution. Hence, new developments of Mg alloys need stringent evaluation of the local galvanic corrosion from elements introduced into the alloys with a focus on enhancing the corrosion resistance to pitting [1]. The effects of adding rare earth elements to magnesium die casting were reported more than ten years ago. However, the search for lightweight solutions in the automobile industry has created a renewed interest [4, 5].

In the Center for Advanced Vehicular Systems (CAVS) at Mississippi State University, the electrochemistry between a conductor, AE44 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: 4.7%) and rare earth elements (E: 2.56%) on the corrosion mechanisms is also being studied. Previous work using semi-polished and super-polished AE44 Mg demonstrated that pitting is initially the dominant corrosion mechanism [5, 6]. Following the initial exposure to the NaCl aqueous solution, changes in the surface resulted in changes to pit characteristics, as the dominant corrosion mechanism changed [6]. Initially, the dominant form of corrosion was pitting, followed by a change to intergranular corrosion [6]. General corrosion eventually became the dominant corrosion mechanism [6]. While the corrosion mechanisms of AE44 Mg have been previously studied, these results were gathered using polished AE44 Mg coupons with two different roughness. The corrosion effects of 3.5% NaCl on cast AE44 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 AE44 Mg.

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