Comparison of electrochemical corrosion performance of eutectic Al-Si automotive alloy in deep seawater and 3.5% NaCl solution

Abstract

The corrosion of aluminium silicon alloys is of vital importance, as they are frequently exposed to corrosive environments. The alloy is often exposed to seawater in marine applications, and despite having NaCl, the seawater contains some other ions. The purpose of this study is to compare the electrochemical corrosion performance of eutectic Al-Si alloys in seawater with 3.5% NaCl in order to understand the effects of those additional components, and an alloy with trace silicon is also considered for comparison. Electrochemical impedance spectroscopy (EIS) and Tafel polarization methods were implemented for the corrosion analysis. The results of these methods imply that the eutectic alloy is more resistant to corrosion in seawater than in NaCl solution. Some additional elements in seawater produce protective layers on the alloy surface, shielding the alloy from corrosion in seawater. The open circuit potential is shifted towards the nobler direction for the eutectic alloy than in the trace Si-added alloy. The eutectic alloy's corrosion rate and current (Icorr) are higher. Mg2Si precipitates in the eutectic alloy, forming oxide films of MgO and SiO2, protect the alloy from further corrosion. The alloys' surface morphologies were analyzed not only by optical microscope but also by scanning electron microscope. The photos depict reduced damage on the surfaces corroded in the seawater environment compared to the NaCl solution environment. The Si phases are discernable in these images. Additional demolishment and pit formations are identified on the surfaces of the eutectic alloy from SEM inspection. Fewer pits and damages are observed in the case of seawater environments. The EDS analysis shows a higher amount of oxygen for the alloy corroded in NaCl solution, supporting the phenomenon of more damage than in seawater.