Impact of Deep Seawater on the Electrochemical Corrosion Performance of Hypoeutectic, Eutectic and Hypereutectic Al-Si Automotive Alloys

Corrosion of aluminum alloys is a significant concern for shipbuilders to ensure the longevity of ocean vessels and equipment. Hence, the study aims to evaluate the corrosion performance of Al-Si as automotive alloys with varying concentrations of Si in deep seawater using electrochemical impedance spectroscopy and potentiodynamic polarization techniques. The surface condition prior to corrosion, as well as after corrosion, is analyzed using optical and scanning electron microscopy. The entire test is performed at an ambient condition. The results demonstrate that the corrosion resistance improves in alloys with increasing amounts of Si near the eutectic level, after which the corrosion protection deteriorates. Higher Si concentrations show higher values for polarization resistance and open circuit potential. In contrast, corrosion current and corrosion rate values decrease with increasing Si concentration up to eutectic composition. Si assists the formation of SiO2 and MgO films through Mg2Si intermetallics along with the Al2O3 on the surfaces, which aids in corrosion resistance. Optical and scanning electron micrographs reveal the presence of films on the surfaces after corrosion, which exhibit pits and pinholes on the surfaces of the alloys with lower Si concentrations. Hypereutectic alloy is affected by corrosion of its pinhole-like defects. The corresponding EDS analysis also confirms the protective layer of different oxides on the surfaces.