Corrosion performance of high pressure die-cast Al-Si-Mg-Zn alloys in 3.5 wt% NaCl solution

Abstract

In the present investigation corrosion behavior of the Al-Si-Zn alloys with and without the addition of Mg was studied. Microstructures revealed the presence of eutectic Si and Zn phases at the grain boundaries and Mg addition led to the formation of Mg2Si phase. X-ray diffraction results clearly indicated the presence of pure Si, Mg2Si and α-Al phases. Corrosion behavior of all the alloys was investigated in 3.5 wt% NaCl solution using electrochemical and immersion corrosion techniques. Potentiodynamic polarization test results after 48 h of exposure indicated that, the corrosion resistance increased as the Zn content in the alloy was increased, while the addition of Mg decreased the corrosion resistance. Electrochemical impedance spectroscopy (EIS) results as a function of exposure time are also revealed a variation in the surface layer and charge transfer resistance values with and without Mg addition. Corrosion rates calculated from the immersion test further substantiated the electrochemical corrosion results. Corrosion morphologies after immersion test confirmed the dissolution of α-Al matrix and presence of cathodic Si phase at the grain boundaries. The presence of anodic Mg2Si phase in the Mg containing alloy could also be responsible for the decrease of corrosion resistance.