Corrosion studies of high pressure die-cast Al-Si-Ni and Al-Si-Ni-Cu alloys

Corrosion behavior of high pressure die-cast Al-Si-Ni alloys (Al-3Si-1Ni (SN31) and Al-3Si-3Ni (SN33)) with and without copper (Cu) addition has been investigated. Corrosion behavior was assessed in 3.5 wt% NaCl by electrochemical and immersion corrosion techniques. The presence of single phase pure Si and binary Al3Ni phases were confirmed from the X-ray diffraction (XRD) and microstructural studies for SN31 and SN33 alloys. Interestingly, Cu addition (1 at.%) resulted in formation of Al3Ni2 phase along with a marginal decrease in Al3Ni fraction and did not produce any Cu containing phases. Potententiodynamic polarization studies revealed an increase in corrosion current density (icorr) value as the Ni content was increased. However, the addition of Cu to SN31 and SN33 alloys increased their corrosion resistance. Electrochemical impedance spectroscopy studies confirmed the higher charge transfer resistance (Rct) values for the Cu added alloys. Immersion corrosion test results revealed that, the corrosion rate of Cu added alloys were three times lower than SN31 and SN33 alloys. In the SN alloys, increase in corrosion rate is attributed to the increase of Al3Ni phase as the Ni content was increased. The added Cu dissolves in the Al alloys and also reduce the Al3Ni fraction thereby improved their corrosion resistance.