Improving the properties of 211Z Al alloy by enhanced electroless Ni-P-TiO2 nanocomposite coatings with TiO2 sol

Ni-P-TiO2 nanocomposite coatings on 211Z Al alloy substrate were prepared by electroless deposition through adding TiO2 sol into conventional Ni-P electroless plating solution, and the Ni-P coatings on 211Z Al alloy were also prepared as a comparison. The properties of 211Z Al alloy with Ni-P and Ni-P-TiO2 deposits in as-plated and heat-treated states were investigated. It was found that, after heat treatment at 400 °C for 1 h, the microhardness of 211Z Al alloy with Ni-P coatings increased from 608 HV to 929 HV, while that with Ni-P-TiO2 nanocomposite coatings greatly increased from 685 HV to 1325 HV at the same condition. The far higher improvement in the microhardness of heat-treated Ni-P-TiO2 nanocomposite coatings was attributed to the formation of more Ni3P hard phase, finer spherical nodular shaped grain size and the existence of more Ni atoms arranged along (111) close-packed planes. In the salty environment, 211Z Al alloy substrate with Ni-P-TiO2 coatings exhibited higher corrosion resistance than pure 211Z Al alloy substrate.