Preparation and Properties of Ni-P-TiO₂ Composite Coatings

Ni-P-TiO₂ composite coatings were prepared on surface of AZ31B magnesium alloy by adding TiO₂ sol (2-11 mL/L) to Ni-P bath. The corrosion behavior of the composite coating in 3.5% (mass fraction) NaCl solution was evaluated by electrochemical method. The micro morphology and microstructure of the composite coatings were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The microhardness of the composite coatings was tested by Vickers hardness tester. The results show that the size of cauliflower-like particles in microstruture decreased with raising the TiO₂ sol content, and the microhardness and corrosion resistance increased first and then decreased. When the TiO₂ sol content was 8 mL/L, the composite coating had a compact structure without obvious defects and cracks, and its microhardness and corrosion resistance hit the best values. The Ni-P-TiO₂ composite coatings had obvious pitting potential, indicating that the coating had formed a passivation layer in the corrosive medium, which provided a better protective effect for magnesium alloy.