Effect of pH on Microstructure and Properties of Ultrasonic-Assisted Electroless Ni–P Coatings on Titanium Alloy

Ni–P coatings with various pH values (4.5–5.4) were fabricated on Ti-6Al-4V titanium alloy by ultrasonic-assisted electroless deposition. The effect of pH on the deposition rate and phosphorus content was studied, as well as pH on the surface morphology, phase structure and microhardness of the deposited Ni–P coatings. The results showed that the increasing pH enhanced the deposition reaction, and ultrasonic cavitation and mass transfer effect improved the catalytic activity of the substrate surface and the reaction system. So the deposition rate was increased. But the higher pH decreased the stability of the bath and precipitated NiHPO3, resulting in the decrease of deposition rate. With the increase of pH, the phosphorus content decreased at first and then kept stable (17 ± 0.45 wt%), while the microhardness enhanced at first and then kept stable (605 ± 2HV). The ultrasonic-assisted electroless Ni–P coatings had the typical cauliflower-like morphology and microcrystalline structure characteristics.