Effect of ultrasonication on Ni-Mo coatings produced by DC electroformation

The effect of the ultrasound-assisted method on Ni-Mo alloy coatings by direct current (DC) electroformation in a sulphate-citrate bath was investigated with different ultrasound powers. This study focused on the microstructure, molybdenum content, grain size, roughness, micro-hardness and corrosion resistance of the Ni-Mo alloy coatings, which were influenced by the ultrasound parameters. X-ray diffraction results showed that ultrasound modification decreased the grain size of the nanocrystalline alloy coatings. The hardness of the as-plated coatings after ultrasonication was determined by Vickers diamond indentation tests and compared with that of the Ni-Mo coatings in the absence of ultrasound. As a result, the hardness (719 HV) of the former coatings improved significantly. Scanning electron microscopy and atomic force microscopy images showed a smoother coating and a decrease in the surface roughness of the electrodeposited film, as a result of the use of ultrasound. Furthermore, the corrosion resistance behaviour of the coatings was analysed by a Tafel polarisation curve and electrochemical impedance spectroscopic studies in a 3.5 wt% NaCl solution. The ultrasonicated coatings (Ni-Mo) significantly enhanced corrosion resistance (4.1 mu A cm(-2)) compared with those without ultrasound assistance. However, at the maximum ultrasound power (270 W), the molybdenum content, grain size, roughness, microhardness and corrosion resistance of the Ni-Mo alloy coatings all slightly decreased.