Characterization of pulse reverse Ni–Mo coatings on Cu substrate

• Simultaneous effects of anodic duty cycle and anodic current density on "electrocrystallization block" phenomenon were investigated.
• Through using the pulse reverse method, Mo content and porosity of Ni–Mo coatings increase.
• The pulse reverse method can improve the crystalline state of Ni-Mo coating and its crystallite size simultaneously.

The effect of pulse reverse current (PRC) method on Ni–Mo coatings electroplated from chloride solution was investigated by various plating parameters such as plating duration, the anodic duty cycle, the anodic current density and the cathodic current density. By increasing the anodic duty cycle and anodic current density, the Mo content of coatings reached 68 wt.% and 78 wt.%, respectively at cathodic current densities of 500 and 300 mA cm− 2. The Mo content of coatings increases by the preferential dissolution of Ni on the anodic pulse and also by the replenishment of Mo complexes in the diffusion layer near the substrate surface during the anodic pulse. In comparison with the direct current (DC) and the pulse current (PC) methods, the PRC method deposited coatings having grain-refined morphology and more porosity especially at the anodic current density of 200 mA cm− 2. This is related to preferential dissolution of the high stress zones and the increase of Mo oxide. The PRC method made coatings with the crystallite size of 125 Å which is less than 287 Å crystallite size of coatings deposited by the PC method. Moreover, the PRC method made coatings with a higher crystalline state than coatings deposited by the DC method. The chemical composition, the surface morphology and the crystal structure of all deposited Ni–Mo coatings were evaluated by atomic adsorption spectroscopy (AAS), scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods, respectively.