Optimization of plating process and corrosion behavior of nanocrystalline Ni-Mo coatings on pure aluminum

Nanocrystalline Ni-Mo coatings were electrodeposited on pure aluminum and the effect of sodium molybdate content, current density, pH and temperature on the corrosion resistance were investigated by the Taguchi experimental design method. The phase structure, micromorphology and element composition of the Ni-Mo coatings were characterized by XRD, SEM, EDS and TEM. The immersion, Tafel polarization and impedance tests were carried out to study the corrosion resistance of the Ni-Mo coatings in 3.5 wt% NaCl solution. The optimum control parameters were obtained as sodium molybdenum content 10 g/dm3, current density 8 A/dm2, pH 8, and temperature 25 celcius. At the level, the Ni-Mo coating was nanocrystalline structure with a compact and smooth surface. The existence of Mo in coatings could inhibit the pitting of aluminum. The corrosion rate of the best Ni-Mo coating was a quarter of pure aluminum and the corrosion current density was calculated as 1.06 x 10-5 A/ cm2, while the pure aluminum in the same medium was only 4.95 x 10-5 A/cm2. Also, the leveler temperature was a critical factor for the corrosion resistance performance of Ni-Mo coatings.