Study On Pulse Electrodeposition And Property Of (Ni-Co)/Al₂O₃ Composite Coating

The composite coatings prepared by DC (Direct Current), PC (Pulse Current) and PRC (Pulse Reverse Current) methods respectively, evaluated by content of Al₂O₃ in coatings, microhardness and macro-residual stress, to gain the optimal electrolyte composition and operating condition: Ni2SO4 250g/L, NiCl2 45 g/L, HBO3 30 g/L, CoSO4 3 g/L, anion surfactant 0.05 g/L, pH 4.3, stirring speed 250 r/min, temperature 43℃, current density 2 A/dm2, positive work time 3 ms, positive duty circle 0.3, positive pulse mean current density 2.22 A/dm2, reverse work time 3 ms, reverse duty circle 0.1, reverse pulse mean current density 0.22 A/dm2. SEM,TEM,XRD,EDS and XPS were employed to observe the microstructure of composite coatings, Tafel, CV, EIS and wear tests were employed to evaluate the corrosion and wear resistance.Based on the DC diffusion model, we build a duplex diffusion layer model which is composed of inner pulse diffusion layer and outer stable diffusion layer. In inner pulse diffusion layer, the concentration of ions fluctuates with frequency of pulse current; in outer stable diffusion layer, the transport speed of ions is almost stable all the time, the main purpose is to transport the ions from the mass to the pulse diffusion layer. For PC, the pulse diffusion layer only depends on the diffusion coefficient of cations and the pulse length, is not related to the concentration of the mass.For electrodeposition by DC methods, the metals deposite uninterrupted and the particles were also embeded uninterrupted into the coatings; For electrodeposition by PC method, the particles with biggish volume were desorbed from the coatings and returned to the electrolyte again owing to the presence of pulse interval; For electrodeposition by PRC method, the particles carried positive charges are much more easy to desorb from the coatings owing to the effecf of reverse pulse current combined with pulse interval, in addition, the reverse pulse current also could dissovle the metals, further accelerates the desorption of particles, thus the particles size embeded in the coatings by PRC method is the least. With the increasing of particles content in coatings, the three kinds of coatings all exhibit grains fine, indicating that the presence of Al₂O₃...