| Due to its favorable properties such as desirable appearance, high hardness, resistance to wear and corrosive environments etc., Cr-Ni alloy coatings especially obtained by electrodeposition have been being the subject of investigation by numbers of researchers. Environment-friendly and low-toxic trivalent chromium electrodeposition has been being widely studied to replace the heavy-polluting hexavalent chromium electrodeposition both at home and abroad. However, it is of some disadvantage like unstable plating bath, difficult to get thick coatings and so on for trivalent chromium deposition. Owing to its superior properties such as high strength, high melting point, desirable heat stability and wear resistance, nano-ZrO2 particles have been widely used to modify the hardness, abrasive and wear resistance of metal deposits currently. In this work, nano-ZrO2 particles were added to the Cr-Ni plating bath in order to prepare the Cr-Ni-ZrO2 composite coatings with desirable properties by means of co-electrodeposition. Effect of little amount of Ni ions in the bath on trivalent chromium electrodeposition under the condition of low current density was studied. The influence of bath component and process parameters on the co-electrodeposition of Cr-Ni-ZrO2 were also studied. Properties of Cr-Ni-ZrO2 coatings such as surface morphology, composite, organizational structure, corrosion resistance, void fraction and hardness were characterized by means of SEM, EDS, XRD, Tafel plots and electrochemical impedance spectroscopy (EIS) separately. Analyses of the results are as follows:(1) Cr coatings obtained from the plating bath with little amount of nickel ions are of better appearance than that of without nickel ions. The thickness of the coatings obtained from the plating bath with little amount of nickel ions are 0.46～0.93μm, which meets the industrial requirement. Results of cathodic polarization cures show that the LSV of the plating bath shift towards positive side when nickel is added to the bath. That is to say, nickel ions can strengthen cathodic polarization of trivalent chromium and promoting the forming of blue film. It is demonstrated that the addition of little amount of nickel ions has a catalytic effect on chromium deposition.(2) Effects of concentration of auxiliary complexing agent, main salt, nano-ZrO2 particles and additive A on the thickness and appearance of Cr-Ni-ZrO2 coatings were studied by single-factor experiments, and the bath formula was optimized by orthonormal experiments. Effects of tempture, pH, current density, deposition time and stirring speed on the thickness and appearance of Cr-Ni-ZrO2 coatings were studied. The Cr-Ni-ZrO2 composite coatings obtained under the optimum process condition are bright and are of the thickness of 8.5μm. Results of SEM and EDS show that nano-ZrO2 particles are well distributed in the coatings and the surface morphology of Cr-Ni-ZrO2 composite coatings is homogeneous and finely crystallized. However, some ZrO2 particles in the coatings have lager diameter (100nm) than before (30nm), given some of them agglomerate. Mass fractions of the main element in the coating are as follows:Cr 82.6%, Ni 0.41%, Zr 7.65%, and O 9.34%.(3) Compared with Cr-Ni alloy coatings, Cr-Ni-ZrO2 composite coatings take more even and more compact surface. Analysis of XRD demonstrates that structure of Cr-Ni-ZrO2 composite coatings is amorphous. Kinds of performance test show that Cr-Ni-ZrO2 composite coatings possess good adhesion, low void fraction and high hardness (664.6HV). Tafel plots and EIS of Cr-Ni-ZrO2 composite coatings and Cr-Ni-alloy coatings demonstrate that Cr-Ni-ZrO2 composite coatings have much better corrosion resistance than Cr-Ni alloy coatings in both acid and alkaline corrosive medium. However, Cr-Ni-ZrO2 composite coatings are more easily corroded than Cr-Ni-alloy coatings in neutral corrosive medium.(4) Analyses of cyclic voltammogram curves and cathodic polarization cures show that trivalent chromium ions are deposited by a two-step reduction, but the detaied reductions are remaied to study further. Conclusions are made by analyzing cathodic polarization as follows: the nickel ions in the bath barely co-deposited with trivalent chromium ions but tend to boost the deposition of trivalent chromium ions; addition of auxiliary complexing agent A is good for dispersion of the plating bath; the additive A in the bath refines grains of the coatings and brightening the coatings; the addition of nano-ZrO2 particles strengthens the cathodic polarization of trivalent chromium and boosting the deposition rate.
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