Investigation On Pretreatment Technology For Nickel-based Composite Coatings

Nickel coatings have high hardness, and its electrode potential is more positive than steel. It is applied abroad in the bottom and intermediate layers of decorative and protective coatings. Nickel reacts with oxygen in the air and the surface rapidly produces extremely thin oxide film. It easily makes the adhesion of the coating bad. In this thesis, the pretreatment for nickel composite coatings were investigated aimed at good adhesion. Electrochemical work station was used to investigate the property of the coatings. The Ni-ZrO2 composite work pieces were prepared by pulse current electroplating technology. The nickel composite coatings were prepared after different pretreatment technology. Heating experiment was used to investigate the influence of pretreatment on the plating adhesion.Ni-ZrO2 composite coatings were prepared by a series of pretreatment, including acid etching, activation and nickel pre-plating. After another Ni-ZrO2 electroplating, Metallurgical Microscope was used to investigate the morphology of the nickel composite coating after different pretreatment. Heating experiment was used to investigate the influence of different pretreatment on the adhesion. It is found that the optimum acid etching time is 7 min. The acid etching time is too short, the surface oxide layer can't be removed completely and can't form optimum surface roughness; the acid etching time is too long, it will enlarge the corrosion to the work pieces, form gullies on the surface and cause the adhesion drop. The influence of activation on the adhesion is that the adhesion will be enhanced along with the time lengthening. The aim of activation is to remove a thin oxide film on the surface and present the metal crystal texture on the surface. The corrosion of activation to the work pieces is small, so the surface roughness will not be enlarged with the time lengthening. The optimum activation time is 8min. Nickel pre-plating optimum technological parameters are that the current density is 8A/dm2 and the time is 45s. The adhesion is influenced by current density and time through influencing the surface roughness. Increasing current density will increase the nickel pre-plating surface roughness and lengthening the time will reduce the nickel pre-plating surface roughness.Anodic activation instead of chemical etching was used to repair the composite coating which takes copper as substrate. Along with the chemical etching processing, the cupric ions enter the chemical etching solution and the copper is replaced on the nickel surface. It will affect the adhesion and cause the coating exfoliating and blisters. Constant potential anodic activation was used as the pretreatment for the nickel composite work pieces through the comparison of copper and nickel anodic polarization curves. The influence of time and temperature of the anodic activation on the plating adhesion was investigated. The surface cleanness and the plating adhesion are enhanced along with the lengthening of the time and the elevation of the temperature.