Research On Optimization Of Chromium-free Color Passivation Process And Its Performance

Zinc plating is one of the most widely used anti-corrosion measures for metals.In order to further improve the corrosion resistance of the workpiece,passivation treatment should be carried out after electroplating.Hexavalent chromium passivation and trivalent chromium passivation are widely used in industry,but chromate is harmful to human body and environment.In order to reduce the harm of passivation solution,chromium-free passivation gradually replaced chromate passivation.The corrosion resistance of passive film formed by passivation of inorganic salts or organic compounds alone is not ideal.The passivation film obtained by the combination of organic matter and inorganic salt in composite passivation has strong corrosion resistance,some of which are close to or even more than chromate passivation..In this paper,tannic acid-sodium silicate composite passivation is used to improve the corrosion resistance of the passivation film through the synergy of organic and inorganic.The optimum content range of each component of the passivation film was determined by single factor experiment,and then the optimal ratio of the passivation solution was determined by orthogonal experiment.The optimum process for tannic acid-sodium silicate composite passivation is:7 C₇₆H₅₂O₄₆,5g/L Na₂SiO₃·9H₂O,6mL/L TiCl₃,15mL/L H₂O₂,8 mL/L H₃PO₄,4 mL/L HNO₃,7 g/L KCl,the pH of the passivation solution was adjusted to 1.5 with H₂SO₄,the passivation time was 30 s,the passivation temperature was normal temperature,and the drying method was hot air drying.A color passivation film is obtained under this process,and the passivation film has a uniform color distribution.The corrosion resistance and surface morphology of the passivated and unpassivated samples under this process were investigated.The corrosion resistance of the passivated and unpassivated tests was investigated by salt water immersion test,copper sulfate drop test and electrochemical test.It turns out that the corrosion resistance of the passivated sample is much greater than that of the unpassivated sample.The surface morphology,elemental quantitative analysis and passivated functional groups of the passivation film were tested by SEM,EDS and infrared spectroscopy:the composition of the passivation film was finally obtained.The results show that the passivation film has two layers.The inner layer of the passivation film is mainly composed of a grid polymer formed by silicate and a complex formed by tannic acid and metal ions.The outermost layer of the passivation film is mainly composed of zinc oxide,and finally the film formation principle of the passivation film is obtained.The SEM morphology observation shows that the surface of the passivation film has a certain degree of granularity under this process.This process has certain deficiencies and needs to be optimized..The optimum method of process modification is to apply the pressure of passivation liquid on the passivation surface when passivating the workpiece,and to control the pressure change by changing the different depths of passivation workpiece in the passivation liquid.The samples were passivated under different passivation hydraulic pressure,and compared with the traditional passivation process.The corrosion resistance of passive film under different conditions was studied by salt spray test and electrochemical test.The surface morphology of passive film was observed by SEM,and the adhesion of passive film was studied by scratch method.Finally,the corrosion resistance of passivation film was improved when a certain passivation hydraulic pressure was applied,and the corrosion resistance of passivation film was optimized under P4 condition.At the same time,the surface morphology was also improved,and the passive film surface was smooth and compact at P4 pressure.Compared with traditional passivation,under pressure,the adhesion of passivation film has been greatly improved.Under traditional technology,the adhesion of passivation film reaches grade 4,and the adhesion is the worst.Under P4 condition,the adhesion of passivation film reaches grade 1,and the adhesion of passivation film is the best.Therefore,applying certain passivation hydraulic pressure to passivation surface can improve the comprehensive performance of passivation film.