The Research On Trivalent Chromium Black Passivation And Sealant Of Zinc Electroplating Layer

The electroplated zinc black passivation and sealing process can effectively improve the corrosion resistance of metal products,which is widely used in the electroplating zinc process.Hexavalent chromium black passivation technology is mature and perfected,and raw material cost is low.The hexavalent passivation film is black,oily with high corrosion resistance and has self-healing ability after being damaged,so hexavalent chromium passivation technology is often used for post-treatment of zinc plating in industrial production.However,hexavalent chromium has been banned because of its high toxicity.The trivalent chromium black passivation film has good corrosion resistance and the passivation mechanism is similar to that of hexavalent chromium.The toxicity of trivalent chromium is much less than that of hexavalent chromium.Now,the process of hexavalent chromium black passivation technology has been completely replaced by other passivation technologies.However,compared with hexavalent chromium passivation technology,the domestic trivalent chromium black passivation technology still has a certain gap in terms of stability of passivation solution and anti-corrosion property.Optimizing the formulation and application process of passivation solution is an urgent process problem in production.Although there are trivalent chromium passivation solutions with high corrosion resistance in foreign countries,it is difficult to obtain a stable black passivation layer in industrial continuous production because of its harsh operating condition.Therefore,it is necessary to deeply study trivalent chromium black passivation process,which has important practical significance for electrogalvanizing industry production.The passivation solution formulation and operation process significantly affected the stability and corrosion resistance of the passivation layer,and the basic composition and content of trivalent chromium passivation liquid are determined by an optimization test.A large number of single factor tests were carried out on each component in the passivation solution.The effect of each component in the passivation liquid on the corrosion resistance of passivation film was studied by Tafel polarization curve method.Four-factor and three-level orthogonal experiments were designed.The stability and anticorrosion resistance property of passivation film were evaluated by observing method,electrochemical impedance method and potentiodynamic polarization curve method.The optimal composition of the trivalent chromium passivation solution is Cr2(SO4)3 20 g/L,C6H8O7/X 10,Co(NO3)2 15 g/L,H3PO4 20 g/L,p H 2,t 30 s,T room temperature.The trivalent chromium passivation film obtained by the above process is uniform in color.The process is simple and reproducible.The SEM and EDS were carried out to characterize the surface morphology and ingredient of passivation film.The post-treatment was conducted to further improve the corrosion resistance of trivalent chromium passivation film.The effects of sealing treatment on the corrosion resistance of electrogalvanized trivalent passivation film were evaluated by electrochemical impedance method and lead acetate test.The results show that black and bright passivation film with strong corrosion resistance can be obtained after post-treatment with suitable sealant.Compared with the non-sealed passivation film,the corrosion resistance is significantly improved.