High Corrosion Resistant Trivalent Chromium Black Passivation Process Research

The traditional black passivation process of hexavalent chromium on zinc coating has uncomplicated formulation and simple operation.passivation film not only has excellent corrosion resistance and is decorative,but also has self-repair ability.But this process uses hexavalent chromium,which is poisonous,results in serious pollution of the environment,poses a great threat to human health.Countries around the world have promulgated strict laws and regulations,aiming at limiting the use of hexavalent chromium strictly.The structure and performance of trivalent chromium passivation film are close to hexavalent chromium,but the toxicity is only one percent of hexavalent chromium.Therefore trivalent chromium passivation is a preferred substitute process of hexavalent chromium passivation process.At present,trivalent chromium blue and white passivation process is stable,the performance of passivation film has reached even more than hexavalent chromium passivation film,which have large-scale commercial applications.However,the studying on trivalent chromium black passivation process is lagging behind relatively.Compared to hexavalent chromium black passivation,the stability of domestic trivalent chromium black passivation,film black appearance and the corrosion resistance have a certain gap.Compared with domestic product,Japan Dipsol company has increased the stability,black appearance,corrosion resistance considerably.However,the the price is expensive,which enterprise can not afford.In this paper,the effects of basic components,process parameters and the concentration of the passivation on the passivation film properties were investigated.The film-forming mechanism and corrosion mechanism of trivalent chromium black passivationwere discussed.By using the process,the appearance and corrosion resistance ofpassivation film are close to foreign Dipsol company products.In this paper,the best process of trivalent chromium black passivation was determined by using the methods of visualization,electrochemistry,neutral salt spray test in preliminary screening,single factor experiment and orthogonal experiment mainly according to the appearance and corrosion resistance of the passivation film.The specific process is45.6g/LCr Cl₃·6H₂O,14g/L complexing agent A,7g/L complexing agent B,22g/LNa H₂PO₄,13.7g/LCoCl₂·6H₂O,1.3g/L NiCl₂·6H₂O,0.005mol/L Organic sulfide,0.4ml/LNano-silica solution;p H value1.8-2.0,temperature 40-50?,passivation time 35-45s.The trivalent chromium black passivation film obtained by this process has uniform blackness,good adhesion,small Tafel corrosion current,corrosion rate is low,high corrosion resistance.The passivation film without sealing process reaches more than 60h by means of neutral salt spray Experimental,which is close to foreign company.Meanwhile,this process is stable,easy to maintain and can be used for actual production.The element composition and elemental valence of trivalent chromium black passivation film were investigated by means of EDS and XPS.The mechanism of film formation of trivalent chromium black passivation film was discussed.The corrosion resistance and microstructure of trivalent chromium black passivation film,trivalent chromium black passivation sealed film and hexavalent chromium black passivation film were studied by means of SEM and electrochemistry.The corrosion mechanism of trivalent chromium black passivation film was discussed.Results shows that the trivalent chromium black passivation film mainly uses zinc-chromium-phosphorus as the skeleton,and the metal sulfide-the elemental metal are dispersed in in the passivation film to show black by the synergistic effect,which have low solubility in water,play a role in inhibiting the dissolution of zinc and mechanical isolation.