The Research Of Trivalent Chromium Passivation On Zinc Coating

The Cr(Ⅵ) passivation is applying very widely in passivation technics on zinc and zinc alloys. But the Cr(Ⅵ) has great toxicity with the environment and can bring cancer to the workers. It is very dangerous in the producing process and the dispose process of waste water and residue. So, to find a Cr(Ⅵ)-free passivation technics which not only can protect the environment but has the analogous capability to the Cr(Ⅵ) passivation. Cr(Ⅲ) passivation possesses as good performance as Cr(Ⅵ) passivation, but the toxicity realated to Cr(Ⅲ) passivation is only one percent of the latter one, thus Cr(Ⅲ) passivation will be the most acceptable technics instead of Cr(Ⅵ) passivation and attracts many researchers'attention.A new Cr(Ⅲ) passivation technics is found which can protect the environment and be the good substitute of Cr(Ⅵ) passivation. The new technics can form a Cr(Ⅲ) conversion layer which can present a corrosion protection of above 72h in the salt spray test.Through a period of exploring and researching process, we find a new colorized trivalent chromating technology. In this processing solution, Cr(Ⅲ) which is offered by Cr2(SO4)3·6H2O is the main element of the conversion layer; malonic acid can form a water-soluble complex with Cr(Ⅲ); NaNO3 is the oxidant; Ni2+ present in an amount effective to activate the bath and improve the corrosion resistance of the passivate film; Cl- is the accelerant of metal deliquescence which quicks the formation of passivate film and shorts the passivation time by accelerating anode's deliquescence; The organosilicon topcoating liquid H is used to improve the corrosion resistance of the passivate film. According to orthogonal method and one-way method, the optimal process conditions of a Cr(Ⅲ) passivate solution was obtained: 3mL/LCr2(SO4)3·6H2O, 5g/L malonic acid,5g/LNaNO3,0.25g/LNi2+,0.3g/LCl-,0.6 mL/Ltopcoating liquid H,pH1.6,passivation temperature 25℃,passivation time 80s. The correlation among the passivating parameters and the morphology and corrosion resistance of the electroplated zinc coating were explored. The formation mechanism of passivate film were also discussed.According to national standard method, we test the capability of the passivate film. The result indicate that the capability of the film is satifacted. The passivate film has decorative colorized, and can pass 79h NSS test. But the chroma and corrosion resistance of the Cr(Ⅲ) passivate film are inferior to Cr(Ⅵ) passivate film, so the thchnic should be improved more.