Study Of Chromium-Free Passivation Process For Zn-Ni Alloy Coatings

Passivation process is necessary to make Zn-Ni alloy have excellent corrosionresistance and decoration performance. Traditional passivation process is chromatepassivation. But chromate is poisonous, so it will do harm to the environment andhuman beings. As a result, it has been strictly forbidden at home and aborad.Nowadays the researches on Zn-Ni alloy passivation process are only a few. Andthe chrome-free passivation process is not reported. Therefore developing apassivation process that is low or no pollutional and has good performance is veryimportant.Comparing the advantages and disadvantages of some passivation processes,Molybdate passivation system is chose. It is used in Zn-Ni alloy iridescent andbright blue passivation. At last iridescent passivation solution and technologyconditions were determined: Na2MoO₄ 12.5g/L, Na2SiO₃ 5g/L, Na3PO₄ 12g/L,NaNO₃ 25g/L, CH₂(COOH)₂ 24g/L, pH 3.5, temperature 48℃, passivation time125-130s, aging time 10s. Using this passivation process, the obtained passivationlayer has well-proportioned color. The color is orange when we see it in the indoornatural light, and the color become pink and yellow when we see it in the sun. Thebright blue passivation process is: Na2MoO₄ 25g/L, Na2WO₄ 20g/L, Na2SiO₃ 5g/L,Na3PO₄ 12g/L, NaNO₃ 25g/L, CH₂(COOH)₂ 24g/L, pH 3.5, temperature 48℃,passivation time 110s. the color of passivation layer is well-proportioned birghtblue.SEM image shows that Molybdate passivation system obtained passivationlayer has many microcrack, and the number of microcrack decreases when somesodium tungstate is added. XPS result indicates iridescent passivation composed byMo(OH)₃, MoO₃/MoO₄2-, MoO₂, Mo(OH)₂, MoCl3, MoO₃/SiO 2, ZnSiO 4, Zn3(PO₄)₂,ZnO, Zn(OH)₂, Zn(H₂PO₄)₂, ZnO/SiO₂, and besides above compounds bright bluepassivation also composed by WO₃, H₂WO₄, WOx. EIS test shows that theelectrochemical corrosion resistance of iridescent passivation is 2082Ω·cm², andthe electrochemical corrosion resistance of bright blue passivation is 2584Ω·cm².They are both higher than chromate passivtion resistance. Based on above analysis,the formative mechanism of molybdate passivation is studied. The compound of Mo,P and Si composited deposit, then forms passivation.