| Chromate passivation is the mostly widely used and the highest cost-effective treating technology in hot dip galvanized industry. But the chromate is carcinogenic, and white particles will appear on products surface after passivation solutions has been used for a period of time. For the particles will affect products’ appearance, it is necessary to replace the passivation solutions. The non-chromate application technologies are immature, therefore seeking an effective method to extend the life of chromate passivation solutions even realizing the zero-emission goal is an urgent task.It has been found that the appearence of partickes is caused by the increasement of zinc content in passivation solutions. This paper aims at studing how the zinc in chromate passivation solutions affects the passivation and what’s the mechanism; how to regenerate the chromate passivation solution containing zinc. Studied the impact of zinc on the formation and composition of chromate coating by SEM, EDS, XRD, XPS. Studied the corrosion resistance of chromate coating by NSS, EIS and Tafel polarization curve. The results are:1、The zinc in solutions makes the particles appear on the coatings, the higher zinc content the more particles. The results of EDS analysis show that the particles’ Zn / O atomic ratio is about 1:2. The results of XRD analysis show that the zinc in solution may not change the matters of the coatings, and the compositions of the coatings mainly are Zn, Cr compounds. The results of XPS analysis show the O content and Zn / Cr atomic ratio of coating both increase, which indicates there will have more zinc compounds in the coatings when the chromate passivation solutions contain zinc. The results of NSS、EIS、Tafel polarization curve test all show that the zinc contributes to get a better corrosion resistance.Propose an impact mechanism of zinc in chromate passivation solutions on chromate passivation: The zinc contributes to form a thicker and more dense coating; In the air forming-coating stage, the zinc will react with other ions to form a outer coating whose density is different from its inner layer.2、Explored a EDTA titration method to deteminate the zinc concentration. Removed the Cr(VI) by 717 anion exchange resin, then titrate the concentration of zinc by EDTA disodium using chrome black T as indicator. This method is simple、cheap and repeatable and the testing error can be control under 5%. This method can be used in the industry to deteminate the zinc content beteew 50~1400mg/L.3、Removed the zinc in chromate passivation solution using 732 cation resin by static adsorption treatment. The treatment can remove zinc ions selectively and did not affect the Cr(VI) content. The results of FTIR comfirm that the resin’s exchange position is sulfoic acid group, therefore the exchange won’t bring other ions in but H+, which make the p H decline.The exchange rate of the 732 cation exchange resin is fast. The adsorption behavior of the resin for Zn(Ⅱ)obeys monomolecular layer model, and the saturated adsorption amount is about 85.6 mg/g. The absorption and desorption properties of the resin are good which means the resin can be recycled. The best p H for absorption ranges from 3 to 6. |
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