| The hot-dip galvanized films is applying very widely as the corrosion protection coatings of materials, it is usually to participate in the corrosion reaction as the sacrificial anode. But under the outside environment, the zinc as the active metals is easy to be corroded and form the white corrosion products in the surface, leading to that the appearance quality become bad. Therefore, the passivation treatment is often carried out on the galvanized layer to enhance the inoxidability and appearance quality. However, the conventional passivation treatment on the galvanized layer is the Cr(Ⅵ) passivation, which is very poisonous for the natural environmental and human being. So developing the environment-friendly passivation solution is the focus in the academic.In this article, The basic composition of Cr(Ⅲ) colorized passivation solution is studied through orthogonal test and extreme difference analysis methods, by using the single factor test method researched the working condition (pH, passivation temperature, passivation soaking time, stop time and drying conditions), and the optimum adding amount of rare earth cerium additive in Cr(Ⅲ) passivation solution is studied. The corrosion resistance of Cr(Ⅲ) passivation coating, Cr(Ⅲ) adding Ce4+passivation coating and Cr(Ⅵ) passivation coating are comparative studied by lead acetate spot test, the neutral salt spray test, immersion test, electrochemical analysis tests. The morphology and composition of passivation films were researched through metallographic microscope, SEM and EDS. Abrasion resistance and thermal stability of different passivation films were researched through adhesion test and thermal performance test examines. Besides, the formation and corrosion resistance mechanisms of Cr(Ⅲ) passivation coating were also discussed.This study indicates that the basic composition of Cr(Ⅲ) colorized passivation solution is:7.5g/L Cr(NO3)3,6g/L C6H8O7,1g/L NaNO3,1g/L CoSO4,2g/L NaCl. The optimum process parameters:pH2.5, passivation temperature35℃, passivation time90s, stop time15s, hair dryer dry. The optimum quantity of cerium additive in the passivation solution is1.5g/L.It can be seen that the Cr(Ⅲ) passivation coating adding cerium has better appearance quality than the pure Cr(Ⅲ) passivation coating. From metallographic images we can know that two types of passivation coatings are the amorphous coatings, and the passivation coating containing cerium is more uniformity and brighter; The SEM images show that the surface of Cr(Ⅲ) passivation coating has some micro-crack, and the Cr(Ⅲ) passivation coating containing cerium is compact and no crack; EDS analysis shows that the main composition of Cr(Ⅲ) passivation coating are:Zn, Cr, O, C, and the main composition of Cr(Ⅲ) passivation coating containing cerium are Zn, Cr, O, C, Ce, This shows that rare earth cerium additive take participate in the film forming reaction.The result of lead acetate spot test shows that the black spots of Cr(Ⅲ) passivation coating containing cerium is the latest, and the time is101s. Salt spray test and NaCl solution immersion test show that the corrosion area of Cr(Ⅲ) passivation coating containing cerium are less than chromate passivating treatment and Cr(Ⅲ) passivation coating in the same treatment time. And according to the potentiodynamic polarization curves analysis, the corrosion potential of Cr(Ⅲ) passivation coating containing cerium is maximum, and the corrosion current of Cr(Ⅲ) passivation coating containing cerium is minimum.The film forming mechanism of Cr(Ⅲ) passivation coating shows that passivation time and pH have a great influence on the comprehensive performance of passivation coating. The corrosion mechanism of Cr(Ⅲ) passivation coating shows that the Cr(Ⅲ) passivation coating isolate the Zn to external environment. |
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