| The Hot-dip galvanizing coating(HDG) is widely used in industrial production, while it is easy to produce white rust that affects the corrosion resistance. Traditional thromates are the most effective way used in anticorrosive treatments for HDG,However, hexavalent chromium is carcinogenic to people and contaminated to the environment, so the chromium free conversion coatings become a develop trend. In this paper, a kind of passivation treatment which consists of film-forming agent(metavanadate), forming accelerator(fluorotitantic acid) were prepared,obtain continuous and uniform metavanadate-hexafluorozirconic acid conversion coating on HDG. The microstructure and growth of the metavanadate hexafluorozirconic acid conversion coating on HDG steel were investigated by SEM, EDS, XPS, XRD, neutral salt spray(NSS), electrochemical impedance spectroscopy(EIS) and the electrochemical polarization(Tafel) aimed to examine the corrosion resistance.The optimal process parameters of the metavanadate-hexafluorozirconic acid conversion coating based on orthogonal test and single factor experiment are as follows: Na VO3 5 g/L, H2 Zr F6(45wt%) 6g/L, p H 1.5, treating time 20 min, temperature 30℃. When the treating time is 1 min,the grain boundary of zinc is clearly visible. With increasing immersion time, the thickness of metavanadate-hexafluorozirconic acid conversion coating increase. A continuous and uniform conversion coating is formed on hot-dip galvanizing when treating time is 20 min.Prolong the treating time to 45 min, which resulted from the crack propagation and quality decrease of the conversion coating, turnup with flaking off in the localized area afterwards.The results of EDS, XPS, XRD analysis show that the metavanadate-hexafluorozirconic acid conversion coating contains Zn, O, V, Zr, F elements, and the surface of the conversion coating is composed of Zn F2, Zn O, Zr O2,VO2,V2O5 and its compound.The results of NSS analysis show that with the increase of treating time,the corrosion resistance of samples appear increase at first then decrease.When the treating time is 20 min,the salt spray corrosion resistance of the basic conversion coating is greatly improved over the zinc layer. After three cycles of neutral salt spray corrosion, the sample only appears 5% white rust. The results of Tafel analysis show that self corrosion potential shuffle after conversion coating treated,the branch of anode moving scope is more obvious, the corrosion current density of the conversion coating was the lowest one and corrosion current density decreases the order of two majorities compared with HDG. The impedance values for the coatings initially increase and the decreased with increasing immersion time. The corrosion resistance property is found be best up to the immersion time of 20 min that is exceed to the hot-dip galvanized coating, electrochemical impedance increases the order of two majorities compared to untreated HDG sample. The results of immerse corrosion process can be summarized as follows: corrosive medium and electrolyte solution infiltrate the coating then diffuse into the gap in the microcracks when the conversion coating immersed from 30 min to 24 h; inner layer was destroyed and electrolyte solution diffuse into the inner layer when the coating immersed from 48 h to 144 h; the corrosion of zinc became the dominate reaction after immersed 192 h, the conversion coating is out of protection.The scratching corrosion test showed that the metavanadate-hexafluorozirconic acid conversion coating had good self-sealing ability.Adhesion and wettability of the metavanadate-hexafluorozirconic acid conversion coating were studied to characterize the physicochemical properties of the coatings.Contact angle of conversion coating sample increase then HDG,transforming from wetting to non wetting, which delay the electrolyte solution diffuse into inner layer damaging zinc effectively. The performance of conversion coating increased at first then reduced, excellent performance appear when treating time reach 20 min. |
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