| Chromates are widely used in anticorrosive treatments for zinc and zinc coating. However, hexavalent chromium presents very high toxicity and carcinogenic nature, and has a bad impact on environment. For this reason, an intense research effort is being undertaken to replace chromates passivation by new more environmentally friendly non-chromate treatments. In this paper, KMnO4is chosen as a main salt of the permanganate solution and the optimal process parameters of the permanganate conversion coating for hot-dip galvanized (HDG) steel has been systematically studied. Then citric acid and KH2PO4were added into the permanganate solution as a compositon of the coating. The HDG samples were dipped into the improved permanganate to obtain the improved permanganate conversion coating, which has a better corrosion resistance. The process technologies, microstructure, growth and corrosion resistance of the basic permanganate conversion coating and improved permanganate conversion coating on HDG steel were investigated by SEM, EDS, XPS, XRD, IR, electrochemical polarization, electrochemical impedance spectroscopy (EIS) and neutral salt spray (NSS).The optimal process parameters of the basic permanganate conversion coating are as follows:KMnO420g/L, pH2.5-3.0, T40℃,/60min. The optimal concentration of citric acid and KH2PO4additives to the permanganate solution are20g/L and50g/L. It is shown that the thickness of the basic permanganate conversion coating increase with the increase of treatment time. The continuous convertion coating is formed on the surface of zinc when treatment time is60min. After treated for60min, the inhomogeneous of the coating increased. The improved permanganate conversion coating may grow faster with the addition of citric acid and KH2PO4. The continuous, compact and complete convertion coating is formed on the surface of zinc when treatment time is10min. With the increase of treatment time, the thickness of the improved permanganate conversion coating increased. After treated for20min, the cracks and warps will appear, and the granular attachments in the coating increased.The results of XPS, XRD and IR analysis show that the basic permanganate conversion coating contains Zn, O and Mn elements, and the surface of the conversion coating is composed of the oxides of Zn and Mn. The improved permanganate conversion coating is composed of the compound of P, O, Mn, the oxides of Zn and Mn and citric acid salt. The analysis results show that citric acid and KH2PO4engage in the formative process of the conversion coating.The results of NSS analysis show that the corroded area of the HDG reaches85%in the1th cycle while the basic permanganate conversion coating reaches40%and the improved permanganate conversion reaches5%. Therefore, the corrosion resistance of the improved permanganate conversion coating is more outstanding than that of the basic permanganate conversion coating.Compared with HDG, the corrosion current density of the basic permanganate conversion coating decrease from12.06μA·cm-2to3.96μA·cm-2, the polarization resistance increase from0.614kΩ·cm2to2.139kΩ·cm2, the impedance resistance increase from1kΩ·cm2to2.5kΩ·cm2. and its corrosion resistance is superior to the basic permanganate t conversion coating. The corrosion current density of the improved permanganate conversion coating declined to0.034μA·cm-2, the polarization resistance and the impedance resistance increased to29.237kΩ·cm2and27.77kΩ·cm2. The two coatings can control anode and cathode reaction. The corrosion resistance of the improved permanganate conversion coating is greatly improved.The corrosion process of the improved permanganate conversion coating is divide into three stage:outside layer dissolve and electrolyte solution diffuse into the inner layer when the improved permanganate conversion coating immersed from20min to12h; the coating was destroyed when the coating immersed24h; the corrosion of zinc became the dominate reaction when the coating immersed48h-72h. |
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