Chromium-free Passivating On Galvanized Sheet And The Research Of Corrosion Resistance

In order to eliminate hazards of hexavalent chromium to humans and the environment, it was necessary to find new passivation process to replace chromate passivation. This paper studied two methods of chromium-free passivation, i.e, inorganic rare earth passivation film and organic silane passivation film, used for the galvanized layer.The single rare-earth films optimum process determined by orthogonal experiments was that Ce (NO)₃:25g/L, H₂O₂ 20ml/L, H₃BO₃: 2.0 g/L, processing time: 3min, temperature: 45℃, pH:3. It studied the effect of cerium salt concentration, pH, temperature, H₂O₂ content and processing time on the film-forming by Pb (AC)₂·9H₂O drop test and scanning electron microscopy. A kind of new passivation on the galvanized layer was formed by the cerium salt complexed with KH-570. Scanning electron microscopy showed the complexed passivation film some micro-cracks. Electrochemical tests of polarization curves and AC impedance show that the corrosion resistance of modified rare earth film was better than a single rare earth passive film as well as galvanized layer. X-ray diffraction analysis (XRD) and X-ray photon (XPS) spectroscopy analyzed the composition and structure of rare earth passive film. The results showed that the coating mainly was composed of oxides and hydroxides of cerium and zinc.Tafel polarization curves determined the optimum formula for the passivation solution was V (BTESPT): V (ethanol): V (H₂O) = 4.5: 5.5: 90. It Studied the modified effect of inorganic inhibitor Ce(NO)₃ and nano-SiO₂ particles on BTESPT film, Tafel polarization curves showed that optimum amount of the Ce(NO)₃ and nano-SiO₂ particles were 5.5×10-5M and 15ppm, and that the modified effect of Ce(NO)₃ was better than that of nano-SiO₂ particles. Besides, it also studied that the role of KH-550 surface pretreatment in two kinds of modified passive film. Electrochemical impedance tests show that the corrosion resistance of modified films pretreated by KH-550 was better than that of modified film because KH-550 made the modified films thicker and promoted the BTESPT film forming on the galvanized sheet.