| Galvanized sheet is widely used in machinery, construction, transportation and other fields. However, the surface is rotted with white rust in the process of storage and long-distance transportation, causing by the damp heat, which directly affects the corrosion resistance and decorative appearance. In order to improve the corrosion of galvanized sheet, chemical passivation treatment should be carried out. Hexavalent chromium acid salt is used in traditional passivation process, with excellent adhesion, high corrosion resistance and good self-healing capability. However, this method has caused serious environmental problems and it is poisonous to human, which limited the application of hexavalent chromium passivation process. However, other passivating processes haven’t reached the levels of hexavalent passivation.Based on this, a trivalent chromium passivation agent was developed in this paper. A dense layer of passivation film was formed on the surface of galvanized sheet with the roller coating way and back curing. The passivation film was combined to substrate closely, which can isolate the oxygen and water molecules and slow down the occurrence of electrochemical corrosion to improve the corrosion resistance. At the same time, the film was formed by trivalent chromium, which greatly reduces the toxicity caused by hexavalent chromium and environmental pollution.Through orthogonal test, chromic anhydride, sodium potassium tartrate, inorganic acid, cobalt sulfate salt and nano silica sol were used to prepare the trivalent chromium passivation agent in this paper, and passivation process parameters were discussed by single factor experiment. Neurtal salt spray test(NSS), copper sulfate intravenous drip experiment, saline soak weightless experiment, Tafel polarization curve, AC impedance spectroscopy(EIS) characterization methods were used to study the resistance of passivation film. Scanning electron microscopy(SEM) and X-ray spectrometer(EDS) were used to analyses the passivation membrane morphology and elements. Ultimately the optimal formula of trivalent chromium passivation agent wes determined. The passivation process parameters were:curing temperature 80℃, curing time 25 seconds. The copper sulfate intravenous drip experiment and NSS results showed that passivation membrane had excellent corrosion resistance when the membrane weight was up to 20mg/m2; black time could be up to 350 seconds and white rust area was under 5% after 120 hours continuous spray. Weightlessness experiment showed that the corrosion rate was reduced from 1.551×10"5 g/(cm2h) to 7.581 X 10-7 g/(cm2h). Tafel polarization curves showed that the corrosion current density was decreased obviously from 8.458 X10-4 A/cm2 to 6.356 X 10-6 A/cm2, and the corrosion potential was moved about 50mv, from 1.015V to 0.965V. EIS spectra showed that the capacitive reactance arc was increased obviously after being passivated by trivalent chromium passivation agent. The real part of impedance Z’rised markablely from 850ohm/cm2 to 8000ohm/cm2, imaginary part-Z" rised strikingly from 300ohm/cm2to 3500 ohm/cm2. SEM showed that passivation membrane was completely attached to the zine layer, with little granular material on it. EDS showed that the passive film was mainly composed of C, O, Si, P, Cr and Zn. This trivalent chromium passivation agent prepared in this paper has excellent corrosion resistance and low cost. What’s more, the passivation process is consistent with the existing process, and it has a good application prospect and popularization value. |
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