A Research Of Galvanized Layer Silicate Chromium-free Black Passivation Process

Electro-galvanized is one of the important metals anti-corrosion measures, but the electroplating industry’s pollution comes mainly from the passivation treatment after plating. Currently, hexavalent chromium salt and trivalent chromium salt passivation are widely used for galvanized layer of passivation on the market. The hexavalent chromium salt passivation process have been very mature、stable、low cost, and the passivation film is relatively thick、uniform and has good corrosion resistance. Because hexavalent chromium has a good oxidation resistance, the zinc coating would be passivated again when the passive film was local corroded, with good self-healing ability to repair; trivalent chromium salt passivation is currently the research focus. That not only possess the nature of hexavalent chromium salt passivation, and the toxicity is one percent of hexavalent chromium. But trivalent chromium salt passivation film is far poorer than hexavalent chromium salt on the corrosion resistance,and the salt of the trivalent chromium will be converted to the hexavalent chromium in the placement process, resulting excessive levels of hexavalent chromium in the film and greatly limits the circulation of products. Phosphide technology is one of relatively mature the metal surface treatment technologies. The phosphide technology itself non-toxic low-pollution fine nature of the acclaimed, but most of the phosphate coating show gray or gray-black at room temperature,and need long processing time, and the decorative appearance and efficient production can not meet the requirements. Therefore, study the passivation process that has a good decoration and good corrosion resistance, low-pollution or no pollution, not only solve the problems of chromium salt passivation, but also make phosphide at room temperature. That is great significance for electroplating industry achieve the European Union green technology indicators and promote of environment-friendly production.On the basis of silicate passivation process research and study of low temperature phosphating, the paper studied silicate chrome-free black passivation process, and passivation effect, and the passive film of the corrosion resistance, and the passivation solution of performance. Galvanized layer silicates the best ratio of chromium black passivation solution, and its optimum conditions were as follows: Na2SiO3·9H2O,16-24g/L; HNO3,11-15ml/L; NaH2SO4,15-22g/L;ZnSO4,8-14g/L; FeSO4,0.5-1.5g/L; NaHSO4,16-24g/L; passivation time60-120S, PH value of1.8-3; passivation temperature of15-35℃; closed5S; dry conditions with blow dry, drying drums or natural drying. By scanning electron microscope to observe the passive film’s appearance and organizational structure obtained that the surface becomes more smooth after passivation, beause the uncoated defects is significantly reduced, and the surface quality has been greatly improved; electrochemical workstation test the polarization curves, impedance spectroscopy, corrosion current density obtained that the surface passivation coating anti-corrosion potential was shifted positively and the anti-corrosion resistance was significantly improved, and the corrosion current density was significantly reduced; Analysis obtained by XPS and EDS test, the passive film containing the elements Zn, O, N, S, P, Si, Fe,which Zn mainly exists to Zn+and P mainly exists to P5+and Si mainly in the form of SiO2and Si2O32-.From the industrial experiments obtained that the passivation process can form uniform black and shiny appearance of the passive film on the acidic roll-plated galvanized, hanging plating galvanized alkaline plating zinc plating, hanging plating galvanized; the passivation solution is valid in about half a year quitely trivalent chromium salt passivation process; by bulk passivation obtained that per liter passivation solution passivation screws, nuts and other plated approximately13.86kg,0.673m2, and this life is a little better than trivalent chromium salt passivation process.The silicate chrome-free black passivation film, is not only black shiny galvanized surface, good corrosion resistance, but also solve the chromate environmental pollution and carcinogenic from the fundamental,and the cost of the passivation process is far below the trivalent chromium salt passivation process. Therefore, the silicate chrome-free black passivation process has a good prospect on the galvanized market.