The Research Of Galvanized Layer Silicate Chromium-free Color Passivation Process

The pollution of electroplating industry is very serious, which mainly comes from the passivation solution.The application of chromate passivation solution containing Cr6+has been severely restricted at home and abroad because of it’s toxic and serious environmental pollution. So far, no a completely chromium-free passivation technology can replace the traditional chromate passivation to realize industrial production.In view of this, a chromium-free color silicate passivation process was studied in this paper. This passivation took the sodium silicate solution as raw solution. By changing the composition of the passivation solution and process factors we got the different colorful appearance and corrosion resistance of passive film in orthogonal tests and single factor experiments. The corrosion resistance of passive film was measured in the neutral solution spray test, copper sulfate drip experiment, and electrochemical test. The optimal formulations and process conditions of sodium silicate colorful passivation we finally got were as follow:Na2Si03:15-25g/L, H2SO4:4-7ml/L, H2O2:15-20ml/L, CuSO4:0-2g/L, Zn (NO3)2:10-20g/L. Passivation temperature:30℃-40℃, PH values:1.8-2.8, passivation time:30S-40S, drying temperature:room temperature, drying method: centrifuge or fan dry.The appearance of this galvanizing colorful passive film prepared by the process is bright, light and uniform.Samples which were measured by5%NaCl neutral solution spray test appeared the white rust after85hrs averagely and got a better corrosion performance than trivalent Chromate just quiting to hexavalent Chromate.At the same time, SEM microscopic observation showed the surface morphology of the silicate colorful passive film.Element valence and composition of the silicate colorful passive film were measured by XPS, EDAX respectively. We obtained the formation mechanism of silicate colorful passive film from above tests which combined with the metal color and light refraction to explain the reason why the silicate colorful passive film showed the different colors.Industrial test of the silicate colorful passivation process was introduced at the last. The maintenance and stableness of passivation solution as well as the appearance and corrosion resistance after expanding production were studied. The results showed that after expanding production, the passivation solution was quite stable and the stability would be improved by removing ferrous sulfate, sodium formate, zinc sulfate and other compositions or by adding sodium silicate solution. Galvanized samples passived by this silicate chromium-free colorful passivation solution were bright, light, uniform and well Corrosion resistant performance.In summary, the silicate chromium-free colorful passivation technology is clean, stable, inexpensive, corrosion-resistant and simple to operate. By further optimization,it is expected to replace the traditional chromium colorful passivation process and has a broad market prospect.