Study On Chromate-free Passivation Process For Electrolytic Manganese

Manganese is an important metal element which has been extensively used in industry, especially as a basic material in metallurgical industry. In order to ensure the commercial value and functional performance of manganese, passivation treatment is required to enhance the capacities of electrolytic manganese. However, the Cr6+ in chromate passivation liquids is a kind of strong carcinogenic substance. The gas produced during the operation and the washing waste water used in the passivation process lead to severe environmental problems. Therefore, the technology of chromate-free passivation for electrolytic manganese metal has been a hot research topic with academic value and practical significance.Based on the earlier stage research, this study established a proper preparation technology of manganese metal, and compared three kinds of chromate-free and phosphate-free passivation technologies and selected the silicate passivation technology which showed better effect in optimization experiment. The study was performed by the orthogonal experiment and the single factor test. The effect of passivation solution components, pH value and passivation time in corrosion resistance of passive film were discussed. The optimum composition of the passivation solution was as follows: Na2SiO3 20 g/L, Na2B4O7·10H2O 10.5g/L, H8N2O6S 11g/L, complexing agent A 9g/L, complexing agent B 9g/L, the pH value of passivation solution 2.4～3.2, passivation temperature 20℃～40℃, passivation time 15s～25s.An evaluation system of anticorrosion effect that was suited to the electrolytic manganese was established through screening. The passive film and unpassive film were studied by appearance inspection, ventilated coupon test, HNO3-K2MnO4 solution drop test, dipping in salt solution test, colorimetric analysis test and Tafel polarization curve test. The test results indicated that the silicate passive films could obviously improve the anti-corrosive performance and protect the manganese in a certain degree. At the same time, the microscopic appearance of the silicate passive film had been investigated by SEM techniques. Through EDS analysis, it is found out that he passivation film contains elements such as Mn, Si, O, C (probably have B), and so on. Based on the results of the study, the mechanism of silicate passive film formation and corrosion resistance had been speculated.This study established a reasonable, simple and effective experimental scheme for passivating electrolytic manganese to study the non-toxic, non-pollution silicate passivation technology. The combination of this technology and humidity controlling could get the similar appearance of passivation effect and protective lifetime by chromate passivation. Therefore, the silicate passivation as a newly-developed measure for anticorrosion technology deserve further investigation for its advantages, such as low cost, non-pollution and convenient.