| Traditionally, hexavalent chromate was used in passivation of the galvanized steels. However, chromate is not environmentally friendly due to its toxicity and carcinogenicity, which greatly restricts its application when the people’s health is of great concern. Therefore, it is impeding to explore the chromate-free passivation techniques, and, among them, silicate is one of the most potential alternatives. Unfortunately, passivation process in the silicate is inefficient and unstable. In order to solve this problem, a new promoting agent of film-formation was explored in this thesis, and this agent can accelerate the deposition rate and can improve the corrosion resistance of the formed passive film. Meanwhile, the process of passivation and mechanisms of film-formation in the silicate solvent have been discussed in detail, and also discussed is the anti-corrosion performance of this film. Furthermore, industrialized application of this passivating technique is successfully effected and a great economic benefit is obtained.At the beginning parts of the thesis, the optimized compositions and the operating conditions of the passivating solution have been determined as follows:SiO32-is4g/L, H2O215ml/L, NO3-20g/L, SO42-9g/L, film promoting agent6g/L, pH value1.5to2.5, passivation time10～30s, temperature15～40℃, slot time5s, light time about5s. It was found that pH values, immersion time and temperature all influence markedly the corrosion resistance of passive film.The mechanism of silicate passive films formation was studied. In the formation of silicate passive films, four processes took place, which include the formation of silicon dioxide gels, the dissolution of zinc coating, the formation of alkaline thin film and the formation of silicate passive films in succession. Thermodynamics and quantum chemical calculations showed that there exist crystal defects on the surface of zinc coating, which are the electrochemically heterogeneous parts with high energy and tend to form the anode areas of micro-cell corrosion compared with the rest parts of the coating. Metal zinc was oxidized to zinc ion at the anode, and hydrogen peroxide was reduced at the cathode. Hydrogen peroxide adsorbed on the galvanized zinc surface and breaked into OH" which stayed the two-phase interface. Subsequently, the surface pH value increased because of the formation of alkaline film, which is a prerequisite condition for the formation of passive film. The hydroxyl could combine with the zinc ion to produce zinc hydroxide, immediately followed by dehydration to zinc oxide. On the other hand, silicate hydrolyzed to silicon dioxide gels in the acidic passivating solution in pH values of1.5～2.5. And then, silicon dioxide gel reacted with zinc ion and hydroxyl to form zinc silicate. As the whole, ZnO, ZnSiO3, other zinc compounds and the filling colloidal SiO2fine particles co-deposited on the surface of galvanized zinc, and the silicate passive film formed.Electrochemical tests indicate that the passive films formation process undergoes three stages. In initial stage (0～30s), the passive film grows rapidly, but the film is not complete, and some holes are distributed in the film. In the subsequent stage (30～120s), the film growth rate decreases, but membrane performance is improved continuously, the surface of film become smooth and its corrosion resistance increased. When the passivation time exceeds120s, corrosion resistance decreases with time instead due to the films’ overlap.Because of silicate passive film’s structural and electrochemical character which was able to significantly improve the corrosion resistance of galvanized zinc. It was found that silicate passivated films formed by the accumulation of small particles, which can effectively isolated the corrosive medium due to its structural integrity, finesse and uniform distribution. Electrochemical researches indicated that the electrochemical impedance of silicate passivated films is780Ω·cm2in the NaCl medium, which is larger than zinc coating. As the electrochemical corrosion inhibitor, the passive film hinders the charge transfer process. The corrosion potential and current are-0.8828V and6.156×10-6Amp·cm-2, respectively. Especially, the silicate passive films corrosion rate is0.0183g·m-2·h-1. By the scanning electrochemical microscopy characterization of silicate passivated film was researched. Compared with galvanized zinc and chromate passivation film, no regions of high electrochemical activity exist on silicate passivated film surface. The rate of electron transfer across the substrate/electrolyte reduced, and as result the dissolution of the substrate is significantly inhibited. Parameters from Tafel curve slope showed that, compared with the zinc coating, silicate passivated film polarized both the cathodic process and the anodic process to the greater extent, and the ba is180.663mV, significantly greater than the98.579mV of bc, indicating that the corrosion processes are controlled mainly by the anodic polarization.In industrial application of this silicate passivating technique, galvanized steel parts in complicated shapes can be well passivated, and the passivation film looks mirror-like, uniform, without stripping phenomenon; Results of corrosion resistance test of this passive film fully reach the industrial standards. pH values of the passivating solution in the long-term continuous operation change little, indicating of good stability and easy maintenance; As production volume increases, the content of silicate, sulfate and nitrate in passivation solution decrease, and in order to ensure product quality, these can be supplied by adding sodium silicate, sulfuric acid and sodium nitrate after some operation time to regulate the compositions of the passivating solution. During operation, the appropriate method for the preparation of passivating solution, stirring, passivation temperature, dipping time, drying temperature, etc, can effectively extend the life of passivating solution.In summary, the technique of silicate-contained passivating solution in the present study shows environmentally friendly, low cost, excellent product performance, wide application, and great market competitive advantage, and it is expected to produce enormously economic and social benefits. |
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