| Process and mechanism of electrodeposited Zn-Fe-Si02 composite coatings were studied, including some contents as follow:Potential-pH plots of Zn-H2O and Fe-H2O systems were drawn, and the thermomechanical analysis was made. Results showed that the region of zinc and iron's codeposition was wide, which provided theoretic basis for electrodepositing Zn-Fe alloy. But the balance potential of zinc was greatly different from iron's, so in order to make zinc and iron codepositing, some complexant must be added into electroplating solution.The technics of electrodepositing Zn-Fe-SiO2 composite coatings was confirmed: FeSO4-7H2O 120~200g/L, ZnSO4-7H2O 30~70g/L, (NH4)2SO4 80~120g/L, C6H8O7-H2O 30~50g/L, H3BO3 30g/L, ascorbic acid Ig/L, SiO2 20~60g/L, addition propriety, value of pH 2~4, time of electroplating 60min, current density 4~8A/dm2, temperature 15~25 , intensity of agitation 200rpm.Researches on the process of electrodeposited Zn-Fe-SiO2 composite coatings indicated that composite coatings with different contents could be obtained by means of appropriate parameters. The concentration of main-salt was the primary influence on the content of iron in the composite coatings, and the adding of FeSO4-7H2O could help to increase the content of iron, while the adding of ZnSO4-7H2O helping to reduce the content of iron. The adding of (NH4)2SO4 could help to increase the content of iron, while the adding of C6H8O7-H2O helping to reduce the content of iron, but both of the two influence were weak. Temperature had little and irregular effect on the content of iron. Increasing the value of pH could decrease the content of iron, while helped to increase the content of SiO2. Current density influenced the component! of composite coatings greatly. When current density was 6A/dm2, the content of iron was at the lowest level, while the content of SiO2 was maximal. The adding of SiO2 could help to increase the content of SiO2 to some extent, but which had little effect on the contents of zinc and iron.The corrosion resistance of Zn-Fe-SiO2 composite coatings (Fe7. 20-12. 09%.SiO20. 40-0.51%) were studied, and the result showed that Zn-Fe-SiO2 composite coatings had high corrosion resistance without passivation. In diluent HC1 solution, the corrosion resistance of composite coatings was equal to Zn-Fe coatings, and it was determined mainly by the content of iron, being raised with increasing the content of iron, but the content of SiO2 had little effect on this. In 5%NaCl solution, the corrosion resistance of composite coatings was better than that of Zn-Fe coatings, being about 1.5-4 times of that of Zn-Fe coatings, and it was much better than that of zinc coatings, being about 3-20 times of that of zinc coatings. The corrosion resistance in 5%NaCl solution decreased with the increasing of content of iron and increased with SiO2 .The conception of micro-double-electric layer was brought forward for the first time, and theory of the process of SiO2 particle's codeposition with Zn-Fe alloy was studied from the points of micro-double-electric layer on the surface of SiO2 particle, electric field's intensity between the cathode and solution, and cathodic polarization. The results showed that SiO2 particle could show electropositive with adsorbing some cations, this helped it to be transported and adhered to cathode and codeposite with the basic metal. Electric field's intensity between the cathode and solution influenced the composite codeposition greatly, and this influence could be embodied by current density. When zinc and iron codepositing, the existing of zinc made iron's deposition potential lower, and the existing of iron made zinc's deposition potential higher, this led to zinc, which had lower potential originally, deposited first, so a conclusion could be drawn that Zn-Fe's codeposition belonged to abnormal codeposition. Si02 particle had little effect on the electrochemical process of Zn-Fe's codepositing at the cathode. |
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