Study On Process, Mechanism And Application Of Electrodeposited Zn-Fe-SiO₂ Composite Coatings

Enormous economic lost and hazard are caused by metal corrosion. Even though electroplating zinc coating has been applied widely to protect steel from corrosion, there are disadvantages such as low corrosion resistance, high hydrogen embrittlement, pollution of passivation, etc. The research results show that corrosion resistance of low-iron zinc alloy coating (Fe%wt<1% ) is high after passivation, while high-iron zinc alloy coating (Fe%wt>1%) , which is not needed passivation, is not satisfactory.Process and basic theory of electrodeposited Zn-Fe-SiO₂ composite coating have been studied in order to develop innocuous technique which can produce high corrosion resistance coating. The research covers lab-scale experiment, scale-up experiment, mid-scale experiment, deposition mechanism and corrosion mechanism. Furthermore, application outlook of this technique has been analyzed impersonally.The main subjects and results in this dissertation are as follows:High-iron Zn-Fe-SiO₂ composite coatings can be obtained in sulfate bath, and component scopes of the coatings are Fe 2³% and SiO₂ 0.4⁰.6%. Composition of the electrolyte and operating conditions are as follows: FeSO₄·7H₂O 20⁴0 g/L, ZnSO₄·7H₂O 40⁶0 g/L, (NH₄)₂SO₄ 50-70 g/L, citric acid 20-40 g/L, Al₂(SO₄)₃·12H₂O 6-9 g/L, SiO₂ 20-40 g/L, ascorbic acid 2-3 g/L, additive ZFS-1 2-4 ml/L, additive ZFS-2 15-20 ml/L, additive ZFS-3 4-8 ml/L, pH 2.4-2.8, current density 2-3 A/dm², temperature 15-35 ℃. FeSO₄·7H₂O, ZnSO₄·7H₂O, pH value, temperature and current density have great effect ? on component, appearance and corrosion resistance of coating. Citric acid and Al₂(SO₄)₃·12H₂O have little effect on component, appearance and corrosion resistance of coating, except their concentration being too low. (NH₄)₂SO₄ and additive ZFS-3 have little effect on component of coating, but they have great effect on appearance and corrosion resistance of coating. SiO₂ and additive. ZFS-2 have little effect on iron content and appearance of coating, but they have great effect on silica content and corrosion resistance of coating. Additive ZFS-1 has little effect on component and corrosion resistance of coating, butit has great effect on appearance of coating. Ascorbic acid has little effect on silica content of coating, but it has great effect on iron content, appearance and corrosion resistance of coating.Low-iron Zn-Fe-SiO2 composite coatings can be obtained in chloride bath, and component scopes of the coatings are Fe 0.2-0.8% and SiC>2 0.4-0.6%. Composition of the electrolyte and operating conditions are as follows: FeSCy7H2O 6-10 g/L, ZnCl2 80-100 g/L, KC1 200-230 g/L, SiO2 1-3 g/L, ascorbic acid 0.8-1.2 g/L, additive ZFS-1 8-12 ml/L, additive ZFS-2 8-10 ml/L, additive ZFS-3 15-25 ml/L, pH 3.5-5.5, current density 1-2 A/dm2, temperature 15³5°C. FeSCV7H2O, ZnCb, pH value, current density and temperature have great effect on component, appearance and corrosion resistance of coating. Additive ZFS-3 has little effect on component of coating, but it has great effect on appearance and corrosion resistance of coating. Ascorbic acid has little effect on silica content and corrosion resistance of coating, but it has great effect on iron content and appearance of coating. Additive ZFS-2 has little effect on iron content and appearance of coating, but it has great effect on silica content and corrosion resistance of coating. SiC>2 has little effect on iron content of coating, but it has great effect on silica content, appearance and corrosion resistance of coating. KC1 and additive ZFS-1 have* little effect on component and corrosion resistance of coating, but it has great effect on appearance of coating.Corrosion resistance, adhesion, porosity and hydrogen embrittlement of Zn-Fe-SiO2 composite coatings have been tested systematically, and these data have been compared with the properties of Zn and Zn-Fe alloy. Besides performance of Zn-Fe-SiO2 composite coatings^ current efficiency, deposition rate and throwing power of the electrolyte have been tested too. The results show that corrosion resistance and all other performance of Zn-Fe-SiC>2 composite coatings are better than Zn and Zn-Fe alloy's, and all the performance of Zn-Fe-SiC^ composite electrolyte are desirable. Therefore, the Zn-Fe-SiC>2 composite electrodepositing technique is superior to Zn and Zn-Fe alloy electroplating.Comparing sulfate system with chloride system in property of coating, capability and cost of electrolyte, operating condition and stability of technics by scale-up experiment, it can be seen that chloride system has more advantages. In the mid-scale experiment with chloride system, current density, distance between electrodes, stir, number and distributing pattern of anodes and additives have great effect on the appearance of coating, so the operating must be normative. During the manufacture process, uniform and bright Zn-Fe-SiC>2 composite -coatings can be obtained, the component, thickness and corrosion resistance of different pp|coating keep even, the value of pH, composition and capability of electrolyte keep equably. and maintenance operation is convenient. The results of scale-up experiment show that this technique is innocuous and safe.Additive ZFS-2 has been synthesized, which can promote SiC>2 particles depositing with metal on the cathode. The function of additive ZFS-2 in the process Zn-Fe-SiO2 composite electrodepositing, and theory of SiO2 co-deposition with metal have been studied. Some conclusions have been drawn. SiC>2 particles do not change cathode polarization. Additive ZFS-2 can make SiC>2 showing positive charge by being adsorbed on the particles' surface, and the attraction power between S1O2 particles and cathode is enforced, therefore, SiC>2 particles can be enwrapped in depositing metal on the cathode.The control steps of composite depositing in sulfate and chloride systems with different concentration of particles have been discussed. The experimental data of the two systems agree well with the proposed codeposition mechanism based on a two-step adsorption process. Loose adsorption of particles on the cathode is the control step when particle concentration is low, while strong adsorption of particles on the cathode will be the control step when particle concentration is high, but the turning point corresponds with different value of particle concentration. In sulfate system, the control step will be strong adsorption of particle when particle concentration is more than 10 g/L. In chloride system, the process of composite deposition will be controlled by strong adsorption of particle when particle concentration is more than 4 g/L.Potential-pH diagrams of Zn-H2O and Fe-FbO system have been drawn, and the corrosion process of coatings has been analyzed thermodynamically. In order to study corrosion kinetics, the corrosion rates of Zn-Fe-SiC>2 composite coatings in different medium have been tested, and the static corrosion potential, polarization resistance and corrosion current of coatings in 5% NaCl solution have been determined by electrochemical method. Zn-Fe-SiO2 composite coatings are eroded quickly in acid and neutral salt spray, while slowly in neutral salt solution.and atmosphere. The metal component is the main factor determining the corrosion rate when composite coatings corrode in strong corrosive medium, while the SiO2 content is more important if the medium's corrosive is weak. Experimental data show that SiC>2 particle can make coating static corrosion potential nobler and polarization resistance larger in salt solution, so there is less corrosion current in Zn-Fe-SiCh composite coating. When Zn-Fe-SiO2 composite coatings are eroded in salt solution, S1O2 particle willform corrosion-products with metal, and these corrosion-products can adhere to the surface of coating and decrease the corrosion rate. Morphology of Zn, Zn-Fe, and Zn-Fe-SiO2 coatings show that Zn-Fe-SiO2 composite coating has the finest crystal, so it has the largest corrosion resistance. The component of Zn-Fe-SiC>2 composite is uniform, so probability of local corrosion will be low. Furthermore, SiC>2 particle can diminish the porosity of coating, so the substrate will not be eroded by environment directly.Zn-Fe-SiC>2 composite coatings have been put on a bridge on atmospheric corrosion test for one year. Trial manufacture of Zn-Fe-SiC>2 composite coatings has been put in practice on surface of steel tubes for Yunnan Internal-combustion Engine Plant. In the last part of this dissertation, the application and market of Zn-Fe-SiO2 composite coatings are presented in detail, and the economic and social benefits of industrialization of this technique have been analyzed scientifically.