| Anode steel-claw, which is an important part of prebaked-anode type cell, is an assembly unit for connection of carbon anode and aluminum guide rod. Anode steel-claws are always used in a bad environment of high acid and high temperature for a long time. Therefore, the anode steel-craws can just be used in a limited time because of the corrosion of acid. Furthermore, liquid aluminum can be polluted by corrosion layer, which results in the decrease of the quality of aluminum ingots. Thus, it is very important to find out the corrosion mechanism of the anode steel-claw by acid and find the reasonable anti-corrosion methods. The following investigations have been carried out in the thesis:The corrosion mechanism of the anode steel-claw has been investigated firstly. The results show that the steel-claws are eroded primarily due to oxidation reduction reaction by O2and SO2with the production of Fe3O4and Fe2O3. Furthermore, the steel-claws are also eroded because of washing by the electrolyte liquid. The method of plasma spraying has been employed to improve the corrosion resistance of the steel-claw by preparing Al2O3ceramic coatings on the surface of anode steel-claw in the thesis.The designed orthogonal experiments have been carried out in order to investigate the parameters of plasma spraying Al2O3ceramics coating. The best parameters have been obtained as following by the study:spraying distance of100mm, spraying power of45kW, the gas flow of2000L·h-1, feeding voltage of18V.Adhesive strength, microstructure and corrosion resistance of gradient coating of aluminum-coated nickel/Al2O3ceramic and NiCrAl/Al2O3ceramic have been studied. The results show that the adhesive strength of Al2O3ceramic coatings increase because of the base coating, especially for aluminum-coated nickel/Al2O3ceramic adhesive with an average adhesive strength of40.35MPa. The average adhesive strength for NiCrAl/Al2O3ceramic is37.20MPa higher than that for Al2O3ceramic coatings of23.37MPa. The microstructure of gradient coating for aluminum-coated nickel/Al2O3ceramic is the best. The combination surface is smooth, spraying material melt is good and there are less coating porosities.In the microstructure of gradient coating for NiCrAl/Al2O3ceramic microstructure, there are less defects, the combination surface is smooth and there are less coating porosities. But in the microstructure of gradient coating for Al2O3ceramic coating, there are more defects and coating and there is a small amount of micro cracks. The corrosion resistance for gradient coating of aluminum-coated nickel/Al2O3ceramic is best with coating corrosion weightlessness of5.3mg, followed by the gradient coating of NiCrAl/Al2O3ceramic coating with corrosion weightlessness of6.1mg. And the corrosion resistance for Al2O3ceramic coating is worst with corrosion weightlessness of9.2mg.The phase composition, deposition rate, porosity rate and thermal shock resistance for Al2O3ceramic coatings have also been studied. The results show that the80%of the Al2O3phase in the Al2O3ceramic coatings are mainly composed of the steady state α-Al2O3and the rest is γ-Al2O3. The deposition average rate and porosity rate for spraying Al2O3ceramic coating is43.1%and11.2%respectively. The coating thermal shock is43times with spraying of35kW power,57times with40kW and50times with45kW.Finally, the preliminary experiment of the Al2O3ceramic coating on the anode steel-claw industrial application has been carried out. The steel-claws have been installed in an aluminum manufacturer in Yunnan province and have been used in two electrolytic cycle though the treatment of sand blasting, cleaning processing and spraying the DH-2008plasma spraying system Al2O3ceramic coating. The Results show that the steel-claws are eroded in a smaller degree. So the plasma spraying Al2O3ceramic coating on surface of steel-claw plays an obvious rule of protection of steel-claw. |
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