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| Electrolytic Coating of Aluminum on Iron and Steel and on Superalloys in Fused Salt Bath |
| Shih Shengtai Feng Mingren Cao Tielian Ma Zhangyuan and Zhang Cailin (Shanghai Institute of Metallurgy Chinese Academy of Sciences) |
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Abstract Aluminum coatings can greatly improve oxidation resistance of iron and steel and hot-corrosion resistance of superalloys. They are usually carried out industrially by pack cementation or hot dip process. In pack cementation, the articles to be coated are packed in a mixture of powdered ferro-aluminum and aluminum oxide and heated to 850-1000℃ for 5 to 24 hours. This process involves much labor and prolonged time. The hot dip process on the other hand, though simple and rapid, suffers from such drawbacks as surface roughness, dissolution of iron in aluminum and losses of aluminum through oxidation and evaporation. Fused salt electrolytic coating of aluminum is carried out as follows. Molten aluminum is connected as anode, and is covered by a molten salt mixture (1: 1 NaCl+KC1 with some cryolite) in which the work pieces are embedded. The work pieces are connected as cathode. Direct current is passed on, and Al passes into Al~(+++) ions in the fused salt, which upon discharge. deposit on the cathode, and diffusion takes place at the process temperature. The work pieces are not in contact with molten aluminum so that dissolution by the latter is out of the question. As molten aluminum is covered with salt, evaporation and oxidation losses are minimized. The surface is smooth and no post heat treatment is needed. The thickness of electrolytic Al coating increases with coating temperature, time and current density. The Al uptake depends, however, mainly on current density and time, and is independent of temperature. Faraday's law is obeyed, and the current efficiency amounts to about 85%. The structure of the coating as revealed by electron probe, X-ray diffraction and metallographic techniques shows the presence of a very thin film of pure Al at the outermost surface,and then the following phases in succession,namely: Al_3Fe Al_6Fe_2, AlFe and AlFe_3. Oxidation resistance of iron and steel is greatly enhanced with electrolytic Al coating, and hot-Corrosion resistance of Ni-base superalloys is also much improved by such coatings.
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Received: 25 August 1981
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Cite this article:
Shih Shengtai Feng Mingren Cao Tielian Ma Zhangyuan and Zhang Cailin (Shanghai Institute of Metallurgy Chinese Academy of Sciences). Electrolytic Coating of Aluminum on Iron and Steel and on Superalloys in Fused Salt Bath. J Chin Soc Corr Pro, 1981, 1(4): 1-7.
URL:
https://www.jcscp.org/EN/ OR https://www.jcscp.org/EN/Y1981/V1/I4/1
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[1] S. T. Shih (石声泰) De Re Metallica (A Montana School of Mines Publication Vol. XXll, No. 2, 5 (1956)
[2] S. T. Shih (石声泰) Paper presented at NACE International Conference on High Temperature Corrosion, March 2-6 (1981) San Diego,California U. S. A.
[3] 石声泰,闻立昌:钛在熔盐中的阴极保护,(全国腐蚀和防护科学技术会议报告集)170(1964)
[4] N. C. Cook, Proceedings of the International Conference, Protection against Corrosion by Metal Finishing, Basel, 22-25 November, 157(1966) |
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