熔盐电解法渗铝的研究

在钢铁和高温合金上渗铝,可以大大提高这些材料抗高温氧化和热腐蚀的能力。目前工业上常用的渗铝方法有固体包装法和热浸法。固体包装法是将工件埋在含有铝铁合金粉和氧化铝粉的混合物中,加热到850°至1000℃,保温5至24小时而获得渗铝层。此法不需特殊技术,但要先制备铝铁合金,而装箱、进炉出炉,卸出工件劳动较多,因为连箱加热,渗铝速度慢,因之所需时间很长,为其缺点。热浸法是将工件浸于熔融铝浴中而获得渗铝层,速度较快且操作简便,但铁在铝液中有一定程度的腐蚀溶解,工件渗铝后表面常不均匀,有铝堆集,而且由于金属铝的蒸发和氧化,铝的损耗也较大,此外渗铝层脆性较大.需要进行渗后热处理,这些都是热浸法不足之处。 熔盐电解法渗铝是在坩埚(石墨或金属)中盛金属铝(可用铝屑),上面放1:1(分子比)的NaCl+KCl混合盐,另加少量冰晶石。坩埚和铝接为阳极,工件接为阴极。在惰性气氛保护下加热使盐熔化,然后通电流,铝作为阳极被熔盐腐蚀而生成铝离子,在阴极(工件)上沉积。由于工件不在铝浴中,故不被铝溶解,且受到阴极电流的保护作用,故不被熔盐腐蚀。铝上有熔盐复盖,蒸发和氧化均较小,故铝的损耗亦小。渗铝层较均匀,渗后不必另行热处理.沉积的铝量主要决定于电流密度与时间,服从法拉第定律,与温度无关.电流效率约在82约91％间,平均约85％。沉积的铝在高温下与底金属相互扩散而形成渗铝层,其厚度决定于温度、时间及电流密度。用电子探针分析、X-射线衔射分析及金相检查,得知纯铁上电解渗铝层最外表面有一薄层金属铝,往内为Al_3Fe、Al_5Fe_2、AlFe及AlFe_3。 钢铁经熔盐电解渗铝后,抗高温氧化性能大为提高。高温合金用此法渗铝后,抗热腐蚀性能亦显著改进。

Electrolytic Coating of Aluminum on Iron and Steel and on Superalloys in Fused Salt Bath

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.