Coating Process of iron siliside on iron for oxidation resistance

开发了一种在低碳钢表面涂覆Fe3Si-SiO_2复合涂层的工艺，以提高其耐腐蚀和抗氧化性能。利用熔盐在铁片上沉积硅化铁，然后在低温下用臭氧气体氧化铁片表面，考虑到蒸发和氧化的最小化以及在空气中的稳定性，选择了由KCl-NaC l-NaF-Si-SiO_2-Na_2SiF_6组成的熔盐。在纯铁和低碳钢表面化学镀1小时即可形成厚度超过100μm的平整的Fe_3Si层。当硅化物层生长如此迅速时，熔盐被困在硅化物层与衬底的界面上。当以铁硅合金为衬底时，未发现缺陷。这些发现表明，从纯硅到熔融盐的硅离子供应如此之快，导致了缺陷。包覆机理由不成比例反应和扩散组成。考虑到生长机理，用铝硅液态合金代替纯硅，在铁表面成功地镀制了α-(Fe，Si)合金，且涂层中不含任何缺陷。随后，熔盐被替换为含有纯硅的传统盐。该层在层内存在浓度梯度，在界面处没有缺陷。通过合金化硅来控制活性或降低盐中的硅离子浓度，可以获得健康的涂层，同时在钼、铌、镍等难熔金属上形成抗氧化性的硅化物涂层。一个新的发现是，臭氧气体吹气可以通过产生熔融的CrO_3来形成Cr2O_3层。

A process to coat Fe_3Si-SiO_2 composite film on the mild steel was developed for corrosion and oxidation resistance. Using the molten salt, the iron silicide was deposited on the iron sheet, and its surface was then oxidized by ozone gas at the low temperature.The molten salt consisting of KCl-NaCl-NaF-Si-SiO_2-Na_2SiF_6 was chosen because of the minimized evaporation and oxidation, and stability in air. Electroless coating on the pure iron and on the mild steel formed the smooth Fe_3Si layer over 100μm thick within 1 hour. The molten salt was trapped at the interface between silicide layer and the substrate when the growth was so rapid. No defect was fund when the iron-silicon alloy was supplied as the substrate. These findings showed that such a rapid supply of silicon ions from pure silicon to the molten salt caused the defects. The coating mechanism consisted of disproportional reaction and diffusion. Considering the growth mechanism, the α-(iron, silicon) alloy was successfully coated on iron and the layer did not contain any defects, when the aluminum-silicon liquid alloy was used instead of pure silicon. Subsequently the molten salt was replaced to the conventional salt containing pure silicon. The layer had the concentration gradient in the layer and no defects at the interface. The healthy coating was achieved by alloying silicon to control activity, or by reducing silicon ion concentration in the salt.Similarly, the silicide layer for oxidation resistance was formed on the refractory metals such as molybdenum, niobium and nickel. A new finding was that ozone gas blowing could form Cr_2O_3 layer by producing the molten CrO_3.