Coating Process of iron siliside on iron for oxidation resistance

研究了在低碳钢表面涂覆Fe_3Si-SiO_2复合膜的工艺。利用熔盐将硅化铁沉积在铁片上，然后在低温下用臭氧气体氧化其表面。选择由KCl-NaCl-NaF-Si-SiO_2-Na_2SiF_6组成的熔盐是因为其蒸发和氧化最小，并且在空气中稳定。在纯铁和低碳钢表面进行化学涂层，在1小时内形成了厚度超过100μm的光滑Fe_3Si层。由于生长过快，熔盐被困在硅化物层与衬底之间的界面处。采用铁硅合金作为衬底，未发现缺陷。这些发现表明，硅离子从纯硅到熔盐的快速供应导致了缺陷。涂膜机理由非比例反应和扩散组成。考虑到生长机理，当用铝硅液态合金代替纯硅时，α-（铁、硅）合金成功地包覆在铁表面，且包覆层不含任何缺陷。随后，熔盐被替换为含有纯硅的常规盐。层内具有浓度梯度，界面处无缺陷。通过合金化硅来控制活性，或通过降低盐中的硅离子浓度来获得健康的涂层。同样，在钼、铌、镍等难熔金属上也形成了抗氧化的硅化物层。新的发现是，臭氧气吹可以通过产生熔融的Cr_2O_3形成Cr_2O_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.