Silicide coating on refractory metals in molten salt

For better oxidation resistance of refractory metals in air, the electroless coating of silicide in the molten salt was developed in open air at 973-1173 K. The molten salt consists of NaCl, KCl, Na2SiF6 and Si powder, where the proportional reaction between Si and Si4+ ions forms Si2+ ions. Si2+ deposits on the metal substrate and forms the metal silicide. The deposited silicide layers were classified into two categories depending on the metal substrates: (1) Nb, Mo and Cr mainly formed silicon-rich disilicide (MSi2) layer. (2) Fe, Ni and Co formed silicon-poor silicide layer (MSin, n<2), such as Fe3Si. This difference was described by the diffusivity of Si through the silicide layer into the metal.