The Mechanism of Deposition of Titanium Coatings from Fused Salt Baths

The mechanism of the titanizing process, as described previously, consists mainly in the direct collision of Ti particles dispersed in a molten salt bath ( , , or both together) with the surface of the metallic or ceramic object submerged. The Ti particles stick to the surface, and, if the temperature is high enough (e.g., for Fe above 910°C) a part of the Ti deposited diffuses into the substrate forming a diffusion layer. Its thickness conforms to Rhines' diffusion equation. By the titanizing method described not only can Fe, low carbon steel, Co, Ni, and noble metals be coated, but so can ceramic materials such as porcelain and alundum. Coatings on Fe can also be produced in fused salts ( or ) containing lower Ti chlorides in larger amounts but no metallic Ti. In this case the coating is produced by an exchange mechanism, e.g., (coating on excess Fe), or by Ti formed in the disproportionation reaction. The collision mechanism and the exchange mechanism may go on simultaneously with more active metals. However, not more than 4.0% of the total Ti coating is produced by the latter mechanism. The lower Ti chlorides in the fused salt baths are formed by the reaction ; the Na evaporates partially at the working temperature of above 900°C.