Low-pressure gas-phase pack cementation coating of complex-shaped alloy surfaces

Abstract

A new pack cementation process is reported where the substrates are treated in the gas phase, over a pack, in a semi-open chamber maintained under reduced pressure. A stable halide is always present in the condensed state in the chamber and the process is controlled by the metal activities in the pack, the total pressure and the temperature of the treatment. It is applied here to the aluminization of massive substrates and external and internal surfaces of pure nickel and nickel alloy tubes, but it can be extended to various geometries such as complex-shaped substrates or porous materials. The conditions necessary to increase the gaseous transport rate are deduced from a gaseous diffusion model which takes into accout the Knudsen diffusion contribution at the lower pressures, inside tubes or narrow cavities. The intrinsic diffusion rates in the gas phase are compared with the intrinsic solid state diffusion rates under various conditions of aluminization ranging from high to low aluminium activity, total pressure and temperature ranging from 0.3 to 760 Torr and from 670 to 950 °C respectively. It is shown how these parameters can be varied in conjunction to obtain a uniform coating on a given geometry. Finally, parts to be coated are classified into two groups depending on whether the Knudsen diffusion is negligible or not. This process extends to the transport of other metals and some co-transport.