Deposition of aluminium on magnesium by a CVD process

The feasibility of Al deposition on Mg and Mg-alloys by means of Fluidized Bed CVD (FBCVD) or Pack Bed CVD (PBCVD) processes is evaluated by means of thermochemical computations and experimentation. For the thermodynamic analysis, a closed system consisting of Al2O3 as filler, Al as donor, halide or halide compound as process activator and Ar as inert gas was considered. Different activators were evaluated in the temperature range from 280 to 420 degreesC. Among them NH4Cl, and iodine are the best candidate compounds due to the relatively high partial pressures of the formed aluminium halides. The analysis indicates that the deposition of aluminium on magnesium takes place through a displacement reaction mechanism on the surface followed by limited inward diffusion of Al. Pack bed experiments have shown that by use of NH4Cl activator, aluminium is deposited locally. Iodine activator results in more even but porous coatings. The process is simple and promising for the formation of corrosion and wear resistant coatings on magnesium. However, in order to provide a long-term protection, the coating should be deposited under conditions that eliminate through-thickness pores. Further developments are required in order to achieve dense, pore-free coatings. (C) 2003 Elsevier B.V. All rights reserved.