On the aluminisation of stainless steel by CVD in fluidised beds

The effect of coating parameters such as bed composition, H2/HCl reactive gas ratio and temperature is investigated so as to obtain Al diffusion coatings on AISI 304 stainless steel substrates by chemical vapour deposition in fluidised bed reactors (CVD-FBR). It will be shown that thicker coatings are obtained when the beds are composed of AlCl3 (s) instead of pure Al powders. An increase in the reactive gas mixture also produces thicker coatings but some porosity may be found occluded in the proper coatings. Finally, moderate temperatures and times seem to be the most suitable to produce the best coatings avoiding the presence of sharp coating/substrate interfaces. A post-coating heat treatment brings about double-layered coatings based on FeAl and FexNiyAl containing Cr derivatives.