Iron aluminide coatings on ferritic steels by CVD-FBR technology

The deposition of Al on ferritic steels coupons by chemical vapor deposition in fluidized bed reactors (CVD-FBR) was studied. Before performing the experiments, thermochemical calculations were used to obtain the initial parameters of processes. These calculations were made by means of the Thermocalc software. The thermodynamic system studied consisted of Al powder as donor, HCl and H2 as gaseous activators and Ar as fluidizing inert gas. The results indicate that the most important precursor aluminide chlorides formed in this system, in the temperature range from 400 to 600 °C, were AlCl3, Al2Cl6, AlHCl2 and AlCl. Aluminium diffusion coatings were obtained on ferritic steel (P-92). The effect of heat treatment under inert atmosphere was also been studied. Diffusion heat treatment was performed at 700 °C for 2 h in order to allow the phase transformation of Fe2Al5 to phases with lower aluminium content. Morphology and composition of coatings were characterized by different techniques, such as optical microscopy (OM), scanning electron microscopy (SEM), electron probe microanalysis, and X-ray diffraction (XRD). The results are discussed in relation to the diffusion occurring during the heat treatment of Al coatings obtained at low deposition temperatures and short deposition times.