On the selection of halide activators for the formation of hybrid Ni-aluminide/Ni coatings on creep resistant ferritic steels by low temperature pack cementation process

Thermochemical calculations were made and experiments were subsequently conducted to verify the thermochemical predictions in an attempt to establish a basis for selecting suitable halide activators for forming Ni-aluminide/Ni hybrid coatings on creep resistant steels by pack cementation at temperatures below 700 °C. The pack powder mixtures analysed and used in subsequent experiments consisted of Al, Al2O3 and one of chloride or fluoride salts which included NH4Cl, AlCl3 (anhydrous), NaCl, NH4F and AlF3. Pack Al content was varied from 0.5 to 16 wt% to show its effects on aluminising kinetics at constant temperature and time. The aluminised layers were analysed using XRD, SEM and EDS. It was demonstrated that the partial pressure of AlX (X = Cl and F) vapour species generated in packs can be used to measure the activating strengths of different chloride and fluoride salts. This is particularly true in the case of chloride salt activated packs in which aluminising kinetics was shown to be higher in packs activated by salts with higher activating strengths. Fluoride salts are not suitable activators in the low temperature range concerned because a solid AlF3 phase layer would deposit on the surface, which would then prematurely terminate the aluminising process.