This is a detailed study aimed to understand the effects of pack composition on the formation and growth of aluminide coatings on alloy steels by pack aluminisation at 650° C, a temperature below the melting point of Al (660° C), using pack powders consisting of Al as depositing source, a halide salt as an activator and Al2O3 as inert filler. The packs activated by ACl(3), NH4Cl, AlF3 and NH4F were used to investigate the effects of the type of halide salt on the coating formation and growth process and subsequently to identify the most suitable activator for pack aluminising alloy steels at 650° C. The effects of pack Al content on the rate of coating growth were then studied by varying the pack Al content from 1.4 wt% to 10 wt% whilst fixing the pack activator content at 2 wt%. It was observed that among the halide salts studied, AlCl3 is the only suitable activator for pack aluminising alloy steels at 650° C and the rate of coating growth increases with the pack Al content. The equilibrium partial pressures of vapour species generated at the deposition temperature in packs activated by different types of halide salts were calculated and the results were discussed in relation to the observed deposition tendency of packs activated by different types of activators. A vapour phase transportation model was applied to elucidate the relationship between the rate of coating growth and the pack Al content. It was also demonstrated that by combining the low temperature pack aluminising parameters identified in this study with electroless or electro Ni plating, coherent nickel aluminide coatings free of microcracking can be produced on alloy steels at 650° C. (C) 2005 Springer Science + Business Media, Inc.