Abstract: Aluminum-chromium diffusion coatings on pure nickel substrate were formed by two-step pack cementation process. It was found that existed many voids in the coatings which were formed by first aluminizing and then chromizing. It could be understood that aluminum in the coatings reacted with HCL gas decomposed from activator NH₄CL and formed ACL₃,which evaporated and resulted in formation of the voids.Oxidation resistance of Al-Cr coating with these voids was reduced signigicantly.