The present invention relates to a process for forming on the surface of a metal part a protective coating containing aluminium and zirconium, in which process said part and a cement made of an aluminium alloy are brought into contact with a gas at a treatment temperature in a treatment vessel, the gas comprising a carrier gas and an activator, the activator reacting with the cement to form a gaseous aluminium halide that decomposes on the surface of the part, depositing metallic aluminium thereon, the activator containing a zirconium salt such as ZΓO (¾ obtained from granules of a zirconium salt), disassociation reactions of said zirconium salt taking place within a disassociation temperature range with formation of a Zr metal coating on the surface of the part, the assembly comprising the part, the cement and the zirconium salt granules is progressively heated in the chamber from room temperature up to the treatment temperature, the process being characterized in that the treatment chamber is maintained at an overpressure with no carrier gas flow throughout the temperature range corresponding to the disassociation reactions of the zirconium salt.