A method for surface treatment of a part made of aluminum, magnesium, or one of the alloys thereof, to protect the part from corrosion. The method comprises consecutively immersing the part in a first aqueous bath containing a corrosion-inhibiting metal salt and an oxidizing compound, and a second aqueous bath containing an oxidizing compound and a corrosion-inhibiting rare-earth salt. The method can be carried out for the chemical conversion of aluminum or the alloys thereof, and of magnesium or the alloys thereof, on parts that have not been previously treated, or after anodizing the part to seal the anodic layer.