A method of treating metallic workpieces with an anodizing solution, compositions of the anodizing solution and the coatings prepared with this anodizing solution for anodizing metallic surfaces, especially surfaces of magnesium, magnesium alloys, aluminum and aluminum alloys, are disclosed. The compositions are basic aqueous solutions comprising a water-soluble inorganic hydroxide, phosphorus and oxygen containing anions, at least one surfactant and an alkaline buffer based on at least one alkaline hydrolyzed silane, on at least one alcohol showing at least one alkaline radical group or on a mixture of them.