Abstract: A process is disclosed for treating the surface of a valve metal such as aluminum to form a two layer protective coating thereon using an anodizing bath consisting essentially of an aqueous solution having a concentration ranging from about 0.001 molar to a saturated solution of a monomeric phosphorus-containing compound selected from the class consisting of a 1-30 carbon water soluble phosphonic acid, a 1-30 carbon water soluble phosphinic acid, and mixtures thereof. The valve metal surface is anodized in the anodizing bath while maintaining a voltage selected from a range of from about 1 to about 400 volts until the current density falls to a level indicative of the fact that a nonporous valve oxide layer has been formed on the valve metal surface and a reaction product from the monomeric phosphonic/phosphinic acid compound is chemically bonded to the oxide layer.