Twice-anodized aluminum article having an organo-phosphorus monolayer and process for making the article

A method for producing a triplex coating on an aluminum substrate comprising,

• sequentially anodizing said substrate in first and second anodizing procedures,
• said first procedure comprising anodizing said substrate in a first aqueous acid electrolyte selected from the group consisting of phosphoric acid, phosphorous acid, oxalic acid, chromic acid, and sulfuric acid to produce (i) a porous anodic oxide layer having open pores and a passivation layer forming bottoms thereof, forming a singly-coated substrate on an outer surface thereof;
• and thereafter,
• in a second anodizing procedure comprising, anodizing said singly-coated substrate in a second aqueous organophosphorus-containing electrolyte consisting essentially of a substantially water-soluble organophosphorus compound having a head portion containing an O-P linkage bondable to Al through its O atoms, and a remaining tail portion of said compound having at least two directly linked carbon atoms which are bonded to the P atom of said linkage through a C atom, so as to generate (ii) an organo-phosphorus monomolecular essentially continuous monolayer on said porous anodic oxide layer outer surface, and simultaneously to produce (ii) a barrier layer of non-porous aluminum oxide under said porous anodic oxide layer;
• said non-porous barrier oxide layer being generated in a thickness of about 14 .ANG./volt under said porous anodic oxide layer and integrally with unoxidized aluminum, said O-P linkage being chemically bonded to Al atoms in said outer surface, said porous anodic oxide layer having a P/Al ratio of 0.5 or less;
• and
• said barrier layer having a P/Al ratio in a range from 0.0001 to 0.06.