Distribution of Phosphate Ions in the Porous Anodic Oxide Film Formed on Aluminium in Phosphoric Acid Solution

Abstract

A porous oxide film having parallel pores is produced when aluminium is anodized in such a solution as phosphoric acid and a thin compact oxide layer called the barrier layer exists at the bottoms of pores, The thickness of pore-wall, which is a half of the distance between the adjacent pores, is nearly the same as that of the barrier layer, b (Fig. 1). lt has been known that a small amount of phosphate ion is included in the film as the result of coordination of these iions towards aluminium ions in the oxide.In this investigation, the distribution of phosphate ion across pore-wall( =1000A) was measured for the film formed at 100 V (vs. SCE) in 4% HsPO kept at 250C, a technique of dissolving the film in 2 mol/l H2SOG solution being utilized. The dissolution of. the filrp proceedsYff the surface of pore-wall in such a manner that pore widens evenly. all thro.ugh ihe depth of the pore (Fig. 1), so that the distribution of, phosphate ion was determined-by measur the time-variations of the amounts of phosphate and aluminium ions in solutio4 during dissolution (Fig. 2). The concentration of phosphate izefo at 900 A (Fig. 4). The average concentration of phosphate be abeut O.14 mg-PO/rng-Al. The to that across the pore-waH, in view o eiectronmicroscepy. The mechani sm of film growth explaining described. nearly constant irrespective of, but increases gradually and decreases nearly equal to in the film was estimated to distribtition of phosphate across the barrier laYer was similar of the results of examination of the film by ESCA and the phosphate distribution is described.