Ahstrati-Porous
anodic Al,4 flhns were investigated for their behaviour during hydrothermal treatment in H,O at 100°C and a mechanism for the process of oxide hydration was proposed. The films were prepared galvanostatically in a 15% w/v HsSO, bath at 20, 25 and 30°C and at 5, 15 and 35 mA cm-'. The resulting thicknesses were between 1.5 and 120pm. It was revealed that the maximum amount of H,O uptake, on prolonged hydration and retained by the oxides, is directly related to the Pore void volume of initially produced dry films. Their ratio, M, m /v, is always close to 1 g cme3 for all films having a mean thickness-of Q 75 pm.-Additionally, dry or'hydrated A&O, examined by XRD and ir spectroscopy techniaues failed to reveal the nresence of anv crvstalline form of ALO,. AlOOH and AhOH), . The absenck of any crystalline hydraied forms is consisient with the dependence of mw,,, on v supporting the hypothesis of a microcrystalline nature of films. An A&O, hydration mechanism was proposed including a "reaction" between H,O and A&O, pore wall surface. It results in the addition of OH groups and chemisorbed molecular H,O on microcrystallite surfaces effecting the separation of microcrystallites and physically adsorbed molecular H,O on the hydroxylated/hydrated surfaces in the intercrystallite spaces causing the swelling of the moderately coherent hydrated oxide layer on pore walls. Ultimately, the process results in the depletion of A&O, at points where it has become emaciated, ie at the mouths of conical pores in the cases of the maximum limiting film thicknesses attained and in the production of a hydrated layer, on pore walls, of variable thickness sufficient to fill up pores along a distance from the pore base, depending on structural film features.