A Tracer Study of Porous Anodic Alumina

Abstract

The present study employs a tungsten tracer incorporated into the aluminum substrate to investigate the development of porosity in anodic alumina formed in phosphoric acid electrolyte. An unusual inversion of the tracer distribution is revealed as the tracer layer traverses the barrier region. Although initially incorporated into the barrier layer at locations beneath pore bases, associated with the scalloped metal/oxide interface, the tracer at these locations subsequently lags behind that found at the cell walls. The behavior is contrary to expectations of a field-assisted dissolution model of pore development, with usual migration behaviors of film species in the barrier layer. However, the findings are consistent with pore formation due mainly to flow of alumina from the barrier layer toward the cell walls, driven by film growth stresses. Flow of film material can also account for the presence of phosphorus species in the film and the increased thickness of the film relative to that of the oxidized metal. Anodic alumina films are used extensively in protection and functionalization of aluminum alloys, in electronics through aerospace to architecture. The films are usually formed in aqueous electrolytes, with two morphological types recognized that depend upon composition of the electrolyte, pH, current density, voltage, temperature, etc. Fig. 1. 1-4 Barrier films consist of compact, amorphous alumina of uniform thickness, up to a few hundred nanometers. Porous films comprise a thin barrier layer next to the metal and an outer layer of porous alumina, up to tens of m thick. 2,3 The pores are of approximately cylindrical section and extend from the film surface to the barrier layer. The thickness of the barrier layer and the diameter of the pores are related to the forming voltage, with ratios of 1n m V 1 , while the thickness of the porous layer depends primarily upon the anodizing charge for a particular current density. The porosity has often been explained by massively increased dissolution of the alumina at the pore base under the high electric field of the barrier layer. 5 An early suggestion was made of a role of oxide flow in the dissolution process, with flow occurring due to electrostriction stresses, estimated to be of the order 100 MPa and sufficient to deform oxides. 6