Tracer studies relating to alloying element behaviour in porous anodic alumina formed in phosphoric acid

The behaviours of hafnium, molybdenum and hafnium tracer species in porous anodic alumina films are compared in order to investigate the relationship between the migration rate of alloying element species and their amounts and distributions in the films. The study employs substrates consisting of a thin layer of Al–Hf, Al–Mo or Al–Nd alloy deposited by magnetron sputtering on to electropolished aluminium, with a top layer of aluminium deposited above the alloy layer. The substrates are then anodized at constant current density in phosphoric acid to incorporate the tracer species into the porous anodic film. The tracer distributions in the film are investigated by scanning electron microscopy, and the amounts of tracer quantified by Rutherford backscattering spectroscopy. The distributions of the species within the films differ for the three tracer species. Molybdenum species, which migrate slower than Al3+ ions, are retained within the films, since they are unable to migrate to the pore base regions of the films. In contrast, hafnium and neodymium species, which respectively migrate at rates similar to and faster than Al3+ ions, are able to reach the pore base regions where they are lost to the electrolyte; the loss of the faster migrating-neodymium species is greater than that of hafnium species. The behaviours are related to the influence of flow of the alumina within the barrier layer on the transport of the tracer species.