Impurity-driven defect generation in porous anodic alumina

We report that the morphologies of porous anodic alumina films are affected significantly by the presence of small amounts of copper impurity in the aluminium substrate, resulting in generation of defects during anodizing in phosphoric acid over a wide range of conditions. The copper is accumulated at the metal/film interface and transported preferentially to triple points of the alumina cells, where oxygen gas is generated leading to development of gas-filled voids.