The Mechanism Regulating Film Thickness During Porous Anodic Oxide Growth on Aluminium: Transition from Homogeneously Thick Films During Soft Anodizing to Nodules Formation During Hard Anodizing

The growth of porous anodic film during sulphuric acid anodizing has been studied with the aim of investigating the mechanism resulting in uniform film thickness under conventional anodizing conditions and localized film growth under hard anodizing conditions. Based on the field-assisted flow model, it is proposed that the film comprises a rigid part, the cell walls, and a barrier layer plasticized by the combined effect of the electric field, the electrostriction stresses and the ionic migration process. Under conventional anodizing conditions, the film is uniform in thickness because the rigid part regulates the growth rate at the plasticized barrier layer by providing a mechanical connection between neighbouring pores, and hence a mechanical constraint. Vice versa, under hard anodizing conditions, cracking of the rigid part of the film due to rapid film growth rate leads to loss of the regulation mechanism provided by the mechanical constraint and hence localized film growth and formation of nodules.