Transformation of porous structure of anodic alumina films formed during galvanostatic anodising of aluminium

A method was developed revealing even slight variations of structural features around the barrier layer of porous anodic alumina films formed in H2SO4 electrolyte at conditions different from those of best ordering. This is based on the combination of a suitable structural–kinetic model with the average size of imprints of barrier layer units on the Al surface determined by FESEM. The galvanostatic growth of films in steady state is accompanied by rise of anodic potential with time depending on temperature. Simultaneously structural features around the barrier layer like cell width, pore base diameter and barrier layer thickness increase with time and anodic potential. The ratios of their values remain constant in time but depend on temperature. A proportionality rule thus applies explained by demand for constant Al3+ and O2− transport numbers in steady state. The enlargement of the above structural features occurs simultaneously with termination of some pores due to their narrowing resulting from drop of ionic conductance in pores and/or barrier layer. The surface density of cells/pores across the film increases to film surface, where it becomes highest, established around the start of second transient stage. The porosity near barrier layer remains constant with time but increases with temperature. The anodising ratio and average field strength across the barrier layer generally slightly increases and decreases respectively with time while the relevant trends are affected by temperature. FESEM examination of film surfaces showed structure affected by metallurgical texture, altered however to deeper layers, and surface structure changes due to chemical attack of oxide by the electrolyte.

► During prolonged galvanostatic anodising of Al anodic potential rises in steady state. ► Cell width, barrier layer thickness and pore base diameter parallel widen. ► Their ratios and porosity near barrier layer are independent of time. ► These structural parameters diminish and porosity increases with temperature. ► Pore surface density falls toward the barrier layer due to termination of some pores.