The aluminium oxide obtained by anodization in acidic solutions has a highly ordered nanoporous structure consisting of straight parallel pores. The oxide layer has good electrical insulating properties on planar surfaces. However, the electrochemical formation of nanoporous film on convex surface induces effects such as crack formation and deterioration of the insulating properties. The present work is focused on investigating the conditions of growth of a thick oxide layer on aluminium wires with a diameter of 0.2 mm to 0.5 mm. In order to prevent cracking, the oxide growth was carried out by reducing the anodization voltage according to a mathematical model in view of obtaining conical pores. In this study, the minimum thickness at which a crack-free oxide layer grows was determined.