The microhardness and fretting wear resistance of anodic oxide layers, produced on commercially pure aluminium by potentiostatic anodizing
in sulphuric acid under conditions of controlled convection and heat transfer in a reactor with a wall-jet configuration, were evaluated as a function
of the electrolyte temperatures in a wide range from 5 °C up to 55 °C. Additionally, information on the microstructure of the anodic films was
acquired by FE-SEM analyses whereas image analysis of high-resolution surface images yielded quantitative information on the evolution of the
surface porosity as a function of the electrolyte temperature. Hence measured mechanical properties were directly related to the corresponding
microstructure. The microhardness of the anodic films progressively decreased with increasing electrolyte temperatures whereas the wear
resistance remained constant for the lower considered temperatures from 5 °C to 25 °C, followed by a decreasing wear resistance with increasing
electrolyte temperature from 25 °C onwards. Both mechanical properties displayed an important decrease when the electrolyte temperature was
raised from 45 °C to 55 °C. FE-SEM analyses indicated the formation of porous oxides with initially equal pore diameters at the metal-oxide
interface, though pore widening due to chemical dissolution of the oxide by the electrolyte led to films with cone-shaped pores. This phenomenon
became more pronounced with increasing electrolyte temperature and towards the surface of the anodic layer. The deterioration of the
microhardness with increasing electrolyte temperatures could mainly be attributed to the increase of the porosity in the outer region of the oxides
since the rate of microhardness reduction is almost synchronous with the rate of porosity increase. In contrast, the variation of the wear resistance
with increasing anodizing temperature indicates that the degradation of the wear resistance does not only depend on the oxide porosity and is also
affected by other characteristics of the oxide.