Structure and Density of Anodic Oxide Films Formed on Aluminum in Oxalic Acid Solutions

Abstract

Electropolished 99.99% Al foil coupons were. galvanostatically anodized in C₂H₂O₄ solutions by applying a fixed amount of charge (7.2 Coulombs/m²). The morphology of the formed oxide (cell size, 2R, pore diameter, 2r, and barrier layer thickness, δ_b) was examined as functions of the acid concentration (2-8%), temperature (10-40°C) and current density (1-180Am^-2), by using electronmicroscopy and electrochemical techniques developed in a previous investigation. The morphology of the oxide films is solely the function of the anodizing voltage, E_a, while E_a is affected by the solution concentration, temperature and anodizing c.d. The density of oxide, ρ, is estimated to be 3.0-3.5, as being the weight of film divided by the volume of oxide. The value of ρ was found to increase with decreasing c.d. and increasing temperature and solution concentration.