Barrier Oxide Growth and Dissolution on Aluminium in Aqueous Media

• Main Author: Giskeødegård, Nils Håvard
• Format: Dissertation
• Language: English

The oxide formed during anodization of aluminium has been a subject of attention since the beginning of the 20th century, due to its protective properties. The oxide consists of two layers, a dense layer adjacent to the metal (barrier oxide) and a hydrated and porous outer layer. Most of the available studies deal with oxide growth in oxygen containing gaseous environment and high voltage anodizing in aqueous solutions. The subjects of interest in the present study are the growth kinetics, and the physical and electrochemical properties at macroscopic scale of the barrier oxide formed at low temperature and potential in aqueous environment. The motivation for the present work is to obtain comprehensive information about barrier oxide growth and dissolution rates, as well as barrier oxide properties, on aluminium by using three experimental approaches, viz., chronoamperometry, electrochemical impedance spectroscopy (EIS) and visual spectroscopic ellipsometry (VISSE), simultaneously and investigate to what extent the results from each can be evaluated in a complementary manner. In addition, VISSE should be able to provide useful information about the growth and dissolution of the hydrated layer and its properties. Another incentive for the work is to develop a quantitative method to investigate the effect of trace elements in aluminium, especially the low melting point elements such as Pb, on the electrochemical and corrosion properties (passivity) of the oxide. The materials investigated were pure Al and Al containing 20 ppm of Pb, the latter representing an impure example. The test solution consisted of mixtures of glacial acetic acid and sodium acetate in the pH range 5.6‑6.3. It was exposed to an air conditioned laboratory, maintained at 22°C. The experimental method involved stepwise change of the applied potential and monitoring the ensuing time-dependent change by current measurement and VISSE at each step. EIS was used to calibrate the steady-state barrier-film thickness at the end of each step. The dependence of steady state barrier-oxide thickness on the applied potential, as obtained by EIS, was 1.1 nm/V, in agreement with the well-accepted data for the experimental conditions specified above. In addition, EIS was used to evaluate the steady-state resistance of the barrier oxide. For barrier-film growth, VISSE and chronoamperometry gave the same growth rates under identical experimental conditions, as expected. Thus, the use of the two methods sim