(A) study on the cathodic corrosion and anodic oxidation of pure aluminium n aqueous solutions

Abstract

The present work involves the studies on the cathodic corrosion and anodic oxidation of pure aluminium in aqueous acidic, neutral and alkaline solutions. In chapter III, faradaic reactions during cathodic polarization and their effect on the cathodic corrosion of pure aluminium have been investigated in aqueous acidic, neutral and alkaline solutions. Potentiodynamic polarization curves for natural oxide-covered pure aluminum showed two Tafel slopes which arise from the change in faradaic reaction from oxygen reduction to water reduction with decreasing applied potential. It was found that the corrosion of pure aluminium arises from the attack by hydroxide ions which are produced by faradaic reactions of water reduction and/or oxygen reduction and it is coupled with the hydrogen evolution reaction. It appeared that the oxide film is spontaneously formed and exists on the surface of pure aluminium still even under cathodic polarization in aqueous solutions. On the basis of the experimental results, the cathodic corrosion mechanism was suggested to proceed by the chemical dissolution of the oxide film at the oxide/electrolyte interface and simultaneous formation of the oxide film at the aluminium/oxide interface. Chapters IV - VIII are concerned with the anodic oxidation of pure aluminium in aqueous acidic and alkaline solutions. The dissolution kinetics of the barrier layer in porous anodic oxide film during anodizing of pure aluminium in acidic solutions has been investigated as a function of applied potential and pH of the solution in chapter IV. The real dissolution rate increased with increasing relative change of potential drop to the initial applied potential, implying that the real dissolution of the barrier layer proceeds via an electrochemical dissolution reaction at the barrier layer/solution interface(field-assisted dissolution). The electrochemical dissolution rate did not change with pH, suggesting that the electrochemical dissolution of the barri...