Abnormal polarization change in anodic dissolution of aluminium at high current densities

Abstract

Two types of anodic oxidation of aluminum have been recognized in the past: a) oxidation requiring a steady increase of voltage drop across the interface up to a few hundred volts in order to overcome the resistance of increasingly thick compact oxide layer and maintain a steady oxidation current, and b) a porous oxide layer forming oxidation resulting in layers of significant thickness, rather regular structure and with good adsorbing qualities (representing the basis of decorative anodization processes). The third type of oxidation is recognized, following seemingly normal anodic dissolution kinetics. This type of dissolution has been obtained in chloride, chlorate, and perchlorate solution and in some other electrolytes. In this third type, it has been found recently, however, that a careful elimination of pseudo-ohmic overpotential leads to an unusual type of polarization behavior, whereby after reaching some current density range, the polarization of the aluminum-electrolyte interface alone, decreases with increasing current density in an approximately linear. It is the purpose of this short communication to report in more detail about this phenomenon which is tantamount to the appearance of negative equivalent resistance at the surface.