Factors Affecting the Growth of Porous Anodic Oxide Films on Zirconium

Abstract

Although anodization of zirconium and its alloys generally results in the formation of barrier‐type films, porous anodic oxide films can be produced in some electrolytes and by some preparation techniques. A survey of common electrolytes reveals that those containing nitrate, chromate, dichromate, and phosphate ions commonly lead to porous anodic films. An electron microscope study of the films produced in these electrolytes, and others giving barrier‐type films, has shown that no single explanation fits all instances of porous film formation. While incorporation of the electrolyte anion in the oxide may be the primary factor influencing the formation of porous oxide films, it appears to operate primarily by affecting the local transport of oxygen ions through the films. This effect may be achieved by influencing the crystallization of the oxide during growth so that in some instances local oxidation and cracking occur at grain boundaries, while in other instances it leads to the general development of a porous structure.