The Amorphous to Crystalline Transformation of Anodic Aluminum Oxide during Anodization in an Ammonium Citrate Electrolyte

Abstract

The growth of crystalline anodic aluminum oxide during anodization in an aqueous ammonium citrate electrolyte has been investigated. Crystalline sites were observed to nucleate only during the early growth of the oxide film, and their number was dependent upon the electrolyte temperature and the crystallographic orientation of the electropolished aluminum substrate. As anodization continued, the crystalline sites grew larger but did not increase in number; eventually, the individual sites grew together to form a continuous crystalline sheet. It is postulated that the initial crystalline sites grow radially outward by field‐assisted transformation of the surrounding amorphous oxide into crystalline oxide. Transition layers observed between the amorphous and crystalline areas of the film are interpreted as intermediate stages of the amorphous to crystalline transformation.