Structural Features of Oxide Coatings on Aluminum

TLDR

Anodic oxide coatings on aluminum are composed of close‑packed hexagonal cells, each containing a single pore. The study presents dimensions and formulas for calculating cell size and pore volume of anodic coatings formed in sulfuric, oxalic, chromic, and phosphoric acid electrolytes. Electron microscopy was used to investigate the structural features of the porous anodic oxide coating on aluminum. Pore size depends on electrolyte type but not on forming voltage, while wall and barrier thicknesses are mainly governed by forming voltage with minor electrolyte influence.

Abstract

The structural features of the porous type of anodic oxide coating applied to aluminum have been investigated with the electron microscope. These coatings consist of close‐packed cells of oxide, predominately hexagonal in shape, each of which contains a single pore. Pore size is a function of the electrolyte used and is independent of forming voltage. Wall thickness and barrier thickness are primarily a function of forming voltage and are affected to a minor degree by the electrolyte type. Pertinent dimensions of anodic coatings formed in sulfuric acid, oxalic acid, chromic acid, and phosphoric acid electrolytes are presented, and formulas are given for calculating the cell size and pore volume of these coatings.