Hard Anodizing

Abstract

Thick anodic oxide films were produced on aluminium using the sulphuric acid hard anodizing process. Low acid concentrations, low temperatures, high current densities and short processing times minimised oxide dissolution and gave the least porosity. Scanning electron microscopy and potential-time measurements were used to study the anodizing process. Flaws in the oxide initially present on the aluminium surface were randomly distributed and provided low resistance paths for the anodizing current. Nucleation at flaws produced localised growths of anodic oxide which in cross-section were lens shaped nodules. Internal growth stresses cracked the outer layer of the nodules and caused characteristic rosette patterns. Anodizing became less and less localised as the nodules, or hillocks, grew and merged to cover the whole surface. The results established that the hard anodizing process was essentially the same as conventional sulphuric acid anodizing with an inner barrier layer and an outer porous layer of oxide. The nucleation and growth of oxide in the initial stages of hard anodizing remained localised for a longer period and the pore volume was smaller than for conventional anodizing. The results enabled re-interpretation and a new explanation of the results of Csok4n and others whose fibre theory, now shown to be incorrect, differed from the Keller-Hunter-Robinson model which was further developed quantitatively by Wood and O'Sullivan. Pretreatment affected the initiation of anodizing and during its study icing defects (electropolishing defects) were observed when electropolishing under specific conditions. These were investigated, found to occur when the electrolyte and operating conditions were on the borderline between electropolishing and anodizing, and a mechanism for their formation proposed.