The anodic oxidation of valve metals—II. The influence of the anodizing conditions on the transport processes during the anodic oxidation of zirconium

Abstract

The transport numbers for metal and oxygen in anodic ZrO 2 have been measured as a function of the oxide film thickness, growth rate, electrolyte composition, metal surface finish and electric field using 222Rn as an inert marker. The field in the oxide has also been determined as a function of current density and electrolyte composition. The metal transport number apparently diminishes with increasing oxide thickness but this is shown to arise from the formation of a hydrated layer on the oxide surface which leads to greater energy loss of the α-particles and apparently greater burying. However, this effect only has significance for thin films. For thick films the measured transport numbers are independent of thickness but show increases from very low values as the current density increases and with changes in the anodizing electrolyte in the sequence: sodium hydroxide; ammonium borate; sodium sulphate. The maximum metal transport number observed was 0.22 for 0.5 M sodium sulphate at 50 mA current density. The field in the oxide also increased with increase in current density and with electrolyte in the same sequence.