Film growth and alloy enrichment during anodizing AZ31 magnesium alloy in fluoride/glycerol electrolytes of a range of water contents

The effect of anodizing AZ31 magnesium alloy in a fluoride/glycerol electrolyte containing from 1 to 40 vol.% water on the compositions of the anodic film and the underlying alloy has been investigated. A range of constant current densities and water contents of the electrolyte were employed. Scanning and transmission electron microscopy, with energy-dispersive X-ray spectroscopy, X-ray diffraction and ion beam analyses were used to analyse the anodized alloy. Barrier films, containing fluoride and oxide species, were formed above the matrix, at high current efficiency; a porous film was formed above Al-Mn intermetallic particles. The current density and the water content of the electrolyte affected the O:F atomic ratio and the ionic transport in the films. Significant enrichments of zinc and aluminium were generated in a thin layer of the matrix immediately beneath the film under all conditions of anodizing, with the enrichment of zinc reaching 6.2 × 1015 at cm−2 in the enriched alloy layer. The cause of the enrichments and the consequences to film growth are discussed.