Anodizing of pure magnesium in KOH-aluminate solutions under sparking

Anodizing of magnesium was studied in solutions containing 3 M KOH + 0.6 M KF + 0.21 M Na3PO4, with and without addition of 0.4 M and 1.1 M of alumina te. At constant applied current, the process occurs in two stages. First, t he voltage rises linear ly until an initial breakdown occurs. A thin and co mpact oxide film is formed at this stage. In the second stage an intense co ntinuous spark is maintained on the metal surface. A thick oxide film is fo rmed at the site of sparking. The formation of the film is accompanied by v oltage oscillation and gas evolution. The anodic films formed under continu ous breakdown conditions are rough and porous and enriched in aluminum. Ano dic films formed in a bath containing 1.1 M aluminate at 15 mA/cm(2) are co mposed of a single oxide phase MgAl2O4 spinel. (C) 1999 The Electrochemical Society. S0013-4651(98)09-036-3. All rights reserved.