The behaviour of second phase particles during anodizing of aluminium alloys

Several model second phase particles and a practical alloy (AA7075-T6) have been anodized in chromic and sulphuric acid to disclose the relation between particle composition, electrolyte nature and oxidation behaviour. Generally, magnesium-containing second phases are readily oxidized, because the presence of sufficient magnesium hinders the formation of a stable oxide, while copper- and iron-containing particles oxidize at reduced rates and support a relatively stable oxide. At low potential, in chromic acid, the oxidation rate of magnesium-containing particles is reduced, due to passivation induced by chromate anions. Conversely, for particles containing only copper and/or iron, chromate anions increase the oxidation rate.