The voltage¯time behaviour for porous anodizing of aluminium in a fluoridecontaining oxalic acid electrolyte

A systematic investigation has been undertaken of voltage¯time behaviour for galvanostatic porous anodizing of aluminium in 0.4 moldm-3 H2C2O4 modified by the addition of variable amounts of NH4F, in circumstances of varying current density and stirring conditions. Both the concentration of NH4F in the electrolyte and stirring of the fluoride-containing (FC) electrolyte appeared to affect the voltage¯time response for anodizing aluminium, causing a decrease in the forming voltage. The voltage-reducing effect is further substantially enhanced while the solution is stirred. Despite a tenfold decrease in the anodizing voltage in the FC electrolyte, no apparent changes in pore morphology are determined from SEM observation. The phenomena described are explained by considering formation of FC electrolyte-derived species and their incorporation into growing oxide film.