ENHANCED PITTING CORROSION-RESISTANCE OF ALUMINUM-ALLOY-7075 IN THE PRESENCE OF OXALATE ANIONS

The presence of oxalate in chloride (Cl-)-containing corrosive aqueous media was found to protect aluminum (Al) alloy AA 7075 (UNS A97075). The effect of 0.05 nn sodium oxalate on induction times for the initia tion of pit formation was measured on AA 7075 specimens. The variance and the mean value of the induction times measured increased with the time of specimen immersion at the open-circuit potential (OCP). Statis tical analysis was done by potentiostatic measurement of several speci mens using electrochemical instrumentation controlled by a personal co mputer. Pit generation rates were found to be inversely proportional t o the duration of the immersion period at OCP. Therefore, it was concl uded that prolonged exposure of the tested specimens to sodium oxalate solutions enhanced their resistance to corrosion. Impedance measureme nts confirmed this finding. The increased resistance was attributed to the retardation of the process by which the surface protective layer would have been destroyed. This retardation may have been ascribable t o the formation of surface complexes between Al3+ and oxalates from th e bulk solution. Polarization of the specimens at anodic potentials re duced the corrosion resistance of AA 7075, possibly because of the des tabilization of the surface complexes formed between Al and the oxalat e ions.