Electronic Properties of Passive Films Grown on Al 7075 in Solutions Containing Oxalate and Chromate

Electronic properties of passive layers grown anodically on Al 7075 (UNS A97075) in chromate and oxalate solutions during polarization at 500 mV_SCE were investigated using electrochemical impedance spectroscopy (EIS). Impedance results were analyzed in terms of capacitance-vs-frequency plots during reverse polarization from 500 mV_SCE to more negative potentials. Plots yielded capacitance values dependent upon both frequency and applied potential. Increases in capacitance with decreasing potential were attributed to width variations of a space charge inside the passive film. Mott-Schottky plots gave slopes and intersection potentials dependent upon the imposed alternating current (AC) signal frequency. Data were interpreted on the basis of the amorphous semiconductor/electrolyte junction theory. Differences were found in semiconducting properties of the passive layers formed in solutions containing chromate and oxalate ions. These differences were related to the anticorrosive resistance toward pitting, since it is well known that chromate is a more effective inhibitor than oxalate. The oxide developed in the presence of chromate ions exhibited less noble flat-band potentials and lower average densities of states.