IMPEDANCE SPECTRA FOR ALUMINUM-7075 DURING THE EARLY STAGES OF IMMERSION IN SODIUM-CHLORIDE

The impedance spectra obtained at short exposure times for Al 7075-T6 immersed in 0.5 N NaCl do not agree with the pitting model proposed by Mansfeld and co-workers. Instead of a transmission line impedance, an inductive loop. is observed at low frequencies. This result is due to the non-linearity of the system at short exposure times, when E(corr) is close to E(pit). The application of the anodic portion of the ac signal causes this non-linear behavior. The observed impedance is therefore a system response and not a true impedance. For longer exposure times agreement with the pitting model is observed.