Inhibition of copper corrosion by N-phenyl-1,4-phenylenediamine(NPPD) has been investigated in de-aerated, aerated, and oxygenated aqueous 0.50 M HCl solutions by using potentiodynamic polarization, potentiostatic current-time, electrochemical impedance spectroscopy, and weight-loss measurements, along with scanning electron microscopic (SEM) and energy dispersive X-ray (EDX) experiments. Potentiodynamic polarization measurements showed that the NPPD molecules significantly decrease cathodic, anodic, and corrosion currents in all these solutions. Potentiostatic current-time measurements as well as SEM and EDX investigations of the copper surface revealed that NPPD suppresses the copper dissolution current due to its adsorption on the copper surface as a Cu(I)-NPPD complex. Impedance measurements also supported the results obtained from both the potentiodynamic and potentiostatic experiments. The inhibition efficiencies measured from polarization, electrochemical impedance spectroscopy (EIS), and weight-loss experiments are all internally consistent with each other. These results together showed that NPPD is a good mixed-type inhibitor for copper corrosion in all solutions studied. (c) 2006 Elsevier Ltd. All rights reserved.