Utilizing electrochemical impedance spectroscopy (EIS), we characterize the passive film properties of alloy 22 during immersion in low pH nitrate and chloride solutions. In pure HCl, the passive film grows thinner with increasing acid concentration. In contrast, in HNO3, the passive film corrosion protection properties are enhanced, which leads to low corrosion rates, even at pH < -0.5. The combined influence of both HCl and HNO3 in contact simultaneously with the alloy 22 surface shows multiple phases in the passive film properties depending on the pH. EIS results show that the passive film changes either thickness and/or composition as the system is driven chemically through different corrosion states, including: active. passive, active/passive and transpassive. (c) 2006 Elsevier Ltd. All rights reserved.