Passive films formed on Fe-18Cr alloy in a borate buffer solution were studied using impedance spectroscopy and photoelectrochemical response. The passive film was composed of an inner oxide and covering hydroxide layer. Both layers exhibited n-type semiconductor properties with an optical bandgap energy, E-g, of 3.4-3.5 and 2.4 eV, respectively. Based on the X-ray photoelectron spectroscopy results of the film structure, an electronic energy band structure model for the bilayer semiconductor was proposed. Photoirradiation altered the energy band structure, which resulted in a photocurrent transient and changes in the capacitance of the space charge layer. The differences between passive films on Fe-Cr alloys formed in a borate buffer solution and a sulfuric acid solution are discussed in terms of film growth kinetics and corrosion behavior. (C) 2004 The Electrochemical Society.