The life time of stabilized ferritic stainless steels used as car exhaust parts (manifolds, tubes) can be largely reduced in the presence of water vapour, by breakaway oxidation when big iron-containing oxides nodules rapidly grow in place of the thin chromia-rich protective scale. This phenomenon occurs after a certain incubation time and was shown to be influenced by the composition of the stainless steel, particularly by its silicon content. In this paper, a detailed photoelectrochemical characterization of the oxide scales prior to breakaway is presented in function of the chromium content. The silicon-rich oxide formed in contact with the steel during thermal oxidation at 900degreesC is shown to exhibit a different PEC signature when formed in oxygen or in water vapour (150 mbar).