The present paper investigates the effect of exposure time, chloride and sulfate additions on the corrosion mechanism of low-alloyed steel in a cladding flaw of a nuclear reactor pressure vessel using in-situ electrochemical impedance spectroscopy coupled to exsitu characterization of the oxides by surface analytical techniques. A quantitative interpretation of impedance spectra using the Mixed-Conduction Model for oxide films formed in high-temperature water allows for a discrimination between the rates of inner layer formation and cation transmission through that layer. The values of the inner layer thickness and cation release estimated from impedance measurements are in good agreement with those stemming from ex-situ analysis. At short exposure times, higher film formation and cation release rates of LAS are measured in the presence of chloride and sulfate additives. However, the effect of exposure time itself appears to be stronger than that of the impurities, and the protective ability of the oxides at longer exposure times starts to dominate the overall corrosion process. (C) 2016 The Electrochemical Society. All rights reserved.