The passive-film on interstitially hardened AISI 316L stainless steel was studied in 0.1 M NaCl solution using electrochemical impedance spectrometry (EIS) and surface analytical techniques. The interstitially hardened AISI 316L was achieved via low-temperature paraequilibrium carburization. EIS experimental data were fitted and analyzed by an equivalent-circuit model that incorporated two time constants: one representing the properties of the carburized layer and one representing those of the passive film. To validate the model a series of samples with varying carburized layer thickness were prepared by successive polishing. In addition to EIS, these samples were analyzed by XPS (X-ray photoelectron spectrometry) to determine the chemical composition of the passive film and carburized layer. A consistent correlation was observed between the EIS response and the chemical compositions of the passive film and carburized layers. Supporting data from long term immersion tests are also reported. (C) 2015 Elsevier Ltd. All rights reserved.