Properties of the Passive Film Formed on Interstitially Hardened AISI 316L Stainless Steel

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.