In this work, impedance spectroscopy was used to follow the time-dependent modifications of the surface layers formed on a carbon steel exposed to a water with high content in salts (typically: Tunisian south-west waters). Several layers were identified starting from green rust (Fe(II)–Fe(III) hydroxysulfates), with the intermediate formation of black magnetite, and the final formation of an outermost layer of brown oxyhydroxide of the FeOOH type. The relevance of the equivalent circuit describing the overall behavior was established through selective modifications of the stratified layers and the response of the impedance diagrams to these modifications. Diffusion phenomena through the layers were shown to play a determining role in the behavior of the system.