Corrosion can be not only associated with a tremendous economic loss but also with safety hazards. Coating can isolate the exposed area from the aggressive environment, while some coatings impair the corrosion resistance during manufacturing process. It is of importance to investigate the corrosion mechanism and evaluate the properties of coatings to impact on corrosion. Electrochemical impedance spectroscopy (EIS) is the most widely used method to evaluate coatings. Corrosion properties of a ferritic stainless steel with and without oxide films formed at elevated temperatures in air were investigated during exposure to NaCl solution. Surface characterization and chemical composition of the inorganic coating were studied. A correlation between the spectroscopic characteristics of oxide films and electrochemical parameters was established with the aim to shed light on the effect of oxidation on the corrosion behaviour of ferritic stainless steel. The water uptake and interfacial delamination of a polymer-coated metal system exposed to NaCl solution were investigated. The electrochemical impedance spectra were analysed using different equivalent circuit models to derive the time-dependence of the parameters of the coating, delaminated area and interfacial processes. Sulphuric acid solutions with different concentrations were applied as corrosive environment to accelerate the degradation of polymer-coated metal systems. A new parameter fp derived from the changing rate of phase angle method was used to monitor the coating degradation and to determine the breakpoint frequency. The frequency fp could be a good approach to evaluating the extent of the delamination of polymer-coated metal systems at the medium and later exposure stage.