Electrochemical impedance spectroscopy study on multi-layered coated steel sheets

Electroplating technique is used to coat nickel over the coated substrates like galvannealed and galvanized, and uncoated substrate of cold rolled closed annealed steel sheets as well. These coated substrates are exposed to 3.5% aqueous NaCl solution in a flat cell to carry out the corrosion tests. The effect of nickel coating on galvannealed, galvanized and cold rolled closed annealing steel sheets is studied for their anti-corrosive performance by electrochemical impedance spectroscopy. Electrochemical impedance data were analysed by using Bode and Nyquist plot by considering the metal-coating-electrolyte interface as an electrical equivalent circuit model. The circuit elements i.e. polarisation resistance and coating capacitance are determined to evaluate the corrosion behaviour of these multilayered coatings. The changes in the impedance characteristics of the systems are found to occur as a function of exposure time in all the three cases. These results have showed differences in the protective characteristics of the three systems, where the nickel coated galvannealed exhibited superior corrosion resistance compared to nickel coated cold rolled closed annealed and galvanised.