Two microwave absorbing coatings (MACs) are applied onto magnesium alloy substrate and are studied using electrochemical impedance spectroscopy (EIS) during immersion in 3.5% NaCl solution. The diffusion behavior of electrolyte through the MACs was firstly studied. The result shows that electrolyte transfers significantly fast in carbonyl iron loaded coating than barium ferrite loaded coating. In order to account for the distribution of different absorbents on corrosion process, a modified equivalent circuit with a distribute transmission line model was proposed to fit EIS spectra of MACs. Simulated EIS diagrams agree very well with experimental spectra of 25% and 50% PVC MACs. Moreover, simulation data was studied to analyze degeneration of two MACs. The result shows that the corrosion of Cl particles plays an important role for failure of Cl coating, but the degeneration of BF coatings which were slower than Cl coating was ascribed to defects within the coating. (C) 2013 Elsevier Ltd. All rights reserved.