EIS study of effective capacitance and water uptake behaviors of silicone-epoxy hybrid coatings on mild steel

•Six equivalent circuit models were used to analyze corrosion behaviors of silicone-epoxy coatings on mild steel.•The water uptake was estimated from the Brasher–Kingsbury relation using the three capacitances.•Using CeffectB calculated was an effective way of estimating the coating thickness.•Time constant for silicone-epoxy coatings on mild steel was a surface distribution. Electrochemical impedance spectroscopy (EIS) of bare type A3 mild steel and silicone-epoxy hybrid coatings/mild steel system in 5wt.% NaCl solution has been investigated, respectively. The analyses of impedance models and water uptake behaviors of silicone-epoxy hybrid coatings have been studied during the aging process of the coatings. Taking into account the diffusion of oxygen, water and corrosion products as well as the formation of the passive film, six distinguishing impedance models were used to fit the EIS spectra of the coatings on type A3 mild steel. The water uptake was estimated from the Brasher–Kingsbury relation using CPE and two effective capacitances (CeffectB and CeffectH) which were also used to calculate the coating thicknesses. A comparison between the coating thicknesses calculated from the effective capacitance CeffectB and thickness gauge measurements suggested a surface distribution of time constant on the surface of the coatings. The use of CeffectB allows estimating the “true” values of water uptake in the silicone-epoxy hybrid coatings.