A method to select the optimal equivalent electrical circuit applied to study corrosion system of composite coating on magnesium alloy

Mg-Al layered double hydroxides/micro-arc oxidation (Mg-Al LDHs/MAO) composite coating on AZ31 magnesium alloy is fabricated by hydrothermal treatment. Corrosion system is described by ideal equivalent circuit, in which each layer or interface is regarded as the corresponding electronic component. The optimal equivalent circuit is determined by the minimum total error area of electrical parameters. Error areas of phase angle and modulus for experimental and fitting data are calculated for the selected four different kinds of circuit models such as EC1, EC2, EC3 and EC4. The results show that circuit model EC2 is the best because its total error is 1/2, 2/19 and 1/15 of error values for other three kinds of circuit models, respectively. This work reveals that the optimal equivalent electrical circuit can well reflect corrosion system of Mg-Al LDHs/MAO composite coating with the help of electrochemical impedance spectroscopy (EIS) and scanning electron microscope (SEM).