EIS (Electrochemical Impedance Spectroscopy) technique has been used in many areas such as biosensors, battery development, fuel cell development etc. Many efforts have also been made to apply this technique to paint/coating evaluations. The epoxy industrial coatings containing fillers or ceramic beads are commonly used in chemical processing, Mining/Mineral & Ore processing, Oil/gas, Power (fossil, hydroelectric, and nuclear), water/waste water, etc. These coatings provide excellent corrosion and erosion resistance for metal and concrete surfaces. EIS is a useful tool to investigate corrosion/erosion resistance of these protective coatings under various environment conditions.

EIS can measure impedance of coatings and phase angle of imposed AC voltage/current. The changes in impedance and phase angle can be used as indicators of coating structure changes caused by corrosion, erosion, water penetrations, etc. There are various ways to analyze EIS data to correlate the data with the performance of paint/coating products but these methods all have limitations when used to evaluate high performance protective coatings. In this presentation, we discussed the limitations of commonly used EIS data analysis methods. A modified CPE model is proposed to evaluate high performance protective coatings on water/chemical penetrations and/or surface deteriorations due to corrosion and/or erosion.


EIS data fitting models are composed of resistors, capacitors, and other electronic components. These electronic elements can be used to simulate coating films, defects or damages of coating samples. In many models, a perfect coating film is considered as a capacitor.

Resistors and Capacitors in EIS Data Fitting Model

Resistors are electronic components which have constant electrical resistance. In both DC (direct current) and AC (alternating current) circuits, the impedance of a resistor can be calculated by (inline-equation). Resistors always have constant impedance which are independent of frequency of AC voltage in AC circuits.

Capacitors are different from resistors. In a DC circuit, current flow stops when a capacitor is fully charged, so the impedance is infinite. In an AC circuit, the impedance (also called reactance) of a capacitor can be calculated by equation: (inline-equation), where C is the capacitance of the capacitor, f is frequency, ω is angular frequency by radians. From this equation, the impedance of a capacitor is inversely proportional to the frequency of the AC power supply: (inline-equation). At extreme high frequency, the impedance equals 0. At extreme low frequency (an AC circuit), the impedance is infinite, which means that there is no current flow through the capacitor.

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