Electrochemical impedance measurements performed in a combination of three and four-electrode arrangement allows the parallel impedance to be obtained, which corresponds to the current flow parallel to the metal-coating interface.
In the three-electrode arrangement the classical barrier property is measured because all the current flows through the coating towards the metallic substrate. The parallel impedance is obtained by combining the three-electrode measurement with the four-electrode where a part of the current is forced to circulate also through the coating but parallel direction to the metal-coating interface.