The selected equivalent circuit is shown in Fig. . The circuit elements include the solution resistance Rs, the resistance of the oxide/hydroxide surface layer or protective film R1 , the constant phase element CPE1 to address the capacitive response of such films , the charge transfer resistance R2, and constant phase element for the electrochemical double layer CPE2 . The capacitive component of the impedance response was assigned by two parameters of the constant-phase element CPE (Y0 and n), defined as: