Theoretical Interpretation on Impedance Spectra for Anodic Iron Dissolution in Acidic Solutions Containing Hydrogen Sulfide

Nyquist impedance diagrams for dissolution of iron in acidic solutions with hydrogen sulfide (H₂S) display an inductive loop in the fourth quadrant and two capacitive loops in the first quadrant at lower anodic potentials. The charge-transfer resistance (R_t) was larger than polarization resistance (R_p). A general reaction model originating from chemisorption followed by two successive charge-transfer steps was established to examine the proposed mechanism. Theoretical derivation showed the model could be used to interpret impedance results very well when charge-transfer coefficients of the two charge-transfer steps were approximately the same and when the Faradaic desorption step was the rate-determining step. At higher anodic potentials, the Nyquist diagrams comprised an inductive and a capacitive loop, indicating a change in the reaction mechanism. This type of impedance plot could be interpreted by a model similar to the Bockris mechanism.