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 q uadrant and two capacitive loops in the first quadrant at lower anodic potentials, The charge-transfer resistance (R-t) was larger than pola rization resistance (R-p). A general reaction model originating from c hemisorption followed by two successive charge-transfer steps was esta blished to examine the proposed mechanism. Theoretical derivation show ed the model could be used to Interpret impedance results very well wh en charge-transfer coefficients of the two charge-transfer steps were approximately the same and when the Faradaic desorption step was the r ate-determining step. At higher anodic potentials, the Nyquist diagram s comprised an inductive and a capacitive loop, indicating a change In the reaction mechanism. This type of impedance plot could be Interpre ted by a model similar to the Bockris mechanism.