An AC impedance study of the anodic dissolution of iron in sulfuric acid solutions containing hydrogen sulfide

The anodic dissolution of iron in sulfuric acid solutions with H2S was investigated by means of ac impedance and steady-state polarization curves. The results show that H2S can accelerate markedly both the anodic dissolution and cathodic hydrogen evolution in most cases, but it can also exhibit a strong inhibiting effect upon iron dissolution under certain special conditions. Usually, the Nyquist diagrams measured at lower positive potentials consist of two overlapping capacitive loops, one of which is a characteristic capacitive loop caused by the adsorption of H2S on the electrode surface. At higher positive potentials, the original characteristic low frequency capacitive loop disappears, and the complex plane impedance diagrams for the iron electrode consist of an inductive loop and a capacitive loop. Based on the impedance behaviour of iron in the presence of H2S, two reaction models were proposed to interpret its dissolution mechanism in acidic solutions with H2S. (C) 1998 Elsevier Science S.A. All rights reserved.