The influence of pH and the concentration of hydrogen sulfide (H2S) on corrosion of iron in acid solutions was studied using a potentiostatic polarization method. The alternating current (AC) impedance technique also was used to characterize the active dissolution process of iron. Results showed the dissolution process was accelerated by H2S. The anodic dissolution current (ia) increased with pH and H2S concentration with reaction orders of about npH=nH2S=0.25 when the ratio of H2S concentration and hydrogen ion (H3O+) concentration was < 101.5. ia reached a maximum and became independent of pH and [H2S] when [H2S]/[H3O+] > 101.5. The Nyquist diagram corresponding to the active dissolution process in the Tafel range exhibited two capacitive loops in addition to the well-known, high-frequency capacitive loop. A mechanism was proposed to explain the results in which H2S chemisorbed first on the electrode surface and then catalyzed the anodic dissolution of iron in two discharging steps.

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