Contribution of Diffusing Hydrogen to Anodic Processes and Pitting for Iron in Chloride-Bearing Bicarbonate Solutions

Abstract

The effects of diffusing hydrogen atoms on anodic processes and pitting corrosion for iron in chloride-bearing bicarbonate solutions were studied with a dual cell for realizing the hydrogen diffusing and electrochemical measurements simultaneously. A high concentration of chloride in solution, precleaning, and diffusing hydrogen in iron can move the open-circuit state from a passive state to an active dissolution state. Potentodynamic anodic polarization curves show that the effect of diffusing hydrogen is strongly dependent on the chloride concentration in the solution. The diffusing hydrogen atoms enhance the anodic reaction before the oxygen evolution potential when the chloride concentration is low while they enhance the anodic reaction in the overall potential range when the chloride concentration is high. In addition, diffusing hydrogen atoms slow down the anodic reaction in the first current plateau regime while not significantly affecting the other potential regimes if the chloride concentration is sufficiently high. Comparisons of the results from the hydrogen-diffusing electrodes with those from the precleaned electrodes facilitate clarifying the roles of diffusing hydrogen atoms in anodic reactions. For the hydrogen-diffusing electrode, the occurrence of the active dissolution regime is mainly from the surface cleaning effect, and the enhanced anodic reaction in the transition regime, prepassive regime, passive film growth regime, passive regime, and in some cases, transpassive regime, is due to both the electrode kinetics and the surface cleaning effect. Diffusing hydrogen would retard the anodic reaction in a specific potential regime through its combined effect with high concentration chlorides. More severe pitting is generally observed for the precleaned electrodes and the hydrogen diffusing than for the noncharged electrode. Depending on the applied potential and time as well as the chloride concentration, the hydrogen-diffusing electrode would show more extensive or severe pitting than the precleaned electrode, with some exceptions showing less severe pitting.