Open circuit, steady-state dissolution

When active iron dissolution in H2SO4, pH O, occurred at a potential 60 mV cathodic to the open-circuit potential, at which hydrogen evolution prevailed, the electrode surface remained practically undamaged and retained its bright appearance after immersion for 24 h at this potential, and the steady-state current was reached only after a 20-h polarization. According to Keddam et this means that the interaction between iron and hydrogen, which is a very slow process, controls the electrode kinetics. 

The characteristic kinetic quantities reported by various authors for iron corrosion and active dissolution in aqueous chloride solutions in the region of potential in which chloride ions exert an inhibitory effect (i.e., negative to the potential of unpolarizability) are presented in Fig. 30. At open circuit, A °VApH ranges between -40 (reported by Bala ) and -63 mV dec" (reported by McCafferty and Hackerman ), and A log / °VApH = -0.6 0.1. Steady-state anodic Tafel slopes near 60 mV dec VoH == U and values of Vq" between -1 and -0.5 have been more frequently reported, but the scatter in the of data is evident. 

The electrochemical mechanism of dissolution is illustrated schematically by the simplified polarization diagram shown in Fig. 2. The open circuit potentials of the cathode process, E, and the anodic process. Eg, are the equilibrium potentials of the corresponding partial reactions of Eqs. 28 and 29. The dissolution current corresponds to the steady-state rate of dissolution. The corresponding dissolution potential of the dissolving solid lies between the equilibrium values of the cathodic and anodic reactions. From the figure, it also follows that conditions which shift the point of intersection of the anodic and cathodic polarization curves by decreasing their slopes, lead to an increase in dissolution rate. Conversely, an increase in the slopes of the curves lowers the dissolution rate. 

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