The electrode kinetics of the deposition and dissolution of iron

Abstract The rate of the deposition and dissolution of Fe has been measured as a function of potential for constant solution compositions: Fe2+ concentration at constant pH; pH at constant Fe2+ concentration; and the presence of the anions SO42−, Cl−, ClO4−, Ac− and NO3− at constant Fe2+ concentration. The method of measurement was that of galvanostatic transients. The rate of the hydrogen evolution reaction, both from H3O+ and H2O, has been examined during these transients, as a function of potential and pH. The ohmic correction to the total potential was the subject of separate measurements. Cathodic transients manifested regions of H+ discharge from H3O+, Fe2+ deposition and H+ discharge from H2O. Anodic transients exhibited a characteristic maximum, removed by successive anodic pulses. Capacitance values, calculated from initial gradients of potential/time transients, are some five times higher than those expected. Transition times are consistent with Sand's equation in respect of Fe2+ deposition. Steady state anodic Tafel lines have slopes of about 2 3 RT/F; the slope becomes RT/2F in impure solutions. The cathodic slope (after correction for the partial current density due to H discharge) is apparently RT/F in (total) current density regions above those corresponding to the limiting current density for iH. When account is taken of the pH change at the cathode surface due to H discharge, the cathodic Tafel slope becomes 2RT/F. ( ∂ log i o ∂ log a Fe 2+ )pH = 0.8; ( ∂ log i o ∂ log a OH − ) aFe2+ = 1; ∂ e corr ∂ pH =0.06; ∂ log i corr ∂ pH = −0.48; at pH = 3 and cFe+2 = 0.5 M, the velocity of Fe2+ + 2e0− ⇌ Fe depends upon the anion present in the order ClO4− > SO42− > Cl− > Ac− > NO3−. The deposition of H from H2O is slower than that from H3O+ to an amount equivalent to a heat of activation for proton discharge which is 9 kcal/mole greater for H2O than from H3O+. It is shown that the linear dependence of the reaction rate at constant potential and cFe2+ upon pH cannot be due to dissolved H in the Fe. General kinetic expressions are deduced for an overall electrode reaction M2+ + 2e0− M, the mechanism of which involves OH− ions: they relate Tafel slopes of anodic and cathodic partial reactions to the reversible potential, corrosion potential and corrosion rate. Application of them to data in the literature suggests that for Fe2+ + 2e0− ⇌ Fe, αa = 3 2 or 5 2 and αc = 1 2 or 3 2 respectively. Seven paths and mechanisms for this reaction are proposed and their kinetic consequences evaluated. Attention is particularly drawn to inconsistencies in one previous suggestion. If the anodic Tafel slope is RT/2F and a cathodic Tafel slope of RT/2F, an order of reaction 2 with respect to Fe2+ and 1 with respect to OH−, would also be expected, if a multielectron transfer reaction is not acceptable. A mechanism consistent with the results reported here is: Fe + OH− ⇌ FeOH + e0−; FeOH → FeOH+ + e0−; FeOH+ ⇌ Fe++ + OH−. This path is near in energy requirements to another, in which ba = RT/2F. Anion effects, except that of NO3−, may be consistent with a model in which specific adsorption of the anion changes the area free for ion-metal exchange. The difference in the heat of activation of hydrogen deposition on Fe from H3O+ and H2O is consistent with rate-determining proton discharge.