Anodic Dissolution of Iron in Acidic Sulfate Electrolytes: II . Mathematical Model of Current Oscillations Observed under Potentiostatic Conditions

Abstract

A mathematical model describing sustained current oscillations observed in the iron‐sulfuric acid system, under potentiostatic conditions, is presented. This model assumes that the sustained oscillations are due to a continuous cycling of a portion of the electrode between the active and the passive states. The electrode surface is covered by a porous ferrous sulfate film. Transient changes in the potential and concentration profiles in the pores of the salt film and in the diffusion layer are responsible for the continuous cycling. A one‐dimensional model that describes these processes is presented. Results from this model are compared to the experimental current‐time curves. The calculated current‐time curve shows oscillatory behavior. The characteristics of the calculated current‐time curve differ from experimental results. The calculated ferrous sulfate film thickness is substantially thinner than expected from a steady‐state analysis and from the previously reported work of others. Due to lack of quantitative agreement between experimental and calculated results, two modifications to the present model are proposed in a qualitative manner. These modifications suggest that one may wish to: (i) remove the requirement of passivation, and assume that the current oscillations are caused by continuous cycling in the fraction of the electrode surface area that is covered by ferrous sulfate; (ii) assume that the current oscillations are caused by continuous cycling in the porosity of the salt film in combination with changes in the amount of active area. Both proposed modifications would include kinetics of salt‐film precipitation and dissolution. It is anticipated that finite precipitation and dissolution kinetics will increase the calculated salt‐film thickness. Further work would be required to incorporate either modification into the model in a quantitative way.