A mathematical model for the electrochemical pickling of steel

In industrial electrolytic pickling, a steel strip with oxidized surfaces is passed through an aqueous electrolyte between a configuration of electrodes, across which a potential difference is applied. The strip is thereby indirectly polarized, and electrochemical reactions at the strip surface result in the dissolution of the oxide layer and the evolution of hydrogen and oxygen. To obtain a better understanding of this process, we derive in this paper a mathematical model for predicting the potential, current density and ionic species distributions in a vertical pickling cell, as well as the oxide dissolution rate at the steel strip. The model is two dimensional, steady state and isothermal, and is based on the conservation equations for ionic species in dilute solution, involving convection, diffusion, migration and reaction. Kinetic Tafel expressions for the electrochemical gas evolving reactions at the lead anode, stainless steel cathode and at the bipolar steel strip surface are introduced. The derived model comprises six ionic species; numerical solutions for a full version and two reduced versions of this model are then obtained. Finally, the implications of the results for the actual pickling process are discussed. (c) 2007 The Electrochemical Society.