Modelling of copper etching in aerated chloride solutions

The effect of dissolved oxygen on the rate of chemical etching of copper by acidic cuprous chloride solution was investigated numerically for the case of a rotating disc electrode (RDE). Copper dissolution in aerated acidic cupric chloride solutions of composition 3.5 M CuCl₂ + 0.5 M HCl + 0.5 M KCl and 3.5 M CuCl₂ + 0.5 M HCl was investigated with consideration of the instantaneous homogeneous reaction 4 CuCl^2-₃ + 4H⁺ + O₂ ⇌ 4 CuCl^-₃ + 2 H₂O which takes place within the mass transport boundary layer. It was assumed that CuCl^2-₃ and O₂ react instantaneously, resulting in the formation of a reaction plane which separated the mass transport boundary layer into two regions: a region adjacent to the electrode where oxygen was depleted, and a region adjacent to the bulk where CuCl^2-₃ was absent. A one-dimensional mathematical model was developed to predict the position of the reaction plane. The model accounted for diffusion, migration and fluid flow generated by an RDE and included nine species and five heterogeneous electrochemical reactions. The homogeneous redox reaction served to regenerate the chemical etchant solution and thus enhance the etch rate. With increasing oxygen concentration and rotation rate the reaction plane was found to move closer toward the electrode and thus the etch rate of copper increased. Addition of KCl increased the Cl^- content in solution so as to shift the reaction plane toward the bulk.