The influence of Cl− on the electrochemical dissolution of cobalt white alloy containing high silicon in a sulfuric acid solution

Investigations were conducted on the electrochemical dissolution of cobalt white alloy and the anodic passivation in a sulfuric acid solution. The galvanostatic and potentiodynamic experiments were carried out using PARSTAT 2273. The composition and structure changes of alloy anodes were analyzed by Scanning Electron Microscopy (SEM), Energy Dispersive Spectrometry (EDS) and X-ray Diffraction (XRD). The metal contents in the electrolyte were measured by Atomic Absorption Spectrophotometry (AAS). The results showed that the cobalt white alloy was composed of the Co–Fe alloy matrix phase and the Cu alloy precipitate phase. This multiphase alloy did not dissolve uniformly and was subjected to anodic passivation due to the high silicon content. The passivation occurred due to the generation of colloidal SiO2, which covered the anode surface and hindered the dissolution of the alloy. However, the polarization curves indicate that Cl− could accelerate the electrochemical dissolution of cobalt white alloy. Cl− could be preferentially adsorbed on the anode surface and undermine the SiO2 coating and the passive film on the alloy surface. NH4Cl significantly increased the dissolution rate of the Co–Fe alloy phase by 4–6 times and improved the anode current efficiency by 27.83%. The electrochemical dissolution was selective, which facilitated the separation of valuable metals. Fe and Co were dissolved into the electrolyte while Cu deposited on the cathode. The anode current efficiency in the electroleaching process was 54.6%. The calculated recoveries of Co and Fe were respectively 100.77% and 98.56% after 22 h of electroleaching, indicating that the Co–Fe alloy phase was preferentially dissolved.