Investigation of anodic dissolution of cobalt in alkaline solution

Electrochemical Machining (ECM) can be a promising option for finishing cemented carbides. Cobalt is the most important binder material in those materials e.g. WC- 10Co. This work is focused on the anodic dissolution of cobalt in different electrolytes. The basic solution was sodium nitrate (pH5.6) which is commonly used as an ECM-electrolyte. Since tungsten carbide as hard phase is only soluble in alkaline solutions the authors investigated the electrochemical behavior of cobalt in alkaline solutions. The use of additives suppresses the passive film formation and enhances the machining process. Organic solvents improve the surface quality. Co can be easily electrochemical machined in common sodium nitrate electrolytes. The current efficiency is only slightly lower than 100% which might be caused by a temporarily oxygen evolution. Co is completely passive in pure NaOH electrolytes The high current density measured in the electrochemical experiments seems to be completely consumed by the oxygen evolution. A mixed electrolyte consisting of sodium nitrate and ammonium suppresses the passivation of Co in alkaline solution and allows the machining process The surface smoothness can be enhanced by using solvent mixtures with higher viscosity.