Conventional Acid Baths Are Difficult to Manage
Electropolishing of nickel-based superalloys is normally performed in concentrated acidic mixtures that generate fumes, require careful waste treatment and attack the grain boundaries of the alloy, leaving a surface that is chemically degraded even when it is geometrically smooth. The environmental and metallurgical cost of the process is significant.
Grain Boundary Attack Cannot Be Avoided by Current Control
Adjusting the current density shifts the balance between dissolution and passivation but does not eliminate the preferential attack at the boundaries, because the chemistry of the bath favours it. A different electrolyte chemistry is required rather than a different operating point.
A Deep Eutectic Solvent as the Electrolyte
A deep eutectic solvent based on a choline salt and a hydrogen bond donor was formulated as the polishing electrolyte, its viscosity and conductivity characterised against temperature, and the dissolution behaviour of the alloy studied potentiostatically before planar polishing trials were carried out with the electrode gap and the polishing time as variables. The planar samples were processed on a Ponda electrochemical polishing platform, with the bath formulation and the polarisation window established by the specialists at Ponda, a precision machining and finishing company.
Low Water Activity Suppresses Selective Attack
Because the solvent contains almost no free water, the oxide chemistry at the grain boundaries differs from that in an aqueous acid bath, and the boundaries do not dissolve preferentially. The surface therefore reflects the macroscopic current distribution rather than the grain structure.
A Smooth Surface With Intact Boundaries
Roughness reached 0.05 micrometre on Hastelloy X planar samples, and metallographic examination showed no intergranular attack after polishing, in contrast to acid-bath controls that displayed visible boundary grooves. The solvent could be reused across several batches with only a small loss of removal rate.