Electrochemical etching of metals and minerals using ultrasound in deep eutectic solvents

Abstract

This study compares the anodic dissolution of two iron-based alloys, mild steel and an FeNdB super magnet, with that of a mineral, chalcopyrite (CuFeS₂), in a deep eutectic solvent formed from choline chloride and ethylene glycol under silent and ultrasonic conditions. These three materials have different granular morphologies and heterogeneities. The aim was to show how a combination of ultrasound and electrochemistry can lead to different etching mechanisms through increased mass transport by the mechanical action of bubbles at the electrode surface. Linear sweep and cyclic voltammetries showed that both alloys passivated on the anodic sweep. Using ultrasound significantly decreases passivation, but does not fully prevent it as was previously proposed. Partial passivation produces enhanced pitting and anisotropic etching of all grains. Where the alloy grains are heterogeneous, as is the case with FeNdB, anisotropic etching can lead to surface fragmentation under certain conditions, with partial grains being dislodged from the surface without dissolution. In this case, selective dissolution of the Nd-rich phase was observed. Cyclic voltammetry of chalcopyrite showed several anodic redox processes under silent conditions, but the use of ultrasound led to a linear current–voltage response with a roughly 15-fold increase in the current. Electrochemical etching occurs around grain boundaries, making the material more prone to further ultrasonic fragmentation. This led to a rapid separation of the copper-rich phase from the largely silica-based gangue phase. The surface was inert under just insonation, demonstrating the importance of the anisotropic electrochemical etching process. This approach could be used for the purification of minerals.