Abstract: Anisotropic pitting mechanisms were investigated during electropolishing (EP) of Zr-based bulk metallic glass (BMG) surfaces in a chloride-containing alcohol-based electrolyte. Electrically discharge-machined (EDMed) and mechanically polished (MPed) BMG surfaces were selected as the typical preprocessing surfaces. NaCl-ethylene glycol solution was selected as the electrolyte because of its ability to avoid solvent decomposition and form high-viscosity complexes on surfaces to lower the surface roughness. The surface roughness and morphology of both specimens under varying EP parameters were examined and compared. With the optimized parameters, the recast layer of the EDMed surface was removed, and a decrease of more than 50% in the roughness values was achieved for both types of surfaces. However, anisotropic pitting also occurred on the surfaces subjected to EP. Further findings showed that pitting was induced by the notches distributed in the Cu-rich film covering the EPed surfaces, which resulted from the distinctive dissolution rates of different elements. For the EDMed surface, the notches were related to the Zr-rich crystals observed beneath the recast layer, whereas for the MPed surfaces, such notches appeared directly during the gradual dissolution of the surfaces. The Cu₂O product serving as a protective layer for the covered region was detected at the pitting boundary on both types of surfaces. The region without such protection gradually dissolves at high voltages. In addition, deep slots were formed at the boundaries of the pits on the MPed surfaces under the corrosion of chloride ions, which were concentrated at the boundaries. Finally, dissolution-pitting models were proposed to describe the anisotropic pitting mechanisms on BMG surfaces in non-aqueous solutions.

The full text can be downloaded at https://doi.org/10.1007/s40436-025-00565-w

Key words: Electropolishing (EP), Bulk metallic glass, Electrically discharge-machine (EDM), NaCl-ethylene glycol, Pitting mechanism