Plasma Electrolytic Polishing of TC4 Alloys in Hydrochloric Acid Modified Novel Electrolyte: Effects of Electrolyte Reusability and Polishing Time on Surface Morphology

Plasma electrolytic polishing (PeP) is an anodic surface finishing method that achieves localized dissolution and smoothing of metal surfaces. Unlike conventional fluoride-based electrolytes reported in the literature, the present electrolyte combination incorporates hydrochloric acid (HCl) as an additional constituent to mitigate fluoride retention on the polished surface and modulate electrolyte conductivity during processing. This study systematically investigated the effect of controlled HCl addition (0.1%, 0.3%, and 0.5%) on the polishing efficiency, surface roughness, gloss, and material removal rate (MRR) of TC4 alloy processed by PeP. To assess the long-term stability of the electrolyte combination and its practical viability for repeated industrial application, the effects of electrolyte reusability (first, second, and third use) and polishing time (120, 180, and 240 s) on surface quality were also examined. Owing to the selective dissolution mechanism of PeP, MRR decreased with increasing polishing time. At 240 s, surface roughness improved from 4.5 to 0.558 µm, while gloss increased nearly threefold, from 15.6 to 56.06 GU. Corner diameter increased in regions of initial electrolyte contact, attributed to locally lower electrical resistance and higher current density at the corners. Electrolyte reuse progressively reduced process performance, diminishing roughness and gloss improvements while intensifying corner damage.