Unravelling correlation between electrolyte composition and surface finishing in electropolishing of AISI 316 L stainless steel in H3PO4 - H2SO4 mixtures

A systematic study on AISI 316 L electropolishing (EP) in H3PO4-H2SO4-based electrolytes is presented, aiming to investigate correlations between electrolyte physico-chemical properties and surface finishing and to clarify interfacial phenomena occurring during EP. The effect of electrolyte concentration, [H3PO4]/[H2SO4] ratio and organic additives was considered. Electrical conductivity, density, and viscosity were measured; cyclic voltammetry was run to evaluate EP potential window and plateau current density. Surface finishing was characterised by roughness and brightness measurements; morphology was investigated by optical and electronic microscopy; composition was assessed by energy dispersive X-ray spectroscopy. Best results were obtained in concentrated electrolytes showing lower electrical conductivity, higher viscosity, a wider EP potential window and a lower plateau current density. Lower roughness was obtained when [H3PO4]/[H2SO4] = [H2SO4]/[H3PO4] ≈ 4. Regardless [H3PO4]/[H2SO4] ratio, EP surfaces revealed well-defined crystalline grains, showing different intra-granular microstructure depending on electrolyte composition. Finally, hypotheses on interfacial phenomena governing the process were made.