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

Authors: Matteo Pozzi; Luca Sala; Massimiliano Bestetti; Silvia Franz

Addresses: Department of Chemistry, Materials and Chemical Engineering 'Giulio Natta', SurfaceLAB – Laboratory of Surface Engineering and Applied Electrochemistry 'Roberto Piontelli' Politecnico di Milano, Piazza Leonardo Da Vinci 32, 20133 Milano (MI), Italy; Rösler Italiana s.r.l., via Elio Vittorini 10/12, 20863 Concorezzo (MB), Italy ' Rösler Italiana s.r.l., via Elio Vittorini 10/12, 20863 Concorezzo (MB), Italy ' Department of Chemistry, Materials and Chemical Engineering 'Giulio Natta', SurfaceLAB – Laboratory of Surface Engineering and Applied Electrochemistry 'Roberto Piontelli' Politecnico di Milano, Piazza Leonardo Da Vinci 32, 20133 Milano (MI), Italy ' Department of Chemistry, Materials and Chemical Engineering 'Giulio Natta', SurfaceLAB – Laboratory of Surface Engineering and Applied Electrochemistry 'Roberto Piontelli' Politecnico di Milano, Piazza Leonardo Da Vinci 32, 20133 Milano (MI), Italy

Abstract: 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.

Keywords: electropolishing; EP; AISI 316 L; H₃PO₄; H₂SO₄; cyclic voltammetry; CV; electrolytes physico-chemical properties; electrolyte composition and surface finishing correlation; surface finishing.

DOI: 10.1504/IJSURFSE.2025.147573

International Journal of Surface Science and Engineering, 2025 Vol.19 No.2, pp.145 - 167

Received: 17 Jun 2024
Accepted: 03 Nov 2024
Published online: 21 Jul 2025 *

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