A multi-physics and multi-scale approach to characterize the viscous layer growth formed on SLM 316 L stainless steel during electropolishing in an acid mixture

This study aims to provide experimental evidence of the formation and the growth of a viscous layer during the electropolishing of additively manufactured 316 L stainless steel parts, using a combination of optical characterization techniques. A tertiary current distribution model was subsequently developed to simulate the electropolishing process and predict the evolution of the viscous layer. The results demonstrate that the viscous layer forms and grows within a specific potential window before being disrupted by gas evolution due to solvent oxidation. Schlieren imaging estimates the thickness of the layer to be approximately 1.4 mm after 5 min of polarization at the onset of the polishing plateau—about 1 mm thicker than its natural state without polarization. Particle Image Velocimetry (PIV) confirms the presence of a flow-deprived zone near the surface, roughly 1 mm thick, contrasting with the bulk region where natural convection dominates. A simplified reaction mechanism is proposed, based on experimentally determined electron-transfer kinetics. Metal cations are assumed to be instantly complexed, with the diffusion of the resulting complexes considered equivalent to that of the free complexing agents (phosphates), based on literature values. Using these assumptions, the tertiary current distribution model successfully replicates the growth of the viscous layer. The model's predictions were validated by experimental measurements of metal cation concentrations, supporting the hypothesis that the diffusion of “acceptor” species is the primary driving force behind electropolishing. This work also confirms that phosphate-complexed metal cations diffuse analogously to anions.