Electrochemical Impedance Study of Cast and ALM Stainless Steel Electropolishing in Acids Media and Deep Eutectic Solvents
Authors: Rotty Chloé , Doche Marie-Laure , Mandroyan Audrey , Vivier Vincent , Hihn Jean-Yves 
Published: February 2020

Electropolishing (EP) process is now extensively used as a post-treatment particularly for additive manufacturing parts. This electrolytic dissolution process consists in a controlled dissolution of metallic surfaces leading to both surface roughness decrease and brightness increase, and is usually performed in concentrated acidic media. Interestingly, alternatives eco-friendly electrolytes depending on the metal type have been developed during the last years such as Deep Eutectic Solvents 1,2. Four distinct regions appear on a typical electropolishing polarization curve,3–5namely the active dissolution of the substrate after the oxide layer breakdown, a viscous film formation at the interface electrolyte/substrate (not always present), a limiting current plateau ascribed to a diffusion controlled mechanism (this corresponds to the electropolishing zone), and the solvent oxidation and oxygen evolution domain. Given the complexity of the electropolishing process, many mechanisms have been proposed. Among them, the most commonly accepted explanations consists in the presence of a resistive layer due to dehydratation, oxide formation or saturation of dissolved metal ions at the electrode interface. In this respect, the electropolishing mechanism was investigated by Linear Sweep Voltammetry (LSV) and Electrochemical Impedance Spectroscopy (EIS) for 316L Stainless Steel in phosphoric and sulfuric acid media and choline chloride-ethylene glycol mixtures. Then an extended study has concerned additive manufacturing parts.

The control of the dissolution reaction by diffusion was confirmed with the linear dependence of the limiting current density vs.electrode rotation rate as described by the Levich’s law. Nevertheless, varying the viscosities by changing temperature from 35°C to 70°, show a direct relationship between the diffusion coefficient and the kinematic viscosities, irrespective of the cation concentration at the interface, which suggest a minor role in the diffusion limiting step. This limitation is therefore provided by the diffusion of an acceptor specie from the electrolyte toward the anode surface. To discriminate the role of water or mineral anion, a full descriptive model of the electrochemical behaviour of interface was developed, based on the corresponding hypotheses. This model found its roots in iron corrosion and electropolishing considerations.6Additionally, EIS measurements were carried out in acidic and DES media. This enable to confront experimental results with the theoretical model for the dissolution reaction. In the first case, water was assumed to be the limiting specie, whereas and in the second case an acceptor specie (as phosphates or chlorides) was considered. A similar study has been conducted for both cast and additive manufacturing 316L Stainless Steel in both acidic and DES media. In all cases, the proposed model allows to predict the electrochemical behaviour of the system.

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