Voltammetric and impedance studies of the electropolishing of type 316 stainless steel in a choline chloride based ionic liquid

Abstract

We demonstrate the first practical alternative to the use of phosphoric and sulphuric acid mixtures for the electropolishing of stainless steel. In this paper, efficient electropolishing of type 316 stainless steel is demonstrated in an ionic liquid composed of ethylene glycol (HOCH₂CH₂OH) and choline chloride (HOC₂H₄N(CH₃)₃⁺Cl⁻). Linear sweep voltammetry, chronoamperometry, scanning electron microscopy, atomic force microscopy and AC impedance methods were used to investigate the steel dissolution mechanism and the results are compared to polishing done in aqueous acidic solutions. It is shown that the quality of the polish is related to the breakdown of the oxide film and preliminary data suggest that the polishing process may be controlled by the diffusion of chloride ions. The dissolution is different from that found in aqueous acid solutions, and oxide breakdown is shown to be slower, which can lead to pitting at low current densities.

Introduction

Electropolishing is the controlled corrosion of a metal surface to bring about a reduction in surface roughness. The first systematic study of electropolishing was carried out by Figous and Jacquet and led to a patent in 1930 [1]. The majority of studies have been carried out on stainless steel although metals such as copper, nickel and titanium have also been studied [2], [3], [4]. The current stainless steel electropolishing process is performed worldwide on a commercial scale and is based on concentrated phosphoric acid and sulfuric acid mixtures. The polishing process is thought to involve the formation of a viscous layer at the metal surface and many processes employ viscosity improvers such as glycerol. The practical aspects of electropolishing have been reviewed by Mohan et al. [5] whereas the more fundamental aspects are covered in a review by Landolt [6]. While electropolishing is an extremely successful process there are major issues associated with the technology most notably that the solution used is highly corrosive and toxic with extensive gas evolution occurring during the process. Over the last 20 years ionic liquids have been of considerable interest for a wide variety of electrochemical and synthetic applications [7], [8], [9], [10]. Most of the studies have concentrated on imidazolium and pyridinium cations; however, their high cost and sensitivity to water has made them of little practical use for large-scale applications such as metal finishing. Eutectic mixtures of zinc, tin and iron halides with a variety of quaternary ammonium salts can be formed and used for metal finishing [11], [12], [13], [14]. These ionic liquids are air and moisture stable and the components are widely available and relatively benign allowing their usage in large-scale applications such as metal deposition. Zinc and zinc alloys can be electrodeposited with high current efficiency from these ionic liquids [13], [14]. This technology can be extended to other metals by using hydrated metal salts [15], [16]. For example, mixtures of CrCl₃·6H₂O with choline chloride form a liquid that can be used for the efficient electrodeposition of chromium.

The principle of forming an ionic liquid by complexation of the halide anion has been extended by mixing quaternary ammonium salts with compounds that form hydrogen bonds. This has been demonstrated with a range of hydrogen bond donors including amides such as urea [17] and carboxylic acids such as oxalic acid [18]. We have previously shown [19] that ethylene glycol can also be used as the hydrogen bond donor, and mixtures with choline chloride can be used for the electropolishing of stainless steel. This process has the advantage that high current efficiencies are obtained with negligible gas evolution at the anode/solution interface. The liquid used is also comparatively benign and non-corrosive compared to the current aqueous acid solutions. In the present study we use AC impedance, linear sweep voltammetry and chronoamperometry to investigate the mechanism of electropolishing in these glycol mixtures. The analysis is largely qualitative due to the complexity of the dissolution mechanism and the difficulty of making steady-state measurements on a surface that is etched up to 1 μm during the measurement timescale.