Abstract
ABSTRACTElectropolishing is a common method for decreasing surface roughness and removing surface irregularities. In this paper the electropolishing of nickel and cobalt are successfully demonstrated in a deep eutectic solvent, comprising a 2:1 molar mixture of ethylene glycol and choline chloride. Voltammetric and electrochemical impedance studies were used to characterise the polishing mechanism and show that film formation occurs prior to polishing. Scanning electron microscopy and atomic force microscopy were used to characterise the morphology before and after polishing and 3D optical microscopy was used in-situ to observe film formation during polishing. This study shows that the impact of film formation and subsequently mass transport are responsible for electropolishing of both metals in the choline chloride-based ionic liquid.
Highlights
Electropolishing is essentially the controlled electrochemical corrosion of a metal to bring about a decrease in surface roughness
The DES was prepared by mixing choline chloride (ChCl) (Aldrich, 99%) and ethylene glycol, (EG) (Aldrich, > 99%) in a molar ratio of 1: 2 (1 ChCl: 2 EG)
A very similar shaped voltammogram is obtained with the oxidation peak and on-set voltage for reduction occurring at about the same potentials. This again is very similar to the response obtained for the electrodeposition of cobalt from the same DES.[13] j j
Summary
Electropolishing is essentially the controlled electrochemical corrosion of a metal to bring about a decrease in surface roughness. Electropolishing an item can increase the corrosion resistance of the component as it enables better, more homogeneous passivation and it can decrease wear in moving systems It has most commonly been studied in aqueous solutions primarily using concentrated mineral acids such as sulfuric, hydrochloric and phosphoric acids. While a limited number of studies have investigated pure nickel in aqueous solutions no studies have been published on cobalt, the electropolishing of a cobaltchromium alloy has been patented.[11] It is shown that film formation on the metal surface is important and the mass transportation during the process is the key factor of achieving a polished surface finish which is evidenced by a.c. impedance results
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