Electropolishing in deep eutectic solvents

A fundamental study of electropolishing of stainless steel and nickel based single crystal superalloy CMSX-4 in type III deep eutectic solvent based on choline chloride and hydrogen bond donor i.e. mixture of choline chloride with ethylene glycol in a 1:2 molar ratio was carried out and had been found to be competitive with the current concentrated mixture of inorganic acid electrolytes. Life cycle study was conducted to define the key process controlling factors like electrochemical stability, current efficiency, effect of history of electrolyte, recycling of ionic liquid and its reuse for electropolsihing. The electrochemical techniques like linear sweep anodizing curves, chronoamperometery and galvanostatic studies revealed that electropolishing in 1:2 ChCl:EG proceeded through the formation of viscous layer on the surface of the substrate similar to electropolishing in inorganic acid electrolytes. The optimization of electropolishing process was carried out using the experimental design strategies, Fractional Factorial Design (FFD) and found that electropolishing variables like addition of water, oxalic acid, electropolishing bath temperature, time and potential had positive impact on the surface finish. Surface texture measurements such as surface roughness and surface overlayer morphology of electropolished stainless steel and CMSX-4 was carried out using the microscopic techniques, atomic force microscopy (AFM), scanning electron microscopy (SEM), field emission scanning electron microscopy (FE-SEM) and digital holographic microscopy (DHM) and found to be the function of electropolishing time. Effect of electropolishing on corrosion behaviour of stainless steel was studied using the electrochemical techniques like open circuit potential measurements (OCP), potentiodynamic polarization curves and gravimeteric method showed improvement in the general or pitting corrosion of the workpiece. Nickel based superalloy was also successfully electropolished to remove the casting scales. The dissolution of two phases was found to be the function of electrochemical regime i.e. applied potential and current density.