Nanoscale Electropolishing of High-Purity Aluminum with a Deep Eutectic Solvent

Abstract

Recent advancements in electrochemical polishing have minimized surface roughness and enhanced conductivity properties of high-purity (>99.9% composition) aluminum surfaces in ways that machine polishing and simple chemical polishing cannot compare. Aluminum metals have a high propensity to deliver powerful electric charges with little resistance, thus making them effective for a wide range of electroconductivity experiments. The effects of an acid-free ionic liquid electrolyte solution were tested to determine potential energy thresholds during electropolishing treatments based upon temperature, experiment duration, current, and voltage.High purity aluminum metal specimens were used in electropolishing treatments with an acid-free ionic liquid electrolyte prepared from quaternary ammonium salts as an environmentally friendly alternative to acid-based solutions. An ionic liquid solution was formed from ethylene glycol and choline chloride combined in a 2:1 ratio. Similar ionic liquid solutions have been successfully employed to electropolish metal alloys, but little research has been conducted with this approach to polish rare earth metals like aluminum (1-5). Linear sweep voltammetry and chronoamperometry tests were initially used to determine ideal conditions for electrochemical polishing, while Atomic Force Microscopy revealed nanoscale effectiveness of the ionic liquid relative to an industry standard acid polishing treatment of 1 M Phosphoric Acid. Surface characterization via root mean squared roughness before and after electrochemical polishing treatments in 10x10 µm sample regions were used to estimate polishing efficiency throughout a 15-minute treatment period at 70 deg C and an average voltage of 2V.Results from electropolishing treatments in the ionic liquid electrolyte revealed a significant change in root mean squared roughness from 159.31 nm to 26.649 nm and resulted in an overall mass loss of 0.0394 g. A change of 132.661 nm during the 15-minute treatment resulted in an electropolishing efficiency of 83.272% and a shiny mirror finish 6x smoother than the same aluminum surface prior to treatment.Comparatively, the phosphoric acid electropolishing treatment yielded only a nominal improvement in root mean squared roughness of 97.786 nm, yet resulted in a greater mass loss of 0.0458 g for a lower smoothing efficiency rating of 38.546%. The electropolishing rate via the Phosphoric Acid solution is greater, yet the efficiency is less favorable due to significant pitting at low current densities (Figure 1). Hydrogen contamination was observed to adversely impact the overall root mean squared roughness of all sampled areas, yet this may be remedied via costly degassing treatments at temperatures of greater than 800 deg C to achieve more comparable results to the ionic liquid solution by mitigating some of the pitting and bubbling caused by hydrogen attack. In this study, a green polishing solution was prepared from simple inorganic salts, alkyls, and other organic solvents. This eco-friendly solution produced polished surfaces superior to those surfaces treated with industry standard acid electrochemistry treatments of 1M H3PO4.