Leicester Ionic Liquids Group
Green Solutions
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Research

The main research interests of the group are firmly based around green chemistry with particular emphasis on electrochemical processes. It is active in developing novel solvent systems with industrial applications such as metal deposition and dissolution. It collaborates strongly with industry and much of the work to date has been in the development of novel processes using ionic liquids.

Electropolishing

The process of electropolishing is one of controlled dissolution of a metal surface to bring about a reduction in surface roughness and hence an increase in optical reflectivity.

AFM images (inserts) and SEM images of unpolished (left) and polished (right) stainless steel.

The present stainless steel electropolishing process is performed worldwide on a commercial scale and is based on mixtures of concentrated phosphoric and sulfuric acids. While electropolishing is an extremely successful process there are major limitations and practical issues associated with the technology; most notably that the solution used is highly corrosive and toxic, extensive gas evolution (with associated low current efficiency) also occurs during the process.

We have developed a new electropolishing process using one of our ionic liquids formed from choline chloride and ethylene chloride. This process has three main advantages over the commercial alternative:

We have used AC impedance, linear sweep voltammetry and chronoamperometry to investigate the mechanism of electropolishing in these glycol mixtures. The surface morphology has also been investigated using Scanning Electron Microscopy, SEM with Energy Diuspersive X-ray Analysi, EDX and Atomis Force Microscopy, AFM.

Electropolishing demonstrator unit (part of the Ionic Liquids Demonstrator ild).

SEM and EDX analysis

The surface morphology and composition of polished and unpolished regions of the steels was compared by preparing steel samples with regions masked off using an acrylic resin insoluble in the ionic liquid. The samples were then subjected to the electropolishing regime and the mask was subsequently removed with acetone. This methodology facilitates both qualitative and quantitative comparison of composition and morphology over a spatially confined region of the same surface. One of these masked samples was imaged in an SEM. The magnitude and type of grain boundary pattern are characteristic of stainless steel sheet. In the SEM image (below) both polished and unpolished regions are clearly visible, separated by a boundary defined by the mask. The polished region appears very much smoother with no trace of the grain boundary pattern seen in the unpolished region. Since one of the motivations for the commercial electropolishing process is to improve corrosion resistance of the substrate it was important to establish that the polishing mechanism in the ionic liquid did not result in gross changes of the elemental composition at the interface i.e. dealloying. Data obtained from energy dispersive X-ray analysis (EDX), using a beam spot on each side of the boundary show clearly that the composition of the alloy is unchanged by the polishing process and therefore that no dealloying occurs.

(a) Photograph of a sample of stainless steel 304 masked (with acrylic resin) and polished with ca. 1 mm stripes showing unpolished (u) and polished (p) regions. The mask was removed with acetone (b) Scanning electron micrograph of the unpolished region of the sample shown in (a), high magnification insert. (c) Scanning electron micrograph showing the transition between polished (top) and unpolished (bottom) regions of the sample.

AFM analysis

In order to study the surface morphology and roughness of the steels before and after polishing and in an attempt to quantify the etch rate we examined the surfaces using contact and tapping mode AFM. The figure below shows height contrast images of a sample of stainless steel 304 recorded across the boundary between polished and unpolished areas over a length scale of ca. 74 - 72 mm. The height contrast projection, shows clearly defined areas of morphology in much the same way as the corresponding SEM image. The unpolished area of the surface shows the same surface roughness and grain boundary structure as that observed in the SEM, the surface is very rough (giving rise to its optically dull finish), with feature sizes of up to 500 nm, it is also very flat; this is no doubt a consequence of the production technique (rolling). In contrast, the polished region has a bright optical finish, it is very smooth but there are large scale fluctuations in height.

Resonant mode (ca. 300 kHz) AFM images (recorded in air at a scan rate of 0.5 Hz, 256 lines) at the interface between polished and unpolished regions, (a) tip-height contrast projection and (b) 3-D surface map showing the corresponding height scale, (c) slice through the surface at X = 35.6 mm showing the tip height data for a single line scan.