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.
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
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:
- high current efficiencies are obtained;
- gas evolution at the anode/solution interface during polishing is negligible;
- the liquid used is comparatively benign and non-corrosive compared to the current aqueous acid solutions.
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
(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.
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.