Electropolishing (EP) is a surface finishing process in which the surface roughness is eliminated through selective electrochemical dissolution. The metal to be polished is immersed in an electrolyte, which must be carefully chosen for each system, anodically polarized and dissolved by applying a specific amount of current for a fixed period of time. The process can provide a bright, clean and smooth appearance. Further, EP surfaces could have additional properties such as easy to clean and maintaining, improved passive oxide layers thereby increasing corrosion resistance, reduction of bacterial growth and reduction of friction for contact surfaces.
The development and enhancement of the EP process was widely studied in the thirties by Jacquet Citation[1,2]. This research was probably the beginning of the industrialization of EP which is now a well-known industrial surface finishing process. There are companies worldwide offering sub-contract services for EP, primarily on stainless steels.
In the last few decades, EP seems to have been rediscovered mainly due to the significant increased demand for nano-smooth, super clean, homogeneous, corrosion resistant and biocompatible surfaces, for new applications and technologies. Some examples of the current interest in EP are:
Optical devices such as lasers, solar cells, interferometers, and astronomer's instruments need a mirror-like finish on Aluminum, due to the high reflective and low absorption properties of this material. This could be achieved easily by EP, and some investigations have proved that EP can be used as a method for the preparation of metal mirrors, in the same way as the PVD process, which is an expensive technology commonly used in this field Citation[3]. PVD also requires vacuum and could have a size limitation because of the size of the chamber that will be needed to accommodate the part.
Nanotechnology. Anodic alumina templates containing ordered nanopores are being widely used now for the development of functional nanostructures such as computing networks, memories and nanoscale sensors. The long range pore order requires the starting Alumimum surface to be extremely smooth. In this context, EP has proven to be one of the most suitable methods for surface planarization prior to anodization Citation[4].
High vacuum and nuclear applications. The process of achieving the desired vacuum conditions in high vacuum and nuclear applications could be reduced due to occluded gases in the surface of the equipment. EP could effectively remove the bulk of these gases, allowing high vacuum to be achieved in much shorter times. On the other hand, projects such as, Continuous Electron Beam Accelerator Facility upgrade, X-ray Free Electron Laser linac, and International Linear Collider, need high performance on Nb cavities. EP has become the reference process to reach the surface requirements of these cavities Citation[5].
Biomedical implants. NiTi alloys possess special mechanical properties and good biocompatibility, hence they act as base material for the production of vascular stents. Braiding is a promising alternative for the machining of certain NiTi stents. However, EP treatments are needed for the braided stents are required in order to achieve a medical-grade surface finish. The thermally-grown oxide resulting from the shape–setting heat treatment, following the braiding must be removed by EP which is also required to achieve optimum corrosion resistance Citation[6].
Prototyping methods. EP could be utilized in many prototyping methods, including investment casting, photochemical machining, injection molding, laser cutting, metal stamping, 3D printing, direct metal laser sintering and electrical discharge machining. The application of EP in metal parts produced by these methods could improve the surface roughness leading to an ultraclean surface and enhanced corrosion resistance. In the case of Selective Laser Melting, which is an advanced additive manufacturing technology that fabricates parts in a layer-wise manner, the relatively poor surface finish of these parts requires surface finishing techniques. Conventional processes usually are not practical due to the high geometric complexity of the parts. Hence, there is a necessity to investigate the use of EP as a method of improving the surface finish.
From the above, it is clear that EP is an advantageous process for these technologies or applications; nevertheless, there is still a necessity to continue investigating in this area, in order to solve the two main challenges of the process:
| • | The achievement of enhanced surface properties, like corrosion resistance or biocompatibility, while attaining very smooth surfaces (even with nanoroughness). | ||||
| • | The eco-friendliness of the whole process, mainly avoiding the use of toxic and aggressive electrolytes. | ||||
In the case of improving the safety of the process, especially regarding the EP of materials like Ti and its alloys or Nb, that need aggressive and toxic electrolytes for the optimum EP, different strategies are being considered. The two approaches are the use of less dangerous non-conventional electrolytes like ionic liquids and, on the other, the application of bipolar current pulses during the process.
Pulse reverse EP process of NiTi and other metals and alloys containing Ti, Mo, and Nb Citation[8–10] have also been recently investigated. In direct contrast with conventional EP, the pulse reverse process does not involve high viscosity electrolytes, or the addition of aggressive chemicals like HF to remove passive films. For these materials, the application of the anodic pulses leads to a rougher surface due to the non-uniform breakdown of the passive film. In order to depassivate this surface in a controlled way, cathodic pulses could be interspersed within the anodic pulses, effectively consuming the metal from the film. Even though promising results have been obtained through this approach, additional work is required to elucidate the mechanistic aspects of the process, to optimize the surface roughness of different materials.
Apart from the application of current pulses, and with the goal of reducing the toxicity of current EP chemicals, ionic liquids are being evaluated as electrolytes for this process. These electrolytes provide significant advantages such as high solubility of metal salts, high electrical conductivity and high electrochemical stability, that make them ideal for metal processing. Many studies have been carried out demonstrating the feasibility of using ionic liquids as electrolytes in EP for a variety of different stainless steels, Ni/Co alloys Citation[11], Cu Citation[12] or Ti Citation[13], but there is still work to do with the aim of industrializing this process.
To sum up, EP is an old industrial process which seems to have been revived due to the benefits that it could provide to applications and technologies that have arisen in the last few decades. This situation has led to the emergence of novel EP processes, based on the use of an external magnetic field, the application of bipolar pulses or the use of non-conventional electrolytes. The optimization of these technologies could add an extra value to the whole process, either by improving the electropolished surface properties or by reducing the toxicity of current electrolytes which are significant accomplishments towards the goal of improved surfaces.
Dr. Gemma A. Vara
Edgar J. Butrón
Dr. M. Belén García-Blanco
References
- Jacquet P. A., French Patent 707526, 1930.
- Jacquet P. A.: ‘Electrolytic method for obtaining bright copper surfaces’, Nature, 1936, 135, 1076, 29.
- Adelkhani H., Nasoodi S. and Jafari A. H.: ‘A study of the morphology and optical properties of electropolished aluminum in the Vis-IR region’, Int. J. Electrochem. Sci., 2009, 4, 238–246.
- Alam K. M., Singh A. P., Bodepudi S. C. and Pramanik S.: ‘Fabrication of hexagonally ordered nanopores in anodic alumina: An alternative pretreatment’, Surf. Sci., 2011, 605, (3-4), 441–449.
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- Kassab E., Marquard A., Neelankantan L., Frotscher M., Schreiber F., Gries T., Gomes J. and Eggeler G.: ‘On the electropolishing of NiTi braided stents-challenges and solutions’, Materialwissenschaft und Werkstofftechnik, 2014, 45, (10), 920–929.
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- Taylor E. J., Hall T., Inman M., Snyder S. and Rowe A.: ‘Electropolishing of niobium SRF cavities in low viscosity aqueous electrolytes without hydrofluoric acid’. Proceedings of SRF2013, 2013, Paris, France.
- Abbott A. P., Ryder K. S. and Konig U.: ‘Electrofinishing of metals using eutectic based ionic liquids’, Trans. Inst. Met. Finish., 2008, 86, (4), 196–204.
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- Uda T., Tsukimoto K., Nakagawa H., Murase K., Nose Y. and Awakura Y.: ‘Electrochemical polishing of metallic titanium in ionic liquid’, Mat. Trans, 2011, 52, (11), 2061–2066.