Application of Pulse Techniques for the Electropolishing of Titanium and Gold Alloys in Deep Eutectic Solvents
Authors: Nascimento M. Lucia , Ispas Adriana , Bund Andreas 
Published: February 2024

Electropolishing is widely used industrially to create smooth and bright surface finishings. This technique is particularly relevant for components with complex geometries, especially in the onset of additive manufacturing. (1, 2) As E. J. Taylor showed for different types of materials, pulse techniques can be used to control the surface modification and drive the electrochemical process towards a specific desired result. (3)
The changes in the surface layers and concentration profiles caused by shifts in polarization lead to results difficult to attain in constant current/potential conditions, especially in metals that passivate and alloys containing metals with dissimilar electrochemical stabilities.
Titanium and noble metal-alloys are materials used in a variety of applications. The passivation layer of titanium is very difficult to remove without using strong acids or fluoride-containing compounds. In the case of noble-metal alloys, the problem lies in the non-uniform dissolution of the material under anodic polarization and in the cyanide-containing electrolytes and strong acids which are used for electropolishing.
Water-free solvents have been described by Jacquet for this purpose (4). Ionic liquids and deep eutectic solvents, DES, are increasingly being used to work in potential windows otherwise not reachable in aqueous solutions. Reports indicate that a mixing of choline chloride and ethylene glycol can be used for electropolishing of titanium and other metals (5, 6) and that both dissolution and recovery of Au and Pd is feasible in ionic liquids and deep eutectic solvents. (7, 8)
Using DES for electropolishing applications offers therefore an opportunity to substitute dangerous substances, such as cyanide and fluoride compounds. This could increase the scope of application of electropolishing in terms of work safety, recycling and disposal.
The aim of this work is to show the interplay between pulse parameters (frequency, pulse height and reverse pulses) and the viscosity of the solution. Together with temperature and convection, pulses can be used to selectively control the dissolution of the metal towards the desired surface finish. Using this principle, an average roughness of 20-30 nm was achieved for titanium samples with complex geometry, both under laboratory conditions as well as in a pilot plant. Furthermore, it will be shown how pulses can be used to control the dissolution of electrochemical less noble elements in gold alloys.

Acknowledgements: The project is financially supported by the German Federal Ministry for Economic Affairs and Climate Action (ZIM Program).

References

W. Han and F. Fang, International Journal of Machine Tools and Manufacture,
139, 1–23 (2019).

E. J. Taylor, H. McCrabb, H. Garich and T. Hall, A pulse/pulse reverse electrolytic approach to electropolishing and through-mask electroetching PF Products Finishing, Electroplating, www.pfonline.com (published on 26.09.2011).

E. J. Taylor, M. Inman, T. D. Hall, S. Snyder, J. Mammosser and F. Furuta, ECS Meet. Abstr., vol.
MA2017-01(24), 1197 (2017).

P. A. Jacquet, Trans. Electrochem. Soc.,
69(1), 629 (1936).

W. O. Karim, J. A. Juma, K. M. Omer, Y. M. Salih, K. H. Aziz and S. B. Aziz, Electrochemistry,
88(5), 447–450 (2020).

A. P. Abbott, G. Frisch, J. Hartley, W. O. Karim and K. S. Ryder, Progress in Natural Science: Materials International,
25(6), 595–602 (2015).

E. Billy, E. Chainet and F. Tedjar, Electrochimica Acta,
56(28), 10340–10346 (2011).

P. Giridhar, K. A. Venkatesan, B. P. Reddy, T. G. Srinivasan and P. R. Vasudeva Rao, Radiochimica Acta,
94(3), 131–136 (2006).