Surface Finishing of Strongly Passive Biomedical Materials, such as Nitinol, in Low- and Non-Acid Electrolytes
Authors: Inman Maria E. , Taylor E. J. , Hall Timothy D , Garich Holly , Lucatero Savidra 
Published: February 2020

A process for surface finishing of biomedical alloys is described. For strongly passivating metals, electropolishing under direct current (DC) electric fields in a simple electrolyte can lead to a roughened surface. To remove the passive film, aggressive chemicals are added to the electrolyte, for example, hydrofluoric acid may be used to depassivate the surface.[1]
A pulse/pulse reverse waveform can be used to electropolish passive materials, by tuning anodic pulses to control mass transport and current distribution, interspersed with cathodic pulses to depassivate the surface.[2]-,[3],[4],[5]Off-times inserted between the anodic and cathodic pulses facilitate replenishment of reacting species and removal of by-products and heat. The cathodic pulse eliminates the need for HF or other chemicals to remove the surface oxide. While the exact mechanism of depassivation is unknown at this stage, we speculate that the cathodic pulses remove the oxide either by direct electrochemical reduction or indirect chemical reduction, and restore the virgin metal surface prior to the next anodic pulse. The amplitude of the cathodic pulses required for depassivation is material specific, and appears to be based on the strength of the passive film.
The inclusion of cathodic pulses and off-times suggests that the overall process would be slower than a conventional DC process, which is undesirable for industrial implementation. However, the maximum current density during the anodic pulse is higher than the DC limiting current density. Therefore, the overall removal rate of a pulsed process can rival or exceed that of a DC process despite a duty cycle that is less than 100%, while enjoying enhanced process performance.
Studies in near-neutral aqueous solutions have demonstrated the feasibility of eliminating toxic solutions from manufacturing of biomedical devices. The best processing conditions to date for Nitinol in 17 w/w% H2SO4 has resulted in a final surface Ra of 0.19 µm. We have electropolished Ti-15Mo and Ti-Al-Nb alloys in 10 w/w% H2SO4 or 150 g/L Na2SO4to an Ra of 0.12 µm and 0.23 µm, respectively. We have done preliminary work on Ti-6Al-4V, achieving a near-mirror like surface finish.
For electropolishing of stainless steels in an aqueous NaCl-NaNO3 electrolyte, a waveform sequence of short anodic pulses with interspersed cathodic pulses for 30 s followed by a waveform with long anodic pulses and interspersed cathodic pulses for 15 s has achieved a final Raof 0.026 µm (Figure 1: SS316 surface before and after FARADAYIC® Electropolishing). Finally, we have electropolished molybdenum to a mirror-like surface finish, and investigated on cobalt-chrome alloys and rhenium alloys.
In summary, we have described a pulse reverse electropolishing process that may be applied to biomedical alloys using low viscosity aqueous electrolytes, devoid of aggressive chemicals for depassivating the surface. Consequently, the safety and chemical handling and disposal issues associated with pulse reverse electropolishing are minimized. In addition, the need for active chilling is minimized by the appropriate use of off-times and we anticipate that the process reproducibility and robustness are greatly enhanced.

Acknowledgements: The authors acknowledge the financial support of Faraday corporate, NIH Grant No.1 R43 HL095216-01A1, DOE P.O. No. 594128 and DOE Contract No. DE-SC0004588.
References

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155, D563-568 (2008).
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[3] C. Zhou, E.J. Taylor, J. Sun, L. Gebhart, R. Renz, U.S. Patent 6,402,931, 6/11/02.
[4] M. Inman, E.J. Taylor, A. Lozano-Morales, T.D. Hall, H.M. Garich, Electrochemical System and Method for Machining Strongly Passivating Metals U.S. Patent Appl. 13/153,874 Jun. 11, 2010.
[5] E.J. Taylor, Sequential Electromachining and Electropolishing of Metals and the Like Using Modulated Electric Fields, U.S. Patent No. 6,558,231 May 6, 2003.