Alloys that oxidize easily such as those containing titanium or chromium present a challenge to electropolishing because the polarization that dissolves the metal species produces positive ions, these oxidize and form stable surface layers of metallic oxides that prevent further dissolution. This is usually overcome with the use of acid solutions that dissolve the metallic oxide. This thesis aims to shift the primary control of the electropolishing e_ect from electrolyte variables to a combination of potential variation and hydrodynamic interference. Traditionally this is achieved with one continuous mass removal process that operates after a steady state of dissolution is established, generally requiring hydro_uoric or phosphoric acid to achieve titanium dioxide breakdown. The resulting concentration gradient is heavily a_ected by electrolyte variables such as viscosity and electrical resistance, while the electrical polarization is constrained by the metallic oxide reaction rate which creates a complex net of interdependent variables that can be di_cult to tune. A rapidly changing electric _eld was applied to modulate the alloying element dissolution rates. In tandem with the electropolishing development, stages prior to the electropolishing step were selectively removed to simplify the process. Utilizing a three electrode system and an external potentiostat controller to permit greater _exibility, a variety of alternating current pulsatile waveforms were investigated and the resulting e_ect on surface topology was observed using SEM and AFM microscopes. Di_erential pulse voltammogram yielded a feedback parameter on surface composition, and various pulse parameters were adjusted to optimize for surface smoothness, and identify the primary control variable. An electropolishing method is presented which achieves a :50% reduction in the Sa surface roughness value to an area average of 45 nm on a laser cut tubular stent geometry. It is shown that this method can be adapted to eliminate the need for chemical etching or mechanical polishing prior to electropolishing. The resulting polished surface displays corrosion resistance equivalent or better than other electropolished Nitinol surfaces from literature with a breakdown potential >1V vs SCE, and a similarly high repassivation potential. Balancing the charge in the anodic and cathodic pulses was the key to minimizing the resulting surface roughness, and eliminating micropits. Nitinol is a nearly binary alloy of NiTi and a charge transfer ratio of 1 yielded the smoothest surfaces at current densities around :1 A/cm2. The initial surface condition was found to be irrelevant to electropolishing control with respect to oxide composition, provided enough mass was removed to fully dissolve the initial layers of mixed composition.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/36760 |
| Date | 20 July 2022 |
| Creators | Cloete, Jeran Andre |
| Contributors | Bezuidenhout, Deon, Levecque, Pieter |
| Publisher | Faculty of Health Sciences, Division of General Surgery |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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