Electrolytic polishing, also known as electropolishing, is a metal finishing process that uses an electrolytic cell to remove surface material from a metal specimen, resulting in a smooth, shiny, and level surface. In this process, the metal specimen is connected to the positive terminal of an external power supply and immersed in an electrolytic solution, which serves as the cathode.
When a direct current is applied to the cell, anodic dissolution occurs at the surface of the metal specimen, which results in the removal of surface material through a process of oxidation and reduction. This process results in the formation of a layer of viscous material on the specimen surface, which is composed of products that result from the reaction between the metal and the electrolyte.
- Electrolyte.
- Cathode.
- Work-piece to polish (Anode).
- Particle moving from the work-piece.
toward the cathode. - Surface before polishing.
- Surface after polishing.
The layer of viscous material is essential for proper electropolishing because it acts as a barrier, preventing the electrolyte from attacking the metal surface directly. Instead, the electrolyte reacts with the viscous layer, which results in a leveling and brightening of the specimen surface. This process is particularly effective for soft metals, single-phase alloys, and alloys that work harden readily.
However, electropolishing can have some disadvantages, such as preferential attack in multiphase alloys caused by differences in electrical potential between phases, and chemical attack of nonmetallic inclusions by the electrolyte. This can result in an uneven or damaged surface, which can compromise the integrity of the specimen.
To minimize these disadvantages, it is essential to choose the appropriate electrolyte and operating conditions. The electrolyte should have a high electrical conductivity and be able to dissolve the viscous layer effectively. The operating conditions, such as current density, temperature, and pH, should be carefully controlled to ensure optimal results.
Overall, electrolytic polishing is a useful process for preparing metal specimens for analysis, as it results in a smooth, shiny, and level surface. By selecting the appropriate electrolyte and operating conditions, the disadvantages of electropolishing can be minimized, resulting in high-quality specimens suitable for analysis.
Electropolishing is a process commonly used to improve the corrosion resistance of metals and alloys, including NiTi. The process involves removing the native oxide layer and forming a homogeneous TiO2 layer at the surface, which has a uniform thickness and chemical composition. The surface topography of the electropolished material is a subject of debate, with some studies reporting nanometer size roughness and others finding the surface roughness to be comparable with that achieved with mechanical polishing. Nanometer size surface roughness has been shown to improve blood compatibility, which is why many endoluminal stent materials are electropolished prior to implantation.
In addition to improving blood compatibility, electropolishing has been found to decrease the nickel content at the surface of the material. One study found that the surface nickel concentration decreased from 11.5% to 1% as a result of electropolishing. The breakdown potential was also improved, as shown in Fig. 11.4, and significantly increased compared with stainless steel. This increase in breakdown potential suggests that the material is more resistant to corrosion after electropolishing.
Another study found that electropolishing increased the pitting corrosion potential of NiTi stents in Hank’s solution compared with untreated or heat-treated NiTi stents, thus improving the corrosion resistance. The oxide layer formed through electropolishing was approximately 34 Å thick, and the improved corrosion resistance might be a result of a more uniform oxide layer with lower nickel content rather than a result of film thickness. The Ni-to-Ti ratio at the surface of the electropolished sample was low, around 0.05, prior to the corrosion tests and decreased slightly after immersion.
It is important to note that nickel-leaching rates, surface roughness, and biocompatibility were not assessed in all studies. Proper selection of the electrolyte and operating conditions during electropolishing can minimize any potential disadvantages, such as preferential attack in multiphase alloys caused by differences in electrical potential between phases and chemical attack of nonmetallic inclusions by the electrolyte.
The study by Armitage et al. examined the effect of electropolishing on the surface topography, chemical composition, wettability, and cell toxicity of NiTi surfaces, and found that electropolishing did not offer any advantage in terms of biocompatibility or hemocompatibility compared to mechanically polished surfaces. However, this study did not include any corrosion testing, so it is unclear whether electropolishing affects the corrosion resistance of NiTi surfaces.
X-ray photoelectron spectroscopy (XPS) analysis showed that electropolishing decreased the nickel concentration at the surface and detected the presence of nickel hydroxide, which was not present on the mechanically polished specimen. Atomic force microscopy (AFM) showed an increase in surface roughness and an attendant increase in surface wettability on the electropolished sample compared with the mechanically polished sample, which the authors attributed to the difficulty in obtaining optimal electropolishing conditions.
Cell testing with mouse fibroblasts and human venous endothelial cells showed no cytotoxicity due to any of the examined specimens and no significant differences in cell morphologies between samples. Platelet adhesion studies revealed no significant difference between the two types of surfaces.
However, the study did not examine the thickness of the oxide layer or the ion release rate of the electropolished samples. These parameters are important for determining the usefulness of electropolishing in improving the corrosion resistance of NiTi surfaces.
It is important to note that the results of this study suggest that electropolishing can significantly alter the surface composition and properties of NiTi alloys. The TiO2 layer formed as a result of electropolishing may improve the biocompatibility and corrosion resistance of NiTi surfaces. However, the high concentration of nickel near the surface of the electropolished sample may still lead to significant nickel release, which could be a concern for certain biomedical applications. It is also important to assess the cytotoxicity, breakdown potential, and mechanical properties of electropolished NiTi surfaces before using this technique in clinical applications. Overall, while electropolishing may have potential advantages for improving the properties of NiTi surfaces, further studies are needed to fully evaluate the effectiveness and safety of this technique.