The physical and chemical properties of titanium components, such as plates and rods, are significantly influenced by surface reaction layers. These layers, including surface pollution and defect layers, must be thoroughly removed before processing. This article explores the primary surface polishing techniques for titanium plates and rods, including blasting, pickling, chemical polishing, and electrolyte polishing.
1. Blasting
Blasting is an effective initial treatment for titanium castings. The recommended media for blasting titanium are white and rigid materials, and the blasting pressure should be controlled below 0.45 MPa. Excessive pressure can cause sand particles to create sparks on the titanium surface, potentially leading to secondary contamination and affecting surface quality. The blasting duration typically ranges from 15 to 30 seconds and aims to remove only the surface sand and minor oxidation layers. Any remaining surface reaction layer should be addressed with subsequent chemical methods.
2. Pickling
Pickling, or acid washing, is a crucial step for the complete removal of surface reaction layers after blasting. Acid washing effectively eliminates the reaction layer without introducing additional contaminants. Two common acid solutions used for titanium are:
HF-HCl System: While effective, this method can absorb hydrogen, which may affect the titanium's properties.
HF-HNO3 System: This method helps control hydrogen absorption by adjusting the concentration of HNO3, typically between 15%-30%, with HF concentrations around 3%-5%. This approach also helps in lightening the surface.
After blasting, pickling ensures the thorough removal of any remaining surface reaction layers.
3. Chemical Polishing
Chemical polishing achieves a smooth surface through redox reactions in a chemical medium. This method is advantageous for its ability to polish complex structures without requiring intricate equipment. Chemical polishing is particularly suitable for titanium components with complex geometries. However, controlling the process parameters can be challenging. A typical chemical polishing solution for titanium includes:
HF (Hydrofluoric Acid): Acts as a reducing agent that dissolves titanium to level the surface. Concentration is generally around 10%.
HNO3 (Nitric Acid): Functions as an oxidizing agent to prevent excessive dissolution and hydrogen absorption while providing a bright finish.
Chemical polishing requires high solution concentration, low temperature, and short processing time (1 to 2 minutes).
4. Electrolyte Polishing
Electrolyte polishing, also known as electrochemical polishing, involves using electrochemical reactions to achieve a smooth surface. This method is challenging for titanium due to its low conductivity and strong oxidation characteristics. Hydro-acid electrolytes like HF-H3PO4 or HF-H2SO4 are generally ineffective because they lead to immediate oxidation of titanium.
However, using waterless chloride electrolytes at low voltage has shown promising results for polishing titanium, achieving a mirror-like finish on small test pieces. For complex parts, full polishing remains a challenge, though adjustments in cathode shape and additional methods may improve results. Further research is needed to enhance the effectiveness of electrolyte polishing for complex titanium components.
Conclusion
The surface treatment of titanium plates and rods involves a combination of physical, chemical, and electrochemical methods to remove surface reaction layers and defects. Blasting and pickling address initial contamination, while chemical and electrolyte polishing refine the surface finish. Each method has its advantages and limitations, and selecting the appropriate technique depends on the specific requirements of the titanium components.
