Due to the excellent corrosion resistance of
titanium itself,
titanium rods generally do not undergo surface treatment to further improve their corrosion resistance, but in order to prevent the overall corrosion of titanium in non-oxidizing acid aqueous solutions such as hydrochloric acid and sulfuric acid Crevice corrosion and pitting corrosion in NaCl aqueous solution, sometimes by surface treatment. Atmospheric oxidation treatment: Titanium is placed in a high-temperature atmosphere to thicken the oxide film, and its film thickness increases with the increase of temperature and the extension of time. Atmospheric oxidation treatment is effective for both general corrosion and crevice corrosion of titanium. The method is relatively simple, but the durability is not very reliable. This is because atmospheric oxidation treatment only thickens the oxide film. Pure titanium in a corrosive environment, the thickened oxide film Thinning over time eventually leads to corrosion. The maintenance time of its corrosion resistance is determined by the atmospheric oxidation treatment conditions (T, t) and the harshness of the corrosion environment. It is difficult to predict this time. This method is generally not used for component materials that require long-term stable operation.
The disadvantage of titanium is that it has poor wear resistance and is prone to pitting and other defects on the surface. At present, it is difficult to apply to sliding mechanical parts. Various surface treatment methods are currently being actively researched and developed. The methods suitable for titanium surface treatment include wet coating method represented by Cr plating and
Ni, thermal diffusion method, surfacing method and sputtering method, etc. The more recent and more advanced methods are CVD, PVD, PCVD surface strengthening method. 1. Wet coating: mainly use Cr and Ni-P methods, (it is difficult to directly plate Cr on titanium rods, usually Ni is first plated on titanium rods, and then Cr is plated. The electrolytic method has a fast film forming speed and a thickness of several microns, (decorative It is an effective wear-resistant surface treatment method. 2. Thermal diffusion method: widely used in the hardening treatment of steel materials such as carburizing, nitriding and boronizing thermal diffusion processes, and recently also used Yu titanium. Mainly introduce that the ion nitriding method is different from gas nitriding. It uses glow discharge plasma to destroy the oxide film on the titanium surface. High efficiency. Titanium at 850 degrees, the thickness of the nitride film increases from 0.7um to 5.0um, the surface hardness reaches 1200-1600Hv, and has good film resistance. 3. Overlay welding method: use plasma transferred arc to carry out overlay welding on the surface of titanium plate Hardening modification also has excellent wear resistance. The method is simple, and the processed material does not need to be exposed to the entire high temperature, which can prevent the decline of mechanical properties, but requires secondary processing. It is only used to deal with thicker large workpieces. 4. Splashing Injection method: The method of spraying the dripping molten metal on the surface of the material to be treated by using the high-speed air jet of the plasma flow does not require vacuum, and can be processed in the atmosphere, with high production efficiency. But the adhesion of the coating is not enough.
Precious metal coating: The corrosion resistance of titanium is maintained by the oxide film formed on the surface. The formation reaction of this oxide film is generally expressed by the following formula: Ti+2H2O→TiO2+4H+ +4e This reaction is an anode reaction, so as long as the potential of titanium is increased, this reaction can be further carried out to the right, which means The stability of the titanium oxide film and the improvement of corrosion resistance. However, to increase the titanium potential, a high voltage must be applied from the outside by the counter electrode. At the same time, it is difficult to apply a uniform voltage when the area is large, so it is not often used. Generally, precious metals do not corrode under harsh environments, and show high potentials. Taking advantage of this point, coating the surface of titanium with noble metal will increase the potential of titanium to the expensive side (the direction of high potential), thereby improving its corrosion resistance. Among noble metals, cheaper Pd, Ru or their oxides (PdO, RuO2) are usually used for titanium coating. Coating noble metals or their oxides on titanium rods is extremely effective in improving its corrosion resistance, and the corrosion resistance of coated materials can rival that of Ti/FONT>0.15Pd alloys. The disadvantage is that the precious metal film will peel off from the titanium surface when it is used in fluid or solid-containing fluid for a long time, although this peeling is rare. At present, Japan is developing a coating method with good density, but the cost is higher. Gas method, because it must be heated at a temperature much higher than the phase transition point of titanium, the product cannot meet the requirements of use due to changes in structure and shape; while CVD, PVD, and PCVD methods require special equipment and large-scale equipment that can be mass-produced is being developed Medium, high cost. (These treatment methods are rarely used to improve corrosion resistance, and are sometimes used to improve wear resistance. Pb+, Pt+ implantation method (ion beam, electron beam) ion implantation surface modification is very effective for improving corrosion resistance, but the cost is higher , is under study and has not yet been put into practical use.
