Due to the excellent corrosion resistance of titanium itself, titanium rods generally do not undergo further surface treatment to improve their corrosion resistance. However, in order to prevent comprehensive corrosion of titanium in non oxidizing acid aqueous solutions such as hydrochloric acid and sulfuric acid, which are more susceptible to corrosion, and to prevent interstitial corrosion and pitting corrosion in NaCl aqueous solutions, surface treatment methods are sometimes used. Atmospheric oxidation treatment: When titanium is placed in a high-temperature atmosphere, it thickens the oxide film. The film thickness increases with the increase of temperature and time. Atmospheric oxidation treatment is effective for both comprehensive and interstitial corrosion of titanium, and the method is relatively simple, but its durability is not very reliable. This is because atmospheric oxidation treatment only thickens the oxide film. In corrosive environments, the thickened oxide film on pure titanium becomes thinner with time, ultimately leading to corrosion. The duration of its corrosion resistance is determined by the atmospheric oxidation treatment conditions (T, t) and the severity of the corrosive environment, and it is difficult to predict this duration. This method is generally not commonly used for component materials that require long-term stable operation.
The disadvantage of titanium is its poor wear resistance and the tendency to produce defects such as pits on the surface. Currently, it is difficult to apply to sliding mechanical components. Currently, we are actively researching and developing various surface treatment methods. The methods suitable for titanium surface treatment include wet coating method represented by Cr and Ni plating, thermal diffusion method, surfacing method, and sputtering method. The more recent and advanced methods are CVD, PVD, and PCVD surface strengthening method. 1. Wet coating: Mainly using Cr and Ni-P plating methods (it is difficult to directly coat Cr on titanium rods, usually Ni is first coated on the rods, and then Cr. The electrolytic method has a fast film forming speed, thickness of several micrometers, and decorative coating is only 1um). It is an effective wear-resistant surface treatment method. 2. Thermal diffusion method: Widely used in the hardening treatment of steel materials, including carburization, nitridation, and boronization, and more recently also used in titanium. The main introduction is that ion nitriding is different from gas nitriding in that it uses glow discharge plasma to destroy the oxide film on the surface of titanium. Therefore, the pre-treatment of nitriding does not require mechanical grinding or acid washing to remove the oxide film, and the nitriding efficiency is high. Titanium has a nitriding film thickness increased from 0.7um to 5.0um at 850 degrees Celsius, with a surface hardness of 1200-1600Hv and excellent film resistance. 3. Surfacing method: The use of plasma transfer arc for surface hardening modification of titanium plates also has excellent wear resistance. The method is simple, and the treated material does not need to be exposed to the entire high temperature to prevent a decrease in mechanical properties, but requires secondary processing. Only applicable to handling thicker and larger workpieces. 4. Sputtering method: The method of using a high-speed air jet of plasma flow to spray molten metal onto the surface of the treated material, without the need for vacuum, can be processed in the atmosphere, and has high production efficiency. But the tightness 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 represented by the following equation: Ti+2H2O → TiO2+4H++4e This reaction is an anodic reaction. Therefore, increasing the potential of titanium can further cause this reaction to proceed in the right direction, which means that the stability and corrosion resistance of titanium oxide film are improved. But to increase the titanium potential, it is necessary to apply a high voltage from the opposite electrode and from the outside. At the same time, it is also difficult to apply a uniform voltage when the area is large, so it is not often used. Generally, precious metals do not corrode in harsh environments and exhibit high potentials. By utilizing this, coating the surface of titanium with precious metals, the potential of titanium is directed towards the more expensive side (in the direction of higher potential), thereby improving its corrosion resistance. Cheaper Pd, Ru, or their oxides (PdO, RuO2) are commonly used for titanium coating in precious metals. Coating precious metals or their oxides on titanium rods can improve their corrosion resistance and be effective. The corrosion resistance of coating materials can rival that of Ti/FONT>0.15Pd alloys. The disadvantage is that during long-term use in fluids or fluids containing solids, the precious metal film will peel off the surface of titanium, although this peeling is rare. Currently, Japan is developing coating methods with good compactness, but the cost is higher. The gas method requires heating at a temperature much higher than the titanium phase transition point, resulting in changes in structure and shape that cannot meet the requirements of the product; The CVD, PVD, and PCVD methods require special equipment and large-scale equipment that can be mass-produced is under development, with high costs. These treatment methods are rarely used to improve corrosion resistance, and are sometimes used to improve wear resistance. Pb+, Pt+implantation methods (ion beam, electron beam) ion implantation surface modification is very effective in improving corrosion resistance, but the cost is higher. It is currently being studied and has not yet been practical.
