The reaction of titanium alloy materials such as titanium rods and tubes in air is usually with three non-metallic elements: oxygen, nitrogen, and hydrogen, and their reaction process is closely related to temperature.
Titanium reacts with oxygen in the air slowly below 100 ℃, and only undergoes surface oxidation at 500 ℃. As the temperature increases, the surface oxide film begins to dissolve in titanium, and oxygen begins to diffuse towards the internal lattice of the metal. However, at 700 ℃, a large amount of oxygen has not yet entered the internal lattice of the metal. When the temperature exceeds 700 ℃, the diffusion of oxygen into the metal accelerates, and the surface oxide film loses its protective effect at high temperatures.
The reaction between titanium and oxygen depends on the form and temperature of titanium. Powder titanium can undergo intense combustion or explosion in air at room temperature due to static electricity, sparks, friction, and other effects. However, dense titanium is very stable in air at room temperature. When dense titanium is heated in the air, it begins to react with oxygen. Initially, oxygen enters the surface lattice of titanium, forming a dense oxide film. This surface oxide film can prevent oxygen from diffusing internally and has a protective effect. Therefore, titanium is stable in air below 500 ℃. The color of its surface oxide film is related to the formation temperature. It is silver white below 200 ℃, light yellow at 300 ℃, golden yellow at 400 ℃, blue at 500 ℃, purple at 600 ℃, red gray at 700-800 ℃, and gray at 800-900 ℃. In pure oxygen, the starting temperature for the intense reaction between titanium and oxygen is lower than in air, and titanium burns in oxygen at around 500-600 ℃.
At room temperature, titanium does not react with nitrogen, but at high temperatures, titanium is one of the few metal elements that can burn in nitrogen. The combustion temperature of titanium in nitrogen is about 800 degrees Celsius, and the reaction between molten titanium and nitrogen is very intense. In addition to titanium nitride (Ti3N, TiN, etc.), the reaction of titanium with nitrogen can also form Ti-N solid solution. When the temperature is 500-550 ℃, titanium begins to obviously absorb nitrogen and form interstitial solid solution; When the temperature reaches above 600 ℃, the nitrogen absorption rate of titanium increases. In the Ti-N solid solution, because nitrogen enters the titanium lattice in the form of titanium nitride, the titanium phase transition temperature increases, and nitrogen is also the stabilizer of titanium. At 1050 degrees Celsius, the maximum solubility (mass fraction) of nitrogen in medium is 7%, and at 2020 degrees Celsius, the maximum solubility (mass fraction) in titanium is 2%. However, the rate of nitrogen absorption by titanium is much slower than its rate of oxygen absorption. Therefore, titanium mainly absorbs oxygen in the air, and nitrogen absorption is secondary.
Titanium reacts with hydrogen to form TiH, TiH2 compounds and Ti-H solid solution. Hydrogen can dissolve well in titanium, and 1mol of titanium can absorb almost 2mol of hydrogen. The rate and amount of hydrogen absorption by titanium are related to temperature and hydrogen pressure. Titanium has a hydrogen absorption capacity of less than 0.002% at room temperature. When the temperature reaches 300 degrees, the hydrogen absorption rate of titanium increases; The maximum value is reached at 500-600 degrees. Subsequently, as the temperature increases, the amount of hydrogen absorbed by titanium decreases, and when it reaches 1000 degrees Celsius, most of the hydrogen absorbed by titanium is decomposed. An increase in hydrogen pressure can accelerate the rate of hydrogen absorption by titanium and increase the amount of hydrogen absorbed. On the contrary, under reduced pressure, titanium can be dehydrogenated. Therefore, the reaction between titanium and hydrogen is reversible. The reaction of titanium and hydrogen does not form a film on the surface. Because the hydrogen atom is small, it can quickly diffuse to the depth of the titanium lattice to form interstitial solid solution. The dissolution of hydrogen in titanium can reduce the phase transition temperature of titanium. Hydrogen is the stabilizer of titanium.
