Differences for Titanium Alloy in Air And Temperature
The reaction of titanium alloy materials such as titanium rods and titanium tubes in air usually reacts with three non-metallic elements: oxygen, nitrogen and hydrogen. Their reaction process is closely related to temperature.
The reaction of titanium with oxygen in air is very slow below 100°C, and only the surface is oxidized at 500°C. As the temperature rises, the surface oxide film begins to dissolve in titanium, and oxygen begins to diffuse into the metal's internal lattice, but at 700°C, oxygen has not yet entered the metal's internal lattice in large quantities. When it exceeds 700°C, 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. Powdered titanium can burn or explode violently in the air at room temperature due to static electricity, sparks, friction, etc. However, dense titanium is very stable in the air at room temperature. When dense titanium is heated in the air, it begins to react with oxygen. Initially, oxygen enters the titanium surface lattice to form a dense oxide film. This surface oxide film can prevent oxygen from diffusing inward and has a protective effect. Therefore, titanium is stable in air below 500°C. The color of its surface oxide film is related to the formation temperature.It is silvery white below 200°C, light yellow at 300°C, golden yellow at 400°C, blue at 500°C, purple at 600°C, reddish gray at 700-800°C, and gray at 800-900°C. In pure oxygen, the starting temperature of the violent reaction between titanium and oxygen is lower than that in air, and titanium burns in oxygen at about 500-600°C.
Titanium does not react with nitrogen at room temperature, but at high temperature, titanium is one of the few metal elements that can burn in nitrogen. The combustion temperature of titanium in nitrogen is about higher than 800 degrees. The reaction between molten titanium and nitrogen is very intense. In addition to the formation of titanium nitrides (Ti3N, TiN, etc.), the reaction between titanium and nitrogen also forms a Ti-N solid solution. When the temperature is 500-550 degrees, titanium begins to absorb nitrogen significantly to form an interstitial solid solution; when the temperature reaches above 600 degrees, the speed of titanium absorbing nitrogen increases. In the Ti-N solid solution, nitrogen enters the titanium lattice in the form of titanium nitride, thereby increasing the phase transition temperature of titanium. Nitrogen is also a stabilizer for titanium.
Nitrogen is also a stabilizer for titanium. The maximum solubility (mass fraction) of nitrogen in titanium is 7% at 1050 degrees and 2% at 2020 degrees, but the speed at which titanium absorbs nitrogen is much slower than its speed at absorbing oxygen. 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 be well dissolved in titanium, and 1 mol of titanium can absorb almost 2 mol of hydrogen. The speed and amount of hydrogen absorption by titanium are related to temperature and hydrogen pressure. The amount of hydrogen absorbed by titanium at room temperature is less than 0.002%. When the temperature reaches 300 degrees, the speed of hydrogen absorption by titanium increases; it reaches the maximum value at 500-600 degrees.
As the temperature rises, the amount of hydrogen absorbed by titanium decreases. When the temperature reaches 1000 degrees, most of the hydrogen absorbed by titanium is decomposed. The increase in hydrogen pressure can accelerate the speed of titanium absorbing hydrogen and increase the amount of hydrogen absorbed. On the contrary, titanium can be dehydrogenated under reduced pressure. Therefore, the reaction between titanium and hydrogen is reversible. The reaction between titanium and hydrogen does not form a film on the surface, because the hydrogen atom is small in size and can quickly diffuse into the depth of the titanium lattice to form an interstitial solid solution. The dissolution of hydrogen in titanium can reduce the phase transition temperature of titanium. Hydrogen is a stabilizer for Ti.
