The reaction of titanium alloy materials in the air, such as titanium rods, titanium tubes, etc., is usually related to the three non-metallic elements of oxygen, nitrogen and hydrogen. Their reaction process is closely related to temperature.
Titanium reacts very slowly with oxygen in the air below 100 degrees, and only the surface is oxidized at 500 degrees. As the temperature increases, the surface oxide film begins to dissolve in the titanium, and oxygen begins to diffuse into the metal. However, at 700°C, oxygen does not enter the inner lattice of the metal. When the temperature exceeds 700°C, the diffusion of oxygen to the metal is accelerated, and the surface oxide film loses its protective effect at high temperatures.
The reaction of titanium with oxygen depends on the shape and temperature of titanium. Powder titanium will burn or explode violently under the action of static electricity, sparks and friction in the air at room temperature. However, dense titanium is stable in air at room temperature. When the dense titanium is heated in air, it starts to react with oxygen. First, oxygen enters the crystal lattice on the titanium surface to form a dense oxide film. The oxide film on the surface can prevent oxygen from diffusing into the interior and has a protective effect. Therefore, titanium is stable in the air below 500°C. The color of the surface oxide film is related to the formation temperature. Below 200 degrees is silvery-white, 300 degrees is light yellow, 400 degrees is golden yellow, 500 degrees is blue, 600 degrees is purple, 700-800 degrees is red-gray, and 800-900 degrees is gray. In pure oxygen, the initial temperature ratio of titanium to oxygen is lower than the initial temperature ratio in air. At about 500-600°C, titanium burns in oxygen.
Titanium does not react with nitrogen at room temperature, but at high temperatures, titanium is one of the few metal elements that can burn in nitrogen. When the combustion temperature of titanium in nitrogen is higher than 800°C, the reaction between titanium and nitrogen is very violent. The reaction of titanium and nitrogen can not only generate titanium nitride (ti3n, Ti N, etc.), but also form a Ti-N solid solution. When the temperature is 500-550℃, titanium starts to absorb nitrogen and form an interstitial solid solution; when the temperature exceeds 600℃, the nitrogen absorption rate of titanium increases. In the Ti-N solid solution, nitrogen enters the titanium lattice in the form of titanium nitride, the phase transition temperature of titanium increases, and nitrogen is also a stabilizer of titanium. The maximum solubility (mass fraction) of nitrogen in the air is 7% at 1050°C and 2% at 2020°C. However, the absorption rate of nitrogen by titanium is much slower than the absorption rate of oxygen. Therefore, titanium mainly absorbs oxygen in the air, while the absorption of nitrogen is secondary.
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