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Introduction to titanium: Metal titanium is a metal with great potential. Due to its unique properties and characteristics, it will play an irreplaceable role in the future development of the national economy. The 10-kilometer-thick layer of the Earth’s surface contains 0.6 percent titanium, which is 61 times more than that in copper; it ranks tenth in terms of abundance in the crust (the elements in order of abundance in the crust are: oxygen, silicon, aluminum, iron, calcium, sodium, potassium, magnesium, hydrogen, titanium). Pick up any handful of soil from the ground – it contains a few thousandths of titanium, and titanium ores with reserves of over 10 million tons are not rare in the world. Titanium has a hardness similar to that of steel, yet its weight is almost half that of steel of the same volume. Although titanium is slightly heavier than aluminum, its hardness is twice that of aluminum. Titanium has poor thermal and electrical conductivity, which is similar to or slightly lower than that of stainless steel. Titanium exhibits superconductivity, with the superconducting critical temperature of pure titanium being 0.38–0.4 K. Titanium is malleable; the elongation rate of high-purity titanium can reach 50-60%, while its reduction in area can reach 70-80%. However, it has low strength and is not suitable as a structural material. The presence of impurities in titanium has a significant impact on its mechanical properties; in particular, interstitial impurities (oxygen, nitrogen, carbon) can **increase the strength of titanium while significantly reducing its ductility. The excellent mechanical properties of titanium as a structural material are achieved by strictly controlling the appropriate levels of impurities and by adding alloying elements. Now, titanium is widely used to replace steel in space rockets and missiles. According to statistics, more than 1,000 tons of titanium are used each year in space exploration worldwide. Extremely fine titanium powder is an excellent fuel for rockets; therefore, titanium is known as the metal of space. However, titanium is highly reactive and can react with non-metals such as O2, N2, H2, S, and halogens when heated. Liquid titanium can dissolve almost all metals, allowing it to form alloys with a variety of metals. Titanium added to steel produces titanium steel that is tough and elastic. Titanium forms interstitial compounds or intermetallic compounds with metals such as Al, Sb, Be, Cr, Fe, etc. Therefore, the biggest drawback of titanium is its difficulty in extraction; very stringent conditions are required to produce pure titanium. Due to titanium’s strong reactivity at high temperatures, care is taken during both smelting and casting to prevent these elements from \"attacking\" titanium. When titanium is smelted, air and water must certainly be kept away at all costs; even alumina crucibles, which are commonly used in metallurgy, are prohibited, as titanium will draw oxygen from alumina. Today, titanium is purified by reacting magnesium with titanium tetrachloride in an inert gas—helium or argon. As a result, the cost of producing pure titanium will be very high. Therefore, overall, titanium is mainly used in applications where symmetry is not a high requirement. For example, aerospace, aviation, navigation, deep-sea exploration, etc. Corrosion of titanium at room temperature: Titanium is a very reactive metal; as a result, its equilibrium potential is low, giving it a strong thermodynamic tendency to corrode in media. But in reality, titanium is very stable in many media; it is corrosion-resistant in oxidizing, neutral, and weakly reducing media. This is because titanium has a strong affinity for oxygen; in air or oxygen-containing environments, a dense, strongly adherent, and highly inert oxide layer forms on the surface of titanium, protecting the titanium matrix from corrosion. It can even heal or regenerate quickly even due to mechanical wear. This indicates that titanium is a metal with a strong tendency to passivate. Therefore, in an aerobic environment, titanium has strong corrosion resistance. 1. Corrosion by acids (a) Hydrochloric acid: at room temperature, concentration