Corrosion resistance characteristics of titanium
Titanium is a metal with a strong tendency to passivation. It can quickly form a stable oxidizing protective film in the air and in an oxidizing or neutral aqueous solution. Even if the film is damaged for some reason, it can quickly and automatically recover. Therefore, titanium has excellent corrosion resistance in oxidizing and neutral media.
Due to the great passivation performance of titanium, in many cases, when it comes in contact with dissimilar metals, it does not accelerate the corrosion, but may accelerate the corrosion of dissimilar metals. For example, in a low-concentration non-oxidizing acid, if Pb, Sn, Cu or Monel alloy is contacted with titanium to form a galvanic couple, these materials will corrode faster, and titanium will not be affected. In hydrochloric acid, when titanium is in contact with low-carbon steel, new hydrogen is generated on the surface of titanium, which destroys the oxide film of titanium, which not only causes hydrogen embrittlement of titanium, but also accelerates the corrosion of titanium. This may be due to the high degree of hydrogen on titanium.
The iron content in titanium has an impact on the corrosion resistance of some media. The reason for the increase in iron is that in addition to the raw materials, it is often that the contaminated iron penetrates into the weld bead during welding, which increases the local iron content in the weld bead. Time corrosion has a non-uniform nature. When using iron to support titanium equipment, iron contamination on the iron-titanium contact surface is almost unavoidable and accelerated in the iron-contaminated area, especially in the presence of hydrogen. When the titanium oxide film on the contaminated surface is mechanically damaged, hydrogen will penetrate into the metal. According to conditions such as temperature and pressure, hydrogen will diffuse correspondingly, which will cause different degrees of hydrogen embrittlement of titanium. Therefore, titanium should be used in medium temperature and medium pressure and hydrogen-containing systems to avoid surface iron contamination.
Under normal circumstances, titanium does not undergo pitting corrosion. Titanium also has anti-corrosion fatigue stability.
Titanium has better crevice corrosion resistance, especially Ti-0.3Mo-0.8Ni and Ti-0.2Pd alloys. Therefore, Ti-0.3Mo-0.8Ni and Ti-0.2Pd alloys are widely used as sealing surface materials for container equipment to solve Crevice corrosion on the sealing surface of the equipment.
Application of Titanium
Due to the excellent corrosion resistance of titanium materials, titanium materials are widely used in petroleum, chemical industry, salt making, pharmaceuticals, metallurgy, electronics, aviation, aerospace, marine and other related fields.
Titanium has excellent corrosion resistance to most salt solutions. For example, titanium is more corrosion-resistant than high-chromium-nickel steel in chloride solutions and has no pitting corrosion. However, the corrosion rate is higher in aluminum trichloride, which is related to the production of concentrated hydrochloric acid after the hydrolysis of aluminum trichloride. Titanium also has good stability to hot sodium chlorite and various concentrations of hypochlorite. Therefore, titanium materials are widely used in vacuum salt production and bleaching powder industry.
Titanium has good corrosion resistance to most alkaline solutions. Titanium is relatively stable in sodium hydroxide and potassium hydroxide solutions with a concentration of less than 50%. If the alkali solution contains chloride ions or chlorides, its corrosion resistance even exceeds that of nickel and zirconium. But when the temperature and concentration increase, the corrosion will increase. At present, the chlor-alkali industry is the largest application field of domestic civilian titanium materials.
Titanium is not resistant to corrosion in dry chlorine and has the risk of fire, but it has high stability in wet chlorine, exceeding zirconium, Hastelloy C and Monel, and even in sulfuric acid, hydrochloric acid and sulfuric acid saturated with chlorine. It is also stable in media such as chloride, so titanium is the material of choice for key equipment for the production of titanium dioxide by sulfuric acid. Because titanium has good corrosion resistance in hydrocarbons, it is also good even when it contains acid and chloride impurities. Therefore, titanium materials are also widely used in organic chemicals, such as PTA (purified terephthalic acid), PVA (vinylon) and so on.
Titanium materials have excellent corrosion resistance in seawater, so titanium materials are also widely used in marine fields such as offshore oil drilling platforms and seawater desalination.
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