Titanium metal is a kind of 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 ten-kilometer-thick stratum on the earth’s surface contains six thousandths of titanium, 61 times more than copper, and ranks tenth in the crust (ranking elements in the crust: oxygen, silicon, aluminum, iron, calcium, sodium, Potassium, magnesium, hydrogen, titanium). Just grab a handful of soil from the ground, which contains a few thousandths of titanium. Titanium ore with reserves of more than 10 million tons in the world is not rare. The hardness of titanium is about the same as that of steel, and 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 has superconductivity. The superconducting critical temperature of pure titanium is 0.38-0.4K. Titanium has plasticity. The elongation rate of high-purity titanium can reach 50-60%, and the reduction of area can reach 70-80%, but its strength is low and it is not suitable for structural materials. The presence of impurities in titanium has a great impact on its mechanical properties, especially interstitial impurities (oxygen, nitrogen, carbon) can greatly increase the strength of titanium and significantly reduce its plasticity. The good mechanical properties of titanium as a structural material are achieved through strict control of the appropriate impurity content and the addition of alloying elements. Now, in space rockets and missiles, large amounts of titanium are used instead of steel. According to statistics, the world’s annual titanium used for space navigation has reached more than 1,000 tons. Very fine titanium powder is also a good fuel for rockets, so titanium is known as cosmic metal and space metal.
But titanium is very active and can interact with non-metals such as O2, N2, H2, S and halogen when heated. Liquid titanium can dissolve almost all metals, so it can form alloys with a variety of metals. Titanium steel made by adding titanium to steel is tough and elastic. Titanium and metal Al, Sb, Be, Cr, Fe, etc. form interstitial compounds or intermetallic compounds. Therefore, the biggest disadvantage of titanium is that it is difficult to extract. To extract pure titanium requires very harsh conditions. Because titanium has a very strong chemical ability at high temperatures, people are careful to prevent these elements from attacking when smelting or casting. “titanium. When smelting titanium, air and water are of course strictly forbidden. Even the alumina crucible commonly used in metallurgy is also forbidden to use, because titanium will take oxygen from the alumina. At present, people use magnesium and titanium tetrachloride to interact with inert gas-helium or argon to extract titanium. In this way, the cost of preparing pure titanium will be very high.
Therefore, in comprehensive consideration, titanium is mainly used in occasions where the symmetry cost is not high. For example, aerospace, aviation, navigation, deep-sea exploration, etc.
Room temperature corrosion of titanium
Titanium is a very active metal, so its equilibrium potential is very low, and it has a high tendency of thermodynamic corrosion in the medium. But in fact, titanium is very stable in many media. For example, titanium is corrosion-resistant in oxidizing, neutral and weak reducing media. This is because titanium and oxygen have a great affinity. In the air or in an oxygen-containing medium, a dense, strong adhesion and inert oxide film is formed on the surface of titanium, which protects the titanium matrix from corrosion. Even due to mechanical wear, it will quickly heal itself or regenerate. This shows that titanium is a metal with a strong tendency to passivation. Therefore, in an aerobic environment, titanium has strong corrosion resistance.
1. Corrosion by acid
(A) Hydrochloric acid: At room temperature, hydrochloric acid with a concentration of <5% will not react with titanium at room temperature, and 20% hydrochloric acid will react with titanium at room temperature. When the temperature is high, dilute hydrochloric acid will also corrode titanium. Although titanium is not resistant to corrosion by hydrochloric acid solution, it can also be improved by alloying, anode passivation and adding corrosion inhibitors to improve the corrosion resistance of titanium. Therefore, in production practice, titanium still has use value. (B) Sulfuric acid: At room temperature, titanium has certain corrosion resistance to low-temperature and low-concentration sulfuric acid solutions. About 40% sulfuric acid has the fastest corrosion rate to titanium. When the concentration is greater than 40%, the corrosion rate is reversed when it reaches 60%. Slow down, and 80% reach the fastest. Heated dilute acid or 50% concentrated sulfuric acid can react with titanium to form titanium sulfate. The corrosion resistance of titanium in sulfuric acid can be improved by blowing air, nitrogen, or adding oxidizers or high-priced heavy metal ions into the solution. Therefore, titanium has no much practical value in sulfuric acid. (C) Nitric acid and aqua regia: The dense and smooth surface of titanium has good stability to nitric acid. This is because nitric acid can quickly form a firm oxide film on the surface of titanium, but the surface is rough, especially sponge titanium or powder Titanium can react with secondary and hot dilute nitric acid, and when the temperature rises, both will react with it.
2. Corrosion of titanium by sea water
Titanium is stable in seawater and is not easily corroded by seawater. The reason is that a dense oxide film on the surface of titanium resists the corrosion of the titanium matrix by chloride ions. After testing, titanium has been exposed to seawater at different depths for many years without significant corrosion. Even if there are deposits on the titanium surface, crevice corrosion and pitting will not occur. The presence of sulfides in seawater does not affect the corrosiveness of titanium. In the ocean atmosphere, splash zone and tidal range, titanium does not have corrosion problems. Titanium is also resistant to erosion by high-speed sea water. Frictional particles suspended in seawater are very harmful to copper or aluminum alloys, but have little effect on titanium. Titanium has also been recognized as one of the best anti-cavitation corrosion metal materials in seawater. Because titanium is not toxic to marine organisms, marine organisms attach to the surface of titanium more commonly. There is no crevice corrosion and pitting corrosion on the titanium under the sea creatures, and the surface still maintains the integrity of the corrosion-resistant oxide film. Titanium hardly occurs in seawater pitting corrosion and crevice corrosion, so titanium is the most suitable material for seawater. Because of this good property, titanium is used to make propeller shafts, rigging and heat exchangers for desalination plants; it is also used for cold and water heaters in saltwater aquariums, fishing lines and diving knives. Titanium is used to manufacture housing and other components for marine surveillance deployment, as well as monitors for scientific or military use. The former Soviet Union developed the technology to manufacture submarines mainly from titanium. But when the temperature rises to 90 degrees Celsius, titanium becomes no longer stable in seawater and corrosion begins to occur.
Corrosion of titanium at high temperature
The high temperature corrosion resistance of titanium depends on the characteristics of the medium and the performance of its own surface oxide film. Titanium can be used as a structural material in air or oxidizing atmosphere up to 426 ℃, but at about 250 ℃, titanium begins to absorb hydrogen significantly. In a complete hydrogen atmosphere, when the temperature rises above 316 ℃, titanium It becomes brittle by absorbing hydrogen. Therefore, without extensive testing, titanium should not be used in chemical equipment with a temperature higher than 330 ℃. Considering hydrogen absorption and mechanical properties, the use temperature of all-titanium pressure vessels should not exceed 250 ℃. Titanium is used for heat exchangers. The upper limit of tube temperature is about 316 ℃. Between 500-800°C, experiments have shown that the corrosion rate of titanium is proportional to temperature. After research, as the temperature increases, the integrity of the oxide film on the surface of titanium decreases.
Localized corrosion characteristics of titanium
1. Crevice corrosion
Titanium is particularly resistant to crevice corrosion, and crevice corrosion occurs only in a few chemical media. The crevice corrosion of titanium is closely related to temperature, chloride concentration, pH value and the size of the crevice. According to relevant information, wet chlorine gas is prone to crevice corrosion when the temperature is above 85 ℃. For example, some factories use a packed tower to directly cool the wet chlorine before the cooler to reduce the temperature of the wet chlorine to 65-70 ℃, and then enter the titanium cooler to improve the crevice corrosion resistance, and the effect is also very significant. Practice has proved that: lowering the temperature is an effective way to prevent crevice corrosion. In general, the possibility of crevice corrosion in a narrow gap is much greater than that in a wide gap. In a wide gap, the crevice corrosion of titanium reaches an extreme value. When the gap is very small, because the corrosive medium cannot wet the inner surface of the gap, even if the inner surface is infiltrated, its flow is restricted, and the titanium oxide film is still not destroyed; if the gap is large, the oxygen diffusion is quite rapid, enough to make Titanium passivation. Therefore, when the gap is small or large, it will not cause the occurrence of crevice corrosion.
2. Stress corrosion
Except for a few individual media, industrial pure titanium has excellent stress corrosion resistance, and it is still rare that titanium equipment is damaged by stress corrosion. Industrial passivated titanium can only be used in fuming nitric acid, certain methanol solutions or certain hydrochloric acid solutions, high-temperature hypochlorite, molten salt with a temperature of 300-450 ℃, or NaCl-containing atmosphere, carbon disulfide, n-hexane and dry chlorine. Produce stress corrosion.
When concentrated nitric acid contains more than 6.0% NO2 and less than 0.7% H2O, even at room temperature, industrial pure titanium will experience stress corrosion cracking. Severe stress corrosion and explosion occurred in our country when using titanium equipment in 98% concentrated nitric acid. Although titanium has stress corrosion damage in some special media, compared with other metals, titanium has a good resistance to stress corrosion cracking. When titanium undergoes stress corrosion, with the passivation of the surface, a large tensile stress will be generated, and the dislocation will start to move under a lower external stress. When the local plastic deformation promoted by corrosion develops to a critical state, the stress concentration at the front end of the dislocation cluster is equal to the atomic bonding force, resulting in micro-crack nucleation! After the crack nucleation, the Fermi energy level of the tip matrix is higher than the other cracks In the area, the electrode potential at the crack tip is low, and under the action of the corrosive medium, the metal at the crack tip undergoes anodic decomposition. On the one hand, hydrogen reduces the surface energy of the crack. Under the action of external force, the section will expand in order to balance with the external force. On the other hand, hydrogen increases the Fermi level difference between the crack tip and other areas of the crack, increases the corrosion potential difference, and promotes the development of stress corrosion.
3. Hydrogen embrittlement corrosion
Titanium is an active metal. It reacts with hydrogen not only on the surface but also diffuses into the interior of the titanium. When the hydrogen concentration in the titanium reaches an independent titanium hydride phase, the titanium will be embrittled. Hydrogen may exist in the metal before the titanium is used, or it may be used in hydrogen or hydrogen-containing media to absorb hydrogen. Therefore, in the actual use of titanium equipment, special attention should be paid to hydrogen embrittlement to avoid damage to the equipment.
Generally speaking, the hydrogen in titanium is divided into internal hydrogen and external hydrogen. Internal hydrogen refers to the hydrogen introduced in the process of smelting, thermal processing, heat treatment, pickling, electroplating, etc.; while titanium originally contains no hydrogen or contains little hydrogen, but it is called external due to the introduction of hydrogen from the external environment during use. Hydrogen, specifically, generates active hydrogen atoms on the metal surface through the following ways, and then enters the metal. (1) The medium in which the titanium equipment is located contains molecular hydrogen, such as a high-temperature hydrogen atmosphere. (2) Hydrogen generated by general corrosion or partial corrosion of titanium is absorbed by titanium. For example, crevice corrosion of titanium is often accompanied by hydrogen absorption. (3) Hydrogen produced when titanium and negatively charged metals undergo galvanic corrosion or cathodic protection is overprotected. The latter two types of hydrogen embrittlement of titanium caused by electrochemical corrosion of the cathode are more frequent, and can occur without high temperature and high pressure, and should be paid more attention to.
The hydrogen embrittlement of titanium is affected by many factors, the main influencing factors are hydrogen content, strain rate, stress, stress concentration, medium temperature and environment, etc. When the surface of titanium is contaminated by metallic iron, it will increase the hydrogen absorption of titanium. Because iron can form a corrosive micro-battery with the titanium substrate, nascent hydrogen is produced in the corrosion reaction, which increases the active points and active channels for hydrogen to enter, making hydrogen intrusion easier, and the damaged membrane is not easy to repair. The effect of temperature on the hydrogen absorption of titanium is mainly reflected in increasing the reaction rate of titanium and hydrogen and the diffusion rate of hydrogen in it. At low temperatures, the diffusion rate of hydrogen in titanium is very small. But at higher temperatures (greater than 80 ℃), hydrogen absorption will become obvious. Above 300 ℃, the reaction speed of titanium and hydrogen will increase rapidly, and a large amount of hydride will be generated, which will cause obvious hydrogen embrittlement of titanium. If the temperature exceeds 316 ℃ in a hydrogen atmosphere, titanium equipment is generally not recommended. According to the above influencing factors, methods such as reducing the hydrogen content in the titanium material, increasing the hydrogen solid solubility of the titanium material, reducing the surface pollution of the titanium material, reducing the iron content in the titanium material, eliminating residual stress and other methods, reducing hydrogen embrittlement corrosion happened.
4. Pitting corrosion
The occurrence of pitting corrosion depends on the degree of damage to the oxide film of the parts that can be pitted. This kind of corrosion is easy to occur in the parts with crevices. The passivation film on the surface of the titanium material cannot be self-passivated after being partially destroyed, causing surface electrochemical inhomogeneity, leading to the deep development of corrosion in some parts, forming spot-like local corrosion. For example, when a titanium-made exchanger is used in a zinc chloride solution, pitting corrosion is likely to occur in the part in contact with iron; in a sodium chloride solution, the titanium-made heat exchanger also has slight pitting corrosion; PTFE gaskets and titanium The part where the component forms the gap is the most prone to pitting corrosion; titanium is also corroded in calcium chloride and aluminum chloride solutions, but the corrosion occurs within a certain concentration and temperature range. In addition, due to improper heat treatment , Hot forming and welding discolored parts and iron and other contaminated parts often produce pitting corrosion. High temperature and moderate concentration chloride solution is the main medium for pitting titanium materials, such as 100°C, 25% concentration aluminum chloride solution, 175°C, 75% concentration calcium chloride solution, 103rC, 40% concentration ammonium chloride solution And so on, there have been cases of equipment damage due to pitting corrosion. Generally, the temperature is lower than 80°C, and pitting corrosion is not easy to occur. Iron, copper and other metals contaminate the surface of titanium materials, increasing the tendency of pitting corrosion. The preventive measure is to use pure titanium with high oxygen content, and the titanium equipment should be pickled and treated with atmospheric thermal oxidation before being put into use.
5. Galvanic corrosion
In the electrolyte, titanium and other metals contact to form a galvanic couple, and the metal of low inertness or positive electrode will corrode. Due to the existence of the passivation film of titanium, it is ensured that the titanium becomes the cathode in the galvanic couple and does not corrode. When titanium is used as a cathode, the smaller the surface area of the anode metal, the greater its current density, and the more significant the corrosion. But in hydrochloric acid or sulfuric acid, titanium and aluminum form a galvanic resistance, and the corrosion of aluminum changes the potential of titanium, resulting in rapid corrosion of titanium.
Under different environments and different conditions, the corrosion process and results of titanium are also different. Therefore, in order to exert the best performance of titanium, it is necessary to control the use conditions of titanium.