First, corrosion resistance of titanium in chemical medium
1. Nitric acid
welded titanium tube supplier consider nitric acid to be an oxidizing acid in which titanium maintains a dense oxide film on its surface. As a result, titanium has excellent corrosion resistance in nitric acid. The corrosion rate of titanium increases with the increase of the temperature of nitric acid solution. When the temperature is 190 ~ 240℃ and the concentration is 20% ~ 70%, the corrosion rate can be up to 10mm/a. However, the corrosion of titanium by high temperature nitric acid can be inhibited by adding a small amount of silicon-containing compounds in nitric acid solution. For example, the corrosion rate can be reduced to almost zero when silicone oil is added to a 40% high temperature nitric acid solution. It is also introduced that titanium has high corrosion resistance in 40% ~ 80% nitric acid solution and steam below 500℃. In fuming nitric acid, when the content of nitrogen dioxide is more than 2%, due to the lack of water content causes a strong exothermic reaction, resulting in explosion.
2. Sulfuric acid
Sulfuric acid is a strong reducing acid, and titanium has certain corrosion resistance to low temperature and low concentration sulfuric acid solution. At 0℃, it can resist sulfuric acid corrosion with a concentration of 20%. With the increase of acid concentration and temperature, the corrosion rate increases. Therefore, the stability of titanium in sulfuric acid is poor. Even at the room temperature of dissolved oxygen, titanium can only withstand 5% sulfuric acid corrosion. At 100℃, titanium can resist only 0.2% sulfuric acid corrosion. Chlorine can inhibit the corrosion of titanium in sulfuric acid, but at 90℃, the concentration of sulfuric acid is 50%, chlorine can cause the acceleration of titanium corrosion and even cause fire. The corrosion resistance of titanium in sulfuric acid can be improved by injecting air, nitrogen or adding oxidant or high-value heavy metal ions into the solution. Therefore, titanium in sulfuric acid has no great practical value.
3. Lye
Titanium has good corrosion resistance in most alkaline solutions, and the corrosion rate increases with the concentration and temperature of the solution. In the presence of oxygen, ammonia or carbon dioxide in the lye, the corrosion of titanium will be accelerated. In the lye containing hydrogen oxide, the corrosion resistance of titanium is very poor. But in sodium hydroxide solution corrosion resistance is better than potassium hydroxide, so that in high temperature and high concentration of sodium hydroxide solution also has strong corrosion resistance. For example, the corrosion rate of titanium at 130℃ and 73% sodium hydroxide solution is only 0.18mm/a. Titanium is different from other metals in that it will not produce stress corrosion rupture in sodium hydroxide solution, but long-term exposure may produce hydrogen embrittleness. Therefore, titanium should be used in caustic soda and other concentration alkaline solution temperature should be 93.33℃ or less.
4. Chlorine gas
The stability of titanium in chlorine gas depends on the water content of chlorine. But in dry chlorine gas is not corrosion resistant, and there is a risk of combustion. Therefore, titanium material used in chlorine gas must maintain a certain water content, water content required to maintain titanium passivation in chlorine gas is related to the pressure, flow rate, temperature and other factors.
5. Organic media
Titanium has high corrosion resistance in gasoline, toluene, phenol, formaldehyde, trichloroethane, acetic acid, citric acid and chloroacetic acid. At boiling point and without aeration, titanium will be seriously corroded in formic acid below 25%. In solution containing acetic anhydride, titanium will not only suffer serious overall corrosion but also produce pitting corrosion. For many complex organic media in the process of organic synthesis, such as propylene oxide, phenol, acetone, chloroacetic acid and other chemical media, titanium corrosion resistance is better than stainless steel and other structural materials.
Two, several local corrosion characteristics of titanium
2.1 Slot corrosion Titanium has strong slot corrosion resistance and only occurs in a few chemical media. Titanium slot corrosion is closely related to temperature, chloride concentration, pH value and slot size. According to the relevant information, wet chlorine gas temperature above 85℃ is easy to produce crack corrosion. For example, some factories use a packed tower for direct cooling before the cooler to reduce the temperature of wet chlorine gas to 65 ~ 70℃, and then enter the titanium cooler to improve the crack corrosion resistance, the effect is also very significant. Practice has proved that reducing temperature is one of the effective ways to prevent slot corrosion, and titanium slot corrosion has occurred in high temperature sodium chloride solution. In short, for the parts and components that are easy to produce gap corrosion, such as sealing surface, tube plate and tube expansion joint, plate heat exchanger, tray and tower body contact parts and tower fasteners should be used Ti-0.2Pd titanium alloy, in the design should avoid gaps and stagnation areas. If the fasteners in the tower should not be bolted. Tube plate and pipe expansion and sealing welding structure is better than simple expansion, for flange sealing surface, should not use asbestos pad, should use polytetrafluoroethylene film asbestos pad.
6, high temperature corrosion
The high temperature corrosion resistance of titanium depends on the characteristics of the medium and the performance of the oxide film on its surface. Titanium in the air or oxidizing atmosphere, as a structural material can be used to 426℃, but at about 250℃, titanium began to significantly absorb hydrogen, in a complete hydrogen atmosphere, when the temperature rises to 316℃ above, titanium hydrogen becomes brittle. Therefore, without extensive testing, titanium should not be used for chemical equipment whose temperature is higher than 330℃. From the consideration of hydrogen absorption and mechanical properties, the use temperature of all-titanium pressure vessel should not exceed 250℃, and the upper limit of the use temperature of titanium tube for heat exchanger is about 316℃.
7. Stress corrosion
In addition to individual several media, industrial pure titanium stress corrosion resistance is very good, stress corrosion caused by titanium equipment damage phenomenon is rare. Industrial blunt titanium only in fuming nitric acid, some methanol solution or some hydrochloric acid solution, high temperature hypochlorite, temperature 300 ~ 450℃ molten salt or NaCl atmosphere, carbon disulfide, n-hexane and dry chlorine gas and other media can produce stress corrosion. The stress corrosion rupture tendency of titanium in nitric acid increases with the increase of NO2 content and the decrease of water content. The stress corrosion tendency of titanium reached its maximum in anhydrous nitrate containing 20% free NO2. When the concentration of nitric acid contains more than 6.0%NO2 and less than 0.7%H2O, the stress corrosion rupture of industrial pure titanium will occur even at room temperature. Serious stress corrosion and explosion occurred when titanium equipment was used in 98% concentrated nitric acid. In 10% hydrochloric acid solution, industrial pure titanium is sensitive to stress corrosion rupture. In 0.4% hydrochloric acid and methanol solution, titanium will produce stress corrosion. To sum up, although titanium has the phenomenon of stress corrosion failure in some special media, but compared with other metals, the resistance of titanium to stress corrosion failure is good; Titanium in acid and alkali corrosion resistance is very strong, it can form an oxide film in acid and alkali, but it is also conditional, hope to help you in the use of our materials.