One : Corrosion resistance of titanium in chemical medium
Nitric acid is an oxidizing acid, titanium in nitric acid, its surface maintains a dense oxide film.Therefore, titanium in nitric acid has very good corrosion resistance.The corrosion rate of titanium increases with the temperature of nitric acid solution.The highest corrosion rate is 10mm/a at the temperature of 190 ~ 240℃ and the concentration of 20% ~ 70%.However, adding a small amount of silicon-containing compounds in nitric acid solution can inhibit the corrosion of titanium by high-temperature nitric acid.For example, the corrosion rate can be reduced to almost zero by adding silicone oil to a solution of nitric acid at a high temperature of 40%.Also has the data introduction, below 500℃, titanium has the high corrosion resistance in the nitric acid solution and steam of 40% ~ 80%.In fuming nitric acid, when containing nitrogen dioxide in more than 2%, due to insufficient water content caused a strong exothermic reaction, resulting in an explosion.
Sulfuric acid is a strong reducing acid, titanium has a certain corrosion resistance to sulfuric acid solution at low temperature and low concentration.At 0℃, the corrosion resistance of sulfuric acid can reach 20%. With the increase of acid concentration and temperature, the corrosion rate increases.Therefore, titanium has poor stability in sulfuric acid. Even at room temperature of dissolved oxygen, titanium can only withstand 5% sulfuric acid corrosion.At 100℃, titanium can withstand only 0.2% sulfuric acid corrosion.Chlorine can inhibit the corrosion of titanium in sulfuric acid. However, at 90℃, when the concentration of sulfuric acid is 50%, chlorine will accelerate the corrosion of titanium and even cause fire.The corrosion resistance of titanium in sulfuric acid can be improved by passing air, nitrogen or adding oxidants and high-value heavy metal ions into the solution. Therefore, titanium has little practical value in sulfuric acid.
Titanium in most alkaline solution has a good corrosion resistance, corrosion rate with the concentration and temperature of the solution and increase, in the presence of oxygen, ammonia or carbon dioxide in alkali solution, will accelerate the corrosion of titanium, in the alkali containing hydrogen oxide, titanium corrosion resistance is very poor.However, the corrosion resistance in sodium hydroxide solution is better than that of potassium hydroxide, so it has strong corrosion resistance even in high temperature and high concentration sodium hydroxide solution.For example, the corrosion rate of titanium in 130 ° c and 73% sodium hydroxide solution is only 0.18mm/a. What differentiates titanium from other metals is that it will not produce stress corrosion cracking in sodium hydroxide solution, but hydrogen embrittlement may occur after long-term exposure.Therefore, the application temperature of titanium in caustic soda and other alkaline solutions should be ≤93.33℃.
The stability of titanium in chlorine depends on the water content of chlorine.But it is not resistant to corrosion in dry chlorine and has the risk of causing combustion.Therefore, titanium materials used in chlorine gas must maintain a certain water content, the water content required to maintain titanium passivation in chlorine gas is related to the pressure, flow rate, temperature and other factors of chlorine gas.
Titanium has high corrosion resistance in gasoline, toluene, phenol, formaldehyde, trichloroethane, acetic acid, citric acid, monochloroacetic acid, etc.At boiling point and without aeration, titanium will be severely corroded in formic acid below 25%. In solutions containing acetic anhydride, titanium will not only be severely overall corroded but also will produce pore corrosion.The corrosion resistance of titanium is better than that of stainless steel and other structural materials in the production of epoxy propane, phenol, acetone, chloroacetic acid and other complex organic media that are exposed to in the process of organic synthesis.
Two : Several local corrosion properties of titanium
Crevice corrosion resistance of titanium is particularly strong, only in a few chemical medium crevice corrosion.Gap corrosion of titanium is closely related to temperature, chloride concentration, pH value and gap size.According to the relevant information, the temperature of wet chlorine gas is more than 85℃ easy to produce crevice corrosion.For example, in some factories, a packed column is used to directly cool the wet chlorine gas to 65 ~ 70℃ before the cooler, and then the wet chlorine gas is put into the titanium cooler to improve the anti-crack corrosion, and the effect is also very significant.Practice shows that reducing temperature is one of the effective methods to prevent crevice corrosion.In a word, titanium alloys such as ti-0.2pd should be used in the parts and components that are prone to crack corrosion, such as sealing surface, tube-plate and tube-tube expansion joint, plate heat exchanger, tube-plate and tube-body contact parts and tower fasteners, and cracks and stagnation areas should be avoided in the design.If tower fasteners should try not to use bolt connection.The welded structure of tube plate and pipe with expansion joint and seal is better than that with simple expansion joint. For the flange sealing surface, asbestos pad is not suitable to be used.
High temperature corrosion
The high temperature corrosion resistance of titanium depends on the properties of the medium and the properties of the oxide film on its surface.Titanium in air or oxidizing atmosphere, as a structural material can be used up 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 absorption become brittle.Therefore, without extensive testing, titanium should not be used for chemical equipment whose temperature is higher than 330℃. In consideration of hydrogen absorption and mechanical properties, the service temperature of full titanium pressure vessel should not exceed 250℃, and the upper limit of service temperature of titanium tube for heat exchanger is about 316℃.
In addition to a few individual media, industrial pure titanium stress corrosion resistance is very good, stress corrosion caused by titanium equipment damage phenomenon is rare.Industrial blunt titanium produces stress corrosion only in fuming nitric acid, some methanol solution or some hydrochloric acid solution, high-temperature hypochlorite, molten salt with temperature of 300 ~ 450℃ or medium containing NaCl atmosphere, carbon disulfide, n-hexane and dry chlorine gas.With the increase of NO2 content and the decrease of water content in nitric acid, the stress corrosion cracking tendency of titanium increases gradually.The stress corrosion tendency of titanium reached the maximum in anhydrous nitrate acid containing 20% free NO2.In concentrated nitric acid containing more than 6.0%NO2 and less than 0.7%H2O, even at room temperature, industrial pure titanium will occur stress corrosion cracking.Serious stress corrosion and explosion occurred when titanium equipment was used in 98% concentrated nitric acid.The commercial pure titanium has the sensitivity of stress corrosion cracking in 10% hydrochloric acid solution, and stress corrosion occurs in the solution containing 0.4% hydrochloric acid plus methanol.
To sum up, titanium has a strong corrosion resistance in acid and alkali. It can form oxidation film in acid and alkali, but it is also conditional. I hope it can be helpful to you when you use our materials.