On the corrosion resistance of titanium

1、 Corrosion resistance of titanium in chemical medium

  1. Nitric acid

The surface of titanium in nitric acid is a dense oxide film. Therefore, titanium has excellent corrosion resistance in nitric acid. The corrosion rate of titanium increases with the increase of the temperature of the nitric acid solution. The corrosion rate of the corrosion can reach up to 10 mm / a with the temperature of 190-240 ℃ and the concentration between 20% and 70%. However, adding a small amount of silicon compounds into the nitric acid solution can inhibit the corrosion of titanium by high temperature nitric acid; for example, after adding silicone oil in 40% of high temperature nitric acid solution, the corrosion rate can be reduced to almost zero. It is also reported that titanium has high corrosion resistance in 40% - 80% nitric acid solution and steam under 500 ℃. In the smoke nitric acid, when the nitrogen dioxide is more than 2%, the strong exothermic reaction is caused by the insufficient water content, which leads to explosion.



  2. Sulfuric acid



The sulfuric acid is a strong reducing acid, titanium has a certain corrosion resistance to sulfuric acid solution with low temperature and low concentration. At 0 ℃, the corrosion resistance of sulfuric acid with concentration of 20% increased with the increase of acid concentration and temperature. Therefore, titanium has poor stability in sulfuric acid, and titanium can only resist 5% sulfuric acid corrosion at room temperature of dissolved oxygen. At 100 ℃, titanium can only resist 0.2% sulfuric acid corrosion. Chlorine can inhibit the corrosion of titanium in sulfuric acid, but at 90 ℃ and sulfuric acid concentration of 50%, chlorine will accelerate the corrosion of titanium and even cause fire. The corrosion resistance of titanium in sulfuric acid can be improved by adding air, nitrogen or adding oxidant and high-value heavy metal ions to the solution. Therefore, titanium has no great practical value in sulfuric acid.

  3. Alkali solution

Titanium has good corrosion resistance in most alkaline solutions. The corrosion rate increases with the concentration and temperature of the solution. When oxygen, ammonia or carbon dioxide is present in alkaline solution, titanium corrosion will be accelerated. In alkaline solution containing hydrogen oxide, titanium has poor corrosion resistance. But the corrosion resistance of sodium hydroxide solution is better than that of potassium hydroxide, which makes it have strong corrosion resistance in high temperature and high concentration sodium hydroxide solution. For example, the corrosion rate of titanium in 130 ℃ and 73% sodium hydroxide solution is only 0.18mm/a. Unlike other metals, titanium does not produce stress corrosion cracking in sodium hydroxide solution, but hydrogen embrittlement may occur after long-term exposure. Therefore, the temperature of titanium in caustic and other alkaline solutions should be ≤ 93.33 ℃.

  4. Chlorine

The stability of titanium in chlorine depends on the water content in chlorine. But it is not corrosion resistant in dry chlorine gas, and it is dangerous to cause combustion. Therefore, titanium must keep a certain amount of water in chlorine, and the water content needed to keep titanium passivation in chlorine is related to the pressure, flow rate and temperature of chlorine gas.

5. Organic medium

Titanium has high corrosion resistance in gasoline, toluene, phenol, formaldehyde, trichloroethane, acetic acid, citric acid, monochloroacetic acid, etc. Titanium will be severely corroded in formic acid below 25% under boiling point and without inflation. In the solution containing acetic anhydride, titanium will not only be seriously corroded in all respects, but also pore corrosion will occur. For many complex organic media in the process of organic synthesis, such as propylene oxide, phenol, acetone, chloroacetic acid and other chemical media, titanium has better corrosion resistance than stainless steel and other structural materials.



  II. Some local corrosion characteristics of titanium



  2.1 the crevice corrosion resistance of titanium crevice corrosion is very strong, only in a few chemical medium crevice corrosion. The corrosion of titanium gap is closely related to temperature, chloride concentration, pH value and gap size. According to the relevant data, the crevice corrosion is easy to occur when the temperature of wet chlorine is above 85 ℃. For example, some plants use a packing tower to cool directly before the cooler, which reduces the wet chlorine temperature to 65-70 ℃, and then enters the titanium cooler to improve the anti crevice corrosion, and the effect is also very significant. The practice shows that reducing temperature is one of the effective methods to prevent crevice corrosion, and titanium crevice corrosion has occurred in high temperature sodium chloride solution. In short, titanium alloy ti-0.2pd shall be used for parts and components prone to crevice corrosion, such as sealing surface, expansion joint between tube plate and pipe, plate heat exchanger, contact part between tray and tower body and fasteners in tower. In design, the gap and stagnant zone shall be avoided. If the fasteners in the tower are not connected by bolts as far as possible. The expansion joint and sealing welding structure of tube plate and pipe is better than that of simple expansion joint. For flange sealing surface, asbestos gasket is not suitable, and polytetrafluoroethylene membrane asbestos gasket should be used.



  2.2 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 can be used as structural material to 426 ℃ in air or oxidation atmosphere, but at 250 ℃, titanium begins to absorb hydrogen obviously. In the complete hydrogen atmosphere, when the temperature rises to 316 ℃, the hydrogen absorption becomes brittle. Therefore, titanium should not be used as chemical equipment with temperature higher than 330 ℃ without extensive test. Considering hydrogen absorption and mechanical properties, the temperature of all titanium pressure vessel should not exceed 250 ℃ and the upper limit of titanium pipe used for heat exchanger is about 316 ℃.



  2.3 stress corrosion



  except for some media, the industrial pure titanium has excellent stress resistance and the damage of titanium equipment caused by stress corrosion is rare. Industrial passivation titanium can only produce stress corrosion in smoke nitric acid, some methanol solution or some hydrochloric acid solution, high temperature hypochlorite, molten salt with temperature of 300-450 ℃ or NaCl atmosphere, carbon disulfide, n-hexane and dry chlorine gas. With the increase of NO2 content and the decrease of water content, the tendency of stress corrosion cracking of titanium increases gradually. The stress corrosion tendency of titanium in anhydrous nitric acid containing 20% free NO2 is the highest. When the concentration of nitric acid is higher than 6.0% NO2 and less than 0.7% H2O, the stress corrosion rupture of industrial pure titanium will occur even at room temperature. In China, serious stress corrosion and explosion occurred when titanium equipment was used in 98% concentrated nitric acid. In 10% hydrochloric acid solution, titanium has the sensitivity of stress corrosion cracking, and titanium produces stress corrosion in 0.4% hydrochloric acid plus methanol solution.



In conclusion, titanium has strong corrosion resistance in acid and alkali. It can form oxide film in acid and alkali, but it is also conditional. I hope it will help you when using our materials.