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Corrosion resistance of titanium in chemical media

1, nitric acid
Nitric acid is an oxidizing acid. Titanium in nitric acid maintains a dense oxide film on its surface. So titanium has excellent corrosion resistance in nitric acid. The corrosion rate of titanium increases with the temperature of nitric acid solution. The corrosion rate can reach 10mm/a when the temperature is 190~240℃ and the concentration is 20%~70%. However, adding a small amount of silicon compounds into 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 40 percent high-temperature nitric acid solution. There is also information that titanium below 500C has a high degree of corrosion resistance in 40%~80% nitric acid solution and steam. In fuming nitric acid, when nitrogen dioxide in more than 2%, due to insufficient water content caused a strong exothermic reaction, and lead to the explosion.

2, the sulfuric acid
Sulfuric acid is a strong reducing acid and titanium has a certain corrosion resistance to low temperature and low concentration sulfuric acid solution. Sulfuric acid with a concentration of 20% can be tolerated at 0℃. Corrosion rate increases with the increase of acid concentration and temperature. So the stability of titanium in sulfuric acid is poor, even in dissolved oxygen room temperature, titanium can only withstand 5% of 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 when the concentration of sulfuric acid is 50% at 90℃, chlorine can accelerate the corrosion of titanium and even cause ignition. The corrosion resistance of titanium in sulfuric acid can be improved by adding air, nitrogen or oxidizing agent, high value heavy metal ions into the solution. So titanium has little practical value in sulfuric acid.

3, lye
Titanium has good corrosion resistance in most alkaline solutions, and the corrosion rate increases gradually with the concentration and temperature of the solution. In the presence of oxygen, ammonia or carbon dioxide in the alkali solution, the corrosion of titanium will be accelerated. In the alkali solution containing hydrogen oxide, titanium corrosion resistance is very poor. But the corrosion resistance of sodium hydroxide solution is better than that of potassium hydroxide, even in high temperature and high concentration of sodium hydroxide solution has a strong corrosion resistance. For example, the corrosion rate of titanium in 130C.73% NaOH solution is only 0.18mm/a. Titanium differs from other metals in that it will not produce stress corrosion rupture in NaOH solution, but long-term exposure may produce hydrogen embrittle. Therefore, the temperature of titanium in caustic alkali and other alkaline solutions should be ≤93.33°C.

4, chlorine
The stability of titanium in chlorine gas depends on the water content of chlorine. However, it is not corrosion-resistant in dry argon and may cause combustion. Therefore, titanium used in chlorine gas must maintain a certain water content. The water content required to keep titanium passivated in chlorine gas is related to the pressure, flow rate and temperature of chlorine gas.

5, organic media
Titanium has high corrosion resistance in gasoline, toluene, phenol, formaldehyde, trichloroethane, acetic acid, limonic acid, chloroacetic acid and so on. At boiling point and without aeration, titanium is severely corroded in formic acid below 25%. In solutions containing acetic anhydride, titanium not only suffers severe overall corrosion but also produces pitting corrosion. The corrosion resistance of titanium is better than that of stainless steel and other structural materials in the production of propylene oxide, phenol, acetone, chloroacetic acid and other complex organic media in organic synthesis.
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