In dilute hydrochloric acid, sulfuric acid, and phosphoric acid, titanium dissolves much more slowly than iron. As the concentration increases, especially when the temperature rises, the rate of titanium dissolution is significantly accelerated, and titanium dissolves very quickly in the mixture of hydrofluoric acid and nitric acid. However, except for formic acid, oxalic acid, and a considerable concentration of citric acid among organic acids, titanium will not be corroded. For example, in organic acids such as oxalic acid, butyric acid, lactic acid, maleic acid, hydroxysuccinic acid (benzene fruit acid), tannic acid, and tartaric acid, titanium has strong corrosion resistance.
Nitric acid is an oxidizing acid. Titanium in nitric acid can maintain a dense oxide film on its surface. As the concentration of nitric acid increases, the surface film appears yellowish, light yellow, earthy yellow, and brownish yellow to blue. Various interference colors. The integrity of the oxide film is a necessary condition for maintaining the corrosion resistance of titanium. Therefore, titanium has very good corrosion resistance to nitric acid, and the corrosion rate of titanium increases with the temperature of the nitric acid solution, the temperature is between 190 and 230. C, the concentration is between 20% and 70%, and its corrosion rate can reach up to nearly 10mm/a. Figure 2-12 shows the corrosion rate of titanium in high-temperature nitric acid. However, adding a small amount of silicon-containing compounds to the nitric acid solution can inhibit the corrosion of titanium by high-temperature nitric acid. For example, after adding polysiloxane oil to 40% high-temperature nitric acid solution, the corrosion rate can be reduced to almost zero. There are also information presentations at 500. Below C, titanium has a high degree of corrosion resistance in 40% to 80% nitric acid solution and steam. On the contrary, adding phosphide to nitric acid will accelerate the corrosion of titanium, and this characteristic of titanium can be used to prepare its pickling solution. In fuming nitric acid, when the carbon dioxide content is more than 2%, the insufficient water content causes a strongly exothermic reaction, resulting in volatilization. The possibility of volatilization between titanium and nitric acid is related to the content of N02 and water in nitric acid. As shown in Figure 2-13. However, titanium will not volatilize in nitric acid with a concentration of 80% or lower. The test in 170q2, (20%-80%) HN0, has confirmed this conclusion. The possibility of titanium being used in high-temperature nitric acid above 80% still needs further research for safety considerations. At a temperature below 500°C, titanium is in a molten mixture of nitrates (50% KN03+50% NaN02 and 40% NaN03+7%KN03+53%NaN02) will not have a tendency of the combustion reaction.
Sulfuric acid is a strong reducing acid. Titanium has a certain corrosion resistance to low-temperature and low-concentration sulfuric acid solutions. At 0°C, it can withstand the corrosion of sulfuric acid with a concentration of 20%. Increase. Therefore, the stability of titanium in sulfuric acid is poor. Even at room temperature of dissolved oxygen, titanium can only resist 5% sulfuric acid corrosion. At 100°C, titanium can only resist 0.2% sulfuric acid corrosion. inhibition. But at 90°C, when the concentration of sulfuric acid is 50%, the chlorine will cause accelerated corrosion of titanium, and even cause a fire. The corrosion resistance of titanium in sulfuric acid can be improved by passing air, nitrogen, or adding oxidants and high-priced heavy metal ions into the solution. The main additives that can play a slowing role are high-valent iron, high-valent copper, Ti4+, silver chromate, manganese dioxide, nitric acid, chlorine, and organic corrosion inhibitors, only nitroso compounds, quinones and anthraquinone derivatives, and certain complexes. Composite corrosion inhibitor. Generally speaking, titanium has little practical value in sulfuric acid.
Hydrochloric acid is a reducing acid, and titanium is less stable in hydrochloric acid even at room temperature. The corrosion rate increases gradually with the concentration and temperature of the acid solution. Therefore, titanium is generally suitable for working in 3% and 100°C, 0.5% hydrochloric acid solutions at room temperature. Although titanium is not resistant to the corrosion of hydrochloric acid solutions, it can also be alloyed, anode passivated, and added corrosion inhibitors. To improve the corrosion resistance of titanium. The most effective corrosion inhibitors belonging to the strong oxidizing inorganic compound titanium are nitric acid, potassium dichromate, sodium hypochlorite, chlorine gas, oxygen, and high-priced heavy metal ions (mainly Fe¨, Cu'2+, a small number of precious metals); organic corrosion inhibitors There are oxidizing organic compounds, dichloro compounds, quinone and anthraquinone derivatives, heterocyclic compounds, and complex corrosion inhibitors, so they still have use value in production practice.
Acids are also reducing acids. The corrosion rate of titanium in phosphoric acid is lower than that of hydrochloric acid or sulfuric acid, but higher than that of nitric acid. Titanium is generally suitable for 20. C, 30% or 35°C, 20% aerated or non-aerated phosphoric acid. The corrosion resistance of titanium in phosphoric acid increases gradually with the increase of acid concentration and temperature, which is similar to the situation in titanium hydrochloric acid.
Titanium undergoes the following corrosion reaction in phosphoric acid, namely 2Ti+2H, P04=2TiP04+2H.
Similar to the situation of titanium in sulfuric acid and hydrochloric acid, the addition of oxidants or other corrosion inhibitors to phosphoric acid is beneficial to improve the corrosion resistance of titanium in phosphoric acid. Silver and mercury are also beneficial to improve the corrosion resistance of titanium in phosphoric acid, and nitric acid is also an effective oxidant. Hydrofluoric acid and fluorosilicic acid are the strongest corrosive media, even in very dilute hydrofluoric acid at room temperature, titanium will be severely corroded. Therefore, titanium cannot be used at all in hydrofluoric acid. Titanium is not only corroded rapidly in hydrofluoric acid but also corroded strongly in acidic mediums containing fluorine (such as fluorosilicate and fluoroboric acid). The corrosion reaction of titanium and hydrofluoric acid is Ti+6HF=TiF, +3H. It is a porous corrosion product without any protective effect, so the corrosion develops very quickly. Titanium is more soluble in the mixed acid of hydrofluoric acid, hydrochloric acid, or sulfuric acid. In addition to the corrosion of titanium due to the interaction between concentrated acid and metal, the complexation between F- and Ti4+ accelerates the dissolution of titanium. This reaction is
Ti+6HF=TiF64+2H++2H2 Adding a small amount of soluble fluoride to other acids, such as hydrobromic acid, perchloric acid, formic acid, and acetic acid, increases the corrosion rate of titanium dozens of times. Acidic fluoride solutions, such as NaF, and KHF: also cause severe corrosion of titanium. No ideal corrosion inhibitor has been found in hydrochloric acid.
