Generally speaking, titanium tubes have better corrosion resistance in oxidizing media (such as nitric acid, chromic acid, hypochlorous acid and perchloric acid, etc.). In reducing acid (such as dilute sulfuric acid solution, hydrochloric acid solution, etc.), because the passivation of the oxide film is destroyed, the corrosion rate is relatively fast, and it increases with the increase of temperature and concentration. In reducing acids, the addition of heavy metal salts can play a significant role in corrosion inhibition. The corrosion resistance of titanium-palladium alloy and titanium-nickel-molybdenum alloy is much higher than that of industrial pure titanium.
Titanium tube is the best metal material for heating equipment of nitric acid solution. The titanium heat exchanger is subjected to 60% nitric acid at about 193°C, and no corrosion has been found after many years of use. In the boiling 40% and 68% nitric acid, there was some corrosion at the beginning. After a short time, the passiveness of titanium recovered and the corrosion rate was significantly reduced. It may be related to the corrosion inhibition effect of titanium ions.
The corrosion resistance of titanium in high temperature nitric acid depends on the purity of the nitric acid. Under high temperature pure nitric acid solution or nitric acid vapor, the corrosion is more obvious when the concentration of nitric acid is between 20% and 60%. Various metal ions, such as Si/Cr/Fe/Ti, etc., can slow down the corrosion of titanium in high-temperature nitric acid solutions even if they are used in very low content. In high temperature nitric acid solution, titanium shows stronger corrosion resistance than stainless steel. The corrosion product of titanium (Ti4+) is a very good corrosion inhibitor for nitric acid corrosion.
In sulfuric acid with air at room temperature, industrial pure titanium can only withstand sulfuric acid solutions below 5%; if the temperature drops to about 0°C, the sulfuric acid concentration can be increased to 20%. If the temperature is so high that the solution boils, the sulfuric acid temperature will still corrode even if it drops to 0.5%. When nitrogen is introduced into the sulfuric acid solution at the same temperature, the corrosion rate of titanium is significantly higher than that of air. This corrosion law is basically the same in other reducing inorganic acids.
At room temperature, industrial pure titanium can withstand less than 7% hydrochloric acid solution. The corrosion resistance decreases significantly when the temperature rises. The titanium nickel molybdenum alloy can withstand 9% hydrochloric acid solution. Titanium-palladium alloy can reach 27%. High-priced heavy metal ions, such as iron, nickel, copper, molybdenum, etc., can significantly improve the corrosion resistance of titanium. This is the reason why titanium has been successfully used in hydrochloric acid systems in the hydrometallurgical industry.
At room temperature, industrial pure titanium can withstand less than 30% phosphoric acid solution. When the temperature rises to 60°C, the concentration drops to 10%. At a temperature of 100°C, the concentration of phosphoric acid can only be maintained at about 2%. When the temperature reaches boiling, the corrosion of titanium will not be accelerated.
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