Generally speaking, titanium has relatively good corrosion resistance in oxidizing media (such as nitric acid, chromic acid, hypochlorous acid, and perchloric acid, etc.). In these media, titanium can form a dense oxide film, which effectively prevents further corrosion. However, in reducing acids (such as dilute sulfuric acid solutions, hydrochloric acid solutions, etc.), as the passivity of the oxide film is damaged, the corrosion rate of titanium is relatively fast and increases with the rise in temperature and concentration. In reducing acids, the addition of heavy metal salts can play a significant corrosion inhibition role. For example, titanium-palladium alloys and titanium-nickel-molybdenum alloys, by adding specific heavy metal elements, have significantly improved corrosion resistance compared to commercially pure titanium. This enables these alloys to exhibit more excellent performance in specific corrosive environments.
Titanium is one of the best metallic materials for nitric acid solution heating equipment. When subjected to 60% nitric acid at around 193 °C, titanium heat exchangers have shown no obvious corrosion phenomena after years of use. Even in boiling 40% and 68% nitric acid, although the corrosion rate of titanium may be relatively fast in the initial stage, after a short period of time, the passivity of titanium can be restored and the corrosion rate is significantly reduced. This may be related to the corrosion inhibition effect produced by titanium ions during the corrosion process. In high-temperature nitric acid, the corrosion resistance of titanium depends on the purity of nitric acid. When the concentration of nitric acid is between 20% and 60%, the corrosion phenomenon may be relatively obvious. However, even in nitric acid solutions containing trace metal ions (such as Si, Cr, Fe, Ti, etc.), these ions can also play a role in slowing down the corrosion of titanium. Compared with stainless steel, titanium shows stronger corrosion resistance in high-temperature nitric acid solutions. Moreover, the corrosion product of titanium (Ti4+) itself is an excellent corrosion inhibitor for nitric acid.
In sulfuric acid with air passing through at room temperature, commercially pure titanium can only withstand sulfuric acid solutions with a concentration below 5%. As the temperature drops, the concentration of sulfuric acid that titanium can tolerate will increase to some extent. However, when the temperature rises to the boiling point of the solution, even if the concentration of sulfuric acid drops to 0.5%, titanium will still be corroded. At the same temperature, if nitrogen is passed through the sulfuric acid solution, the corrosion rate of titanium will be significantly greater than that when air is passed through. This corrosion pattern is basically the same in other reducing inorganic acids.
At room temperature, commercially pure titanium can withstand hydrochloric acid solutions with a concentration below 7%. However, as the temperature rises, its corrosion resistance will decline significantly. In contrast, the titanium-nickel-molybdenum alloy can withstand 9% hydrochloric acid solutions, and the titanium-palladium alloy can tolerate hydrochloric acid solutions with a concentration as high as 27%. The addition of high-valence heavy metal ions (such as iron, nickel, copper, molybdenum, etc.) can significantly improve the corrosion resistance of titanium. This is also one of the reasons why titanium can be successfully applied in hydrochloric acid systems in the hydrometallurgical industry.
In addition, at room temperature, commercially pure titanium can also withstand phosphoric acid solutions with a concentration below 30%. However, as the temperature rises, the concentration of phosphoric acid that it can tolerate will gradually decrease. When the temperature reaches 100 °C, the concentration of phosphoric acid can only be maintained at about 2%. However, when the temperature reaches the boiling point, it will not further accelerate the corrosion of titanium.
In conclusion, the corrosion resistance of titanium in different media shows significant differences due to its unique chemical properties and alloying methods. In practical applications, it is necessary to select appropriate titanium materials or alloys according to specific corrosive environments and requirements to meet the usage needs.
