Generally speaking, titanium has better corrosion resistance in oxidizing media (such as nitric acid, chromic acid, hypochlorous acid and perchloric acid, etc.). In these media, titanium is able to form a dense oxide film, which effectively prevents further corrosion. However, in reducing acids (such as dilute sulfuric acid solution, hydrochloric acid solution, etc.), due to the destruction of the passivity of the oxide film, the corrosion of titanium is relatively fast, and increases with the increase in temperature and concentration.
In reducing acid, the addition of heavy metal salts can play an obvious role in corrosion inhibition. For example, titanium-palladium and titanium-nickel-molybdenum alloys show a significant increase in corrosion resistance compared to industrially pure titanium through the addition of specific heavy metal elements. This enables these alloys to exhibit superior performance in specific corrosive environments.
Titanium is one of the best metals for heating equipment in nitric acid solutions. When subjected to 60% nitric acid at around 193°C, titanium heat exchangers have been used for many years without significant corrosion. Even in boiling 40% and 68% nitric acid, titanium corrosion rate may be faster in the initial stage, but 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 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 at 20% to 60%, the corrosion phenomenon may be more obvious. However, even in nitric acid solutions containing trace amounts of metal ions (such as Si, Cr, Fe, Ti, etc.), these ions are able to play a role in slowing down the corrosion of titanium. Compared with stainless steel, titanium shows greater corrosion resistance in high-temperature nitric acid solutions. In addition, the corrosion product of titanium (Ti4+) itself is a very good nitric acid corrosion inhibitor.
In air-ventilated sulfuric acid at room temperature, industrially pure titanium is only resistant to sulfuric acid solutions of less than 5%. As the temperature decreases, the concentration of sulfuric acid that titanium can tolerate increases. However, when the temperature is raised to the point where the solution boils, titanium will still corrode even if the sulfuric acid concentration is reduced to 0.5%. At the same temperature, if nitrogen is passed through the sulfuric acid solution, titanium will corrode significantly faster than if air is passed through. This corrosion pattern is essentially the same in other reducing inorganic acids.
At room temperature, industrially pure titanium can withstand hydrochloric acid solutions up to 7%. However, as the temperature increases, its corrosion resistance decreases significantly. In comparison, titanium-nickel-molybdenum alloys are resistant to hydrochloric acid solutions of 9%, while titanium-palladium alloys are able to withstand hydrochloric acid solutions of up to 27%. The addition of high-valent heavy metal ions (e.g. iron, nickel, copper, molybdenum, etc.) can significantly increase the corrosion resistance of titanium. This is one of the reasons why titanium can be successfully used in hydrochloric acid systems in the hydrometallurgical industry.
In addition, at room temperature, industrially pure titanium is resistant to phosphoric acid solutions of up to 30%. However, the concentration of phosphoric acid it can withstand decreases as the temperature increases. When the temperature reaches 100°, the phosphoric acid concentration can only be maintained at about 2%. However, when the temperature reaches boiling, it does not further accelerate the corrosion of titanium.
In summary, 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 the appropriate titanium material or alloy according to the specific corrosive environment and requirements to meet the needs of use.
