Generally speaking, titanium has good 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, effectively preventing further corrosion. However, in reducing acids (such as dilute sulfuric acid solution, hydrochloric acid solution, etc.), because the blunt nature of the oxide film is destroyed, the corrosion rate of titanium is relatively fast, and increases with the increase of temperature and concentration.
In reducing acids, the addition of heavy metal salts can play a significant role in the corrosion inhibition. For example, titanium-palladium alloy ( like Grade 7 titanium plates or bars) and titanium-nickel-molybdenum ( Grade 12 titanium ) alloy have significantly improved corrosion resistance than industrial pure titanium by adding specific heavy metal elements. This allows these alloys to exhibit more excellent properties in specific corrosion environments. Titanium is one of the best metal materials for nitric acid solution heating equipment. After experiencing about 60% nitric acid of 193 C, the titanium heat exchanger has not found any obvious corrosion phenomenon after many years of use. Even in the boiling 40% and 68% nitric acid, the corrosion rate of titanium may be faster in the initial stage, but after a short time, the blunt nature of titanium can be restored, and the corrosion rate is significantly reduced. This may be related to the corrosion inhibition of titanium ions in the corrosion process.
In high-temperature nitric acid, the corrosion resistance of titanium depends on the purity of the nitric acid. When the nitric acid concentration is at 20%~60%, the corrosion phenomenon may be more 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 Ti corrosion. Titanium shows increased corrosion resistance in high-temperature nitric acid solutions compared to stainless steel. In addition, the corrosion product of titanium (Ti4 +) itself is a very good nitric acid corrosion inhibitor.
In the air-through sulfuric acid at room temperature, industrial pure titanium can only withstand less than 5% sulfuric acid solution. As the temperature drops, the concentration of sulfuric acid that titanium can tolerate increases. However, when the temperature is raised to the boiling solution, the titanium is still corroded even when the sulfuric acid concentration drops to 0.5%. At the same temperature, if the sulfuric acid solution into nitrogen, the corrosion rate of titanium will be significantly greater than that of air. This corrosion pattern is essentially the same in other reducing inorganic acids. At room temperature, industrial pure titanium can withstand less than 7% hydrochloric acid solution. However, its corrosion resistance decreases significantly with increasing temperature. In contrast, titanium-Ni-Mo alloy can withstand 9% hydrochloric acid solutions, while titanium-palladium alloy can tolerate up to 27% solutions. The addition of high-priced 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 used in hydrochloric acid systems in the hydrometallurgy industry.
In addition, industrial pure titanium can also withstand less than 30% phosphoracid solution at room temperature. However, as the temperature increases, the concentration of its tolerable phosphate acid gradually decreases. When the temperature reaches 100, the phosphates concentration can only remain at around 2%. However, when the temperature reaches boiling, it does not accelerate the titanium corrosion further.
The corrosion resistance of titanium in different media shows significant differences due to its unique chemical properties and alloying mode. In practical application, appropriate titanium material or alloy according to the specific corrosion environment and requirements to meet the use requirements.
