Generally speaking, the corrosion resistance of titanium is relatively good in oxidizing medium such as nitric acid, chromic acid, hypochlorous acid and perchloric acid. In these medium, titanium can form a layer of compact oxidation film which can prevent further corrosion effectively. However, in the reduced acid such as dilute sulfuric acid solution and hydrochloric acid etc, due to the passivity destroy of the oxidation film, the corrosion speed of titanium is fast and will be accelerated according to the increase of the temperature and the concentration.
In the reduced acid, adding the heavy metallic salt can play a obvious role in corrosion inhibition. For example, the titanium palladium alloy and titanium nickel molybdenum alloy, by adding a certain heavy metallic elements, will have significant improvement in the corrosion resistance compared with the pure titanium. As a result, these alloys can have a good performance in the specific corrosion environment.
Titanium is one of the best metal materials for the nitric acid solution heating equipment. When subjected to 60% nitric acid around 193℃, the titanium heat exchanger hasn't been found the obvious corrosion after use in many years. Even in boiling 40% and 68% nitric acid, the corrosion speed of titanium may be fast, but after a short time, the passivity of titanium can be restored quickly and the corrosion speed will be reduced obviously. It may be related to the corrosion inhibition produced by titanium ions in the corrosion process.
In nitric acid at high temperature, titanium corrosion resistance depends on the purity of nitric acid. When the concentration of nitric acid is between 20% and 60%, corrosion phenomena is probably obvious. Nevertheless, even in the nitric concentration with trace metal ions such as Si,Cr,Fe,and Ti, these ions can play a role in reducing the corrosion of titanium. Compared to stainless steel, titanium shows greater corrosion resistance in nitric acid solutions at high temperatures. Besides, the titanium corrosion product (Ti4+) itself is a perfect nitric acid corrosion inhibitor.
In the sulfuric acid ventilated with air at room temperature, the pure titanium only can withstand the sulfuric acid solution less than 5%. With the temperature decreasing, the concentration of sulfuric acid solution that titanium can withstand will be improved. However, when the temperature is raised to the boiling of the solution, titanium still can be corrosive even thought the concentration of sulfuric acid solution falls to 0.5%. At the same temperature, if ventilated with nitrogen, the corrosion speed of titanium is faster than that with air. This corrosion rule is basically the same in other reduced inorganic acids.
At the room temperature, the pure titanium is able to withstand the hydrochloric acid less than 7%. However, its corrosion resistance will be decreased obviously with the increase of temperature. By contrast, titanium nickel molybdenum alloy can withstand 9% hydrochloric acid solution and Titanium palladium alloy can withstand up to 27% hydrochloric acid. The addition of high valence heavy metal ions (such as iron, nickel, copper, molybdenum, etc.) can significantly improve the corrosion resistance of titanium which is one of reasons that titanium can be successfully applied to hydrochloric acid system in hydrometallurgical industry.
Besides, at the room temperature, the pure titanium can withstand below 30% phosphoric acid solution. However, the concentration of phosphoric acid it can tolerate will be decreased gradually. When the temperature reaches 100°, the concentration of phosphoric acid only can be maintained at about 2%, but when the temperature reaches boiling state, it can't accelerate the corrosion of titanium.
In conclusion, the corrosion resistance of titanium in different medium will has significant difference because of its special chemical properties and alloying methods. In the practical application, it need to choose proper titanium materials to meet the usage requirements according to the specific corrosive environment and demands.
