A large number of industrially used corrosion resistant materials are still industrial pure titanium. This is because industrial pure titanium has good corrosion resistance to seawater, wet chlorine, and reducing media that are oxidizing, neutral, weakly reducing, and adding corrosion inhibitors; and the overall mechanical properties and processing performance are also superior. The price is also lower than titanium alloys. Therefore, only in the case of industrial pure titanium can not meet the requirements of corrosion resistance, the use of other corrosion resistant titanium alloys, China will be divided into three industrial pure titanium, namely TA1, TA2, TA3.(Grade 1-3) The gap elements of these three kinds of industrial pure titanium are gradually increasing; and the mechanical strength and hardness are also increased gradually. The titanium alloy commonly used in the industry is TA2. This titanium alloy has moderate corrosion resistance and comprehensive mechanical properties. TA3 can be used when the abrasion resistance (titanium and titanium alloys are not wear resistant) and the strength requirements are high. TA1 can be used for better molding performance. Industrial pure titanium at all levels has similar corrosion resistance.
In some cases, especially in strong oxidizing or reducing acids, the metallurgical structure of the heat-affected zone is changed due to iron element impurities in titanium, so that the corrosion resistance thereof is reduced. In this case, it should be taken into account that the content of iron in titanium does not exceed 0.05%.
This titanium alloy has the following three advantages over industrial pure titanium:
a, not only has excellent corrosion resistance to oxidizing media, but also has certain corrosion resistance to weak reducing media. For example, the corrosion resistance of Ti-0.2Pa to nitric acid is as good as that of industrial pure titanium; but the corrosion resistance to dilute hydrochloric acid and dilute sulfuric acid is 500-1000 times higher than that of pure titanium.
b, has good crevice corrosion resistance. Industrial pure titanium is more sensitive to crevice corrosion, and is often accelerated due to the presence of gaps in the titanium equipment. If Ti-(0.15-0.2)Pa is used instead of industrial pure titanium at the crevice, crevice corrosion can be eliminated.
c. The ability to absorb hydrogen is small and it is not easy to produce hydrogen embrittlement. Industrial pure titanium absorbs hydrogen at high temperatures. In particular, the atomic hydrogen generated during corrosion is more easily absorbed, resulting in hydrogen embrittlement. The oxide film formed on the surface of Ti-(0.1-0.20)Pa has higher resistance to hydrogen permeation, so the ability to resist hydrogen embrittlement is greater than that of industrial pure titanium. many.
However, this kind of strong reducing media is not resistant to corrosion; it also contains precious metal-palladium. Its physical, mechanical properties, process performance and industrial pure titanium are similar. Column-tube heat exchangers currently manufactured from this alloy are used in dilute hydrochloric acid and phosphoric acid; in the textile industry they are used to make spinnerets; in all acetaldehyde and terephthalic acid plants flange linings for all titanium equipment Ring and diamond gaskets are also used in this alloy, and its use is good.
Ti-Pa alloy is mainly used in the environment of dilute sulfuric acid, hydrochloric acid and phosphoric acid; it is also used in titanium equipment to prevent crevice corrosion. In the environment of sulfuric acid, hydrochloric acid and phosphoric acid, the corrosion rate increases when the content of Pa is less than 0.05%, and the corrosion rate is almost constant when the content is higher than 0.20%.
This alloy improves the corrosion resistance of the corrosion resistance in a reducing medium; and it maintains the corrosion resistance of industrial pure titanium to oxidizing media such as nitric acid and chromic acid. For example, it has good corrosion resistance in both room temperature 10% sulfuric acid, 10% hydrochloric acid, and moderate concentrations of formic acid and citric acid. It contains less alloying elements; in particular it does not contain rare and expensive palladium. Therefore, its price is only about 10% higher than that of industrial pure titanium. Its processing performance, welding process performance, and industrial pure titanium are close to each other. At a temperature of 200-300°C, the strength is 1.5-2 times higher than that of industrial pure titanium. Because of the advantages of this alloy, it can replace industrial pure titanium in high-temperature seawater, salt water, wet chlorine, and high-temperature, high-concentration chlorides.
The corrosion resistance of Ti-32Mo alloy in boiling 40% sulfuric acid and 20% hydrochloric acid solution is significantly higher than that of industrial pure titanium. It is one of the most resistant titanium alloys to corrosion.
However, its corrosion resistance in oxidizing media is very low
In addition, the excessively high molybdenum content makes the alloy brittle and the process performance deteriorates. In order to overcome the disadvantages of this alloy due to the high molybdenum content, Nb, V, Zr, and Pa were added to the Ti-Mo alloy. This alloy not only maintains the excellent corrosion resistance of the Ti-Mo alloy, but also improves their processing performance. For example, the Ti-15Mo-5Zr alloy not only avoids the alloy embrittlement caused by precipitation of ω phase in the Ti-15Mo alloy, but also improves its cold working process performance and welding performance, and also improves the corrosion resistance of the alloy. For another example, Ti-15Mo-0.2Pa alloy has good corrosion resistance in both oxidizing and reducing media.
This Ti alloy is used as a structural material of the desalination apparatus, and the use temperature can reach about 200°C, while industrial pure titanium can only reach about 120°C. And its crevice corrosion resistance is superior to industrial pure titanium. However, the use of this alloy is much narrower than that of Ti-Pa and Ti-Mo alloys. It is only in the formic acid, hot concentrated magnesium chloride solution corrosion resistance than industrial pure Ti is excellent; but in hydrochloric acid, sulfuric acid corrosion resistance is worse than pure titanium. Therefore, Ti-2Ni alloys are generally limited to use in neutral or rhodium-reducing salt solutions.
Ti-6Al-4V (Gr5 ) Alloy
This is a widely used Ti alloy. Its corrosion resistance is slightly inferior to industrial pure titanium, but it still has satisfactory corrosion resistance to seawater, many acids, alkalis, and other corrosive media. It has good overall mechanical properties. Therefore, such alloys are generally used where high strength or high fatigue properties are required.
The medium that allows the passivation film of Ti to be in a stable state is a titanium corrosion-resistant medium. These media are oxidative, neutral, and weakly reducing.
The various media that damage the passivation film of Ti are titanium-resistant media. These media are strong reducing acids and strong anhydrous oxidants.
In 5% hydrochloric acid solution, the order of corrosion resistance of TA0, BT1-0, Ti-0.2Pa, Zr-O from high to low is Zr-O, Ti-0.2Pa, TA0, BT1-0.