JPH0483837A - Titanium alloy for nitric acid resistant apparatus - Google Patents

Abstract

PURPOSE:To obtain a titanium allay for nitric acid resistant apparatus showing excellent corrosion resistance to oxidizing acid such as high temp. high concn. nitrid acid by adding Ti to a specified amt. of niobium. CONSTITUTION:This alloy is formed of a compsn. constituted of, by weight, 15 to 60% niobium and the balance titanium with inevitable impurities. By adding niobium to titanium, corrosion resistance that niobium possesses is utilized to improve the corrosion resistance of titanium to nitric acid. At the same time, because niobium is a beta stabilizing element, by the addition of niobium to titanium, it can be converted into beta type titanium having good workability. The reason that the lower limit of the content of niobium is regulated to 15wt.% is that, in the case of less than the above, objective corrosion resistance and workability can not be obtd. On the other hand, the reason that the upper limit of the content of niobium is regulated to 60wt.% is that, even the case niobium is added more than the above, the corrosion resistance rather tends to deteriorate.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a titanium alloy for nitric acid-resistant equipment that exhibits excellent corrosion resistance in extremely harsh environments where a highly concentrated aqueous nitric acid solution is present.

[Conventional technology]

Since pure titanium has extremely excellent corrosion resistance, it has come to be widely used as an industrial material in place of conventional corrosion-resistant metal materials. Moreover, in recent years, the environment in which they are used has become increasingly harsh, and there is a desire for further improvement in the corrosion resistance of the materials used.

Pure titanium originally has strong corrosion resistance in oxidizing atmospheres such as nitric acid, but in recent years the concentration of nitrate has become extremely high in nuclear fuel processing plants, solvent extraction plants, etc., and the operating temperature has also increased. Due to high temperatures and the presence of various other ions that degrade corrosion resistance, an environment has arisen in which even pure titanium cannot maintain its corrosion resistance.

[Problem to be solved by the invention]

Therefore, as a nitric acid-resistant material currently in use, there is an alloy made of titanium with tantalum added (Japanese Patent Application No. 4027045), but this adds a large amount of very expensive tantalum (normal commercial products contain 5 wt%). It has the disadvantage that it becomes an expensive material because of the amount of tantalum added, and that its workability deteriorates significantly as tantalum is added.

Other nitric acid-resistant materials include zirconium and stainless steel, but zirconium has the disadvantage that its corrosion resistance deteriorates significantly if even a small amount of halogen ions such as chlorine ions are present, and stainless steel has the disadvantage that ions that increase the potential are present. However, it has the disadvantage that it enters the overpressure dynamic region and corrodes.

The present invention was discovered in view of these circumstances, and provides a titanium alloy for equipment that is resistant to nitric acid and exhibits excellent corrosion resistance, particularly against divisible phenomena such as high-temperature, high-concentration nitric acid. The purpose is to

[Means to solve the problem]

In order to achieve the above object, the present invention provides niobium containing 15-t%
It is characterized in that the content is 60% or less and the remainder is titanium and unavoidable impurities.

The reason why the present invention adds niobium to titanium is to utilize the corrosion resistance of niobium to improve the corrosion resistance of titanium against nitric acid, and at the same time, since niobium is a β-stabilizing element, adding niobium to titanium improves processability. The purpose is to transform it into β-type titanium, which has good properties.

The reason why the present invention sets the lower limit of the niobium content to 15 wt% is because the target corrosion resistance and processability cannot be obtained below this. On the other hand, the upper limit of niobium content was set at 60 tons.
The reason for setting it as yt% is that even if more niobium is added than this, the corrosion resistance tends to deteriorate.

The price of niobium, although not tantalum, is quite high, so the alloy of the present invention is naturally a more expensive material than pure titanium, but it is possible to use scrap Ti-53°5Nb, a superconducting material, at a low cost. Therefore, the alloy of the present invention has a great advantage in terms of price.

〔Example〕

Next, the present invention will be explained based on specific examples.
An alloy of titanium and niobium is melted and produced by arc melting, heated to 900'C, hot rolled to a thickness of 7m, cold rolled to a thickness of 4m, and then processed in an inert atmosphere. After performing heat treatment at 700°C for 3 hours, the sample was heated again to 900°C for solution treatment and rapidly cooled, and this was used as a test material of the alloy of the present invention.

The entire sample material was immersed in a boiling 50% nitric acid aqueous solution,
General corrosion tests were conducted and their cold workability was investigated. The test results are shown in Table 1.

Table 1 Results of corrosion test and cold workability test: X indicates very poor cold workability Δ indicates slightly poor cold workability O indicates good cold workability As can be seen from the corrosion test results in Table 1 of the alloys of the present invention, the corrosion rate of pure titanium without the addition of niobium is as high as 0.59 m/year, whereas the alloys of the present invention have a very good corrosion rate of 0.59 m/year. However, the value is lower than that,
It can be seen that the corrosion resistance against nitric acid is improved by adding niobium.

However, if the niobium content is less than 15 tyt%, the effect of adding niobium is hardly seen, and therefore it is understood that the lower limit of the niobium content needs to be 15 tyt%.

At the same time, when considering cold workability, it is found that the niobium content must be 15 wt% or more.

Furthermore, the composition with the best corrosion resistance of the alloy of the present invention has a niobium content in the range of 20wt% to 30st%, and adding more niobium than this does not improve the corrosion resistance, but on the contrary, it deteriorates. If the niobium concentration exceeds 60-t%, the corrosion resistance will be the same as that of pure titanium. This shows that the upper limit of the niobium content needs to be 60 tgt%.

Next, Table 2 shows the results of adjusting the corrosion rate when chlorine ions were present at the same time.

Table 2 Since scrap gold in nitric acid and chloride ion aqueous solution can be used at low cost, it is also economically advantageous.

As can be seen from this table, even in the presence of chlorine ions, the corrosion resistance of the alloys of the present invention hardly changes, indicating that there is no problem even in the presence of halogen ions. Of course, no pitting corrosion or stress corrosion cracking was observed in this environment.

〔Effect of the invention〕

Claims (1)

[Claims] (1) A titanium alloy for nitric acid-resistant equipment, characterized in that niobium is 15 wt% or more and 60 wt% or less, and the balance is titanium and unavoidable impurities.