JPS6310217B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention exhibits excellent pitting corrosion resistance in an environment where bromide ions exist, particularly in a high temperature and high pressure environment, and has good moldability, which is an essential property as a constituent material for chemical equipment and equipment. This relates to Ti-Mo alloys. [Prior Art] Ti is a metal with good corrosion resistance, especially in environments where halogen ions are present, so it is widely used as a material for process equipment exposed to the above environments. It is becoming. For example, stainless steel is generally used as a material for corrosive environments, but in harsh environments where even stainless steel cannot provide anti-corrosion effects, we have no choice but to rely on Ti or Ti alloys, and now Ti
has become an important material that supports entire industrial processes. However, the corrosion resistance of Ti cannot be said to be perfect at any time and under any circumstances, and because it is used in environments with harsh corrosive conditions, there are various problems with Ti's corrosion resistance. It has been pointed out. The problem with Ti corrosion is that it occurs and progresses locally (i.e. localized corrosion) rather than all over the place, and in particular, crevice corrosion occurs in environments where chlorine ions are present, particularly in high-temperature environments, so special attention must be paid to this. are collecting. The next problem is pitting corrosion in environments where bromide ions are present, and one example is an accident caused by pitting corrosion in a high-temperature, high-pressure reaction vessel using bromide as a catalyst. The former type of crevice corrosion occurs when a very narrow gap structure is formed on the surface of a metal material, whereas the latter type of pitting corrosion does not necessarily require the existence of a gap structure for its occurrence. Rather, it is a phenomenon in which almost the entire surface of the material (for example, 99% or more) is in a healthy, non-corroded state, but only very localized areas are corroded, resulting in perforation. Therefore, the occurrence of pitting corrosion is overlooked without being noticed, and an accident suddenly occurs before any countermeasures can be taken.The importance of establishing means to prevent pitting corrosion is therefore fully recognized. However, the occurrence of pitting corrosion is thought to be due to a completely different mechanism from that of crevice corrosion.
It is said that the methods that have been shown to be highly effective in preventing crevice corrosion cannot be used as is, and that it is necessary to develop unique and effective methods. Measures to prevent pitting corrosion can be broadly divided into approaches from the perspective of controlling the operation of the equipment and approaches that involve improving the material itself. The former approach has a promising result of alleviating the harshness of the operating environment, but this direction of relaxation has its own limitations as it tends to sacrifice the efficiency of the chemical process itself. Moreover, recent chemical processes are progressing toward harsher corrosion conditions, and many situations are encountered that prevent the application of Ti. Therefore, the addition of pitting corrosion inhibitors (so-called inhibitors) has been considered, and anions such as sulfate ions, nitrate ions, and phosphate ions have been found to be effective. However, the addition of an inhibitor causes disadvantages such as a reduction in reaction yield due to contamination of the process itself, and is therefore unsuitable for widespread use. On the other hand, as an approach from the material standpoint, there is a ``nitric acid treatment method for Ti surfaces'' proposed by the present inventors (Japanese Patent Application Laid-open No. 39785/1985). This method is based on anti-corrosion treatment before operation of the equipment, so
Not only is there no negative impact due to contamination of process fluids, but
The pitting corrosion resistance itself is also unique in that it is stably exhibited regardless of the type of halogen ion. However, since the process involves the use of a large amount of nitric acid (particularly hot nitric acid) at the material stage or at the stage after equipment processing, there are some restrictions in actual operation. Incidentally, the mechanism of pitting corrosion of Ti caused by halogen ions is thought to originate from local anode destruction of the passive film, as will be explained in detail later. Therefore, the pitting corrosion resistance of a Ti material should be able to be evaluated based on the breakdown voltage of the passive film, and it can be judged that the higher the breakdown voltage is, the greater the pitting corrosion resistance is. Therefore, this breakdown voltage can also be referred to as the pitting corrosion potential. Regarding the influence of chlorine ions among halogen ions, it is known that the potential for pitting corrosion can be increased by using a Ni-containing Ti alloy [Desalination 3 269-279 (1967)] However, the present invention According to their research, it was found that the pitting corrosion potential of Ni-containing Ti alloys did not increase as expected in an environment where bromine ions were present. [Problems to be Solved by the Invention] It has been found that although chlorine ions and bromide ions are both halogen ions, they exhibit completely different aspects in terms of the role of Ni in preventing pitting corrosion. Therefore, the present inventors elucidated the difference between the pitting corrosion generation mechanism in a chloride ion environment and the bromine ion environment, and also investigated the pitting corrosion prevention function by adding alloying elements. By investigating the differences in the behavior of these alloying elements in the environment and conducting experiments with various alloying elements, we searched for an alloying element that can effectively prevent pitting corrosion in a bromide ion environment. We also investigated moldability, which is an essential characteristic when used industrially as a component material for chemical devices and equipment, and
The appropriate content of O ₂ was clarified. That is, the object of the present invention is to effectively prevent pitting corrosion in the presence of bromide ions,
The present invention also provides a Ti alloy with good moldability. [Means for solving the problem] The alloy suitable for the above purpose contains 0.2 to 3.0% Mo,
Among the impurity elements, the upper limit for Fe is 0.1%, and the O ₂
satisfies the following relational expression for the Mo content (%), O ₂ (%) ≦9/35-1/28・Mo (%) The gist is that the remainder essentially consists of Ti. . [Operation] As mentioned above, the pitting corrosion occurrence and progression mechanism of Ti in an environment where halogen ions are present is that the passive film that should ensure the corrosion resistance of Ti is locally destroyed and bare Ti is exposed. It all stems from this.
This destruction of the passive film occurs when the anodic polarization occurs due to the oxidizing power of the environment, and it is thought that from then on, only the anodic destruction portion will be rapidly corroded. Therefore, in order to understand the pattern of pitting corrosion occurrence more accurately, we tried to understand it in a model using the anode polarization curve (schematic diagram: Figure 6) according to the electrochemical corrosion theory, and found that the following It can be analyzed in various ways. In other words, if you start from a natural potential (immersion corrosion potential) and gradually increase the potential to the positive side, there will be a point where the current, which remains almost flat, will suddenly increase after reaching a certain potential. can be defined as the potential for pitting corrosion to occur, which is determined by the combination of the material and environmental factors. Since anodic polarization does not occur at a potential lower than the pitting potential, the passive film remains healthy and prevents pitting from occurring. ) Destruction of the passive film occurs and pitting corrosion occurs. In other words, the magnitude of the pitting corrosion potential under given environmental conditions is the most important parameter for evaluating pitting corrosion resistance, and this means that as the pitting corrosion potential increases, the pitting corrosion resistance improves. . Therefore, we created test Ti alloys by adding various alloying elements and immersed them in a high-temperature, high-pressure aqueous solution containing bromide ions to measure the pitting corrosion potential of each alloy.We found that the pitting corrosion potential of the Mo-containing Ti alloy was I found that it was particularly high. However, if the Mo content is less than 0.2% by weight, the effect of preventing pitting corrosion is weak, so 0.2% by weight was set as the lower limit. As the Mo content increases, the pitting prevention effect also increases, but the effect reaches saturation when it reaches 3.0% by weight. This is in the passive film.
This is probably because the effect of preventing pitting corrosion is maximized at that point due to the concentration of Mo or the concentration of Mo ions eluted in small amounts near the surface. Since adding more than 3.0% by weight of Mo will have a negative effect on processability and economic efficiency as an industrial material, 3.0% by weight was set as the upper limit. The above-mentioned effect is unique to Mo, and Ni, which has been recognized to have a pitting corrosion prevention effect in a chloride ion environment, was completely ineffective in a bromide ion environment. The difference between chlorine ions and bromide ions and the difference between Ni and Mo can be considered as follows. The pitting potential in bromide ions is considerably lower than that in chlorine ions, and the passive film is more likely to be destroyed. Therefore, when considering pitting corrosion in bromide ions, important factors are not only the properties (structure and composition) of the passive film, but also the nucleation site of pitting corrosion caused by condensation and discharge of bromide ions. However, in chlorine ions, the passive film is destroyed when it grows thick, so the film properties themselves become the dominant factor, and the nucleation site has little effect. On the other hand, since the Ti intermetallic compound acts preferentially as a nucleation site for pitting corrosion, Ni and Co, which are eutectoid alloying elements, easily provide nucleation sites, and themselves form a passive film. Even if it has properties improvement effects, they cancel each other out,
As a result, the effect of improving pitting corrosion resistance is no longer observed. On the other hand, since Mo is a solid solution-forming element and does not provide nucleation sites, it seems that the effect of improving the properties of the passive film is effectively exerted as it is. According to the experimental results of V and W, which are also solid solution forming elements, the pitting corrosion resistance effect was not so remarkable. The reason for this is thought to be that each element exhibits its unique characteristics in functions such as adsorption of bromide ions or discharge suppression, and among solid solution elements, Mo exhibits pitting corrosion resistance in a bromide ion environment. It was surprising to discover that this was a unique ability. By the way, when Mo is added to Ti, the above 0.2 to 3.0
Even if it is added in a relatively small amount on the order of %, there is an unavoidable tendency for the material strength to increase concomitantly and the ductility to decrease to some extent due to the specific effects of Mo. Therefore, in the present invention, in order to ensure moldability as an industrial device material, various studies were conducted and experiments were conducted focusing on the amounts of Fe and O ₂ as impurity elements. First, Ti-Mo alloy with different Fe content (Mo: 0.2 to 3.0
When a bending test was performed on a plate of %), Mo
They found that bendability deteriorates when Fe exceeds 0.1%, rather than depending on the amount of Fe. From this, they learned that sufficient ductility can be obtained for practical use as long as the upper limit of Fe content does not exceed 0.1%. . It is presumed that this restriction on the amount of Fe is related to the precipitation of intermetallic compounds (TiFe). Next, I changed the amount of _O2 .
When we conducted similar studies on Ti-Mo alloys, we found that the upper limit of the O ₂ content has a correlation with the Mo content. That is, it was found that as the amount of Mo increases, the upper limit value of the amount of O ₂ needs to be lowered according to the following relational expression. O ₂ (%) ≦9/35-1/28・Mo (%) Although it is not clear in detail why such constraints from moldability on the amount of O ₂ appear in relation to the amount of Mo, Considering that O ₂ is an α-phase (dense hexagonal crystal) stabilizing element and Mo is a β-phase (body-centered cubic crystal) stabilizing element, it is unlikely that each effect is due to a correlation with each other. It is assumed that there is no such thing. The basic structure of the present invention is as described above, but in order to more reliably obtain good formability as a wrought material among the above-mentioned required properties, it is desirable to appropriately set the final annealing heat treatment conditions. In other words, the appropriate conditions are heating and holding temperature of 700℃.
℃ or more and below the β transformation point (first condition), and the cooling rate is slower than 500℃/min (second condition)
It is something. The reason is that in terms of temperature, 700℃
If it is below the β transformation point (temperature at which α + β two-phase structure transforms to β single-phase structure: it varies somewhat depending on the content of Mo, the main alloying element), then the temperature This is because, when cooled from 100 to 100 mL, a uniform α+β structure is not formed, but a structure containing an acicular α phase, an unstable β phase, etc. is formed, and as a result, moldability is considered to be poor. On the other hand, regarding the cooling rate, if cooling is performed at a rate of 500° C./min or higher, since the alloy contains Mo, it is thought that the moldability will deteriorate due to a kind of quenching effect. [Examples] Example 1 Using sponge Ti, Ti powder, and Mo powder as raw materials, a Mo-containing Ti alloy (Mo content: 0 to 8% by weight) was melted using a vacuum arc melting furnace. The obtained ingot was hot-rolled after hot forging, and further subjected to cold rolling and annealing heat treatment to obtain a 2 mmt alloy plate. cut this
A square plate of 20 mm x 20 mm was obtained, and a Ti lead wire was spot welded to make an electrode for measuring the pitting corrosion potential (for measuring the anode polarization curve). An aqueous solution with a bromide ion concentration of 1% (NaBr equivalent) was placed in an autoclave for electrochemical testing and heated to 140°C.
The electrode was immersed under two conditions: and 200°C, and the pitting corrosion potential was determined. A Pt plate was used as the counter electrode, and an external Ag/AgCl electrode was used as the reference electrode, and the measurement was carried out according to the potential scanning method using an automatic potentiostatic electrolyzer. The results are shown in Table 1.

[Table] As shown in Table 1, when the Mo content exceeds 0.2%, the pitting corrosion generation potential increases rapidly and exhibits a pitting corrosion resistance effect, but this effect reaches saturation at around 3% Mo content. reach Example 2 Using the same test piece as in Example 1, the pitting corrosion potential was determined in the same manner. However, the temperature was 200°C, and the bromide ion concentrations were 0.1% and 5%. The results are shown in Figure 1, and the pitting corrosion potential became significantly higher when the Mo content was 0.2% or more. Example 3 Alloy plates with different amounts of Fe based on Ti-0.5%Mo, Ti-2%Mo, and Ti-3%Mo ( _O2 amount is 0.05 to 0.06
% level), bending R = 2.5 times the plate pressure,
A bending test was conducted with a bending angle of 180°. The results are shown in Table 2 (full results) and Figure 2 (data based on 2%Mo).If the Fe content exceeds 0.1%, cracks will occur at the top of the bend, or
It began to break during 180° bending, and deterioration in workability was observed. It was separately confirmed that there was no significant difference in pitting potential in the Fe 0.1% or less range.

[Table] × Fracture Example 4 Alloy plates with varying amounts of O2 based on Ti-0.5%Mo, Ti- ₂ %Mo, and Ti-3%Mo (Fe amount is 0.04 to 0.05
% level), the same bending test as in Example 3 was conducted. The results are shown in Table 3, and an increase in the amount of O ₂ causes deterioration in bendability, but the upper limit is
It changed depending on the amount. (The more Mo there is, the lower the upper limit is.) In other words, as shown in Figure 3, the upper limit of O ₂ amount is
There is a nearly linear relationship with the amount of Mo, and it is clear that the amount of O ₂ must be kept within this range from the viewpoint of moldability. It was separately confirmed that there was no significant difference in the pitting corrosion potential within this O ₂ content range, and that the effect of Mo was dominant.

【table】

[Table] ○ Good bendability (no cracks)
△Crack occurrence × Fracture Example 5 A cold rolled plate of Ti-2%Mo-0.04%Fe-0.05% _O2 material (β transformation point of this material: 882℃) was annealed under various heat treatment conditions (temperature and cooling rate). After that, a 180° bending test was conducted. At this time, some tests were also conducted on materials whose Fe and/or O ₂ amounts were outside the specified range. The results are shown in Table 4 (full results), Figure 4 (data showing the relationship between heating temperature and bendability), and Figure 5 (data showing the relationship between cooling rate and bendability). It is clear that good bendability can be obtained if the cooling rate is appropriate. However, if the amount of Fe and/or O ₂ was outside the specified range, the workability was poor even if the temperature and cooling rate were appropriate. It has been separately confirmed that the pitting potential of materials within the specified alloy composition ranges hardly changes depending on the annealing conditions.

【table】

[Table] △ Crack occurrence × Fracture [Effect of the invention] The present invention is constructed as described above, and by containing Mo, the pitting corrosion resistance of Ti alloy in a bromide ion environment is significantly improved, and the impurity element of
By specifying the upper limits of Fe and O ₂ amounts, we succeeded in improving moldability without affecting pitting corrosion resistance.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between Mo content and pitting potential in Ti-Mo alloys, and Figure 2 is a graph showing the relationship between Mo content and pitting potential in Ti-Mo alloys.
A graph showing the relationship between bendability and Fe content in a 2% Mo alloy. Figure 3 is a graph showing the correlation between the _O2 content (upper limit) and Mo content and the appropriate range regarding the bendability of a Ti-Mo alloy. , FIG. 4 is a graph showing the relationship between bendability and annealing temperature, FIG. 5 is a graph showing the relationship between bendability and cooling rate, and FIG. 6 is a schematic diagram of an anode polarization curve.

Claims (1)

[Claims] 1 Mo: 0.2 to 3.0% (meaning of weight %, the same applies hereinafter)
and among the impurity elements, Fe: 0.1% or less O ₂ : An amount that satisfies the following relationship with Mo content (%) O ₂ (%) ≦9/35-1/28・Mo (%) A Ti-Mo alloy having excellent pitting corrosion resistance in a bromide ion environment and good moldability.