JPH06306653A - Improvement of corrosion resistance of welded site of stainless steel - Google Patents

Abstract

PURPOSE:To improve corrosion resistance at a welded site and its surroundings by welding an electrode-polished austenite stainless steel, immersing the welded site in an aq. soln. of nitric acid under heating, washing and heat-treating in a specified condition in an inert gas-containing oxygen. CONSTITUTION:The austenite stainless steel which is electrode-polished previously is welded, and the welded site is immersed in the aq. soln. of nitric acid of 20-40% concn. at 60-75 deg.C for several min-several ten min. When the surface treatment by the aq. soln. of nitric acid is ended, the welded site is washed by superpure water, then heat-treated at temp. within a 150-200 deg.C range for 1-2 hour in the inactive gas containing oxygen and dried and oxidized. In this way, the corrosion resistance comparable to that in the conventional method which executes electrode-polish after welding then forms oxidized film is imparted at the welded site and its surroundings.

Description

Detailed Description of the Invention

[0001]

This invention relates to a small amount of water-containing chlorine,
The present invention relates to a method for improving the corrosion resistance of a welded portion of austenitic stainless steel that comes into contact with a bromine-based gas and a heat history portion in the vicinity thereof.

[0002]

2. Description of the Related Art Stainless steel is superior in corrosion resistance to ordinary carbon steel because it has been improved in corrosion resistance by forming a passivation film mainly containing chromium. However, it is known that even when this stainless steel is welded, there is a problem that the heat resistance during the welding lowers the corrosion resistance of the welded portion and its peripheral portion. Therefore, in order to improve this, after the welding, mechanical polishing and subsequent chemical polishing, electrolytic polishing, and other treatments are performed again to re-form the passivation film on the surface whose corrosion resistance is reduced by welding.

In particular, electropolishing is a convenient means for electrochemically electrochemically forming a smooth surface by ionizing and dissolving a metal from a convex portion of the metal surface, and at the same time restoring the corrosion resistance by a passivation treatment such as oxygen oxidation. there were. Therefore, if the structure after welding is simple and the electrode can be introduced up to the welding site, after the surface treatment by post-welding electropolishing, natural oxide film formation and subsequent more appropriate oxidizing conditions as necessary We have obtained a welded stainless steel that can be expected to have sufficient corrosion resistance by measuring the growth of a specific oxide film and forming a passivation film.

[0004]

However, if the stainless steel structure after welding cannot be inserted into the electrode for electrolytic polishing, the above-mentioned prior art cannot be carried out, and it is necessary to adopt an alternative method. . That is, when the above-mentioned passivation film is reformed, it is difficult to apply mechanical polishing when the internal structure of the welding object is complicated, and even if it is applied, it is unavoidable that residual abrasive grains are produced and welded. There was a case where an unexpected obstacle in the usage of the structure was caused. For example, in a gas supply system including a filter 1, a valve 2, a mass flow controller 3 and piping as shown in FIG. 1, when these components and piping are welded and connected, electrolytic polishing after welding is almost impossible,
Even in mechanical polishing, the situation remains the same.

On the other hand, nitric acid treatment or reforming of the passive film on the welded portion by oxygen oxidation is often attempted because it is easy to carry out even if the structure of the welded portion is complicated. However, even with these methods, the corrosion resistance to hydrogen chloride, hydrogen bromide, chlorine, and chlorine and bromine-based gases used in the semiconductor industry cannot be said to be sufficient, which is also a problem.

Under the circumstances described above, the present invention achieves a simple passivation film formation without causing the above-mentioned obstacles and the like, regardless of the shape of the object for forming the passivation film after welding. In particular, it is an object to improve the corrosion resistance of the welded structure to chlorine and bromine-based gas containing a trace amount of water.

[0007]

[Means for Solving the Problems] Even if the structure of the stainless steel welded portion is complicated, it is convenient to adopt a treatment with a fluid in order to perform passivation film formation. However, as described above, the formation of the passivation film by the hot nitric acid aqueous solution after the simple welding of stainless steel was not sufficient from the viewpoint of improving the corrosion resistance to chlorine and bromine gas. Therefore, the present inventor has studied various methods of improving the corrosion resistance applicable to the complex structure of the stainless steel welded portion, and found that the welded portion of the stainless steel material before welding and the weld peripheral portion where heat input is expected are electrolytically polished in advance. As a result, the fact that the nitric acid treatment after welding and the subsequent washing with water and oxidation can form an oxide film having good corrosion resistance to chlorine and bromine-based gas was completed and the present invention was completed.

That is, according to the present invention, pre-electrolytically polished austenitic stainless steel is welded, and the welded portion is immersed in a nitric acid aqueous solution under heating, washed with water, and then in an oxygen-containing inert gas at 150 to 200 ° C. The present invention provides a method for improving the corrosion resistance of an austenitic stainless steel welded portion and its peripheral portion, which is characterized by performing heat treatment for 2 hours to 2 hours. According to the present invention, it is possible to obtain the same corrosion resistance to chlorine and bromine gas as the conventional electrolytic polishing of a welded portion of stainless steel and the subsequent passivation film forming treatment.

The present invention will be described in detail below. The electrolytic polishing solution used in the present invention contains phosphoric acid, sulfuric acid, and chromic acid in an amount of 40 to 4 each.
Those having a ratio of 5%, 34 to 37%, 3 to 4% can generally be used, but the present invention is not limited thereto and a wider range can be selected. For electrolytic polishing, a treatment liquid tank is selected in consideration of the form of the target stainless steel material, and the liquid temperature is 40 to 80 ° C. and the current density is 3
The electropolishing liquid may be circulated under the condition of 0 to 100 A / dm ² . Welding methods for electrolytically polished stainless steel before welding are TIG welding, plasma arc welding,
Nd: YAG welding and electron beam welding can be applied.

After welding, it is necessary to consider the shape of the structure and to select the dipping method so that the nitric acid aqueous solution comes into contact with the entire structure. That is, in order to prevent the dead space from being filled with the aqueous solution, for example, a method of adding the aqueous solution under reduced pressure, shaking in a closed container, or spraying the aqueous solution from a nozzle is used. From the viewpoint of final corrosion resistance, the concentration of the nitric acid aqueous solution is preferably 20 to 40%. When the concentration of nitric acid is lower than this, dissolution of the material is likely to proceed, and when the concentration of nitric acid is high, the effect is not exhibited. The treatment time and treatment temperature with the nitric acid treatment liquid are appropriately selected depending on the concentration of the nitric acid aqueous solution, but are generally several minutes to several tens minutes, and the temperature 60 to
It is recommended to be carried out at 75 ° C and near normal pressure.

After the surface treatment with the nitric acid aqueous solution, it is washed with clean water such as distilled water at room temperature, and it is particularly preferable to use ultrapure water having a low ionic content. The washing time is appropriately selected depending on the structure after welding, but it must be performed until the detection of nitrate ion is no longer recognized. Further, it is more preferable to use an ultrasonic cleaner to enhance the cleaning effect.

For drying and oxidation after washing with water, the treatment time is determined according to the oxygen content in the temperature range of 150 to 200 ° C. in the air or in an inert gas such as nitrogen and argon having various oxygen contents. However, 60 minutes is sufficient when in air. Conditions exceeding 200 ° C can also be adopted,
It is not preferable in appearance because it causes coloring. In the present invention, the chlorine and bromine-based gases refer to semiconductor gases such as chlorine, bromine, hydrogen chloride, hydrogen bromide and various chlorosilanes.

The above-mentioned austenitic stainless steel to be welded and its peripheral portion are electrolytically polished before welding,
Next, after the welding, the austenitic stainless steel welded body obtained by acid treatment with a nitric acid aqueous solution, followed by washing with water and drying in an oxygen-containing gas has improved corrosion resistance to moisture-containing chlorine and bromine-based gases, and its corrosion resistance is austenite. It has been confirmed that it is equivalent to the improvement of corrosion resistance by electrolytic polishing after welding of stainless steel.

[0014]

The present invention will be described in the following examples.

Example 1 In advance, a length of 150.0 mm, an inner diameter of 22.0 mm,
Stainless steel pipe with an outer diameter of 22.5 mm (SUS316L)
Two of them in an electropolishing solution containing 85% phosphoric acid and 96% sulfuric acid in a volume ratio of 1: 1 at a temperature of 40 ° C. and a current density of 100 A / d.
After performing electropolishing treatment for m ² and electrolysis time of 3 minutes, butt the two and rotate them to generate an arc between the tungsten electrode and plasma gas and argon gas as an arc / back shield gas. Plasma arc welding was performed.

After the welding, the connected stainless steel pipe was immersed in a 30% nitric acid solution at a temperature of 70 ° C. for 60 minutes to obtain 18M.
It was washed at room temperature with ultra pure water of Ω · cm until nitrate ions were not detected. Then, after drying this, the temperature in the atmosphere is 150 ° C.
And heat treated for 2 hours. A length of about 3 thus obtained
A 00 mm welded stainless steel pipe was axially cut into four pieces of the same shape. The piece was repeatedly washed with isopropyl alcohol, dried under reduced pressure at room temperature, and impregnated with an aqueous ferric chloride solution (2%, 4%, 8%, 10%) to obtain the same strip-shaped filter paper as the cut piece. Adhere to the inner surface of each cut piece, and place it in an atmosphere with a relative humidity of approximately 100% at 25 ° C.
It was installed for a time and subjected to a corrosion test.

After the lapse of 6 hours, the surface of each cut piece was washed with isopropyl alcohol and dried, and the presence or absence of pitting corrosion was observed under a microscope with the filter paper. The above test for changing the ferric chloride aqueous solution concentration performed here is a pitting corrosion test in which the electrode potential constituted by the hydrogen ion concentration (pH) based on hydrolysis corresponding to the solute concentration is kept constant, The superiority or inferiority of the pitting corrosion resistance of the cut piece can be determined from the critical concentration of ferric chloride that causes pitting corrosion.

As a result of the test, only in the case where a filter paper impregnated with an aqueous ferric chloride solution of 10% was closely attached, the welded portion of the cut piece, and both the heat history portion and the non-heat history portion thereof were detected. Although pitting corrosion was observed, no pitting corrosion was observed at any part in the test in an aqueous solution of 8% ferric chloride or less. As a comparative example, after omitting a series of steps such as acid treatment after welding, washing with water, and drying treatment in an oxygen-containing gas, pitting corrosion was observed in the welded stainless steel pipe at any ferric chloride aqueous solution concentration. It was

Example 2 FIG. 2 shows an all-stainless steel in-line filter for chlorine, bromine, hydrogen chloride, hydrogen bromide, chlorine such as dichlorosilane and trichlorosilane, and bromine type gas having a total length of about 5 to 60 mm. As is apparent from FIG. 2, the all-stainless steel in-line filter has a structure in which it is substantially impossible to insert an electrode for electrolytic polishing into the line.

In welding the component 8 shown in FIG. 2 and the component 10 having the filter portion 11 on the surface 9, the components 8 and 10 are previously mixed with 85% phosphoric acid and 96% sulfuric acid in a volume ratio of 1: 1. After performing electrolytic polishing treatment in an existing electrolytic polishing liquid at a temperature of 40 ° C., a current density of 100 A / dm ² , and an electrolysis time of 3 minutes, both parts are butted and rotated, and a plasma gas and an arc / back shield gas containing argon are used. Plasma arc welding was performed by generating an arc between the tungsten electrode and the gas. After welding, soak the in-line filter in a 30% nitric acid solution at a temperature of 70 ° C for 60 minutes, and
The sample was washed with 8 MΩ · cm ultrapure water at room temperature until nitrate ions were not detected, dried, and then heat-treated in the air at a temperature of 150 ° C. for 100 minutes.

Wet hydrochloric acid gas having a purity of 99.5% was applied to the in-line filter thus obtained at room temperature at a rate of 10 minutes per minute.
After continuing to flow at a rate of 00 ml for 1 hour, the residual hydrochloric acid gas was purged with nitrogen gas and then cut in the axial direction to observe the corrosion state of the welded portion. There wasn't.

[0022]

According to the present invention, it is possible to perform electrolytic polishing before welding even in a welded portion of austenitic stainless steel where the structure after welding is complicated and it is difficult to insert an electrode for electrolytic polishing, and its peripheral portion. The stainless steel structure is electrolytically polished in advance, and after welding, treated with warm nitric acid, washed with water and heated to oxidize, giving the same corrosion resistance as conventional post-welding electrolytic polishing and the subsequent oxide film formation method. There is an effect that is done.

[Brief description of drawings]

FIG. 1 is an overview diagram of a chlorine-based gas supply system showing an example of the presence of many welded pipes.

FIG. 2 is a cross-sectional view of an example of an in-line filter which is a target of the method of the present invention.

[Explanation of symbols]

 1 Filter 2 Valve 3 Mass Flow Controller 4 Weld Section 5 Chlorine Gas 1 6 Chlorine Gas 2 7 Inert Gas 8 Inline Filter Outer 9 Welding Site 10 Filter Support 11 Cylindrical Filter

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Go Wakabayashi 2-3-1 Nishishinjuku, Shinjuku-ku, Tokyo Within Tokyo Electron Co., Ltd. (72) Inventor Shuji Moriya 2-3-3 Nishishinjuku, Shinjuku-ku, Tokyo Within Tokyo Electron Limited

Claims (1)

[Claims] 1. A pre-electrolytically polished austenitic stainless steel is welded, and the welded portion is immersed in a nitric acid aqueous solution under heating, washed with water, and then heated in an oxygen-containing inert gas at 150 to 200.
A method for improving corrosion resistance of an austenitic stainless steel welded portion and its peripheral portion, characterized by performing a heat treatment at 1 ° C. for 1 hour to 2 hours.