JP4678612B2 - Surface-modified stainless steel and processing method for surface-modified stainless steel - Google Patents

Description

  The present invention relates to a surface modified stainless steel that can effectively prevent corrosion resistance, particularly stress corrosion cracking, by surface modifying stainless steel, particularly stainless steel used in nuclear power plants, chemical plants, and the like, and a processing method thereof. About.

  It is well known that the corrosion resistance of stainless steel is due to the action of an extremely thin passivation film formed on the surface, and this passivation film is also easy to use under adverse environmental factors. It is well known that it is destroyed by corrosion and loses its corrosion resistance.

Various stainless steels such as austenitic stainless steel are used for reaction towers and piping of chemical plants such as piping used in nuclear power plants and petroleum refining plants.
However, stainless steel having excellent corrosion resistance is also stress corrosion cracking (hereinafter referred to as “SCC”) if an adverse atmosphere such as external stress or residual stress is applied as tensile stress in an atmosphere of chlorine ions or hydrogen ions. ) Are likely to occur, and various problems are caused by SCC.

Various methods for preventing SCC of stainless steel have been proposed. For example, Patent Document 1 discloses a method of forming a coating film made of chromium on the surface of austenitic stainless steel to prevent corrosion of stainless steel, such as carbon and titanium. A method of supplying a plating solution to the tip of a brush tip tool that becomes an anode of the electrode, conducting a current between the anode and the object to be plated made of stainless steel, and performing electroplating while applying the plating solution to the surface of the object to be plated Has been proposed.
Patent Document 2 discloses a method for preventing SCC of stainless steel by applying a corrosive ion adsorbent on the surface of stainless steel.

Patent Documents 3 to 5 include an electropolishing technique related to the present invention, and Patent Document 3 is a finishing method of a metal workpiece made of SUS630, after cleaning a metal workpiece subjected to machining such as cutting and grinding. Electrolytic deburring, followed by aging treatment, followed by electropolishing, forming a passivated film on the surface of the metal workpiece by electropolishing, and helping to prevent SCC by improving corrosion resistance Has been.
Patent Document 4 relates to a surface treatment for removing a Cr-deficient layer and a work hardened layer, which is a surface layer of a nuclear member, which is considered to be a cause of occurrence of SCC with low stress. This is a nuclear member surface treatment method for performing any one of pickling, grinding, electropolishing, electric discharge machining, surface cutting, surface reduction / softening, wet blasting, and laser machining.
Patent Document 5 has been previously proposed by the present applicant, and fluorine and oxygen are diffused and infiltrated into the surface layer portion of stainless steel in an ionic state to form a fluorine-containing and oxygen-based film, thereby preventing pores. Stainless steel with improved corrosion resistance and a method for producing the same are disclosed.

Japanese Patent Application Laid-Open No. 09-279388 JP 2004-360007 A Japanese Patent Laid-Open No. 2001-3200 JP 2003-202391 A JP 2003-82495 A

Patent Document 1 relates to a plating method and has a problem that it is difficult to uniformly plate the entire surface of stainless steel. Patent Document 2 is a method of applying a treatment agent containing a corrosive ion adsorbent, which is good in workability, but sufficient for preventing SCC under adverse conditions where tensile stress is applied. It is hard to say that there is.
In Patent Document 3, a metal workpiece made of stainless steel is washed, electrolytic deburred, then subjected to aging treatment, and then electropolishing with an aqueous solution of phosphoric acid alone, making the process complicated and expensive. Limited to processing of processed products.
Patent document 4 heat-treats the nuclear member after bending, and performs surface treatment by any one of polishing such as pickling, grinding, electrolytic polishing, electric discharge machining, surface cutting, surface reduction / softening, wet blasting, and laser machining. Although it has been shown to be possible for all technologies, it is difficult to say that the SCC prevention effect has been fully elucidated. There is also a problem that the process is complicated. In Patent Document 5, an electrochemical method is used to form a fluorine-containing and oxygen-based coating layer on the surface of stainless steel to further improve the pitting corrosion resistance by chlorine, thereby preventing SCC. Is not well explained.

  The present invention has been made to solve the above-described problems, and as a result of earnest research based on the above-mentioned Patent Document 5 previously proposed by the present applicant, a special element is included on the surface of stainless steel. An object of the present invention is to provide a surface-modified stainless steel having a dramatically improved SCC resistance and a method of treating the same by diffusing and infiltrating in an ionic state using an electrolytic solution and performing a modification treatment on the surface of the stainless steel. And

  In order to achieve the above object, the invention according to claim 1 is characterized in that boron or boron and fluorine or these and oxygen are diffused and infiltrated into the surface layer portion of stainless steel in an ionic state, thereby containing boron or boron. It is characterized by surface-modified stainless steel in which a fluorine or oxygen-based coating layer is formed to prevent corrosion resistance, particularly stress corrosion cracking.

  Invention of Claim 2 is the processing method of the surface modified stainless steel of Claim 1, Comprising: Stainless steel is made into the direct current anode which made the stainless steel in the direct current | flow anode, or the alternating current pole, or the alternating current on the direct current. Connected to the anode side of the current, boric acid aqueous solution alone or mixed aqueous solution obtained by adding hydrofluoric acid to boric acid or organic acid or inorganic acid or water-soluble salts thereof with boric acid or boric acid A solution in which one or more of hydrofluoric acid or sodium, potassium or ammonium salt thereof is added and added is used as an electrolytic solution, and the stainless steel is subjected to electrolytic treatment, whereby the stainless steel surface layer is boron-containing, boron-containing and fluorine, or these. An oxygen-based coating layer is formed on the surface-modified stainless steel treatment method.

  According to the first aspect of the present invention, it is very easy to perform electrolytic treatment on the surface layer of stainless steel used in piping or reaction towers of nuclear power plants or chemical plants, or where SCC is expected to occur. By such means, boron-containing or boron-containing which is effective for SCC resistance, and fluorine, or these and oxygen can be diffused and infiltrated in an ionic form to form an oxygen-based film on the surface layer of the stainless steel, and the surface can be modified. Therefore, it is possible to obtain a surface-modified stainless steel that can effectively prevent SCC, which has been considered variously, but has been considered extremely difficult.

  According to the invention described in claim 2, in order to form the boron-containing or fluorine-containing and fluorine or the oxygen-based coating according to claim 1, the surface of the stainless steel is electrolytically treated using the most effective electrolytic solution. Surface-modified stainless steel that can prevent corrosion resistance, particularly SCC, can be easily treated.

The applicant of the present invention uses a liquid electrolyte containing a neutral salt of hydrofluoric acid, and surface-treated stainless steel penetrates and diffuses fluorine in addition to oxygen from the surface to a depth of several tens of millimeters. From this, it was clarified that the pitting corrosion resistance by chlorine is improved.
As a result of further research based on this fact, the present invention has found a surface-modified stainless steel that can prevent corrosion resistance, particularly SCC, and a treatment method thereof.

  A preferred embodiment of the present invention uses a 2B material of SUS304, which is a typical stainless steel among austenitic stainless steels, bent into a U shape with a diameter of 16 mm in accordance with JIS standard G0576, and after buckling, Further, it was tightened with bolts and nuts so as to be narrower by 5 mm, and a stainless steel (test piece) was prepared so that a tensile stress was applied to the convex portion of the 2B material. Of course, the 2B material and the bolt and nut are insulated.

  For the above stainless steel bent in a U shape and subjected to tensile stress, boric acid solution alone or a mixed solution obtained by adding hydrofluoric acid to boric acid or organic acid or inorganic acid or its water-soluble salts boric acid Alternatively, an AC / DC multiplying solution in which boric acid and hydrofluoric acid or one or more of sodium, potassium, and ammonium salts thereof are mixed is used as an electrolyte, and the stainless steel to be treated is placed on a direct current anode or on a direct current. While connected to the anode side of the current or one pole side of the AC power supply, it is immersed in the above electrolyte and energized with a sparingly soluble electrode made of stainless steel, graphite, tungsten, molybdenum, or the like as a counter electrode. The immersion electrolytic method is used, or as another method, the stainless steel to be treated is connected to one pole of the power source and placed on the surface of the stainless steel In the state in which a hydrated or non-woven fabric made of natural or synthetic or artificial fibers (hereinafter referred to as “mop”) is interposed between the counter electrode and the electrolyte solution is impregnated in the mop. To move while sliding on the surface of the stainless steel using an electrolytic treatment, or as another method, it is possible to use a direct current anode or the anode side of an AC / DC current obtained by superimposing alternating current on direct current or an alternating current power source The top surface of the stainless steel to be treated connected to one electrode side is covered with a mop soaked with the above electrolyte, and the counter electrode is placed on top of the mop so that the electrolytic treatment is performed, so that boron or boron And fluorine or these and oxygen are diffused and penetrated in the form of ions. Boron or boron and fluorine are penetrated from the surface layer to the inside of several tens of liters, and the corrosion resistance of stainless steel, particularly S In which it allowed to form an excellent surface modification layer C prevention.

In the above electrolytic solution, the boric acid aqueous solution alone is effective from 0.01 Wt% to the saturated concentration, but practically 0.05 to 2.0 W t% is preferable.
Further, hydrofluoric acid is effective up to a saturation concentration of 0.01 Wt% or more, and is practically about 0.05 to 2.0 Wt%.

  Next, as an evaluation test for stress corrosion cracking, the stainless steel surface bent into a U-shape and subjected to tensile stress was subjected to electrolytic treatment as described above, and the surface was modified, followed by 38 according to JIS G0576. To 42 Wt% of a magnesium chloride aqueous solution was immersed in an untreated 2B material that was not subjected to any electrolytic treatment, heated for 30 minutes to several hours, and the state of SCC was observed. Furthermore, a comparison was made with stainless steel subjected to surface modification by electrolytic treatment using a commercially available electrolytic solution.

Untreated stainless steel completely generates SCC in 30 minutes to 1 hour, but the above-mentioned stainless steel surface-modified to boron-containing, boron-containing and fluorine stress-cracked cracks in about 3 to 5 hours Did not occur. However, minute stress corrosion cracking may occur depending on the electrolytic treatment conditions.
Further, it was found that stainless steel subjected to electrolytic treatment using a commercially available electrolytic solution generated SCC in about 30 minutes to 1 hour, and was not significantly different from the case of untreated.
These can be sufficiently observed by visual observation, but can be confirmed by a micrograph.

The materials in the examples described below were all SUS304 2B material of a typical stainless steel type among austenitic stainless steels.
In accordance with JIS G0576, stainless steel with a bending tester bent to a U-shape with a diameter of 16 mm, buckled, and tightened with bolts and nuts to further narrow 5 mm, and tensile stress is added to the convex part (Hereinafter referred to as “test piece”). This test piece was used for all the following descriptions.

  Using an electrolytic solution made of an aqueous boric acid solution adjusted to a concentration of 0.01 to 5.0 Wt%, the entire surface of the test piece was connected to an AC / DC power source and covered with the mop described above for several seconds to several tens of hours. It was slid for 2 seconds and subjected to electrolytic treatment.

Subsequently, the test piece subjected to electrolytic treatment and the test piece not subjected to electrolytic treatment at all were immersed in a boiling magnesium chloride aqueous solution having a concentration of 42 Wt% described in JIS G0576, and the state of stress corrosion cracking was observed. The untreated specimen was partially cracked after 30 minutes, and cracked over the entire width direction after 1 hour and 20 minutes. It broke after 2 hours and 45 minutes.
FIG. 1 shows a photomicrograph of the untreated specimen, which was taken of the cracking situation after 1 hour and 20 minutes.

On the other hand, the test piece electrolytically treated with an aqueous boric acid solution did not crack at all even after 2 hours and 45 minutes had elapsed.
However, when the test piece was further immersed in an aqueous magnesium chloride solution and heated, microcracks occurred after 3 hours and 30 minutes.
FIG. 2 is a photomicrograph showing the state of microcracking in this case.
Then, when various tests were performed by changing the concentration of the electrolytic solution, it was found that the concentration of the electrolytic solution was about 0.05 to 2.0 Wt% and the effect on SCC resistance was great.

Regarding the above-mentioned 42% magnesium chloride, the environment is considered to be too severe with respect to the applied stress. Next, the magnesium chloride concentration was set to 38 Wt%, and the electrolytic treatment was performed under exactly the same conditions as in Example 1, When the test for corrosion cracking was performed, the untreated test piece did not crack even after 30 minutes, and a minute crack occurred after 1 hour, and further cracking occurred across the entire width direction after 3.5 hours. Occurred.
FIG. 3 shows a photomicrograph of the state of cracking after 3.5 hours.

  On the other hand, in the test piece subjected to the electrolytic treatment using the boric acid aqueous solution, cracking did not occur at the time when 5 hours passed in any case of the treatment time of several seconds to several tens of seconds.

A mop soaked with the same electrolyte as in Example 1 was slid using a mixed solution of 0.05 to 2.0 Wt% boric acid aqueous solution and 0.05 to 2.0 Wt% hydrofluoric acid aqueous solution. The electrolytic treatment was performed for several seconds to several tens of seconds. Then, when immersed in a 38 Wt% magnesium chloride aqueous solution together with the untreated test piece and heated, a minute crack occurred after 1 hour for the untreated test piece, but for the test piece subjected to electrolytic treatment, In either case, no cracks occurred even after 5 hours.
FIG. 4 is a photomicrograph showing a situation in which cracking does not occur even after 5 hours when the electrolytic treatment time is 10 seconds.

  Connected to an AC power source using an electrolytic solution in which 0.05 to 2.0 Wt concentration of boric acid and 0.05 to 2.0 Wt% sodium fluoride are mixed with water-soluble salts of organic acids, bent into a U-shape In accordance with the shape of the convex portion of the test piece obtained, a non-woven fabric soaked with an electrolytic solution was covered, and an electrode was placed thereon, and electrolytic treatment was performed for several seconds to several tens of seconds. Thereafter, as described above, the specimens were immersed in a 38 Wt% boiling magnesium chloride aqueous solution and the state of stress corrosion cracking was observed. In all cases, no cracking occurred after 5 hours.

  When the same electrolytic solution as in Example 3 was connected to a DC power source and subjected to immersion electrolytic treatment for several seconds to several tens of seconds, a stress corrosion cracking test was conducted, and cracking occurred even after 5 hours. There wasn't.

Example 6 (comparative example)

In order to see a comparison with a commercially available electrolytic solution, the test piece immersed in the nitric hydrofluoric acid solution A and the acidic B electrolytic solution were connected to a power supply unit consisting of an AC / DC current, and the electrolytic solution was used for the test piece. As a result of conducting a stress corrosion cracking test after moving by sliding with a mop soaked in the water and performing an electrolytic treatment, a minute crack is seen after 50 minutes for A and 60 minutes for the B electrolyte. Later, cracks were seen on one side in the width direction.
FIG. 5 shows a photomicrograph of stress corrosion cracking in the case of nitric hydrofluoric acid treatment A.

  The above is about corrosion resistance, especially stress corrosion cracking, but when tested for pitting corrosion resistance by chlorine, which is thought to be related to stress corrosion cracking, explanation is omitted but stress corrosion cracking Similarly, it was found that pitting corrosion resistance has a great effect.

  As described above, it has been found that the stress corrosion cracking due to the surface modification of the stainless steel according to the present example has a remarkable effect that has not been conventionally shown in any of the electrolytic solutions.

  In the above-mentioned invention, by modifying the surface of stainless steel, the corrosion resistance of one rank or more can be waited and stress corrosion cracking can be prevented. Therefore, piping of nuclear power plants and chemical plants exposed to severe environments, etc. Available to:

It is a microscope picture which shows the condition of the crack after 1 hour 20 minutes progress about the stress corrosion cracking test about the untreated test piece which concerns on Example 1 of this invention. It is a microscope picture showing the state of a microcrack after the stress corrosion cracking test 3 hours 30 minutes about the test piece after the electrolytic treatment which concerns on Example 1 of this invention. It is a microscope picture which shows the crack after the stress corrosion cracking test 3.5 hours about the untreated test piece which concerns on Example 2 of this invention. It is a microscope picture which shows the condition where the crack has not generate | occur | produced about the test piece after the electrolytic treatment which concerns on Example 3 of this invention even after the stress corrosion cracking test 5 hours. It is a microscope picture which shows the micro crack after the stress corrosion cracking test 50-minute progress about the nitric-hydrofluoric-acid solution A which concerns on Example 6 (comparative example).

Claims (2)

Mixing 0.05 to 2.0 Wt% boric acid aqueous solution or 0.05 to 2.0 Wt% boric acid aqueous solution with 0.05 to 2.0 Wt% hydrofluoric acid aqueous solution to the surface layer of stainless steel Electrolytic treatment is performed using a liquid, and boron or boron and fluorine or these and oxygen are diffused and permeated into the surface layer portion of the stainless steel in an ionic state, thereby boron-containing or boron-containing and fluorine or oxygen thereof. Surface-modified stainless steel characterized in that it has a coating layer to prevent corrosion resistance, especially stress corrosion cracking It is a processing method of the surface-modified stainless steel according to claim 1, wherein the stainless steel is connected to a direct current anode or an alternating current pole, or to an anode side of an AC / DC current obtained by superimposing alternating current on direct current, 0.05 to 2.0 Wt% boric acid aqueous solution alone or 0.05 to 2.0 Wt% boric acid added with 0.05 to 2.0 Wt% hydrofluoric acid between other conductive counter electrodes 0.05-2.0 Wt% boric acid or 0.05-2.0 Wt% boric acid and 0.05-2.0 Wt% hydrofluoric acid or its aqueous solution or organic acid or inorganic acid or water-soluble salt thereof A solution in which one or more of sodium, potassium and ammonium salts are mixed and added is used as an electrolytic solution, and stainless steel is subjected to electrolytic treatment to form boron-containing or boron-containing and fluorine or an oxygen-based coating layer thereof on the stainless steel surface layer. Treatment method for the surface modification of stainless steel, which comprises causing made.

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