JP2013185256A - Surface modification treatment method of stainless steel excellent in resistance against chlorine pitting corrosion and resistance against general corrosion and rust preventive property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface modification treatment method by which resistance against chlorine pitting corrosion and stress corrosion cracking property intrinsic to austenitic stainless steel and fatal general corrosion and rusting of martensitic stainless steel is prevented by subjecting austenitic stainless steel raw material used for various kinds of chemical plants and piping susceptible to corrosion trouble due to contact with chlorine gas and chlorine ion and kitchen sink or the like in daily personal life and martensitic stainless steel which features hardness as merit, however, is poor in corrosion resistance and is used for cutting knife, surgical instrument or the like to electrolytic treatment.SOLUTION: Surface modification treatment is performed which extremely improving resistance against chlorine pitting corrosion intrinsic to austenitic stainless steel and suppression of general corrosion and rusting which can be said as fate of the martensitic stainless steel by subjecting a stainless steel to electrolytic treatment with an electrolytic solution prepared by mixing a neutral salt containing a lithium compound, thereby, causing lithium or lithium and oxygen to diffuse or permeate in ion state over an inner part from a stainless steel surface layer part.

Description

  The present invention is a place where troubles due to seawater or chlorine have occurred or are expected to occur, for example, austenitic stainless steel used for chemical plants, aquariums, pools, sinks, etc. Stainless steel that strengthens the passive film of martensitic stainless steel used for instruments, shafts, etc., and significantly improves the suppression of pitting corrosion and rusting, which is the fate of martensitic stainless steel, which is unique to austenitic stainless steel The present invention relates to a method for surface modification treatment of steel.

In light of the fact that welding burn-off cleaning operations after past stainless steel welding depend on nitrofluoric acid, which is a very dangerous poisonous substance, as a result of research, the applicant has electrolyzed safe and harmless neutral salts. Developed an electrolytic burn-off method that uses a liquid and obtained a patent No. 1544367 under the name of “descaling method of alloy steel” to suppress the use of the conventional dangerous poisonous nitrile hydrofluoric acid, making it safer We have developed an electrolyzer that enables harmless burn-out.
The gist of the present invention is a direct current electrolysis method in which glycerol is mixed with a neutral salt solution of phosphoric acid, sulfuric acid, and hydrofluoric acid to form an electrolytic solution, and the treated stainless steel is used as an anode.
According to this method, the above-mentioned burn-out operation can be carried out very effectively. However, since there is a defect that a highly toxic high concentration hexavalent chromium elutes during electrolysis, this type of electrolytic solution is improved and commercially available. The company has developed an innovative and safe electrolyte that does not elute hexavalent chromium even when electrolytic treatment is performed with a simple DC power supply.

  In addition, the development of a revolutionary power supply for electrolytic treatment that has been improved so that hexavalent chromium does not elute completely even if a neutral salt electrolyte that is dangerous due to the elution of commercially available hexavalent chromium is used. Patent No. 1908719 has been acquired under the name of “method of removing scale accompanying welding of alloy steel”. The gist of the present invention is characterized in that an inorganic neutral salt solution is used as an electrolytic solution, and electrolytic treatment is performed with an AC / DC current obtained by superimposing an alternating current with an amplitude equal to or slightly higher than a direct current amplitude on a direct current. Therefore, the harmful hexavalent chromium eluted at the time of electrolysis is immediately reduced to trivalent chromium and has an excellent effect of detoxification.

The present applicant has made it possible to use an alternating current, which has been considered to be inapplicable as an electrochemical technique, because it is not possible to apply a conventional oxygen-based passivation film on the stainless steel surface in a conventional manner. By forming a strong passive film on the surface while maintaining the grace of the steel surface, welding and heat treatment adhered to the surface of the stainless steel while improving rather than impairing the inherent corrosion resistance of the stainless steel. Established a stainless steel surface cleaning and passivation treatment method that removes various foreign substances such as rust, oil, dirt, etc. in one step, and named the “cleaning and passivation treatment method for stainless steel surfaces” Based on the patent No. 3484525.
Patent No. 4218000 “Stainless steel on which fluorine-containing or fluorine-containing / oxygen-based coating layer is formed and its manufacturing method” also has a mechanism for forming a strong passive coating that was unknown in Patent No. 3484525. By elucidating and using electrochemical techniques, fluorine or fluorine and oxygen-based coating layers can be formed by diffusing and infiltrating fluorine or fluorine and oxygen into the surface of stainless steel or its vicinity. Due to the effects that can be obtained by forming a new coating that is superior in corrosion resistance compared to the oxygen-based passivating coating formed on the surface of conventional stainless steel, the inherent properties possessed depending on the type of stainless steel Corrosion resistance is further improved dramatically, especially from the occurrence of pitting corrosion due to chlorine, which is characteristic of austenitic stainless steel, to abnormal corrosion and stress corrosion cracking. Invented significantly improve problems were stainless steel and the manufacturing method.

In general, various stainless steels such as austenitic stainless steel are used for piping used in nuclear power plants and reaction towers and piping in chemical plants such as oil refining plants. Stainless steel with excellent corrosion resistance is also chlorine. If adverse conditions such as external stress and residual stress are applied as tensile stress in an atmosphere of ions, hydrogen ions, etc., stress corrosion cracking (hereinafter referred to as “SCC”) is likely to occur, causing SCC. Various corrosion accidents have occurred.
In Japanese Patent No. 4218000, 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 due to chlorine, thereby preventing SCC. Was not fully explained. As a result of intensive studies based on the above-mentioned Patent No. 4218000 in Patent No. 4678612 “Surface Modification Stainless Steel and Method for Processing Surface Modification Stainless Steel”, the applicant of the present invention, SCC resistance was drastically improved by forming boron-containing or boron-containing and fluorine or their oxygen-based coatings by diffusing and permeating boron or boron and fluorine or these and oxygen in an ionic state. The surface modified stainless steel and its treatment method were established, and new research was conducted over the entire surface modification of stainless steel, and great results were achieved.

Japanese Patent No. 1543867 Japanese Patent No. 1908719 Japanese Patent No. 3484525 Patent No. 4218000 Japanese Patent No. 4678612

In Patent Document 4, the mechanism for forming a strong passive film that was unknown in Patent Document 3 was also elucidated, and a fluorine-containing and oxygen-based film layer was formed on the surface of stainless steel by an electrochemical method to prevent chlorine-resistant pores. Established a method to remarkably improve the corrosion resistance. Furthermore, in Patent Document 5, boron-containing or boron-containing and fluorine or their oxygen-based coating layer is formed on the stainless steel surface layer portion to improve the corrosion resistance, particularly the SCC prevention effect. It was an invention of stainless steel and processing method. In this way, the present applicant has so far mainly prevented pitting corrosion and stress corrosion cracking due to chlorine ions, which are characteristic of austenitic stainless steels such as SUS304 and SUS316, which have excellent corrosion resistance such as general corrosion and rusting. Research and development has been conducted for the purpose of surface modification that dramatically improves the results.
On the other hand, martensitic stainless steels such as SUS410 and SUS420 are used for cutting tools, surgical instruments, bearings, bearings, and the like that require hardness, and have excellent wear resistance, but corrosion resistance is austenitic stainless steel. It is pointed out that it is inferior to. The present applicant performed the surface modification treatment in Patent Document 3, Patent Document 4 and Patent Document 5 on the martensitic stainless steel having inferior corrosion resistance, and confirmed its effect. In martensitic stainless steel, austenite As a result, no dramatic surface modification effect was observed, as confirmed by the ferritic and ferritic stainless steels.

  The present invention has been made in order to solve the above-described problems. Surface modification by an electrochemical method as shown in Patent Document 3, Patent Document 4, and Patent Document 5 previously proposed by the present applicant is proposed. As a result of earnest research based on the quality treatment method, by using an electrolyte containing a specific element on the surface of stainless steel, by reforming stainless steel that has been ionically electrolytically diffused and penetrated, The purpose is to provide a surface modification treatment method that has drastically improved pitting corrosion due to chlorine, which is special to austenitic stainless steel, and prevention of general corrosion and rusting, which is special to martensitic stainless steel. This is completely different from the conventionally known passivation by the use of oxygen, and the present inventors refer to these as ultrapassivation.

  In order to achieve the above object, the invention described in claim 1 is characterized in that 0.05 Wt% or more to saturation concentration of sulfuric acid, phosphoric acid, citric acid, tartaric acid, oxalic acid, malic acid, lactic acid, acetic acid, gluconic acid, glycol The base material is one or more of sodium, potassium and ammonium salts of acid, succinic acid, etc. that act non-oxidatively on stainless steel. An organic acid or inorganic acid salt, or an aqueous solution containing one or more lithium compounds of lithium hydride, lithium hydroxide, and lithium oxide is used as the electrolyte, and stainless steel is used as a direct current anode or alternating current. Alternatively, the lithium can be expanded from the surface of the stainless steel by electrolysis treatment by connecting to the anode side of the AC / DC current with the alternating current superimposed on the direct current. By damaging the stainless steel, it is possible to form a superior anticorrosive film that prevents chlorine pitting corrosion resistance and martensitic stainless steel's fate of general corrosion and rusting, which are the disadvantages inherent to austenitic stainless steel. There is a surface modification treatment method.

  According to the first aspect of the present invention, the present invention relates to austenite used in places where troubles due to seawater or chlorine have occurred or places where such troubles are expected to occur, such as chemical plants, aquariums, pools, sinks, etc. Of martensitic stainless steel used for stainless steel materials, hardened blades, surgical instruments, shafts, etc., lithium salts with organic and inorganic acids or lithium hydride, lithium hydroxide, lithium oxide An aqueous solution containing one or more lithium compounds is used as the electrolytic solution, and lithium is diffused and infiltrated by performing an extremely simple technique called electrolytic treatment, thereby strengthening the anticorrosion coating of each steel type and providing austenitic stainless steel. Significantly improves pitting corrosion and the suppression of general corrosion and rusting, which are the fate of martensitic stainless steel Passivation film that can be formed.

The definition of stainless steel is an iron alloy containing 10.5% or more of chromium, and exhibits excellent corrosion resistance as the chromium content increases. Generally, austenitic stainless steel represented by SUS304 contains 18% or more of chromium, but martensitic stainless steels such as SUS410 and SUS420 contain only 13% of chromium. Martensitic stainless steel is superior in hardness and wear resistance to austenitic stainless steel due to its metal characteristics, but it has been a problem that it has poor corrosion resistance.
Here, the martensitic stainless steel is an electrolyte containing the neutral salt of hydrofluoric acid and boric acid or boric acid and neutral salt of hydrofluoric acid invented by the present applicants in Patent Document 3, Patent Document 4, and Patent Document 5. When the steel is electrolytically treated, the improvement in corrosion resistance is not manifested. Why is the electrolytic treatment with the electrolyte containing the neutral salt containing the lithium compound in the invention of this application against the corrosion and rusting of the martensitic stainless steel? Whether the improvement of the prevention effect is recognized is not clear, but the mechanism is probably due to the formation of different passive films due to the penetration and diffusion of fluorine, boron ions and lithium ions into stainless steel.

  In the present invention, when electrolytic treatment is performed with an electrolytic solution containing a neutral salt containing a lithium compound, not only the improvement of chlorine pitting resistance unique to austenitic stainless steel, which has been clarified so far by the present applicant, In addition to that, the inventors have found a surface modification method for preventing martensitic stainless steel from being totally corroded and rusted.

About the pitting corrosion resistance to austenitic stainless steel, it was performed according to the pitting corrosion test using the ferric chloride solution prescribed | regulated to JIS using 2B material of SUS304 typical in austenitic stainless steel. .
Stainless steel such as sulfuric acid, phosphoric acid, citric acid, tartaric acid, oxalic acid, malic acid, lactic acid, acetic acid, gluconic acid, glycolic acid, succinic acid of 0.05 Wt% or more and saturated concentration or less on the test piece of stainless steel SUS304 It is based on one or more of sodium, potassium and ammonium salts of acids that act non-oxidatively on the base, and in addition to these, lithium salts with organic and inorganic acids or lithium hydride, lithium hydroxide, oxidation An AC / DC current with an aqueous solution containing one or more lithium compounds of lithium as the electrolyte and the stainless steel to be treated as a direct current anode or alternating current pole, or direct current and alternating current superimposed In the state where it is connected to the anode side of the electrode, it is immersed in the above-mentioned electrolyte solution and is made of a poorly soluble electrode such as stainless steel, graphite, tungsten, or molybdenum. As an alternative method, the stainless steel to be treated is connected to one pole of the power source, and between the counter electrode on the surface of the stainless steel, natural or synthetic, artificial While sliding on the surface of stainless steel using a counter electrode in a state where the hydrated fabric or non-woven fabric hydrous material (hereinafter referred to as “mop”) is interposed and the mop is impregnated with the above electrolyte. In another method, the stainless steel to be treated is connected to the anode side of a DC anode or AC / DC current obtained by superimposing an alternating current on the direct current or one pole side of an AC power source. Cover the top surface with a mop soaked with the electrolyte, place a counter electrode on top of it, and perform electrolytic treatment to diffuse and permeate lithium or lithium and oxygen into the stainless steel surface layer in an ionic state. And it forms an excellent surface modification salt Motoana corrosion resistance of austenitic stainless steel.

  In the above electrolytic solution, an aqueous solution of a lithium compound is effective from 0.01 Wt% or more to a saturated concentration, but it has been confirmed that a practical value of 0.05 to 2.0 Wt% is good.

  As a chlorine pitting resistance test for austenitic stainless steel, 10% of the surface of a SUS304 test piece was subjected to electrolytic treatment as described above and the surface was modified, and the untreated test piece was not subjected to any electrolytic treatment. After immersing in a ferric chloride solution for 2 hours and rinsing with water, the surface condition was confirmed. In the end-treated product, a large number of pitting corrosion was confirmed, but in the test piece treated by the present invention, pitting corrosion was observed. Was not recognized at all.

Next, with respect to the general corrosion resistance and rust resistance of martensitic stainless steel, SUS410 2B material, which is a typical steel type among martensitic stainless steels, is used, and the chlorine resistant holes of the austenitic stainless steel described above are used. The same electrolytic treatment as in the food test was performed.
First, as an evaluation test for preventing overall corrosion, a stainless steel surface of SUS410 was subjected to electrolytic treatment as described above and the surface was subjected to modification treatment, and an untreated test piece that was not subjected to electrolytic treatment at all was 0.5 Wt. It was immersed in a sulfuric acid solution at room temperature for 60 minutes and washed with water, and the surface condition was observed and the weight loss before and after immersion was measured to confirm the amount of corrosion. Furthermore, comparison was made with commercially available electrolytic solutions and stainless steel subjected to surface modification by electrolytic treatment using the electrolytic solutions invented in Patent Document 3, Patent Document 4, and Patent Document 5.

  In the untreated stainless steel, the entire surface was corroded and turned black, but in the test piece surface-modified with the electrolyte containing the lithium compound of the present invention, almost no discoloration was observed and the amount of corrosion was also processed. It was very small, about 1% of the product. Moreover, blackening was confirmed also in the stainless steel which performed the electrolytic treatment using the commercially available electrolyte solution, and the corrosion amount was not large different from the case of untreated. Further, in stainless steel that has been surface-modified with fluorine, boron, or an electrolyte containing boron and fluorine invented in the past by the present applicant, discoloration to grayish black was observed, and the corrosion amount was also untreated. Although a slight prevention effect of about 70% was recognized, the effect as much as that treated by the treatment of the present invention was not recognized.

  Subsequently, as an evaluation test for preventing rusting on martensitic stainless steel, 50 cc of a 3% solution of dichloroisocyanurate used in a kitchen or a washroom is placed in a 100 CC plastic container, and the poly container is made of SUS410. Each treated test piece is placed in a 10-liter capacity plastic container containing each treated test piece and hypochlorous acid is volatilized in a 10-liter poly container at room temperature for 3 days. The rust state was observed. As shown above, each of the treated test pieces was subjected to surface modification treatment with the electrolytic solution containing the lithium compound of the present invention, an untreated product that was not subjected to electrolytic treatment at all, a commercially available electrolytic solution, and Patent Literature 3, Patent Literature 4. Stainless steel subjected to surface treatment by electrolytic treatment using the electrolytic solution invented in Patent Document 5 was used.

  As a result of exposing each treated test piece in a hypochlorous acid atmosphere for 3 days, more rusting was confirmed in the test piece treated with a commercially available electrolyte than the untreated one. On the other hand, in the test piece surface-treated with the electrolytic solution of the present invention, rusting was significantly less than that of a commercial product or an untreated product, and a remarkable rust prevention effect was confirmed. In addition, the surface modification treatment with the electrolytic solution disclosed in Patent Document 3, Patent Document 4, and Patent Document 5 previously invented by the present applicant was slightly less rusting than the untreated product. The effects as much as those processed by the inventive process were not recognized.

  About the pitting corrosion resistance to austenitic stainless steel, it was performed according to the pitting corrosion test using the ferric chloride solution prescribed | regulated to JIS using 2B material of SUS304 typical in austenitic stainless steel. .

  Using an electrolyte composed of an aqueous lithium compound solution adjusted to a concentration of 0.01 Wt% to 5.0 Wt%, the entire surface of the test piece was slid for several seconds to several tens of seconds with an electrode connected to an AC power source and covered with the above-mentioned mop. The surface modification treatment was applied.

Next, the surface of the test piece was subjected to electrolytic treatment as described above and the untreated test piece that was not subjected to electrolytic treatment at all was immersed in a 10% ferric chloride solution for 2 hours and washed with water to determine the surface condition. confirmed. A large number of pitting corrosion was confirmed in the untreated test piece, and the corrosion amount was 6.82 (mg / cm ² ).
In FIG. 1, the photograph which image | photographed the condition of the pitting corrosion after a pitting corrosion test about an untreated test piece is shown.

On the other hand, as shown in FIG. 2, the test piece treated with the electrolytic solution containing the lithium compound has almost no pitting corrosion on its surface, and the corrosion amount is 0.39 (mg / cm ² ). It was very small, about 6% of the amount of untreated corrosion.
In addition, when various tests were performed by changing the concentration of the electrolytic solution in the aqueous solution composed of the lithium compound, the concentration of the electrolytic solution was actually about 0.03 Wt% to 2.5 Wt%. It was found that the effect on chlorine pitting resistance was great.

Example 2 (comparative example)

For comparison, the same test as in Example 1 was performed using a commercially available neutral electrolyte A for scoring. However, the test piece treated with a commercially available neutral electrolyte A for scoring was not used. More pitting corrosion was observed than the treated product, and the amount of corrosion was 8.11 (mg / cm ² ), which was higher than that of the untreated product.
In FIG. 3, the photograph which image | photographed the condition after the pitting corrosion test of the test piece processed with the commercially available neutral electrolyte solution A for scoring is shown.

  Next, an aqueous electrolyte containing 0.03 Wt% to 2.5 Wt% lithium compound is used, based on 1.0% Wt% to 20.0 Wt% sodium phosphate and glycolic acid. When connected to an AC / DC power supply and subjected to immersion electrolytic treatment for several seconds to several tens of seconds, a pitting corrosion test was conducted. As with the sliding electrolytic treatment using a mop, almost no pitting corrosion occurred. The amount of corrosion was about 4% of untreated, and the effect was very high.

  Subsequently, with respect to the general corrosion resistance and rust resistance of martensitic stainless steel, the SUS410 2B material, which is a typical steel type, was used, and the mop performed in the above-described austenitic stainless steel chlorine pitting corrosion test was used. The electrolytic treatment was performed using the sliding method used.

  First, as an evaluation test for preventing general corrosion, a stainless steel surface of SUS410 was added to a 0.03 Wt% to 2.5 Wt% lithium salt solution in a phosphoric acid and acetic acid potassium salt aqueous solution of 10.0% Wt% to 25.0 Wt. Connected to an AC / DC power source using an electrolyte solution of an aqueous solution composed of a compound, and the surface treated with the electrolytic treatment as described above for several seconds to several tens of seconds and the untreated test piece without any electrolytic treatment Was immersed in a 0.5 Wt% sulfuric acid solution at room temperature for 60 minutes, washed with water, the surface condition was observed, and the weight loss before and after immersion was measured to confirm the amount of corrosion.

In the untreated test piece, it was observed that the entire surface was corroded and turned black, and the entire surface corrosion was progressing, and the corrosion amount was 1.01 (mg / cm ² ).
In FIG. 4, the photograph which image | photographed the condition after the sulfuric acid solution immersion of an untreated product is shown.

On the other hand, in the test piece surface-treated with the electrolytic solution containing the lithium compound of the present invention, almost no discoloration was observed and no corrosion was confirmed. Further, the corrosion amount was 0.01 (mg / cm ² ), which is very small, about 1% of the untreated product, and it was found that there was a strong inhibitory effect on the overall corrosion of martensitic stainless steel.
In FIG. 5, the photograph which image | photographed the condition after the sulfuric acid solution immersion of the test piece of this invention was shown.

Example 5 (comparative example)

  In order to see a comparison with a commercially available general electrolyte solution or a surface-modified treatment with fluorine, boron or an electrolyte containing boron and fluorine previously invented by the present applicant, In the same manner as in Example 4, the electrolytic treatment was carried out by the mop method using the electrolytic electrolyte B, and further using an electrolytic solution in which boric acid and sodium fluoride were blended with water-soluble salts of organic acids.

The photograph which image | photographed the condition after the sulfuric acid solution immersion in the test piece processed with the neutral electrolyte solution B for commercial scoring was shown in FIG.
As shown in FIG. 6, blackening was confirmed in the test piece subjected to electrolytic treatment using a commercially available electrolytic solution, and the corrosion amount was 1.12 (mg / cm ² ), which was not significantly different from the untreated case. .

Further, when the surface modification treatment was performed with the electrolytic solution containing boron and fluorine previously invented by the present applicant, discoloration to grayish black was observed and the corrosion amount was 0.69 (mg / cm ² ). Although a little prevention effect was recognized, about 70% compared with the untreated product, there was no effect as much as that treated by the treatment of the present invention.
The photograph which image | photographed the condition after the sulfuric acid solution immersion of the test piece processed with the electrolyte solution containing a boron and a fluorine in FIG. 7 was shown.

  Subsequently, as an evaluation test for preventing rust on martensitic stainless steel, 50 cc of a 3% solution of dichloroisocyanurate used in a kitchen or a washroom is placed in a 100 CC plastic container, and the poly container is used in Example 4. Place the test piece treated with the same electrolyte and untreated test piece that was not treated at all in a sealable 10 liter capacity plastic container and place it in a 10 liter plastic container at room temperature for 3 days. In the state where hypochlorous acid was volatilized, the rusting state of each treated specimen was observed.

FIG. 8 shows an untreated product, and FIG. 9 shows a photograph of the situation after exposure in a hypochlorous acid atmosphere for 3 days for the electrolytic treatment test piece of the present invention.
As a result of exposing the test piece in a hypochlorous acid atmosphere for 3 days, significant rust was observed on the entire test piece surface in the untreated product.

  On the other hand, the test piece surface-modified with the electrolytic solution of the present invention clearly has less rusting compared to the untreated product, and a remarkable antirust effect was confirmed.

Example 7 (comparative example)

For comparison, the same test as in Example 6 was conducted using a commercially available neutral electrolytic solution C for scoring and a test piece that had been subjected to electrolytic treatment using an electrolytic solution containing fluorine previously invented by the present applicant. Although it went, in the test piece processed with the commercially available neutral electrolytic solution C for scoring, the rust generation of rust almost equal to that of the untreated product was confirmed.
On the other hand, the surface modification treatment with the electrolyte containing fluorine was slightly less rusting than the untreated product or the test piece treated with the commercially available neutral electrolyte C for scoring, but the treatment according to the present invention. Not as effective as those processed by.
The result of the test piece processed with the commercially available neutral electrolytic solution C for scoring is shown in FIG. 10, and the result of the test piece processed with the electrolytic solution containing fluorine is shown in FIG.

It is the photograph which image | photographed the situation of the pitting corrosion after a pitting corrosion test about the untreated test piece which concerns on Example 1 of this invention. It is the photograph which image | photographed the situation of the pitting corrosion after a pitting corrosion test about the test piece after the electrolytic treatment which concerns on Example 1 of this invention. It is the photograph which image | photographed the situation of the pitting corrosion after a pitting corrosion test about the test piece processed with the commercially available neutral electrolyte solution A for scoring according to Example 2 (comparative example) of this invention. It is the photograph which image | photographed the condition after a sulfuric acid solution immersion about the untreated test piece which concerns on Example 4 of this invention. It is the photograph which image | photographed the condition after sulfuric acid solution immersion about the test piece after the electrolytic treatment which concerns on Example 4 of this invention. It is the photograph which image | photographed the condition after immersion in a sulfuric acid solution about the test piece processed with the commercially available neutral electrolyte solution B for scoring which concerns on Example 5 (comparative example) of this invention. It is the photograph which image | photographed the condition after sulfuric acid solution immersion about the test piece processed with the electrolyte solution containing the boron and fluorine which concerns on Example 5 (comparative example) of this invention. It is the photograph which image | photographed the condition after exposure in a hypochlorous acid atmosphere for 3 days about the untreated test piece which concerns on Example 6 of this invention. It is the photograph which image | photographed the condition after exposure in the hypochlorous acid atmosphere for 3 days about the test piece after the electrolytic treatment which concerns on Example 6 of this invention. It is the photograph which image | photographed the condition after exposure in a hypochlorous acid atmosphere for 3 days about the test piece processed with the commercially available neutral electrolyte solution C for scoring which concerns on Example 7 (comparative example) of this invention. It is the photograph which image | photographed the condition after exposure in a hypochlorous acid atmosphere for 3 days about the test piece processed with the electrolyte solution containing the fluorine which concerns on Example 7 (comparative example) of this invention.

As described above, the present invention improves the resistance to chlorine pitting corrosion by surface-modifying austenitic stainless steel, and various chemical plants and piping that are expected to cause trouble due to seawater and chlorine ions. It can be applied to household sinks and the like to achieve great effects.
In addition, surface modification of martensitic stainless steel can prevent overall corrosion and rusting, so it can be applied to blades, shafts, surgical instruments, etc., and its industrial contribution is extremely high. high.

Claims (1)

  Acts non-oxidatively on stainless steel such as sulfuric acid, phosphoric acid, citric acid, tartaric acid, oxalic acid, malic acid, lactic acid, acetic acid, gluconic acid, glycolic acid, succinic acid, etc. An organic acid or inorganic acid salt of lithium having a concentration of not less than 0.01 Wt% to not more than a saturated concentration, lithium hydride, lithium hydroxide, based on one or more of each of sodium, potassium and ammonium salts of acids An aqueous solution containing one or more lithium compounds of lithium oxide is used as an electrolyte, and stainless steel is used as a DC anode or AC pole, or an AC / DC current anode where DC is superimposed on AC By connecting to the side and performing electrolytic treatment, chlorine pitting resistance and martensite resistance, which are considered to be defects inherent to austenitic stainless steel The surface treatment method of the stainless steel to corrosion and rusting is fate of stainless steel to form an excellent anticorrosive film to prevent it.