JP2005179699A - Method for inhibiting potential of stainless steel from becoming noble - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for inexpensively inhibiting a potential of stainless steel used in fresh water from becoming noble due to the action of microorganisms so as to impart crevice corrosion resistance to the stainless steel, by adding additional metallic elements in a basic composition of SUS304. <P>SOLUTION: The stainless steel includes, by mass%, 0.004-0.05% C, 0.01-1% Si, 0.1-2% Mn, 0.03% or less P, 0.01% or less S, 17-20% Cr, 7.5-10% Ni, 0.05% or less Al, 0.1-0.3% N, 0.005% or less O, and further either group of elements in the following first group or the second group: the first group consisting of 0.5-4% Mo or one or two elements of 0.5-2% Cu and 0.5-3% V in addition to 0.5-4% Mo; or the second group consisting of 0.5-2% Cu, 0.5-2% Cu and 0.5-3% V, or 0.01-0.4% Ti and 0.5-1.5% V. The inhibiting method includes contacting the stainless steel with the fresh water including aerobic bacteria, and keeping it in a natural potential of +150 mV or lower. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention includes river water / lake water / ground water storage tanks, river water / lake water / ground water transport pipes, estuary weirs, sluice gates, industrial water / agricultural water utilization equipment, water / sewage treatment equipment, temperature control circulation The present invention relates to a method for suppressing potential nobleness of stainless steel used in an environment that is in direct contact with fresh water in which microorganisms exist, such as water facilities and industrial wastewater treatment facilities.

Conventionally, tanks for river water / lake water / ground water storage, pipes for river water / lake water / ground water transport, estuary weirs, sluices, industrial water / agricultural water use facilities, water / sewage treatment facilities, circulating water facilities for temperature control Carbon steel, low alloy steel, stainless steel, copper alloy, titanium, etc. are properly used as materials for equipment that comes into direct contact with fresh water, such as industrial wastewater treatment facilities, depending on the salt concentration and temperature conditions.
In addition to this, the present situation is that many materials coated with organic coating, FRP coating, galvanization, etc. are used.

In seawater environments where the salt concentration is high, crevice corrosion and stress corrosion cracking may occur under operating conditions of about 60 ° C or higher. Therefore, 20% Cr-25% Ni-4.5% Austenitic stainless steel such as Mo-1.5% Cu steel, 20% Cr-24% Ni-6% Mo steel, 28% Cr-1.2% Ni-3.5% Mo steel, There are ferritic stainless steels such as 29% Cr-2% Ni-4% Mo steel.
Austenitic seawater resistant stainless steel is expensive because Ni and Mo need to be added at high concentrations, and ferritic seawater resistant stainless steel needs to be added at high concentrations of Cr and Mo. Unless otherwise specified in the present specification,% represents mass%.

  On the other hand, compared to seawater, fresh water such as river water and lake water having a low chlorine ion concentration is general-purpose stainless steel, and relatively inexpensive SUS304 (18% Cr-8% Ni) is used. From the viewpoint of corrosion resistance, it is possible to use seawater resistant stainless steel as described above even in fresh water. However, since seawater-resistant stainless steel is very expensive, there is a significant cost problem in order to use it in fresh water.

  When stainless steel is used in fresh water such as river water and lake water under aerobic conditions where microorganisms can use oxygen, aerobic microorganisms adhere to and grow on the surface of stainless steel to form a biofilm. The The natural potential of stainless steel shifts to a noble potential due to the action of biological metabolites. If there are any aerobic microorganisms in the water in contact with the stainless steel, potential nomination can occur due to the growth of the microorganisms. Since the noble potential becomes a factor that accelerates anodic dissolution of the stainless steel, corrosion such as crevice corrosion is likely to occur in the stainless steel with the noble potential. Such corrosion caused by microbial action is called microbial corrosion.

  Thus, when the potential of stainless steel used in contact with fresh water containing microorganisms becomes noble, it causes corrosion. In addition, the potential nomination not only corrodes the stainless steel itself, but also makes electrical contact with the stainless steel, forming a galvanic cell with an electrochemically base metal such as mild steel and making contact. It is also a factor that further accelerates the corrosion of existing metals. Therefore, establishment of the technique which suppresses the potential nomination of stainless steel is required.

Several methods have been proposed to prevent microbial corrosion that occurs in fresh water.
In Patent Document 1, stainless steel is subjected to electrolytic oxidation treatment and electrolytic reduction treatment by energization in a solution containing hexavalent Cr ions and trivalent Fe ions, thereby reforming the surface of stainless steel, and potential noble. It is a technology that attempts to prevent corrosion by suppressing crystallization.
In Patent Document 2, chlorine ions in water contact with stainless steel are replaced with other anions such as sulfate ions, and chlorine ions flow into the crevice corrosion site in response to crevice corrosion of stainless steel caused by potential nomination. It is a technology that tries to suppress corrosion by suppressing the above.

Patent Document 3 tries to prevent corrosion by adding a bactericidal agent as a water treatment agent to water that is in contact with stainless steel in a cooling water pipe or the like to sterilize or bacterize microorganisms, thereby suppressing microbial action. Technology.
Patent Document 4 is a technique for preventing corrosion by suppressing microbial action by adding Ag, which has known antibacterial properties, to a stainless steel composition.

Patent Document 5 is a technique for preventing corrosion without applying a potential from the outside and forcibly generating potential nomination due to microbial action.
Patent Document 6 and Patent Document 7 are based on the measured values of the noble natural potential based on the measured data of the noble natural potential when the test piece made of stainless steel is immersed in actual river water and lake water. Is a crevice corrosion-resistant stainless steel for fresh water having a high value of crevice corrosion repassivation potential (ER, CREV).
JP-A-7-26395 Japanese Patent Laid-Open No. 10-30196 JP 2001-170687 A JP 2000-129406 A Japanese Patent Laid-Open No. 11-310890 JP 2001-254149 A JP 2002-275589 A Ito et al., “Materials and Environment”, Vol. 50, No. 6, pp. 28-291 (2001)

As described above, in the aerobic environment where microorganisms can use oxygen, in the fresh water such as river water and lake water where aerobic microorganisms exist, corrosion resistance against potential nobleness caused by microbial action, especially most likely to occur With respect to crevice corrosion, it is desired to develop a method for suppressing potential corrosion of stainless steel used in an environment in contact with fresh water and suppressing crevice corrosion.
In addition, the fresh water which this invention contrasts is seawater (chlorine ion density | concentration 19000 mg / L), such as river water, lake water, groundwater, rainwater, and the water and sewage water which uses these, industrial water, agricultural water, and cooling water ) Is a water having a low chloride ion concentration compared to. Therefore, the chlorine ion concentration from pure water to the river water near the river mouth is assumed, and water having a chlorine ion concentration of 0 mg / L or more and 1000 mg / L or less is described as fresh water in the present invention.

  Stainless steel used in freshwater environments with low salt concentrations, such as river water and lake water. Corrosion caused by the nobleness of natural potential (also called corrosion potential or Open Circuit Potential) due to the action of aerobic microorganisms. Although several countermeasure techniques have been proposed for easy crevice corrosion, these have the following problems.

  First, the surface of stainless steel is modified by conducting electrolytic oxidation treatment and electrolytic reduction treatment of the stainless steel of Patent Document 1 by energization in a solution containing hexavalent Cr ions and trivalent Fe ions. Although it is about the technique which suppresses potential nobleness and tries to prevent corrosion, it is not practical to perform such electrolytic treatment on stainless steel, which causes a great cost. In Patent Document 1, it is described that, in a short-term immersion test of only 3 months, potential nobleness suppression and crevice corrosion resistance were effective. However, as shown in Patent Document 6, it is considered that a dipping period of at least one year is necessary in order to examine the action of sufficient potential nobility. It has also not been demonstrated that the effect of surface modification is maintained for a long time.

When the amount of water, such as a cooling water system, is limited for the techniques of changing the composition of water in contact with the stainless steel of Patent Document 2 and Patent Document 3 or trying to suppress corrosion using a water treatment agent such as an antibacterial agent However, it is generally difficult to change the composition of water when it is immersed in river water or lake water, and it is also difficult to introduce a water treatment agent.
Regarding the technology for preventing corrosion by suppressing the microbial action by adding Ag, which is known to have antibacterial properties, to the stainless steel composition of Patent Document 4, if Ag is added, it becomes expensive and the corrosion resistance deteriorates. There is a big challenge to do. Another problem is that the antibacterial action of Ag is effective for microorganisms in freshwater environments, and it has not been proved at all whether microorganism adhesion can be prevented for a long period of time.

  The technique of Patent Document 5 that attempts to prevent corrosion without applying potential from the outside by applying a potential from the outside and forcing potential generation by microbial action is basically a general anti-corrosion technique. In addition to the need for equipment costs for potential application, the operation and maintenance of the potential application apparatus also cost. Therefore, there is a problem that the cost is significantly increased in the long-term use.

Patent Document 6 and Patent Document 7 are based on the measured values of the noble natural potential based on the measured data of the noble natural potential when the test piece made of stainless steel is immersed in actual river water and lake water. Is a crevice corrosion-resistant freshwater stainless steel having a high value of crevice corrosion repassivation potential (ER, CREV).
Patent Document 6 relates to austenitic stainless steel. The Ni content in the steel is specified as 10 to 15%. Since Ni is an expensive element, it is desirable that Ni content can be reduced by lowering the Ni content below 10%.
Patent Document 7 relates to a ferritic stainless steel and has a low Ni content and a low price. However, compared to austenitic stainless steels such as SUS304 and SUS316L that are commonly used in fresh water, ferritic stainless steels are considered to be inferior in crevice corrosion resistance, and have a limited use record and limited use. It becomes.

Accordingly, the present invention relates to a stainless steel for fresh water having excellent crevice corrosion resistance applicable to such a fresh water environment having a low salt concentration, and includes tanks for storing river water, lake water and groundwater, river water and lake water.・ Used as equipment materials for underground water transport pipes, estuaries weirs, sluice gates, industrial water / agricultural water use facilities, water and sewage treatment facilities, circulating water facilities for temperature control, industrial wastewater treatment facilities, etc. We provide stainless steel for fresh water that can ensure longevity and safety over a long period of time without maintenance.
Specifically, based on the basic composition of SUS304 (18% Cr8% Ni) of austenitic stainless steel that is widely used in freshwater environments, the addition of a new metal element suppresses potential nobility due to microbial action, It is an object of the present invention to provide a method for suppressing potential nobleness of stainless steel used in an environment in contact with fresh water, which is excellent in crevice corrosion resistance and economical.

Stainless steel potential nobility is an aerobic environment in which oxygen can be used, and is a phenomenon that occurs in common in fresh water such as river water, lake water, and groundwater in which microorganisms exist, and in seawater. If there is a very small amount of aerobic microorganisms, potential nomination occurs due to growth or the like.
Patent Document 6 reports changes with time in the natural potential when SUS304 is immersed in river water or lake water. In any river water or lake water, it has been reported that the natural potential becomes noble potential as the immersion time elapses and becomes almost constant and settles in about one to two years.

It has been reported that the natural potential becomes noble to about +400 mV (saturated potassium chloride-containing silver-silver chloride electrode standard) in an actual environment. It has also been reported that in ion-exchanged water having no microbial action, the natural potential when SUS304 steel is immersed is about +100 to +150 mV (saturated potassium chloride-containing silver-silver chloride electrode standard). This difference is due to the potential noble action of microorganisms.
Similarly, Non-Patent Document 1 also reports that the natural potential of SUS304 is precious to about +400 mV (silver-silver chloride electrode standard with saturated potassium chloride) due to microbial action in natural seawater. The potential values described in the present invention are all values measured on the basis of a saturated potassium chloride-containing silver-silver chloride electrode.
Therefore, the nomination of the natural potential of SUS304 is a phenomenon that occurs in common in fresh water and seawater.

  For example, when SUS304, which is widely used as a stainless steel for fresh water, is immersed in river water or lake water, the potential becomes noble in 1 to 2 years, but corrosion hardly occurs. When SUS304 is used in an actual structure or the like, it is not easy to evaluate the corrosion resistance by performing an immersion test for a period corresponding to actual use, considering that it will be used for a long period of several decades or more. .

  Therefore, from the above viewpoint, the present inventors have developed various stainless steel test materials having a gap structure over a long period of about two years in natural seawater, which is a more severe corrosive environment than fresh water and in which potential nobleness occurs. The natural potential (called natural potential) was continuously measured and the corrosion resistance was evaluated. This is because it was considered possible to find stainless steel capable of exhibiting sufficient corrosion resistance for use in fresh water by evaluating the corrosion resistance in a corrosive environment worse than fresh water by performing an immersion test in seawater.

  After completion of the immersion test, various stainless steels were evaluated for corrosion resistance. As a result of intensive studies, Mo or one or two of Cu and V in addition to Mo or Mo, or Cu, or Cu and V, or V and V is added to stainless steel having a basic composition of 304 series. It has been found that by adding Ti, potential nomination due to microbial action that occurs in 304 series stainless steel can be suppressed, and good corrosion resistance can be ensured. Therefore, the present invention was completed by specifying an appropriate combination amount of the above elements.

That is, the gist of the present invention is as follows.
(1) In mass%,
C: 0.004 to 0.05%, Si: 0.01 to 1%,
Mn: 0.1 to 2%, P: 0.03% or less,
S: 0.01% or less, Cr: 17-20%,
Ni: 7.5 to 10%, Al: 0.05% or less,
N: 0.1 to 0.3%, O: 0.005% or less, further containing either one of the following group 1 elements or group 2 elements, the balance being iron and unavoidable impurities Stainless steel is brought into contact with fresh water containing aerobic microorganisms and having a chloride ion concentration of 0 mg / L or more and 1000 mg / L or less, and the natural potential is maintained at +150 mV or less based on a silver-silver chloride electrode containing saturated potassium chloride. A method for suppressing potential nobleness of stainless steel.
Group 1: Mo: 0.5-4%
Or, in addition to Mo: 0.5-4%, Cu: 0.5-2%,
V: One or two of 0.5 to 3%.
Group 2: Cu: 0.5-2%,
Or Cu: 0.5-2% and V: 0.5-3%,
Or Ti: 0.01-0.4% and V: 0.5-1.5%.

  According to the present invention, it becomes possible to greatly improve the microbial corrosion resistance in fresh water, and can be used in fresh water, suppress noble natural potential due to microbial action, is excellent in crevice corrosion resistance, and economical. In addition, it is possible to suppress the noble potential of stainless steel and prevent crevice corrosion.

Hereinafter, the present invention will be described in detail.
Patent Document 6 reports changes over time in natural potential when SUS304 steel is immersed in some actual river water and lake water. It has been reported that in any river water or lake water, it becomes noble to a noble potential with the lapse of immersion time and settles to an almost constant value in about 1 to 2 years. It is reported that the natural potential becomes noble up to about +400 mV (saturated potassium chloride-containing silver-silver chloride electrode standard) as measured values.

  On the other hand, in sterilized ion-exchanged water, when the natural potential is measured, it is about +100 to +150 mV (silver-silver chloride electrode containing saturated potassium chloride). This difference in natural potential is due to the potential noble action of microorganisms (Patent Document 6). Moreover, it is also known that the natural potential of stainless steel (SUS304) is precious to about +400 mV due to the microbial action in the natural seawater as well as river water and lake water (Non-patent Document 1). Therefore, the potential nomination of stainless steel (SUS304) is a phenomenon that occurs in common in fresh water and seawater.

  Next, based on the above findings, the present inventors have developed various stainless steel test materials having a gap structure over a long period of about two years in natural seawater, which has a high chlorine concentration and is more corrosive than fresh water. Immersion was performed, and the natural potential (referred to as natural potential) measurement was continuously performed. The shape of the test material is shown in FIG.

  One set of a small test piece 1 of 30 w × 30 l × 2 tmm and a large test piece 2 of 50 w × 90 l × 4 tmm, which are stainless steels of the same composition, are wet-polished on the entire surface (No. 600), and 50 ° C. Was immersed in a 30% nitric acid solution for 1 hour, subjected to passivation treatment, washed with water, and then air-dried. Next, the enamel-coated copper wire 3 was fixed to the upper end of a large test piece of 50 w × 90 lmm by soldering, and was insulation-coated with a silicon sealant. Then, it assembled using the bolt 4 made from polycarbonate, the nut 5, and the washer 6 like FIG. 1 in the state which apply | coated seawater to the test surface.

The test piece assembled in this manner was immersed in seawater in a seawater test tank that continuously took in natural seawater. Measurement of the natural potential (called natural potential) was continued for about two years. A saturated KCl Ag / AgCl electrode was used as a reference electrode.
By the seawater immersion test as described above, the natural potential of SUS304 was nominated to about +400 mV (saturated KCl Ag / AgCl), which is the same level as that measured in river water and lake water in Patent Document 6.

On the other hand, the present inventors have found that the preciousness of the natural potential is greatly suppressed in the stainless steel in which Mo is added to the steel as compared with the stainless steel not containing. Furthermore, it has also been found that excellent corrosion resistance is exhibited by suppressing potential nobleness.
Similarly, for the stainless steel in which one or two of Cu and V were added in addition to Mo in the steel, the effect of suppressing potential nobleness and excellent corrosion resistance were found. Furthermore, even without adding Mo, the effect of suppressing potential nobleness and excellent corrosion resistance were found for stainless steel to which Cu, Cu and V, or Ti and V were added.
Based on this knowledge, the inventors of the present invention limited the elements and amounts of stainless steel that can be sufficiently used in fresh water, and completed the present invention.

The reasons for limiting the constituent requirements of the present invention will be described below.
C is harmful to the corrosion resistance of stainless steel, but a certain content is necessary from the viewpoint of strength. With an extremely low C content of less than 0.004%, the production cost increases. Further, if it exceeds 0.05%, the corrosion resistance is greatly deteriorated, so 0.004 to 0.05% was set.

  Si has a Si content of 1% or less as a component range of normal stainless steel that can be hot-rolled within a range that does not affect the corrosion resistance. Further, if the Si amount is less than 0.01%, the manufacturing cost becomes high, so the content was made 0.01% or more.

  Mn is an austenite stabilizing element and can be added as an alternative to expensive Ni. However, if it exceeds 2%, there is no effect in improving corrosion resistance, and the upper limit of the amount of Mn is 2% or less. Further, if the amount of Mn is less than 0.1%, the manufacturing cost becomes high, so the content was made 0.1% or more.

  P is desirably small in terms of corrosion resistance and hot workability. If it exceeds 0.03%, the hot workability is extremely deteriorated, so the P content is set to 0.03% or less.

  S is an element that significantly affects the hot workability rather than the corrosion resistance, and the lower the amount, the better. Therefore, the S amount is set to 0.01% or less.

  Cr is a basic component of the stainless steel of the present invention, and it is necessary to add 17% or more in order to obtain good crevice corrosion resistance in natural river water. The corrosion resistance improves as the amount of Cr increases, but if it exceeds 20%, the productivity becomes somewhat difficult, and the cost becomes expensive. Therefore, the Cr content range is limited to 17-20%.

  Ni is a basic component of the stainless steel of the present invention together with Cr. In order to facilitate the production of stainless steel plate, the metal structure needs to be an austenite phase, and Ni addition is essential. The minimum amount of Ni for making the steel of the present invention an austenite phase is 7.5%. Moreover, when there is too much Ni amount, a price will become high. The upper limit of the amount of Ni that keeps the austenite phase economically is set to 10%.

  Al is added in a range of 0.05% or less as a deoxidizer. If it exceeds 0.05%, crevice corrosion resistance and hot workability are deteriorated. Therefore, the range of Al content was made 0.05% or less.

  N is a strong austenite-forming element and at the same time an element that inhibits the progress of crevice corrosion occurring in stainless steel. In order to obtain stable corrosion resistance, an N amount of at least 0.1% or more is necessary. Moreover, addition over 0.3% is very difficult on steelmaking, and deteriorates the hot workability of stainless steel. Therefore, the range of N amount is limited to 0.1 to 0.3%.

  O, like S, is an element that significantly affects hot workability, and the lower the better. O was limited to 0.005% or less obtained by a normal stainless steel manufacturing method.

  Mo has an effect of improving pitting corrosion resistance. Particularly in an environment where the chloride ion concentration is likely to be high, this element is important for suppressing pitting corrosion growth. In addition to the above 17-20% Cr and 7.5-10% Ni, the corrosion resistance is improved by adding Mo alone in the range of 0.5-4% or coexisting with V, Cu. . If it is less than 0.5%, the corrosion resistance becomes insufficient, but if it exceeds 4%, the effect of improving the corrosion resistance is saturated and expensive. If the economy is emphasized, it is more preferably 0.5 to 2%.

V, like Ti, is easy to form carbides, has the effect of refining crystal grains and improving intergranular corrosion resistance. Corrosion resistance is improved by adding 0.5 to 3% of V together with Mo and Cu together with 17 to 20% of Cr and 7.5 to 10% of Ni. Moreover, corrosion resistance is improved by adding together with Ti in 0.5 to 1.5% of range.
When V is less than 0.5%, sufficient corrosion resistance cannot be obtained. When adding together with Ti, adding over 1.5% degrades the corrosion resistance. In addition, when it is added together with Mo and Cu, if it exceeds 3%, hot workability is remarkably deteriorated, making it difficult to produce steel and making it economically expensive. If the economy is emphasized, when adding together with Mo and Cu, it is more preferably 0.5 to 2%.

  Cu is known to have an antibacterial action as is Ag. It is also an element that promotes austenitization. Corrosion resistance is improved by adding 0.5 to 2% of Cu together with Mo or V together with 17 to 20% of Cr and 7.5 to 10% of Ni. If the addition is less than 0.5%, sufficient corrosion resistance cannot be obtained. On the other hand, if it exceeds 2%, the corrosion resistance is deteriorated and it is economically expensive.

  Ti is an element that can effectively improve the corrosion resistance. Ti, like V, easily forms carbides and has the effect of improving intergranular corrosion resistance. Corrosion resistance is improved by adding 0.01 to 0.4% Ti together with V together with 17 to 20% Cr and 7.5 to 10% Ni. If the addition is less than 0.01%, sufficient corrosion resistance cannot be obtained. On the other hand, if it exceeds 0.4%, the corrosion resistance is deteriorated and it is economically expensive.

Examples of the present invention will be described below, but the present invention is not limited to these examples.
Table 1 shows the chemical compositions of the inventive steel and the comparative steel. As a comparative steel, SUS304 (18% Cr8% Ni) of sample number 1 in Table 1 was used. A test material was prepared by adding a new metal element based on the basic composition of SUS304, which is a comparative steel.

Each test material was produced as follows.
The raw materials were weighed so that the weight ratios of the components contained in the steel each reached the target value, then melted by an electric furnace vacuum melting method and cast into a mold to prepare an ingot. Then, a soaking treatment is performed at 1150 to 1250 ° C. for 0.5 to 1 hour, heated again to 1250 ° C., hot-rolled to a thickness of 6 mm, heated at 1100 ° C. for 30 minutes, and then subjected to water quenching. A solution heat treatment was performed to produce a crevice corrosion test piece shown in FIG. 1 and subjected to a dipping test.

  The immersion test was carried out by immersing in seawater in a seawater tank in which natural seawater is continuously flowing in, because noble potential occurs as in freshwater, and corrosion occurs more rapidly than in freshwater. . Then, the natural potential of the test piece was measured over time. A saturated KCl Ag / AgCl electrode was used as a reference electrode. The immersion test was conducted for about 2 years. After the test was completed, the weight loss of the large and small test pieces constituting the crevice corrosion test piece due to the corrosion of the test piece and the maximum corrosion depth existing in the large test piece were measured.

For sample numbers 1 to 14 in Table 1, the average value of the natural potential and the measurement result of the weight loss due to corrosion are shown in FIGS. 2 and 3 for the large and small test pieces constituting the crevice corrosion test piece. Moreover, the measurement result of the average value of a natural potential and the maximum corrosion depth which existed in the large test piece is shown in FIG.
Further, for sample number 1 (comparative steel) and sample numbers 15 to 28 in Table 1, the average value of the natural potential and the measurement result of the weight loss due to corrosion are shown in FIG. 5 for the large and small test pieces constituting the crevice corrosion test piece. As shown in FIG. Moreover, the measurement result of the average value of a natural potential and the maximum corrosion depth which existed in the large test piece is shown in FIG.
From the above results, it has been clarified that addition of Mo to steel can suppress potential nomination due to microbial action, and can further improve corrosion resistance. In addition to Mo, it has also been clarified that addition of one or two of Cu and V to steel can suppress potential nomination due to microbial action and further improve corrosion resistance.

For Sample No. 1 (Comparative Steel) and Sample Nos. 29 to 36 in Table 1, the average value of the natural potential and the measurement result of the weight loss due to corrosion are shown for the large and small test pieces constituting the crevice corrosion test piece. 9 shows. Moreover, the measurement result of the average value of a natural potential and the maximum corrosion depth which existed in the large test piece is shown in FIG.
From the above results, it has been clarified that addition of Cu to steel can suppress potential nomination due to microbial action and can further improve corrosion resistance. It has also been clarified that by adding Cu and V in combination to steel, potential nomination due to microbial action can be suppressed, and further, corrosion resistance can be improved.

For sample number 1 (comparative steel) and sample numbers 37 to 45 in Table 1, the average value of the natural potential and the measurement result of the weight loss due to corrosion are shown for the large and small test pieces constituting the crevice corrosion test piece. 12 shows. Moreover, the measurement result of the average value of a natural potential and the maximum corrosion depth which existed in the large test piece is shown in FIG.
From the above results, it has been clarified that the addition of Ti and V in steel can suppress potential nomination due to microbial action and can further improve the corrosion resistance.

For sample number 1 (comparative steel) and sample numbers 46 to 57 in Table 1, the average value of the natural potential and the measurement result of the weight loss due to corrosion are shown in FIG. 14 and FIG. As shown in FIG. Moreover, the measurement result of the average value of a natural potential and the maximum corrosion depth which existed in the large test piece is shown in FIG.
From the above results, it became clear that in stainless steel containing 17 to 20% Cr and 7.5 to 10% Ni, potential nomination due to microbial action can be suppressed and maintained at +150 mV or less, and corrosion resistance can be further improved. .

It is the figure which showed the crevice corrosion test piece used for the immersion test. It is the figure which showed the measurement result of the weight loss amount by the average value of a natural potential and the corrosion of a large test piece about sample numbers 1-14. It is the figure which showed the measurement result of the weight loss amount by the average value of a natural potential, and the corrosion of a small test piece about sample numbers 1-14. It is the figure which showed the measurement result of the average value of a natural potential and the maximum corrosion depth of a large test piece about sample numbers 1-14. It is the figure which showed the measurement result of the weight loss amount by the average value of a natural potential, and the corrosion of a large test piece about sample numbers 15-28. It is the figure which showed the measurement result of the weight loss amount by the average value of a natural potential and corrosion of a small test piece about sample numbers 15-28. It is the figure which showed the measurement result of the average value of a natural potential and the maximum corrosion depth of a large test piece about sample numbers 15-28. It is the figure which showed the measurement result of the weight loss amount by the average value of a natural potential, and the corrosion of a large test piece about sample numbers 29-36. It is the figure which showed the measurement result of the weight loss amount by the average value of a natural potential and corrosion of a small test piece about sample numbers 29-36. It is the figure which showed the measurement result of the average value of a natural potential, and the maximum corrosion depth of a large test piece about sample numbers 29-36. It is the figure which showed the measurement result of the weight loss amount by the average value of a natural potential, and the corrosion of a large test piece about sample numbers 37-45. It is the figure which showed the measurement result of the weight loss amount by the average value of a natural potential, and the corrosion of a small test piece about sample numbers 37-45. It is the figure which showed the measurement result of the average value of a natural potential, and the maximum corrosion depth of a large test piece about sample numbers 37-45. It is the figure which showed the measurement result of the weight loss amount by the average value of a natural potential, and the corrosion of a large test piece about sample numbers 46-57. It is the figure which showed the measurement result of the weight reduction amount of the small value by the average value of a natural potential, and corrosion about sample number 46-57. It is the figure which showed the measurement result of the average value of a natural potential, and the maximum corrosion depth of a large test piece about sample numbers 46-57.

Explanation of symbols

1: Small test piece 2: Large test piece 3: Enamelled copper wire 4: Bolt 5: Nut 6: Washer

Claims (1)

% By mass
C: 0.004 to 0.05%,
Si: 0.01 to 1%,
Mn: 0.1 to 2%,
P: 0.03% or less,
S: 0.01% or less,
Cr: 17-20%
Ni: 7.5 to 10%,
Al: 0.05% or less,
N: 0.1-0.3%
O 2: chlorine containing 0.005% or less, further containing either of the following group 1 elements or group 2 elements, the balance being stainless steel consisting of iron and inevitable impurities, including aerobic microorganisms A method for inhibiting potential nobleening of stainless steel, characterized by contacting with fresh water having an ion concentration of 0 mg / L or more and 1000 mg / L or less and maintaining a natural potential at +150 mV or less based on a silver-silver chloride electrode containing saturated potassium chloride.
Group 1: Mo: 0.5-4%
Or, in addition to Mo: 0.5-4%, Cu: 0.5-2%,
V: One or two of 0.5 to 3%.
Group 2: Cu: 0.5-2%,
Or Cu: 0.5-2% and V: 0.5-3%,
Or Ti: 0.01-0.4% and V: 0.5-1.5%.

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