JP2014218729A - Stainless steel local corrosion suppressing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stainless steel local corrosion suppressing method using chemical species capable of suppressing local corrosion efficiently.SOLUTION: Provided is the stainless steel local corrosion suppressing method used in a wet environment, and a nitrite ion having mol concentration equal to or more than 0.1 times with respect to mol concentration of a chloride ion contained in water content coexists in the water content contacting to the stainless steel.

Description

  The present invention relates to a method for suppressing local corrosion of stainless steel.

  In general, stainless steel is often used as a constituent material for structures that do not allow corrosion. Since stainless steel has a dense protective film (passive film) formed on the surface of the material, it hardly corrodes if used properly.

  However, even stainless steel may corrode depending on the usage environment. For example, in a humid environment in which a structure made of stainless steel (hereinafter, sometimes simply referred to as “structure”) is in contact with seawater (moisture) or the like, chloride contained in seawater in contact with the surface of the structure The passive film of stainless steel may be locally damaged due to the action of substance ions, resulting in local corrosion.

  As used herein, “local corrosion” refers to a phenomenon in which the passive film on the surface of stainless steel is locally damaged, and the new surface exposed at the site where the passive film is damaged rapidly corrodes (dissolves). Point to. Local corrosion is a phenomenon that contrasts with “total corrosion (a form in which the entire surface corrodes almost uniformly)” seen in iron and carbon steel, and the rate of progress in the depth direction as the area of the corrosion site is small. The feature is that is significantly larger than the overall corrosion. In this specification, local corrosion includes pitting corrosion, crevice corrosion, stress corrosion cracking (SCC), and intergranular corrosion.

  Local corrosion proceeds remarkably fast. For example, when pitting occurs, corrosion rapidly expands from the surface of the structure to the depth of the structure, and penetration damage can occur in days to months. Therefore, as an important problem when using stainless steel as an apparatus material, it is possible to suppress local corrosion of stainless steel, and various studies have been made to solve the problem.

  For example, at the design stage of the structure, (1) from among various stainless steels, select stainless steel having high corrosion resistance that can sufficiently withstand the environment in which it is used, or (2) cause local corrosion. Designs that do not have a clearance structure that facilitates the structure, and (3) a structure that suppresses deposits on the surface in order to prevent generation of a clearance are being studied.

  In addition, (4) it is also considered to install an anticorrosion facility on a structure made of stainless steel.

  Furthermore, with respect to the aqueous solution in contact with the structure, (5) removal of chloride ions and (6) removal of dissolved oxygen are performed to remove substances causing local corrosion, or (7) the aqueous solution is cooled. Therefore, studies have been made to reduce the rate of the corrosion reaction and extend the life of the structure.

  However, among the above methods, (1) to (3) can be reflected only at the design stage of the structure, and therefore local corrosion cannot be suppressed for the structure and equipment already used.

  The method (4) is known to be a very effective method for suppressing local corrosion, but cannot be applied when the equipment for cathodic protection cannot be installed due to restrictions on location. Moreover, when a structure is a piping system, it is extremely difficult to suppress local corrosion of the whole area.

  It is difficult to say that the methods (5) to (7) have sufficiently proved the effect of suppressing local corrosion, and since large-scale equipment is required for implementation, adaptation is likely to be difficult.

  On the other hand, the local corrosion control method for adding a chemical (corrosion inhibitor) for suppressing local corrosion to the aqueous solution in contact with the structure is a relatively small-scale addition to the structure already in service. Since it can be applied and local corrosion suppression of the whole structure is possible, several methods are examined (for example, refer patent document 1).

JP 2009-270131 A

  The method described in Patent Document 1 is limited to mild conditions with low corrosiveness, and the effect of suppressing local corrosion is not sufficient, and further improvement has been demanded.

  However, originally stainless steel is a material that should be selected for the grade of steel that does not cause corrosion of the structure, depending on the environmental conditions used, and if corrosion of the structure is a problem, Usually, even if some difficulties are involved, drastic measures such as the above (1) to (3) are selected. Alternatively, a method that is known to be very effective as in (4) above is selected. Therefore, studies have not been sufficiently made on a method for suppressing local corrosion of stainless steel by adding a corrosion inhibitor.

  However, in reality, it is not possible to replace structures that have already caused local corrosion due to stricter conditions than the originally assumed environment or due to location restrictions. Often, there are cases where local corrosion must be suppressed.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a new method for suppressing local corrosion of stainless steel using a chemical species that can easily and effectively suppress local corrosion. And

  In order to solve the above-described problem, one aspect of the present invention is a method for suppressing local corrosion of stainless steel in a wet environment, and in moisture in contact with the stainless steel, the molar concentration of chloride ions contained in the moisture In contrast, the present invention provides a method for suppressing local corrosion of stainless steel in which nitrite ions having a molar concentration of 0.1 times or more coexist.

  In one aspect of the present invention, nitrite is dissolved in the moisture based on the step of measuring the molar concentration of chloride ions contained in the moisture and the measured value of the molar concentration of chloride ions. And a step of coexisting nitrite ions having a molar concentration of 0.1 or more times the measured value.

  In one aspect of the present invention, the molar concentration of the nitrite ions may be one or more times the molar concentration of chloride ions contained in the water.

In one aspect of the present invention, the stainless steel may have a pitting corrosion index determined by the following formula (1) of 18 or more.
[Equation 1]
(Pitting corrosion index) = [% Cr] + 3.3 × [% Mo] + n × [% N] (1)
(In the formula, [% Cr] is the ratio (mass%) of chromium contained in the entire stainless steel, and [% Mo] is the ratio (mass%) of molybdenum contained in the entire stainless steel, [% N] is the ratio (mass%) of nitrogen contained in the entire stainless steel)

In one aspect of the present invention, the molar concentration of chloride ions contained in the water may be 0.6 mol / dm ³ or less.

  In one aspect of the present invention, the moisture temperature may be controlled to 45 ° C. or lower.

  ADVANTAGE OF THE INVENTION According to this invention, the new local corrosion suppression method of stainless steel using the chemical species which can suppress local corrosion simply and effectively can be provided.

It is a schematic diagram which shows the test piece used in the constant potential crevice corrosion test. It is a figure which shows operation in a constant potential crevice corrosion test. It is a potential-current curve in Step 1 of a constant potential crevice corrosion test. It is a graph which shows the corrosion current behavior of Examples 1-3. It is a graph which shows the corrosion current behavior of Example 2 and Comparative Examples 1-3. It is a graph which shows the result of Examples 4-6.

  A method for inhibiting local corrosion of stainless steel according to an embodiment of the present invention is a method for inhibiting local corrosion of stainless steel in a wet environment, and in the moisture in contact with the stainless steel, the molarity of chloride ions contained in the moisture. A nitrite ion having a molar concentration of 0.1 times or more of the concentration coexists.

  As a chemical species capable of suppressing local corrosion in the moisture in contact with stainless steel, the local corrosion of stainless steel can be suppressed by allowing the amount of nitrite ions described above to coexist. Specifically, it is possible to suppress both the occurrence of local corrosion of stainless steel and the progress of local corrosion.

  In this specification, “occurrence of local corrosion” means that the passive film is locally damaged on the surface of stainless steel in contact with moisture, and the state changes from a state where no local corrosion is observed to a state where local corrosion is observed. Refers to that.

  Further, in this specification, “progress of local corrosion” refers to expansion of the range of local corrosion that has occurred in stainless steel. “Expansion of the local corrosion range” includes both the expansion of the local corrosion in the surface direction of the stainless steel surface and the expansion of the local corrosion in the depth direction from the surface of the stainless steel.

  In the method for suppressing local corrosion of stainless steel according to this embodiment, the “wet environment” refers to an environment in which moisture is constantly in contact with a structure made of stainless steel. For example, when drainage is flowing through a pipe made of stainless steel, when storing an aqueous solution in a tank made of stainless steel, or when a frame made of stainless steel is splashed Each environment can be shown, and the method for suppressing local corrosion of stainless steel according to the present embodiment can be widely applied in these environments.

  Examples of stainless steel that can be used in the present embodiment include commonly known stainless steel. Needless to say, the corrosion resistance of stainless steel itself is preferably high in order to suppress local corrosion, and the method for suppressing local corrosion of stainless steel according to this embodiment is pitting corrosion resistance obtained by the following formula (1). The present invention can be suitably applied to stainless steel having an index (PRE) of 18 or more.

[Equation 1]
(Pitting corrosion index) = [% Cr] + 3.3 × [% Mo] + n × [% N] (1)
(In the formula, [% Cr] is the ratio (mass%) of chromium contained in the entire stainless steel, and [% Mo] is the ratio (mass%) of molybdenum contained in the entire stainless steel, [% N] is the ratio (mass%) of nitrogen contained in the entire stainless steel)

  N which is a coefficient of [% N] in the above formula (1) is a value which is about 10 to 30 although it varies depending on a researcher.

  Specifically, it can be suitably applied to stainless steel having corrosion resistance equivalent to or higher than SUS304, which is a stainless steel specified in JIS G4303-1998 “stainless steel rod”.

  If the moisture in contact with stainless steel contains chloride ions exceeding the allowable limit, the passive film on the surface of the structure made of stainless steel is damaged by the action of chloride ions, and local corrosion occurs. And may progress. In the method for inhibiting local corrosion of stainless steel according to the present embodiment, local corrosion is caused by allowing nitrite ions having a molar concentration of 0.1 times or more to coexist with the molar concentration of chloride ions contained in the moisture. It is possible to suppress both the occurrence and progression of

As a method for allowing nitrite ions to coexist, a method for dissolving nitrite in water can be employed. As the nitrite, various salts can be used as long as nitrite ions having a concentration necessary for water containing chloride ions can coexist, such as sodium nitrite (NaNO ₂ ) and potassium nitrite (KNO ₂ ). It can be used suitably.

  The concentration of nitrite ions is preferably equal (1 times) or more to the molar concentration of chloride ions contained in the water. By coexisting such a concentration of nitrite ions, it is possible to further suppress both the occurrence and progression of local corrosion.

  For example, nitric acid ions, sulfuric acid ions, phosphoric acid ions, and molybdate ions are known as chemical species having a local corrosion inhibiting action on stainless steel. Nitrate ions have the following advantages.

  First, nitrate ions and sulfate ions are effective in suppressing local corrosion of stainless steel, but may promote corrosion of structures made of other metal materials such as carbon steel. Therefore, there is a limitation in use in an environment where stainless steel and other metal materials are used in combination.

  In addition, phosphate ions have the property of forming calcium salts by binding with calcium ions. For this reason, when phosphate is added to water with high calcium ion concentration, such as seawater, and phosphate ions coexist, white turbidity and precipitation occur, which can block the cooling water system and transfer heat from the heat exchanger. There is a risk of causing problems such as heat failure.

  Molybdate ion is also known as an effective chemical species for inhibiting the corrosion of carbon steel, so it can be used even in a system in which carbon steel and stainless steel are mixed, and does not produce an insoluble salt. However, the effect of inhibiting corrosion due to the coexistence of molybdate ions is not exhibited unless dissolved oxygen coexists in water, so the effect is low in a reducing environment where the dissolved oxygen concentration is low. In addition, since it is very expensive, it is difficult to apply it to a scene that needs to be consumed in large quantities, such as when applied to a large-scale structure or when applied to a transient cooling water system.

  On the other hand, nitrite ions employed in the method for suppressing local corrosion of stainless steel according to the present embodiment are known to suppress the corrosion of carbon steel and aluminum, while promoting the corrosion of other metal materials. There are few reports. Therefore, nitrite ions, unlike nitrate ions and sulfate ions, can also be used in an environment where a structure including another material such as carbon steel or aluminum is used in combination.

  Also, nitrite ions do not produce a salt that is insoluble in water. Therefore, the malfunction resulting from the salt to precipitate does not arise.

In addition, nitrite ions, unlike molybdate ions, do not require the oxidizing power of dissolved oxygen in water in order to exert a corrosion-inhibiting action, so that a good corrosion-inhibiting effect can be obtained even in a reducing environment. Furthermore, it is much cheaper than molybdate ions, and as described later, the effect of suppressing local corrosion of stainless steel is also high.
Therefore, local corrosion can be effectively suppressed.

The method for inhibiting local corrosion of stainless steel according to the present embodiment is applicable even when the molar concentration of chloride ions contained in moisture is, for example, about the level of seawater. Specifically, if the molar concentration of chloride ions contained in the water is 0.6 mol / dm ³ or less, it can be suitably applied.

  In addition, it is considered that each reaction constituting the occurrence of local corrosion and the progress of local corrosion is likely to proceed kinetically when the temperature of moisture is high. It is thought that it falls. For this reason, it is preferable to manage the temperature of moisture in contact with the stainless steel at 45 ° C. or lower. By managing at such temperature, the reaction rate of the reaction related to local corrosion can be reduced, and the effect of the method for suppressing local corrosion of stainless steel of this embodiment can be enhanced. More preferably, the moisture temperature is controlled to 40 ° C. or lower.

  In the method for inhibiting local corrosion of stainless steel according to the present embodiment, nitrite is added to moisture based on the step of measuring the molar concentration of chloride ions contained in moisture and the measured value of the molar concentration of chloride ions. And dissolving the nitrite ion at a molar concentration of 0.1 times or more of the measured value. By setting it as such a method, it becomes possible to make the nitrite ion required for suppression of local corrosion coexist reliably in the water | moisture content which contacts stainless steel.

  If the molar concentration of chloride ions contained in moisture is known, or if the molar concentration of chloride ions fluctuates, the representative value and the fluctuation range are known. It is better to use nitrite ions in water. For example, when seawater is assumed as the moisture, the molar concentration of chloride ions contained in seawater may vary depending on the ocean area or season, but it is 0.1 times or more based on the upper limit of the fluctuation range. By allowing nitrite ions of a molar concentration to coexist, it becomes possible to reliably coexist nitrite ions necessary for suppressing local corrosion in moisture.

  According to the method for suppressing local corrosion of stainless steel as described above, local corrosion can be easily and effectively suppressed.

  As mentioned above, although the preferred embodiment which concerns on this invention was described, it cannot be overemphasized that this invention is not limited to the example which concerns. Various shapes, combinations, and the like of the constituent members shown in the above-described examples are examples, and various modifications can be made based on design requirements and the like without departing from the gist of the present invention.

[Example]
EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[1. Verification of the effect on the progress of local corrosion (crevice corrosion)]
(Test pieces)
FIG. 1 is a schematic view showing a test piece used in this example, in which FIG. 1 (a) is a side view and FIG. 1 (b) is a front view.

  As shown in the figure, in this example, a flat test piece 1 made of stainless steel of 50 mm × 24 mm × 3 mm was used. As the stainless steel, a commercially available SUS304 steel solution was used, and it was produced by machining. The chemical composition (mass%) of SUS304 is as follows: carbon (C): 0.06%, silicon (Si): 0.44%, manganese (Mn): 1.09%, phosphorus (P ): 0.028%, sulfur (S): 0.005%, chromium (Cr): 18.30%, nickel (Ni): 8.16%.

  The test piece 1 was formed with a bolt hole with a diameter of 6 mm at a position 12 mm from one end in the longitudinal direction, and wet-polished on the surface using 600 mesh water-resistant abrasive paper.

  Such a test piece 1 is sandwiched between a cylindrical polysulfone gasket 2 having a diameter of 20 mm and a thickness of 5 mm and a washer 3, and a bolt 4 is inserted into a bolt hole of the test piece 1, and then a nut 5 is used. Screwed with a torque of 0 N · m. The washer 3, the bolt 4 and the nut 5 were made of industrial pure titanium.

  Furthermore, the lead wire 6 was electrically connected to the other end side of the test piece 1, and the lead wire 6 was covered with an insulating film 7.

(Constant potential crevice corrosion test)
A 0.0563 mol / dm ³ NaCl aqueous solution prepared from deionized water and a special grade reagent (pH of aqueous solution of 40 ° C. = 5.6) was heated to 40 ° C. and degassed by aeration of argon gas, and then the above test piece The lower part of 1 was immersed in an aqueous NaCl solution. The wetted area was about 16 cm ² .

  Further, the Pt counter electrode was immersed, and the following operation was performed using a potentio / galvanostat (manufactured by Solartron, electrochemical measurement system, model number: 1280Z). FIG. 2 is a diagram showing the following operations. FIG. 2A is a graph showing changes in applied potential, where the horizontal axis represents test time (unit: time) and the vertical axis represents potential (unit: V). FIG. 2B is a graph showing the corrosion current measured by the test piece 1 by the potential change shown in FIG. 2A, and the horizontal axis indicates the test time (unit: same scale as FIG. 2A). : Time), and the vertical axis represents the measured corrosion current (unit: μA).

  First, the potentio / galvanostat is set to the potential control mode, and the potential of the test piece 1 immersed in an aqueous NaCl solution at 40 ° C. is swept in the anode direction at a potential feed rate of 30 mV / min to forcibly generate crevice corrosion. (Step 1). When crevice corrosion occurs in Step 1, the current flowing through the test piece (referred to as corrosion current) increases rapidly. Therefore, it is possible to determine whether crevice corrosion has occurred by confirming the behavior of the corrosion current value.

  After the corrosion current reached 1 mA, the potentio / galvanostat was switched to the current control mode, the corrosion current was kept constant at 200 μA for 24 hours, and crevice corrosion was grown (Step 2).

Subsequently, the potentio / galvanostat was switched to the potential control mode, the potential of the test piece 1 was kept constant at 0.3 V (E _hold ), and a necessary amount of drug was added 4 hours after the start of holding the constant potential (Step 3). . Here, the holding potential was set with reference to a report example of the natural potential of SUS304 steel that was in a healthy state after being passivated in neutral natural water. That is, as illustrated in FIG. 2B, if the decrease tendency of the corrosion current is confirmed after the addition of the agent in Step 3, it can be determined that there is an effect of suppressing the progress of crevice corrosion.

The potential of the test piece 1 was measured with reference to the potential of the Ag / AgCl electrode in a saturated KCl aqueous solution at 25 ° C. In this example and the comparative example, all the potentials were standard hydrogen electrodes added with 0.196V. (SHE) standard. For example, the constant potential E _hold is 0.3 Vvs. SHE.

Example 1
Sodium nitrite (NaNO ₂ ) was used as a drug to be added at Step ₃ . The required amount of sodium nitrite is dissolved in an aqueous NaCl solution so that the nitrite ion concentration in the aqueous solution is 0.1 times the 0.0063 mol / dm ³ chloride ion concentration (0.00563 mol / dm ³ ). It was. The pH (40 ° C.) of the aqueous solution after the addition of the drug was measured separately to be 7.1.

(Example 2)
NaNO ₂ was used as a drug to be added at Step ₃ . Example except that the required amount of sodium nitrite was dissolved in the NaCl aqueous solution so that the concentration of nitrite ion in the aqueous solution was 0.0563 mol / dm ³ which is the same concentration (1 times) as the chloride ion concentration. In the same manner as in No. 1, a constant potential crevice corrosion test was conducted. The pH (40 ° C.) of the aqueous solution after addition of the drug was 8.8 when measured separately.

Example 3
NaNO ₂ was used as a drug to be added at Step ₃ . Nitrite ion concentration in the aqueous solution, to the required amount of sodium nitrite as a 10-fold 0.0563mol / dm ³ is a chloride ion concentration (0.563mol / dm ³⁾ was dissolved in aqueous NaCl A constant potential crevice corrosion test was conducted in the same manner as in Example 1 except for the above. The pH (40 ° C.) of the aqueous solution after addition of the drug was measured separately and found to be 9.6.

(Comparative Example 1)
Sodium phosphate (Na ₃ PO ₄ ) was used as a drug to be added in Step ₃ . The same procedure as in Example 1 was conducted except that the required amount of sodium phosphate was dissolved in an aqueous NaCl solution so that the phosphate ion concentration in the aqueous solution was 0.0563 mol / dm ³ , which is the same concentration as the chloride ion concentration. Then, a constant potential crevice corrosion test was conducted. The pH (40 ° C.) of the aqueous solution after addition of the drug was 12.2 as measured separately.

(Comparative Example 2)
Sodium sulfate (Na ₂ SO ₄ ) was used as a drug to be added at Step ₃ . Except that the required amount of sodium sulfate was dissolved in the NaCl aqueous solution so that the sulfate ion concentration in the aqueous solution was 0.0563 mol / dm ³ , which is the same concentration as the chloride ion concentration, in the same manner as in Example 1, A constant potential crevice corrosion test was conducted. The pH (40 ° C.) of the aqueous solution after the addition of the drug was 6.0 when measured separately.

(Comparative Example 3)
Sodium molybdate (Na ₂ MoO ₄ ) was used as a drug to be added in Step 3. Except that the required amount of sodium molybdate was dissolved in an aqueous NaCl solution so that the molybdate ion concentration in the aqueous solution was 0.0563 mol / dm ³ , which is the same concentration as the chloride ion concentration, the same as in Example 1. Then, a constant potential crevice corrosion test was conducted. The pH (40 ° C.) of the aqueous solution after addition of the drug was 7.4 when measured separately.

  FIG. 3 is a potential-current curve at Step 1 of the above-mentioned constant potential crevice corrosion test, where the horizontal axis represents the potential applied to the test piece 1 (unit: V), and the vertical axis represents the measured corrosion current (unit: μA). Indicates. In FIG. 3, the current values measured in Examples 1 to 3 are shown superimposed.

In the potential-current curve of FIG. 3, the stagnation current before the point at which the current rapidly increases (indicated by the arrow with the symbol A in the figure) is about 20 μA to 40 μA, and the stagnation current value is divided by the wetted area. The current density, which was a measured value, was 1.3 μA / cm ^{2 to} 2.5 μA / cm ² . The stagnation current value showed reproducibility in three tests. According to Faraday's law, when this stagnation current density is converted to the corrosion rate of SUS304 steel, it is as small as 0.01 to 0.03 mm / year. No crevice corrosion has occurred yet, and it is considered that healthy passive state is maintained.

Therefore, the current I = 20 μA to 40 μA measured in Step 1 is used in a state where a healthy passive film is formed on the surface of the test piece 1, that is, in a state where a healthy passive state is maintained without causing corrosion. It determines that was a passive holding current _{I pass.}

In the evaluation of the constant potential crevice corrosion test, it was judged that there was an effect of suppressing the progress of crevice corrosion when the measured corrosion current value was reduced by the addition of a chemical. Further, in the test piece 1, the corrosion current is reduced by the addition of an agent, if the current value to be measured becomes equal to or less than the passivation holding current I _pass, crevice corrosion of the test piece 1 stops the progression, again Judged to have returned to a healthy and passive state.

FIG. 4 is a graph showing the corrosion current behavior of Examples 1 to 3 in Step 2 to Step 3 of the above-described constant potential crevice corrosion test, and is a graph corresponding to FIG. The horizontal axis of FIG. 4 indicates that the time when the drug is added is zero. Regions in, are colored strip figures show passivation holding current I _pass obtained in FIG.

As shown in the figure, in Example 1, the corrosion current after adding NaNO ₂ once increased to about 900 μA, but then tended to decrease, and decreased to about 60 μA after 170 hours. Therefore, it is considered that the progress of crevice corrosion is suppressed.

In Example 2, the corrosion current decreased rapidly immediately after the addition of NaNO ₂ and was lower than I _{pass in} about 40 hours. Thereafter, the corrosion current does not exceed I _pass, and it is considered that the progress of crevice corrosion has stopped.

In Example 3, the tendency of decrease in the corrosion current immediately after addition of NaNO ₂ was more remarkable than that in Example 2. After decreasing below I _{pass in} about 3 hours, it decreased to a minute current value of about 1 μA in about 50 hours. Therefore, it is considered that the progress of crevice corrosion has stopped. Further, since the reached corrosion current value is smaller by one digit or more than I _pass , it is considered that the state has shifted to a more stable state than the passive state in the NaCl aqueous solution.

  From these results, when the nitrite ion concentration in the aqueous solution was 0.1 times or more than the chloride ion concentration in the aqueous solution, an effect of suppressing local corrosion of stainless steel could be confirmed.

  In addition, it was found that the higher the concentration of nitrite ions in the aqueous solution, the greater the effect of suppressing the progress of local corrosion, and it is possible to stop the progress of local corrosion by coexisting nitrite ions of equimolar concentration or more. .

  Moreover, FIG. 5 is a graph which shows the corrosion current behavior of Example 2 and Comparative Examples 1-3, and is a graph corresponding to FIG.

In Comparative Example 1, the corrosion current decreased from about 500 μA to about 200 μA after about 5 hours from the addition of Na ₃ PO ₄ , but thereafter changed at about 100 to 200 μA and did not fall below I _pass .

In Comparative Example 2, the corrosion current continued to increase even after the addition of Na ₂ SO _{4 and changed} at a large current value in the vicinity of 1 mA.

In Comparative Example 3, after the addition of Na ₂ MoO ₄ , the corrosion current once increased to about 2 mA, and after about 13 hours, the corrosion current began to decrease and decreased to about 200 μA. However, after that, it changed at about 200 μA and did not fall below I _pass .

  From these results, when compared with the conditions in which ions (chemical species) at the same molar concentration (1 times) as the chloride ion concentration in the aqueous solution coexist in the aqueous solution, the local corrosion inhibitory effect is nitrite ion> phosphoric acid It was found that the order of ion> molybdate ion> sulfate ion was larger. In addition, it was found that the co-existence of equimolar nitrite ions can stop the progress of local corrosion. Compared with the conventional local corrosion inhibition methods using corrosion inhibitors, nitrite It has been found that the method for inhibiting local corrosion of the present invention using ions is very effective for inhibiting local corrosion of stainless steel.

[2. Verification of effects on suppression of occurrence of local corrosion (pitting corrosion)]
(Flat plate test piece)
In this example, a stainless steel flat plate test piece of 15 mm × 15 mm × 10 mm was used. As stainless steel, a commercially available SUS304 steel solution was used in the same manner as the test piece 1. Moreover, a lead wire was electrically connected to one end, and the lead wire was covered with an insulating film.

  As a flat test piece, the surface was wet-polished using 600-mesh water-resistant abrasive paper, and then immersed in 30% nitric acid at 50 ° C. for 1 hour for passivation treatment.

(Pitting corrosion potential measurement)
In accordance with JIS G 0577 “Method for Measuring Pitting Corrosion Potential of Stainless Steel”, the pitting corrosion potential of stainless steel was measured by the following method. Here, the “pitting corrosion potential” is a lower limit potential at which pitting corrosion can occur, and the higher the value, the less pitting corrosion occurs.

First, the flat plate test piece was held in a tetrafluoroethylene jig provided with an opening having a diameter of 1.13 cm so that the surface of the flat plate test piece was in contact with the test solution by 1 cm ² .

  Immediately before the measurement, the surface of the wetted part of the flat plate test piece was dry-polished using 600 mesh abrasive paper and washed with distilled water. The NaCl aqueous solution in which sodium nitrite was dissolved was heated to 40 ° C. and degassed by aeration of nitrogen gas, and then the flat plate test piece was immersed in the NaCl aqueous solution.

Further, after immersing the Pt counter electrode and measuring the natural potential of the flat plate test piece for 10 minutes, the potential of the flat plate piece was measured using a potentio / galvanostat (manufactured by Hokuto Denko Corporation, electrochemical measurement system, model number: HZ-5000). Was swept in the anode direction at 20 mV / min, and the measurement was terminated when the anode current density reached 1000 μA / cm ² .

Based on the provisions of JIS G 0577, the most noble values corresponding to the anode current density of 10 μA / cm ² in the obtained potential-current curve were determined as the pitting potential V _{C and PIT 10} . However, _{VC and PIT10} by the measurement in which pitting corrosion was not confirmed in the optical microscope observation of the flat test piece after the measurement were excluded from the results.

Example 4
With respect to a NaCl aqueous solution having a concentration of 0.00563 mol / dm ^3, the nitrite ion concentration in the aqueous solution is 0 times the chloride ion concentration of 0.00563 mol / dm ³ (no sodium nitrite added), 0.1 A solution in which the required amount of sodium nitrite was dissolved so as to be 1 fold, 1 fold, and 10 fold was used as a test solution, and pitting potential _VC and _PIT10 were measured.

(Example 5)
With respect to a NaCl aqueous solution having a concentration of 0.0563 mol / dm ^3, the nitrite ion concentration in the aqueous solution is 0 times the chloride ion concentration of 0.0563 mol / dm ³ (no addition of sodium nitrite), 0.1 A solution in which the required amount of sodium nitrite was dissolved so as to be 1 fold, 1 fold, and 10 fold was used as a test solution, and pitting potential _VC and _PIT10 were measured.

(Example 6)
Against the concentration of NaCl aqueous solution 0.563mol / dm ^3, nitrite ion concentration in the aqueous solution, 0 times 0.563mol / dm ³ is a chloride ion concentration (sodium nitrite no addition), 0.1 A solution in which the required amount of sodium nitrite was dissolved so as to be 1 fold, 1 fold, and 10 fold was used as a test solution, and pitting potential _VC and _PIT10 were measured.

FIG. 6 is a graph showing the results of Examples 4 to 6, in which the horizontal axis indicates the molar concentration ratio between nitrite ions and chloride ions, and the vertical axis indicates the pitting potential V _{C, PIT10} (unit: V).

  As shown in the figure, in Examples 4 to 6, when nitrite ions having a chloride ion concentration of 0.1 times coexist, an increase in pitting potential is observed, and pitting corrosion is less likely to occur. I understood.

  In Examples 4 to 6, when nitrite ions having a concentration of 1 or more times the chloride ion concentration coexist, pitting corrosion does not occur even if the nitrite ions are made noble up to 1.2V. In the actual environment, it has been found that since there is almost no case of using the material at a potential higher than 1.2 V, it is possible to effectively prevent the occurrence of pitting corrosion.

  In addition, about the pitting corrosion potential when nitrite ion more than 1 times of chloride ion concentration coexists, pituitous corrosion does not occur under these conditions, so nitrite more than 1 time of chloride ion concentration. The pitting potential in the presence of ions is a value greater than at least 1.2V. In FIG. 6, the pitting corrosion potential when nitrite ions having a concentration of 1 or more times the chloride ion concentration coexist is shown at a position of 1.2 V for convenience of illustration, but is larger than 1.2 V for the above reason. In order to show that it is a value, an upward arrow is attached respectively.

  From these results, it is confirmed that the present invention is very effective in suppressing both the occurrence and the progress of local corrosion of stainless steel, as compared with the conventional method of suppressing local corrosion using a corrosion inhibitor. It was.

DESCRIPTION OF SYMBOLS 1 ... Test piece, 2 ... Gasket, 3 ... Washer, 4 ... Bolt, 5 ... Nut, 6 ... Lead wire, 7 ... Insulation film

Claims (6)

A method for suppressing local corrosion of stainless steel in a wet environment,
A method for inhibiting local corrosion of stainless steel in which nitrite ions having a molar concentration of 0.1 times or more with respect to the molar concentration of chloride ions contained in the moisture are coexisting in the moisture contacting the stainless steel. Measuring the molar concentration of chloride ions contained in the moisture;
A step of dissolving nitrite in the water based on a measured value of the molar concentration of the chloride ion and coexisting with a nitrite ion having a molar concentration of 0.1 times or more of the measured value. Item 2. A method for suppressing local corrosion of stainless steel according to Item 1.   3. The method for inhibiting local corrosion of stainless steel according to claim 1, wherein the molar concentration of the nitrite ions is one or more times the molar concentration of chloride ions contained in the moisture. The method for inhibiting local corrosion of stainless steel according to any one of claims 1 to 3, wherein the stainless steel has a pitting corrosion index determined by the following formula (1) of 18 or more.
[Equation 1]
(Pitting corrosion index) = [% Cr] + 3.3 × [% Mo] + n × [% N] (1)
(In the formula, [% Cr] is the ratio (mass%) of chromium contained in the entire stainless steel, and [% Mo] is the ratio (mass%) of molybdenum contained in the entire stainless steel, [% N] is the ratio (mass%) of nitrogen contained in the entire stainless steel) The method for suppressing local corrosion of stainless steel according to any one of claims 1 to 4, wherein the molar concentration of chloride ions contained in the moisture is 0.6 mol / dm ³ or less.   The method for inhibiting local corrosion of stainless steel according to any one of claims 1 to 5, wherein the moisture temperature is controlled to 45 ° C or lower.

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