JP2017181486A - Corrosion evaluation method and corrosion evaluation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a corrosion evaluation method and a corrosion evaluation device capable of evaluating corrosion more conforming to an actual situation.SOLUTION: A corrosion evaluation device comprises: an anode A made of a copper pipe test piece 1 which has a copper oxide (II) coating provided on one surface and also has a water impermeable coating such as a silicone sealant on the other surface; a reference electrode R (hereinafter merely called a standard electrode) as a silver/silver chloride saturated potassium chloride standard electrode which serves as a reference for setting a potential of the anode; a counter electrode C (simulating a health surface of a steel pipe) having a liquid contact surface; a water tank 3 in which aqueous test water is reserved; and a current density measurement circuit 4 which can set the anode A to an arbitrary potential in a state where the anode A, reference electrode R, and counter electrode C are immersed in the aqueous test water in the water tank 3, and measures density of a current flowing between the anode A and counter electrode C.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a pitting corrosion evaluation method and a pitting corrosion evaluation apparatus.

Metal piping is mainly used for piping of equipment in which hot water circulates, and copper is mainly used. Regarding copper pipes, local corrosion (so-called pitting corrosion) may occur in a hot water environment. Therefore, it has been necessary to evaluate the risk of occurrence of such corrosion (corrosiveness).
On the surface of the copper tube in warm water, copper (II) (CuO) is uniformly generated by oxidation of the copper tube itself. Further, it is known that copper (I) oxide (Cu ₂ O, cuprous oxide) exists between copper oxide and copper on the surface layer. These oxide films act as a barrier for oxygen diffusion and suppress corrosion, but when corrosion occurs inside, the environment inside the corrosion is closed by covering with the film. A decrease in pH occurs and the potential of the corroded portion decreases. On the other hand, since the potential of the site where the oxide film is healthy is generally high, macrocells are generated between the corrosion interior and the healthy portion, and corrosion progresses locally at an accelerated rate.

  Concerning components in water quality that affect corrosion, sulfate ions, chloride ions, hydrogen carbonate ions, etc. can be considered, but the effects are not clear. Actually, corrosion may occur even in a water quality that is considered to be less risky at a conventional Mattson ratio (sulfuric acid ion / bicarbonate ion> 1). Therefore, a method for evaluating the corrosivity of water using water that does not have corrosivity as the reference data with the water quality such as pH, conductivity, and chloride ions as variables has been proposed (Patent Document 1). On the other hand, as a method for evaluating pitting corrosion, an anode that is covered with a corrosion product and a cathode that simulates a healthy surface that is not covered with a corrosion product are used as evaluation electrodes. And a method for evaluating the progress of pitting corrosion using the polarization resistance of the anode (Patent Document 2).

JP 2003-75325 A Japanese Patent No. 5348051

However, according to Patent Document 1, it is necessary to know in advance the corrosiveness (presence / absence of corrosion) in an area where the corrosion does not occur (water quality), and new equipment (equipment whose usage is different from before) However, when it is introduced, it takes time to confirm the use record in an area where the equipment is not corroded (water quality), so it is a fact that corrosivity cannot be easily evaluated. Furthermore, even if the use record is confirmed, the corrosion rate is very slow in water quality where corrosion is unlikely to occur. In such a case, it may take several years before the use record can be confirmed. It is very difficult to prepare the data.
On the other hand, in Patent Document 2, the corroded portion that is to be the anode is first forcibly produced, and it is confirmed whether or not the corrosion of the produced corroded portion proceeds. It is not always in a state reflecting the state of the portion where the erosion occurs, and may behave differently from the original pipe surface, and the corrosivity itself to the pipe may not be determined.

  Therefore, in view of the above circumstances, an object of the present invention is to provide a pitting corrosion evaluation method and a pitting corrosion evaluation apparatus that can perform corrosion evaluation in accordance with a more actual situation.

The characteristic configuration of the pitting corrosion evaluation method of the present invention for achieving the above object is as follows:
An anode composed of a copper tube test piece having a copper (II) oxide coating on one side and a water-impermeable coating on the other side, a reference electrode for setting the anode potential, and a counter electrode for current flow are water-based. It is the point which measures the current density which flows when it is immersed in test water and the anode is maintained at the pitting corrosion potential of copper.

  That is, according to the above configuration, since the anode is a copper tube test piece, it is selected as the same material and shape as the copper pipe, and has a copper (II) oxide coating on one surface and the other surface is non-coated. Since it is coated with a water-permeable coating, the coating composition reproduces the corroded surface state of the copper pipe that is about to start pitting corrosion in the actual environment. Evaluation in an approximate environment is possible. When the current density (corrosion current) flowing when the anode is held at the pitting potential is measured, a current flows from a portion of the anode covered with copper (II) oxide. This current is generated by a process that is very close to the state where corrosion progresses locally when the inside of the copper oxide film is pulled up to the pitting potential in the course of corrosion of the actual copper piping. The progress of this can be reproduced more accurately, and an appropriate pitting corrosion evaluation method can be obtained.

  The copper (II) oxide coating may be one in which copper oxide fine particles are deposited and then fixed with aqueous test water.

Specifically, the copper (II) oxide coating can be formed by adhering and depositing copper oxide fine particles on a copper tube test piece and then fixing (fixing) it using aqueous test water.
If comprised in this way, it can be set as the coating | cover nearer to the corrosion of the copper pipe of a real environment.

Further, from the experimental examples described later, it can be evaluated that when the anode holding potential is 200 mV and the steady current density is 10 μA · cm ⁻² or more, there is a high risk of pitting corrosion. Even in the case of copper pipes, the tendency of pitting corrosion can be evaluated.

  In order to achieve the above object, the characteristic configuration of the pitting corrosion evaluation apparatus of the present invention is an anode comprising a copper tube test piece having a copper (II) oxide coating on one side and a water-impermeable coating on the other side. A reference electrode for setting the anode potential, a counter electrode having a wetted surface, a water tank for storing aqueous test water, and the anode, reference electrode and counter electrode immersed in the aqueous test water in the water tank And a current density measuring circuit for measuring the current density flowing from the anode while maintaining the anode at the copper pitting corrosion potential.

  According to the above configuration, the density of the current flowing when the anode made of a copper tube test piece having a copper oxide (II) coating on one side and a water-impermeable coating on the other side is held at the pitting corrosion potential is measured. Therefore, it is possible to reproduce the surface state of the copper pipe where pitting corrosion is about to start, and in evaluating pitting corrosion, evaluation in an environment very close to the actual environment becomes possible. When this anode is immersed in water-based test water and the current density flowing when the anode is held at the pitting corrosion potential is measured, current flows from the portion of the anode covered with copper oxide (II). This current can be said to be generated by a process that is very close to the state where corrosion progresses locally within the copper oxide film during the process of corrosion of the actual copper piping, so the progress of pitting corrosion can be reproduced more accurately. Therefore, an appropriate pitting corrosion evaluation method can be executed.

  Further, a copper having a copper (II) oxide coating on one side and a water-impermeable coating on the other side in a measurement container having an introduction part for introducing aqueous test water and a discharge part for discharging aqueous test water. An anode composed of a tube test piece, a reference electrode for setting the potential of the anode, and a counter electrode having a liquid contact surface for flowing a current are incorporated, and the water system is provided from the introduction part to the discharge part in the measurement container. You may provide the current density measurement circuit which measures the current density which flows from an anode in the state which maintained the anode at the pitting corrosion potential of copper in the state which distribute | circulated the test water.

  In other words, according to the above configuration, the measurement container can perform the pitting corrosion evaluation method in a state where the aqueous test water is distributed over the introduction part and the discharge part, so that it is possible to evaluate the aqueous test water closer to the actual environment. become.

  Furthermore, the measurement container may be interposed in an existing pipe, and the introduction part and the discharge part may be connected to the existing pipe.

  In other words, the measurement container can be evaluated in an environment in which water-based test water that is actually circulated through the existing pipe is connected by connecting the introduction part and the discharge part to the existing pipe. The corrosivity of water-based test water close to the environment can be evaluated.

  Therefore, it became possible to perform corrosion evaluation more in accordance with the actual situation.

Schematic diagram of pitting corrosion evaluation device Schematic of copper tube test piece Graph showing the results of the comparison test Graph showing the results of the comparison test Schematic of the pitting corrosion evaluation apparatus in another embodiment Example of measuring current density when the concentration of sulfate ion and bicarbonate ion is changed, or when the concentration of chloride ion and bicarbonate ion is changed Measurement example of current density when the concentration of sulfate ion, chloride ion, and bicarbonate ion is changed

  Below, the pitting corrosion evaluation method and pitting corrosion evaluation apparatus concerning embodiment of this invention are demonstrated. Preferred embodiments are described below, but these embodiments are described in order to more specifically illustrate the present invention, and various modifications can be made without departing from the spirit of the present invention. The present invention is not limited to the following description.

[Pitting corrosion evaluation device]
As shown in FIGS. 1 and 2, a copper oxide (II) coating 11 is provided on one side and an anode A comprising a copper tube test piece 1 having a water-impermeable coating 12 such as a silicone sealant on the other side, A reference electrode R (hereinafter also simply referred to as a standard electrode) of a standard electrode for silver / silver chloride saturated potassium chloride, which serves as a standard for setting the potential, and a counter electrode C having a wetted surface (simulating the sound surface of a copper tube) The anode A can be set to any potential with the water tank 3 storing the aqueous test water, the anode A, the reference electrode R, and the counter electrode C immersed in the aqueous test water in the water tank 3. The current density measuring circuit 4 that measures the current density flowing between the anode A and the counter electrode C is provided.

[Copper tube test piece]
The copper tube test piece 1 is formed by coating a silicone sealant on one surface (other than the test surface such as the outer surface) of a copper tube 10 that is actually used (covering up to the cut end surface of the copper tube 10), and forming a water-impermeable coating 12 thereon. create. Thereafter, copper (II) oxide fine particles are deposited on the other side (the inner side serving as the test surface), and then moistened with aqueous test water for one day in an environment of high humidity (for example, 40 ° C., 95% RH). By allowing it to stand, a copper (II) oxide coating 11 in which copper (II) oxide fine particles were fixed to the surface of the copper tube was prepared. This was used as the anode A, and was connected to the current density measuring circuit 4 by a conductive wire w for obtaining an electric output from the copper tube 10 portion.
As the counter electrode C, a metal material having a stable oxidation-reduction potential such as a platinum mesh or a SUS mesh is used, and is connected to the current density measuring circuit 4 by a conductive wire w like the anode A.
In this example, a platinum mesh 2 is used as the counter electrode C.

[Current density measurement circuit]
The current density measuring circuit 4 is mainly set to a predetermined copper pitting corrosion potential at the anode A, and a potentiostat 41 that enables measurement of the flowing current, and the obtained current and dimensional information of the copper tube test piece 1. And a calculation unit 42 that calculates the current density from the above.

[Water tank]
The water tank 3 stores the aqueous test water and stirs the inside of the measurement container 30 with the electric heater 31 for setting the temperature of the aqueous test water in the measurement container 30 in which the anode A, the counter electrode C, and the reference electrode R are immersed. The stirring device 32 is provided. As the aqueous test water, water supplied to the copper pipe can be used, and the corrosivity of various water to the copper pipe can be evaluated by measuring the current density.

[Pitting corrosion evaluation method]
In order to evaluate pitting corrosion using the pitting corrosion evaluation apparatus, an anode A comprising a copper tube test piece 1 having a copper (II) oxide coating 11 on one side and a water-impermeable coating 12 on the other side; Then, the counter electrode C having a wetted surface is immersed in water-based test water, and the current density flowing from the anode A is measured in a state where the anode A is maintained at a copper pitting corrosion potential. At this time, for example, when the potential of the anode A is 200 mV (silver / silver chloride saturated potassium chloride standard electrode), if the current density is 10 μA · cm ⁻² or more, it can be evaluated that the risk of pitting corrosion is high. .

[Comparative test]
Prepare a copper tube test piece in which the copper (II) oxide coating in the above copper tube test piece is replaced with copper oxide (I) and a copper tube test piece without a coating (both have a silicone sealant coating). Using tap water with an electric conductivity of about 180 μS / cm as test water, the time-dependent change in current density when the copper tube test piece was held at a potential of 200 mV (silver / silver chloride saturated potassium chloride standard electrode) Asked. Then, as shown in FIG. 3, it was found that the current density settled to a substantially stable value (becomes a steady current) in about 100 hours after the start of the test in any case. The current density at that time varies depending on the type of coating, and it was found that the highest steady current value can be measured when copper (II) oxide is used. Moreover, when the coating material was further variously changed and the steady current value was confirmed, it was as shown in FIG. In FIG. 4, the steady current density was determined as an average current density from 500 hours to 1000 hours after the start of energization. From the figure, it was found that the steady current density of copper (II) oxide was high among various materials, and a steady current density nearly twice that of copper (I) oxide was observed. In addition, although it turned out that a high steady-state current density is obtained also about alumina, it is suitable for performing an evaluation test that copper (II) oxide is used in terms of being a material similar to the copper tube 10 of the base material. It is thought that.
In the above conditions, for example, when the steady current density of 200 mV (silver / silver chloride saturated potassium chloride standard electrode) is about 10 μA · cm ⁻² , the copper tube 10 having a thickness of 1 mm has a service life of about 10 years. Corresponding to the calculated corrosion rate, if it is evaluated as a service life of less than 10 years corresponding to the calculated corrosion rate, it is evaluated that there is a high possibility of pitting corrosion, and the water-based test water is It can be determined that the copper pipe is corrosive.

[Evaluation Example 1]
In FIG. 6, when sulfate ion or chloride ion and hydrogen carbonate ion are present using the copper tube test piece 1 by the above pitting corrosion evaluation apparatus, the concentrations of sulfate ion or chloride ion and hydrogen carbonate ion are respectively shown. A measurement example (evaluation example) of the current density when changed is shown.

  In the measurement example of FIG. 6, the copper A (II) coating 11 is provided on one surface and the anode A composed of the copper tube test piece 1 having the water-impermeable coating 12 on the other surface, and the counter electrode C having a liquid contact surface. Is immersed in an aqueous test water containing sulfate ions or chloride ions and bicarbonate ions, and the current density is measured when the potential of the anode A is 200 mV (silver / silver chloride saturated potassium chloride standard electrode).

In the graph of FIG. 6, the horizontal axis “X” represents the ratio of the ion equivalent of sulfate ion (SO ₄ ²⁻ ) or the ion equivalent of chloride ion (Cl ⁻ ) to the ion equivalent of bicarbonate ion. That is, X is a ratio of ion equivalent of sulfate ion to ion equivalent of hydrogen carbonate ion (ion equivalent of sulfate ion / ion equivalent of hydrogen carbonate ion), or ratio of ion equivalent of chloride ion to ion equivalent of hydrogen carbonate ion. (Ion equivalent of chloride ion / ion equivalent of bicarbonate ion).
In the graph of FIG. 6, the vertical axis “500-1000 h average current density” indicates the average current density (μA · cm ⁻² ) from 500 hours to 1000 hours after the start of energization.

  From FIG. 6, when the current density (corrosion current) is measured by the pitting corrosion evaluation apparatus of this embodiment, the corrosion current changes in the increase / decrease of the ratio of the ion equivalent of chloride ion or sulfate ion to bicarbonate ion. I understand that.

  Specifically, when the ion equivalent of chloride ion or sulfate ion considered to be a corrosion promoting factor is larger than the ion equivalent of bicarbonate ion considered to be a corrosion inhibiting factor, the measured current density is If the ion equivalent of chloride ion or sulfate ion, which is considered to be a corrosion accelerating factor, becomes smaller than the ion equivalent of bicarbonate ion, which is considered to be a corrosion inhibitory factor, the measured current density is reduced. It can be seen that the actual corrosion phenomenon can be accurately evaluated.

  Therefore, according to the pitting corrosion evaluation apparatus of the present embodiment and the pitting corrosion evaluation method realized by the pitting corrosion evaluation apparatus, even when the corrosion inhibition factor and the corrosion promotion factor are present in the aqueous test water, It can be concluded that the corrosion evaluation can be performed according to the situation.

[Evaluation Example 2]
In FIG. 7, the concentration of sulfate ion, chloride ion and bicarbonate ion is changed by the pitting corrosion evaluation apparatus when sulfate ion, chloride ion and bicarbonate ion are present using the copper tube test piece 1. The measurement example (evaluation example) of the current density in the case of making it present is shown.

  In the measurement example of FIG. 7, the copper A (II) coating 11 is provided on one surface and the anode A composed of the copper tube test piece 1 having the water-impermeable coating 12 on the other surface, and the counter electrode C having a liquid contact surface. Is immersed in an aqueous test water containing sulfate ion, chloride ion and bicarbonate ion, and the current density is measured in the case where the potential of the anode A is 200 mV (silver / silver chloride saturated potassium chloride standard electrode).

In the graph of FIG. 7, the horizontal axis “Y” represents a relational expression (function) of the ion equivalent of sulfate ion and the ion equivalent of chloride ion with respect to the ion equivalent of bicarbonate ion. Specifically, Y is a ratio of a value obtained by adding a predetermined value α to an ion equivalent of a chloride ion and an ion equivalent of a sulfate ion to an ion equivalent of a bicarbonate ion ([ion of chloride ion Equivalent + α × Ion equivalent of sulfate ion] / Ion equivalent of bicarbonate ion). The values in FIG. 7 show the case where α is 0.2.
In the graph of FIG. 7, the vertical axis “500-1000 h average current density” indicates the average current density (μA · cm ⁻² ) from 500 hours to 1000 hours after the start of energization.

The above relational expression is based on the so-called “Mattson ratio”, which is known as an evaluation index of water quality, using the ion equivalent of bicarbonate ion, which is a corrosion inhibiting factor, and chloride ion or sulfate ion, which is a corrosion promoting factor. It was examined as a “corrosion index”.
That is, the above relational expression ([ion equivalent of chloride ion + α × ion equivalent of sulfate ion] / ion equivalent of hydrogen carbonate ion) is assumed to be used as a corrosive index as in the so-called “Mattson ratio”. It is.

  From FIG. 7, when the current density is measured by the pitting corrosion evaluation apparatus of this embodiment, even when chloride ions, sulfate ions, and hydrogen carbonate ions are mixed, the corrosion current changes in the increase / decrease of the value of the relational expression. I understand that

  Specifically, when the ion equivalent of chloride ion or sulfate ion considered to be a corrosion promoting factor is larger than the ion equivalent of hydrogen carbonate ion considered to be a corrosion inhibiting factor, that is, When the value increases, the measured current density increases, and the ion equivalent of chloride ion or sulfate ion considered to be a corrosion promoting factor becomes the ion equivalent of bicarbonate ion considered to be a corrosion inhibiting factor. On the other hand, when it becomes small, that is, when the value of the relational expression becomes small, it can be seen that the measured current density becomes small, and the actual corrosion phenomenon can be accurately evaluated.

  Therefore, according to the pitting corrosion evaluation apparatus of the present embodiment and the pitting corrosion evaluation method realized by the pitting corrosion evaluation apparatus, even when the corrosion inhibiting factor and the plurality of corrosion promoting factors are present in the aqueous test water. It can be concluded that the corrosion evaluation can be performed in accordance with the actual situation.

It supplements about the said relational expression.
From FIG. 7, according to the above relational expression, it can be confirmed that the corrosion current can be arranged in a curved line and the corrosivity of the water quality of the aqueous test water can be accurately evaluated.

That is, by using this corrosiveness index, the relationship between the corrosion inhibiting factor and the plurality of corrosion promoting factors can be arranged in a single curve as a corrosion current.
Then, when using the pitting corrosion evaluation apparatus of this embodiment and the pitting corrosion evaluation method realized by the pitting corrosion evaluation apparatus, the anode current density is estimated from the quality of the aqueous test water, that is, the corrosion of the aqueous test water. It is possible to estimate the sex and make it convenient for the user.

Here, the predetermined value α in the relational expression is preferably 0.15 to 0.30, and more preferably 0.18 to 0.25. The predetermined value α is normally set to 0.20. With values in these ranges, the actual water quality corrosivity can be accurately evaluated.
Note that the predetermined value α may be outside the above range depending on the quality of the aqueous test water.

[Another embodiment 1]
As shown in FIG. 5, the pitting corrosion evaluation apparatus is coated with copper (II) oxide on one surface of a measurement container 30 having an introduction part 33 for introducing aqueous test water and a discharge part 34 for discharging aqueous test water. 11 and an anode A made of a copper tube test piece 1 whose other surface is coated with a water-impermeable coating 12, a reference electrode R for setting the potential of the anode, and a counter electrode C having a liquid contact surface are internally provided. And a current density measuring circuit 4 for measuring the current density flowing through the anode while maintaining the anode at a copper pitting corrosion potential in a state where the aqueous test water is circulated from the introduction part to the discharge part in the measurement container 30. It can also be configured. In this case, the pitting corrosion evaluation method can be performed in a state where water-based test water is circulated through the introduction part 33 and the discharge part 34. In addition, if the measurement vessel 30 is interposed in the existing piping and the introduction portion 33 and the discharge portion 34 can be connected to the existing piping, the environment in which the water-based test water that is actually circulated in the existing piping is distributed. Evaluation can be made below.

  In addition, when evaluating in the environment where the water system test water distribute | circulated to the existing piping distribute | circulates, the measurement container 30 can be connected with insulation with respect to the existing piping, and set electric potential is made into the electric potential of existing piping. Thus, when the potential of the existing piping is sufficiently high (for example, exceeding 200 mV), simpler measurement conditions can be realized. In this case, if the measurement container 30 is made of a material such as SUS, the measurement container 30 itself can be used as the counter electrode C.

[Another embodiment 2]
In the previous embodiment, the silicone sealant was used as the water-impermeable coating 12. However, the material is not limited to this as long as it is non-permeable and does not affect the progress of corrosion of the copper tube. Can be used.
In the previous embodiment, the relationship between the current density and the copper tube was exemplified as a criterion for determining the corrosivity that the estimated service life was estimated to be about 10 years. It shall be changed and set as appropriate according to the environment.

  According to the present invention, the corrosiveness of refrigerant water or the like can be evaluated in advance, and can be used effectively, for example, for selecting the material and thickness of a newly installed pipe.

1: Copper tube test piece 3: Water tank 4: Current density measuring circuit 10: Copper tube 11: Coating 12: Water-impermeable coating 30: Measuring vessel 33: Introduction part 34: Discharge part A: Anode C: Counter electrode

Claims (6)

  When an anode composed of a copper tube test piece having a copper (II) oxide coating on one side and a water-impermeable coating on the other side is immersed in water-based test water, and the anode is maintained at the pitting corrosion potential of copper A pitting corrosion evaluation method for measuring the current density.   2. The pitting corrosion evaluation method according to claim 1, wherein the copper (II) oxide coating is one in which copper oxide fine particles are deposited and then fixed with aqueous test water. 3. 3. When the holding potential of the anode is 200 mV (silver / silver chloride electrode standard), it is evaluated that the risk of pitting corrosion is high when the steady current density is 10 μA · cm ⁻² or more. The pitting corrosion evaluation method described in 1.   An anode comprising a copper tube specimen having a copper oxide (II) coating on one side and a water-impermeable coating on the other side, a reference electrode for setting the potential of the anode, and a counter electrode having a liquid contact surface Measure the density of current flowing from the anode while holding the anode at the copper pitting corrosion potential with the anode, the reference electrode and the counter electrode immersed in the aqueous test water in the aquarium. An apparatus for evaluating pitting corrosion comprising a current density measuring circuit.   A copper tube test having a copper (II) oxide coating on one side and a water-impermeable coating on the other side in a measuring vessel having an introduction part for introducing aqueous test water and a discharge part for discharging aqueous test water A state in which the anode of a piece, a reference electrode for setting the potential of the anode, and a counter electrode having a liquid contact surface are provided, and the aqueous test water is circulated from the introduction part to the discharge part in the measurement container Thus, a pitting corrosion evaluation apparatus provided with a current density measurement circuit for measuring the current density flowing from the anode while the anode is maintained at a copper pitting corrosion occurrence potential.   The pitting corrosion evaluation apparatus according to claim 5, wherein the measurement container is interposed in an existing pipe, and the introduction part and the discharge part are connectable to the existing pipe.

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