JP2004069472A - Method for evaluating and controlling local corrosion - Google Patents

Abstract

An object of the present invention is to measure a change in water quality of minute internal water simulating local corrosion of a metal member of a water-based plant using a microelectrode, and evaluate the occurrence and progress of local corrosion.
Artificial pits 151 with a simulated structure of local corrosion at A water-based plant of the metal member is provided, the water quality changes in the internal water X of the artificial pits 151, by measuring with a microelectrode M _1, wherein the metal The occurrence or progress of local corrosion in the member is evaluated, and the occurrence or progress of local corrosion in the metal member is suppressed based on the measurement result.
[Selection diagram] FIG.

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a technique for evaluating the state of local corrosion (pitting corrosion, crevice corrosion) occurring on the surface of a heat exchanger or a pipe in a water system and suppressing the progress of the local corrosion.
[0002]
[Prior art]
Local corrosion occurs in the heat exchangers and pipes of water-based plants, and when it progresses, the depth of pitting (erosion) increases, and eventually the penetration of the water-based plant can lead to a shutdown of the water-based plant. is there. For this reason, there is a demand from related industries for a technology capable of reliably predicting the progress of local corrosion without stopping the operation of facilities of a water-based plant and the passage of water.
[0003]
Here, regarding the local corrosion occurring in the water system, since the mass transfer between the part where the corrosion is progressing and other so-called free surfaces is less likely to occur, the water quality of the local corrosion internal water changes extremely, It is known that in the local corrosion internal water, consumption of dissolved oxygen, concentration of metal ions and chloride ions, and pH reduction due to hydrolysis of metal ions occur.
[0004]
At present, regarding the mechanism of pH decrease and the mechanism of chloride ion concentration in local corrosion internal water, the former is qualitatively understood from hydrolysis of metal ions, and the latter is qualitatively understood from electrical neutral conditions. There has been a report of a change in the water quality of the local corrosion internal water measured in a laboratory using a cell in which the local corrosion is modeled (Hiruru Toru, Corrosion and Corrosion Prevention Committee Research Meeting Materials, 29, 17 (1985); Katsuhisa Sugimoto, Corrosion Protection '86, C-105, (1986)).
[0005]
[Problems to be solved by the invention]
However, in the past, it was only necessary to measure the change in the water quality of the local corrosion internal water with the progress of local corrosion, and to perform a process to actively stop the progress of local corrosion that occurred, and to repair the local corrosion There was no technique to evaluate the process before the completion and the confirmation that the local corrosion was completely repaired by measuring the water quality change, particularly pH and chloride ion concentration of the internal water of the pit.
[0006]
Further, the above report is a result under the condition that the chloride ion concentration of bulk is high (0.5 M NaCl), and the measurement method described in the report is performed under the condition of low chloride ion concentration (several mg / L to several hundred mg / L). ), When applied to an actual cooling water system or boiler water system as it is, measurement of water quality changes (decrease in pH, concentration of chloride ions, consumption of dissolved oxygen) in the artificial pit having a simulated structure of local corrosion It took a lot of time to finish.
[0007]
In other words, when the bulk has a low chloride ion concentration condition, in order to measure the water quality change of the internal water of the artificial pit in a short time, the volume of the artificial pit cell is made as small as possible, and the measurement target is It was necessary to reduce the volume of water inside the cell. However, since it is difficult to miniaturize the electrode structure, there has been a limit to the narrowing of the cell itself.
[0008]
Therefore, the present invention evaluates the occurrence and progress of local corrosion by measuring the water quality change (pH and chloride ion concentration) of micro internal water simulating local corrosion of a water-based plant using a microelectrode. It is an object of the present invention to elucidate the mechanism of the progress of local corrosion and to provide a technique for suppressing or repairing the occurrence or progress thereof.
[0009]
[Means for Solving the Problems]
In order to achieve the above object and solve the above technical problem, the present application employs the following means.
[0010]
First, an artificial pit having a simulated structure of local corrosion in a metal member of a water-based plant is provided, and a change in water quality of the internal water of the artificial pit, that is, at least one of the pH, the chloride ion concentration, and the dissolved oxygen concentration of the internal water. Is provided by using a microelectrode.
[0011]
Specifically, an artificial pit having a simulated structure of a local corrosion state is formed at a predetermined location in a water-based plant. For example, a stainless steel, copper, carbon steel or other steel tube is provided with an artificial pit provided with a simulated structure of local corrosion in a part of a steel tube, and microelectrodes are provided, and an aqueous medium is continuously introduced into the tube. After passing the water, the pH, the chloride ion concentration and the dissolved oxygen concentration of the water inside the artificial pit are measured with a microelectrode and evaluated.
[0012]
According to this evaluation method, a change in the water quality of the local corrosion internal water can be reproduced in a short time. That is, by employing a microelectrode in the electrode structure of the artificial pit, the cell can be narrowed, and the volume of the internal water contained in the cell can be reduced. Can be reproduced in time.
[0013]
Here, a “microelectrode” (microelectrometric or microvoltametric electrode) can be defined as a microelectrode used as a working electrode for polarography or voltammetry. Generally, an electrode having a surface area of about 0.01 to 0.1 cm ² is used. However, the present invention also includes a very small electrode having a surface area smaller than that, for example, about 10 ⁻⁷ to 10 ⁻⁶ cm ² , and the present invention can be widely adopted as long as it can measure a change in water quality of a minute amount of internal water. .
[0014]
At present, this microelectrode has begun to be used in the fields of medicine and neurophysiology, such as concentration analysis in living cells, and in previous studies, dissolved oxygen in the liver, brain cells and skin cell tissues, blood, etc. Concentration, hydrogen ion concentration, oxygen activity, and intracellular and external ions such as sodium, potassium, and calcium were measured, and as a result, the state of the microregion inside and around the cell was clarified. Further, in the field of environmental engineering, the present invention was first applied to a study for quantifying in situ biological metabolic activity and chemical reaction rate in low mud in freshwater bodies. Recently, various microelectrodes (eg, pH, O ₂ , N ₂ O, NO ₃ ⁻ , NH ₄ ⁺ , H ₂ S, H ₂ S, CH ₄ , CO _2, etc.) in a biofilm used for wastewater treatment ) Is being actively researched.
[0015]
However, the technical idea of employing the microelectrode as an electrode disposed in an artificial pit that is a simulated structure of local corrosion of a water-based plant, that is, an electrode used to measure a change in water quality of water inside the artificial pit, The effect of "reproducing a change in the water quality of local corrosion internal water in a short time" obtained by using the microelectrode in the artificial pit could not be predicted by a person skilled in the art, even without the idea.
[0016]
Here, regarding the microelectrode for measuring the pH of the internal water, a triodedecylamine having a hydrogen ion-selective permeability in two aqueous phases separated by a liquid membrane (Liquid Ion-exchanging membrane; LIX). It is preferable to measure the potential difference (electromotive force) generated between the aqueous phases. Here, the electromotive force is expressed by Nernst's formula (1) described below, and the activity, that is, the concentration can be quantified from the measured electromotive force.
[0017]
E = E _o + slog (ai) (1)
[E: electromotive force (mv), E _o : standard electrode potential (mv), s: slope (mv), ai: activity (M)]
[0018]
In the circuit, a potential difference occurs not only between liquid membranes but also on the surface of the salt bridge or the electrode-liquid interface, but the comparison can ignore the change in potential difference due to the same transport number of the electrolyte (for example, KCI) or the concentration change of the substance to be measured By using electrodes, the change in electromotive force can be considered to be due only to the change in concentration between LIXs. For the prototype pH microelectrode, measure the potential with a solution of various pH in advance to create a measurement line, and measure the potential of the pH microelectrode in the water inside the cell of the artificial pit that is a simulated structure of local corrosion. Measure and determine the pH of the water inside the cell based on the measurement line.
[0019]
For the measurement of chloride ion concentration, the potential of an Ag / AgCI electrode (ultrafine Ag / AgCI line) with respect to a saturated KCI silver / silver chloride electrode is measured. The relationship between the electrode potential of Ag / AgCI and the chloride ion concentration has the following formula (2), and the chloride ion concentration can be converted from the measured value of the electrode potential of Ag / AgCI.
[0020]
^{[CI - (mg / L)} ] = 2.2 × 10 5 EXP [-43.9 × E] ··· (2)
E: Electrode potential of Ag / AgCI (V)
[0021]
Next, for the microelectrode for measuring the dissolved oxygen concentration, the oxygen concentration is quantified by a current value generated when oxygen is reduced on the surface of platinum or gold coated on platinum. By applying a constant voltage at which a limit current specific to oxygen is obtained, a limit current proportional to the oxygen concentration is obtained, and the oxygen concentration can be quantified. Here, in order to prevent the sensitive part from directly contacting the solution to be measured, a method of isolating the sensitive part from the solution via an oxygen-permeable film may be adopted.
[0022]
Next, in the present application, an artificial pit having a simulated structure of local corrosion in a metal member of a water-based plant is provided, a change in water quality of water inside the artificial pit is measured using a microelectrode, and based on the measurement result, Provided is a method for suppressing local corrosion, which suppresses occurrence or progress of local corrosion generated in the metal member.
[0023]
In this method, based on the measurement results of the pH, chloride ion concentration, and dissolved oxygen concentration obtained from the measurement operation using the artificial pit, local corrosion or heat exchange or piping in facilities of an actual water-based plant or Estimate or predict progress and reduce actual local corrosion.
[0024]
For example, by adding a predetermined amount of a local corrosion terminator according to the measurement result to a predetermined location of the water-based plant, the pH of the internal water in the fine pores or gaps of the local corrosion is actually generated in the water-based plant. And concentration of chloride ions can be suppressed. That is, the phosphate acts as a pH buffering agent in the aqueous medium, and also exhibits the effect of changing the rust membrane formed as the local corrosion progresses to cation permeability, thereby preventing chloride ions from entering. Exhibits the effect of blocking.
[0025]
Here, examples of the “local corrosion terminator” include a mixture of a “phosphoric acid agent” and a “metal salt agent”. Phosphoric acids include phosphonic acids such as orthophosphoric acid, hexametaphosphoric acid, pyrophosphoric acid, tripolyphosphoric acid, aminotrimethylphosphonic acid, 2-phosphono-1,2,4-tricarboxybutane, and hydroxyethylidenephosphonic acid, and their water-soluble compounds. And the like. Examples of metal salt drugs include zinc salts, molybdates, chromates and the like.
[0026]
These agents may be added to the target aqueous system after being mixed in advance, may be added at the same time, or each agent may be added separately. In use, either manual or automatic can be adopted.
[0027]
When a mixture of a phosphoric acid-based drug and a metal salt-based drug is used, a water-soluble polymer is preferably used in combination in order to disperse calcium phosphate and calcium carbonate. As the water-soluble polymer, a known scale dispersant such as a partial hydrolyzate of polyacrylamide can be used.
[0028]
In addition, according to the above method, the effect of suppressing the occurrence or progress of local corrosion should be confirmed by continuously monitoring water quality changes (pH, chloride ion concentration, dissolved oxygen concentration) in the water inside the artificial pit. Can be. In other words, when the effect of suppressing the occurrence or progress of local corrosion in a water-based plant is good, it is reflected in the measurement results that the pH of the water inside the artificial pit decreases, the concentration of chloride ions and the consumption of dissolved oxygen are suppressed. Is done.
[0029]
As described above, the present invention has the technical significance of providing a method capable of monitoring the occurrence or progress of local corrosion in a metal member of a water-based plant in real time and further suppressing the occurrence or progress of the local corrosion. are doing.
[0030]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, a preferred embodiment of the present invention will be described with reference to the accompanying drawings. Note that the present invention is not limited to the following embodiments.
[0031]
First, FIG. 1 is a diagram showing the configuration of the entire water system in which the method for evaluating local corrosion according to the present invention can be suitably performed.
[0032]
An inlet header 2 and an outlet header 3 are provided at the left and right ends of the heat exchanger denoted by reference numeral 1 in FIG. 1 in the water-based plant, respectively. A tube 4 is provided. Liquid W ₁ to be cooled around the heat transfer tube 4 is passed through. Reference numeral W ₂ represents a liquid which is cooled by the heat exchanger 1.
[0033]
Cooling water C ₁ flowing out of a cooling tower indicated by reference numeral 5 is supplied to the inflow side header 2 by a pump 6. The water C ₂ discharged from the outlet header 3 after passing through the heat transfer tube 4 passes through a local corrosion monitoring device 8 disposed in a steel tube 7 connected to the outlet header 3, and the cooling tower 5 is cooled. Will be returned to The material of the tube 7 is arbitrarily selected.
[0034]
The monitoring device 8 includes an artificial pit on which a micro electrode is arranged. This artificial pit is formed in a part of the tube 7 and has a minute communication hole communicating with the internal cavity of the tube 7. That is, the communication hole has a structure simulating pitting corrosion of local corrosion. The number of the tubes 7 and the monitoring devices 8 to be installed is three in FIG. 1, but can be appropriately selected.
[0035]
In the process for continuously passed through the water C ₂ as the test water in the tube 7, detects the potential change of the cell internal water notched by the voltmeter 82, detected value E (see FIG. 1) FIG. It is sent to an arithmetic unit indicated by reference numeral 9 in 1. The arithmetic unit 9 calculates the pH, the chloride ion concentration, and the dissolved oxygen concentration of the internal water flowing into the micropores of the artificial pit according to a predetermined arithmetic expression.
[0036]
Here, a data signal S relating to the pH of the internal water of the artificial pit, the chloride ion concentration, and the dissolved oxygen concentration calculated by the arithmetic unit 9 is sent to the control unit 10 to stop the local corrosion of the predetermined concentration stored in the tank 11. The agent 12 is added via a pump 13 to a pipe 14 through which the cooling water C1 flows. The control unit 10 controls the addition amount of the local corrosion stopper 12 according to the data signal S.
[0037]
By adding a predetermined amount of the local corrosion terminator 12 to the aqueous system by the above method, the pH of the internal water is reduced and the concentration of chloride ions in the micropores or gaps of the local corrosion is actually generated in the aqueous system plant. Can be suppressed.
[0038]
This is because the local corrosion terminator 12 acts as a pH buffering agent in the aqueous medium, and also acts to alter the rust membrane formed as the local corrosion progresses to cation-permeable, thereby forming fine pores or This is because it exerts an effect of preventing chloride ions from entering the gap.
[0039]
Further, the monitoring device 8 can also continuously monitor and confirm the water quality change (pH, chloride ion concentration, dissolved oxygen concentration) of the water inside the artificial pit for the effect of suppressing the occurrence or progress of local corrosion. it can.
[0040]
That is, when the occurrence or suppression of the occurrence of local corrosion in the water-based plant is observed by the addition of the local corrosion stopping agent 12, the pH in the water inside the artificial pit decreases, the concentration of chloride ions, the dissolution of chloride ions increase. A measurement result that the consumption of oxygen is suppressed is obtained from the monitoring device 8.
[0041]
【Example】
Example 1 (Reproduction test of local corrosion).
[0042]
FIG. 2 is a diagram illustrating a basic configuration of the artificial pit 15 which is a simulated structure of local corrosion. Using the artificial pit 15 shown in FIG. 2, an experiment was conducted to reproduce the local corrosion of carbon steel in the actual cooling water system. The internal volume of the cell 15a of the artificial pit 15 is 0.04 cm ³ , the test electrode area is 0.035 cm ² , and the communication hole 15b between the internal water X and the external solution Y of the cell 15a is φ1 mm. To the test piece 15c carbon steel, +600 mV vs. Ag / AgCI / sat. Anodic polarization with a potential of KCI was performed. As the test solution Y, simulated cooling water for actual equipment was used. 1 is a potentiostat, and 15e is Ag / AgCl / sat. The KCL reference electrode, reference numeral 15f, represents a platinum plate for determining the dissolved oxygen of the solution Y.
[0043]
Here, FIG. 3 is a diagram illustrating a configuration of the artificial pit 15 when measuring the pH of the water X inside the cell. Reference numeral M ₁ denotes a microelectrode for pH measurement. The tip of the microelectrode M1 is inserted into the cell 15a. Note that reference numeral 15g denotes Ag / AgCI / sat. Reference numeral 15h denotes a microelectrode M1 and Ag / AgCI / sat. Each represents a digital voltmeter conducted to the KCI electrode.
[0044]
FIG. 4 is a diagram illustrating a configuration of the artificial pit 15 when measuring the chloride ion concentration of the water X in the cell. Reference numeral 15i in FIG. 4 denotes an Ag / AgCl microelectrode having a diameter of φ1 mm.
[0045]
FIG. 5 is a diagram illustrating a configuration of the artificial pit 15 when measuring the dissolved oxygen in the water X inside the cell. Code M ₂ shown in FIG. 5 shows a microelectrodes for dissolved oxygen measurement cell internal water X. Reference numeral 15j is a small current meter for conducting the microelectrode M ₂ (picoammeter).
[0046]
FIG. 6 shows the results of measuring the pH, the chloride ion concentration, and the dissolved oxygen concentration of the internal water X in the cell 15a using the artificial pits 151 to 153 having the structures shown in FIGS. 3, 4, and 5, respectively.
[0047]
FIG. 6 (A) is a diagram (graph) showing the time-dependent change of the pH of the water inside the artificial pit in Example 1, and FIG. 6 (B) is the time-dependent change of the chloride ion concentration of the water inside the artificial pit in Example 1. FIGS. 6A and 6B are diagrams (graphs) showing the change over time of the current value (dissolution current value) generated when the dissolved oxygen in the water inside the artificial pit in Example 1 is reduced.
[0048]
As shown in FIG. 6, the pH of the solution before the start of the test was 8.5, the chloride ion concentration was 200 mg / L, and the dissolved oxygen concentration calculated from the current value was 5.6 mg / L. The pH dropped to around 3 after 24 hours. The chloride ion concentration increased immediately after the start of the test, and was concentrated to 35,000 mg / L after 48 hours. The dissolved oxygen was almost reduced after 24 hours. As described above, with the progress of local corrosion, a decrease in pH of the water X inside the cell, concentration of chloride ions, and consumption (reduction) of dissolved oxygen were confirmed, so that local corrosion of the artificial pit 15 was simulated. The effectiveness as a structure was proved.
[0049]
Example 2 (water quality change suppression verification experiment using a specific local corrosion stopper).
[0050]
Next, at the start of the local corrosion reproduction test in the above experiment, an experiment was conducted to verify whether or not the progress of local corrosion was suppressed by injecting a phosphoric acid-based agent and a zinc salt-based agent. Was. FIG. 7 shows the results of this experiment. The local corrosion inhibitor used in this experiment was a mixture of a phosphate-based drug and a zinc salt-based drug, and the concretely used phosphate-based drug was hexametaphosphoric acid 100 mg PO ₄ / L, zinc salt The system drug is zinc chloride 20 mg Zn / L.
[0051]
FIG. 7 (A) is a diagram (graph) showing the change over time in the pH of the water inside the artificial pit when the local corrosion stopping agent is injected, and FIG. 7 (B) is a graph showing the change in the water inside the artificial pit when the local corrosion stopping agent is injected. FIG. 7 (C) is a diagram showing a change over time in the concentration of a substance ion, and FIG. 7 (C) is a diagram showing a change over time in a current value generated when dissolved oxygen in the water inside the artificial pit is reduced when a local corrosion inhibitor is injected. (Graph).
[0052]
As can be easily understood by comparing and contrasting FIGS. 6 and 7, the injection of the phosphate-based drug and the zinc salt-based drug lowers the pH of water X in the cell, concentrates chloride ions, and consumes (reduces) dissolved oxygen. Was found to be suppressed respectively. That is, the local corrosion terminator has a pH buffering property, and has an effect of preventing chloride ions from penetrating by changing the rust membrane formed as the local corrosion proceeds to cation permeability. As a result, it was found that the decrease in pH, concentration of chloride ions, and consumption (reduction) of dissolved oxygen in local corrosion were effectively suppressed.
[0053]
【The invention's effect】
In the method for evaluating local corrosion and the method for suppressing local corrosion according to the present invention, micro-internal water (artificial pit) simulating local corrosion (pitting corrosion, crevice corrosion) generated on the surface of carbon steel or other steel equipment in an aqueous system. By measuring the pH, the chloride ion concentration and the dissolved oxygen concentration of the internal water), it is possible to monitor the occurrence or progress of local corrosion. In addition, a process to actively stop the progress of local corrosion that occurred in water-based plants is performed, the process until the local corrosion is repaired, confirmation that the local corrosion has been completely repaired, and local corrosion Elucidation of the mechanism of the progress phenomenon can be achieved by monitoring changes in water quality inside the artificial pit.
[0054]
Since the method according to the present invention employs the microelectrode, the cell volume of the artificial pit can be reduced, so that the change in the water quality of the water inside the cell can be reproduced in a short time. In other words, by employing a microelectrode in the electrode structure of the artificial pit, the cell can be narrowed. As a result, the capacity of the internal water contained in the cell can be reduced, and the water quality change of the local corroded internal water can be reduced. Can be reproduced in time.
[0055]
Further, according to the present invention, since the state of local corrosion of the water-based plant can be grasped in real time, the remaining life of the water-based plant can be estimated.
[0056]
Local corrosion can be monitored while the operation of the water-based plant is continued, and a penetration or leakage accident caused by the progress of local corrosion can be prevented.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of an entire water system in which a method for evaluating and suppressing local corrosion according to the present invention can be suitably performed. FIG. 2 A diagram showing a basic configuration of an artificial pit (15) used in an embodiment. FIG. 4 is a diagram showing the configuration of an artificial pit (151) when measuring the pH of water X inside the cell. FIG. 4 is a diagram showing the configuration of an artificial pit (152) when measuring the chloride ion concentration of water X inside the cell. FIG. 5 is a diagram showing the configuration of an artificial pit (153) when measuring the dissolved oxygen in the water X in the cell. FIG. 6 (A) shows the change over time of the pH of the water (X) in the artificial pit in Example 1. Representation diagram (graph)
(B) Diagram showing the change over time in the chloride ion concentration of the water (X) inside the artificial pit in Example 1 (graph).
(C) A diagram (graph) showing a temporal change of a current value (dissolution current value) generated when dissolved oxygen in the water (X) in the artificial pit in Example 1 is reduced.
FIG. 7 (A) is a graph showing a time-dependent change in pH of water (X) in an artificial pit when a local corrosion inhibitor is injected (graph).
(B) Diagram showing the change over time of the chloride ion concentration of water (X) in the artificial pit when a local corrosion inhibitor is injected (graph).
(C) A graph showing a change with time of a current value generated when dissolved oxygen in water (X) in an artificial pit is reduced at the time of injecting a local corrosion inhibitor (graph).
[Explanation of symbols]
8 Monitoring device 12 (with artificial pit structure) 12 Local corrosion stopper 15 (151, 152, 153) Artificial pit 15a Cell 15b Communication hole C ₂ Cooling water (discharged from heat exchanger 1) (test water to be monitored)
M (M ₁ , M ₂ ) Microelectrode X (cell) internal water

Claims (4)

A method for evaluating local corrosion, comprising: providing an artificial pit having a structure for simulating local corrosion in a metal member of a water-based plant, and measuring a change in water quality of water inside the artificial pit using a microelectrode. The method for evaluating local corrosion according to claim 1, wherein at least one of pH, chloride ion concentration, and dissolved oxygen concentration of the internal water is measured by the microelectrode. An artificial pit having a simulated structure of local corrosion in a metal member of a water-based plant was provided, and a change in water quality of the water inside the artificial pit was measured using a microelectrode. A method for suppressing local corrosion, comprising suppressing the occurrence or progress of local corrosion. 4. The method according to claim 3, wherein a local corrosion inhibitor is added to the water-based plant according to the measurement result.

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