JP4019352B2 - Evaluation method and control method of local corrosion - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technique for monitoring local corrosion (pitting corrosion, crevice corrosion) generated on the surface of a heat exchanger or piping in an aqueous system.
[0002]
[Prior art]
When local corrosion occurs and progresses in the heat exchangers and piping of the water-based plant, the depth of pitting corrosion (erosion) increases, and if it eventually penetrates, the operation of the water-based plant may be stopped. There is. For this reason, there is a demand from the related industry for a technology that can reliably predict the progress of local corrosion without stopping the operation and water flow of the facilities of the water-based plant.
[0003]
Here, regarding the local corrosion that occurs in the water system, since the mass transfer between the part where the corrosion is progressing and other so-called free surface becomes difficult to occur, the water quality of the local corrosion internal water changes extremely, It is known that in locally corroded internal water, consumption of dissolved oxygen, concentration of metal ions and chloride ions, and pH decrease due to hydrolysis of metal ions occur.
[0004]
At present, regarding the mechanism of pH reduction and concentration of chloride ions in locally corroded internal water, the former is understood qualitatively from hydrolysis of metal ions, and the latter from electrical neutral conditions. There are reports of laboratory changes in water quality changes in these locally corroded internal waters using a cell that models localized corrosion (Toru Water Flow, Corrosion and Corrosion Protection Committee Committee Meeting, 29, 17 (1985); Sugimoto, K., Corrosion Protection '86, C-105, (1986)).
[0005]
[Problems to be solved by the invention]
However, in the past, only by measuring the water quality change of local corrosion internal water accompanying the progress of local corrosion, the local corrosion that occurred is actively stopped and the local corrosion is repaired. There has been no technique to evaluate the process until it has been confirmed and that the local corrosion has been completely repaired by measuring the water quality changes in the pitting internal water, especially the pH and chloride ion concentration.
[0006]
Therefore, the present invention evaluates the occurrence and progress of local corrosion by monitoring the water quality change (pH and chloride ion concentration) in the micro internal water that simulates local corrosion in an aqueous plant, and also progresses the phenomenon of local corrosion. It is an object of the present invention to provide a technique for elucidating the mechanism of the above, and suppressing or repairing the occurrence or progression thereof.
[0007]
[Means for Solving the Problems]
In order to achieve the above object and solve the above technical problem, the present invention adopts the following means.
[0008]
First, an artificial pit provided with a simulated structure of local corrosion is provided in a metal member of an aqueous plant, and an aqueous medium is continuously passed through the metal member provided with the artificial pit so that the pH and chloride of the internal water of the artificial pit The occurrence or progress of local corrosion in the metal member is evaluated by measuring the ion concentration.
[0009]
Specifically, an artificial pit having a simulated structure of a local corrosion state is formed at a predetermined location of the water-based plant. For example, an artificial pit having a structure simulating local corrosion is provided in a part of a steel tube made of stainless steel, copper, carbon steel or other steel, and an aqueous medium is continuously passed through the tube to Changes in water pH and chloride ion concentration are monitored.
[0010]
The method of measuring the pH of the artificial pit internal water is not particularly limited, but a method of measuring the potential of the antimony electrode inserted into the artificial pit internal water and converting the electrode potential to pH can be adopted.
[0011]
The electrode reaction of antimony is basically an equilibrium reaction between surface antimony oxide (Sb ₂ O ₃ ) and hydrogen ions (H ⁺ ), and is represented by the following equation.
Sb ₂ O ₃ + 6H ⁺ + 6e → 2Sb + 3H ₂ O
[0012]
The formula for converting the antimony electrode potential into pH is expressed as the following formula (1).
E (mV VS.Ag/AgCl/sat.KCl)=−44−59×pH (1)
[0013]
Therefore, if the potential of the antimony electrode provided in the artificial pit is measured, the pH of the water inside the artificial pit can be measured. A saturated potassium chloride / silver / silver chloride electrode is used as a reference electrode. A calibration curve is prepared by measuring the potential of the prototype antimony electrode in advance with solutions of various pHs, and the pH in the artificial pit is obtained from the potential of the antimony electrode in the artificial pit and the calibration curve.
[0014]
Next, regarding the measurement of the chloride ion concentration in the water in the artificial pit, the potential of the Ag / AgCl electrode (extra fine Ag / AgCl line) with respect to the saturated potassium chloride / silver / chloride electrode is measured. There is a relationship represented by the following formula (2) between the electrode potential of Ag / AgCl and the chloride ion concentration, and the chloride ion concentration can be determined from the measured electrode potential of Ag / AgCl. Note that E in the following formula represents an electrode potential (V) of Ag / AgCl.
Cl ⁻ (mg / L) = 3.4 × 10 ⁵ EXP [−43.4 × E] (2)
[0015]
Next, in the present application, the pH and chloride ion concentration of the internal water of the artificial pit provided with the simulated structure of local corrosion in the metal member of the water-based plant are measured, and the local corrosion generated in the metal member based on the measurement result Provided is a method for suppressing the occurrence or progression of
[0016]
In this method, based on the measurement results of pH and chloride ion concentration obtained from the monitoring work using the artificial pit, local corrosion or progress in the equipment such as heat exchangers and pipes of an actual water plant is estimated or progressed. Predict and suppress real local corrosion.
[0017]
For example, by adding a predetermined amount of phosphate according to the measurement result to a predetermined location in the aqueous plant, the internal water in the local corrosion micropores or gaps actually generated in the metal member in the aqueous plant PH reduction and chloride ion concentration can be suppressed. In other words, phosphate acts as a pH buffer in an aqueous medium, and also acts to alter the rust membrane produced with the progress of local corrosion to cation permeability, thereby invading chloride ions. Demonstrate the action. In addition, the addition of the phosphate to the water-based plant can be selected either manually or automatically.
[0018]
Moreover, according to the said method, the inhibitory effect of generation | occurrence | production or progress of a local corrosion can also be confirmed by monitoring the water quality change (pH and chloride ion concentration) of the water inside an artificial pit. That is, when the effect of suppressing the occurrence or progression of local corrosion in the water-based plant is good, it is reflected in the measurement result that the decrease in pH of the artificial pit internal water and the concentration of chloride ions are suppressed.
[0019]
As described above, the present invention has the technical significance of providing a method capable of monitoring in real time the occurrence or progress of local corrosion in an aqueous plant and further suppressing the occurrence or progress of local corrosion. .
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Next, preferred embodiments of the present invention will be described with reference to the accompanying drawings. In addition, this invention is not limited to the following embodiment.
[0021]
First, FIG. 1 is a diagram showing a configuration of an entire water system that can suitably carry out the local corrosion evaluation method according to the present invention, and FIG. 2 is a diagram showing an artificial pit used in the method and a configuration of its periphery.
[0022]
An inflow side header 2 and an outflow side header 3 are provided at the left and right ends of the heat exchanger indicated by reference numeral 1 in FIG. A tube 4 is provided. The liquid W ₁ to be cooled is passed around the heat transfer tube 4. Reference numeral W ₂ represents a liquid which is cooled by the heat exchanger 1.
[0023]
Cooling water C ₁ discharged from the cooling tower indicated by reference numeral 5 is supplied to the inflow header 2 by a pump 6. The water C ₂ discharged from the outflow header 3 after passing through the heat transfer tube 4 passes through the local corrosion monitoring device 8 disposed in the steel tube 7 connected to the outflow header 3, and then the cooling tower 5. Will be returned to. The material of the tube 7 is arbitrarily selected.
[0024]
Here, the configuration of the monitoring device 8 will be described with reference to FIG.
[0025]
The monitoring device 8 is provided with an artificial pit indicated by reference numeral 81. The artificial pit 81 is formed in a part of the tube 7 and includes a minute hole 81 a communicating with the internal cavity 71 of the tube 7. That is, the micro holes 81a have a structure simulating local corrosion pitting corrosion. In addition, although the number of installation of the tube 7 and the monitoring apparatus 8 is three places in FIG. 1, it can select suitably.
[0026]
Next, in the artificial pit 81, an antimony electrode 83 derived from a voltmeter 82a disposed outside the tube 7 and an Ag / AgCl electrode 84 derived from a voltmeter 82b disposed also outside the tube 7, respectively. It penetrates the thermosetting resin body 85 arranged in the micro hole 81a and protrudes into the micro hole 81a so as to be provided.
[0027]
Further, on both sides of the artificial pit 81, two saturated KCl silver-silver chloride electrodes 86, 86 respectively connected to the voltmeters 82a, 82b protrude from the internal cavity 71 of the tube 7 and are opposed to each other. ing. Note that the saturated KCl silver-silver chloride electrodes 86 and 86 function as reference electrodes.
[0028]
In the process of continuously passing water C ₂ as test water into the tube 7, the potential change of the internal water X is detected by a voltmeter 82 (82 a, 82 b), and the detected value E (see FIG. 1) is 1 is sent to the arithmetic unit indicated by reference numeral 9 in FIG. In the arithmetic unit 9, the pH of the internal water X flowing into the artificial pit 81 (the minute hole 81 a) according to the expressions (1) and (2) already described in the item “Means for Solving the Problems” and Find the chloride ion concentration.
[0029]
Here, the data signal S relating to the pH of the internal water X of the artificial pit 81 and the chloride ion concentration calculated by the arithmetic device 9 is sent to the control unit 10, and the phosphate solution 12 having a predetermined concentration stored in the tank 11. Is added to the pipe 14 through which the cooling water C1 is passed through the pump 13. The controller 10 controls the amount of phosphate solution added according to the data signal S.
[0030]
By adding a predetermined amount of phosphate to the water system by the above method, the pH reduction of the internal water and the concentration of chloride ions in the micropores or gaps of local corrosion are suppressed in the actual water system plant. can do.
[0031]
This is because phosphate acts as a pH buffer in an aqueous medium, and also acts to change the rust film produced with the progress of local corrosion to cation permeability, thereby forming fine pores or gaps. This is because it exerts the action of preventing the intrusion of chloride ions into the glass.
[0032]
Further, the monitoring device 8 can continuously monitor and confirm the water quality change (pH and chloride ion concentration) of the internal water of the artificial pit 81 with respect to the effect of suppressing the occurrence or progression of local corrosion.
[0033]
That is, when the occurrence or progression of local corrosion in the aqueous plant is suppressed by the addition of phosphate, the pH drop in the internal water of the artificial pit 81 is suppressed, and the concentration of chloride ions is also suppressed. The measurement result is obtained from the monitoring device 8.
[0034]
【Example】
Hereinafter, an embodiment of the present invention will be described with reference to FIGS.
[0035]
Example 1.
Actual cooling water was used as a test solution. The tube 7 was a carbon steel tube. The test liquid (C ₂ ) was continuously passed through the carbon steel tube 7 by being circulated by a pump. FIG. 3 shows the results of measuring changes over time in the pH and chloride ion concentration in the artificial pit 81. 3A is a graph (graph) showing a change with time in chloride ion concentration in the internal water X of the artificial pit 81, and FIG. 3B is a change with time in pH of the internal water X in the artificial pit 81. It is a figure (graph) showing.
[0036]
The pH in the artificial pit 81 before the start of the test was about 8.5 and the chloride ion concentration was 200 mg / L. However, after about one month, the pH dropped to 3, and the chloride ion concentration was 35000 mg / L. Concentrated to L. Thereby, the pH drop of the internal water X in the artificial pit 81 accompanying the progress of the local corrosion and the concentration of chloride ions are confirmed, and it can be seen that the local corrosion is progressing.
[0037]
That is, it is clear that the monitoring device 8 can grasp the change in pH of the internal water X of the artificial pit 81 and the chloride ion concentration with time, and thus the local corrosion evaluation method according to the present invention was found to be effective. .
[0038]
Example 2.
Based on the same method as in Example 1, test water flow into the carbon steel tube 7 was started, and when changes in pH and chloride ion concentration of the internal water X in the artificial pit 81 were confirmed, phosphorus was added. Experiments injecting acid salts were conducted. 4A is a graph (graph) showing a change in chloride ion concentration in the internal water X of the artificial pit 81 with time, and FIG. 4B is a change in pH in the internal water X of the artificial pit 81 with time. It is a figure (graph) showing.
[0039]
As shown in FIG. 4, it was confirmed that addition of phosphate suppresses a decrease in pH and concentration of chloride ions in the artificial pit 81. Therefore, it is understood that phosphate has an ability to moderate pH, and has the effect of blocking the entry of chloride ions by changing the rust membrane produced with the progress of local corrosion to cation permeability. (See FIGS. 3 and 4 for comparison).
[0040]
【The invention's effect】
In the method for evaluating local corrosion and the method for suppressing local corrosion according to the present invention, minute internal water (artificial pits) simulating local corrosion (pitting corrosion, crevice corrosion) generated on the surface of carbon steel and other steel equipment in an aqueous system. By monitoring the pH of the internal water) and the chloride ion concentration, it is possible to monitor the occurrence or progress of local corrosion.
[0041]
In addition, the local corrosion that has occurred in the water-based plant is actively stopped, the process until the local corrosion is repaired, confirmation that the local corrosion has been completely repaired, and the local corrosion It is possible to elucidate the mechanism of the progress phenomenon by monitoring the water quality change in the artificial pit.
[0042]
Furthermore, according to the present invention, since the state of local corrosion of the aqueous plant can be grasped in real time, the remaining life of the aqueous plant can be estimated.
[0043]
It is possible to monitor local corrosion while continuing the operation of the water-based plant, and to prevent a penetration / leakage accident caused by the progress of local corrosion.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of an entire water system that can suitably implement a local corrosion evaluation method according to the present invention. FIG. 2 is a diagram showing a configuration of an artificial pit (81) used in the method and its surroundings. (A) The figure (graph) showing the time-dependent change of the chloride ion concentration of the water in the artificial pit in Example 1.
(B) The figure (graph) showing the time-dependent change of pH of the artificial pit internal water in Example 1.
4A is a graph (graph) showing a change over time in the chloride ion concentration of water in an artificial pit in Example 2. FIG.
(B) The figure (graph) showing the time-dependent change of pH of the artificial pit internal water in Example 2.
[Explanation of symbols]
7 Tube 8 Monitoring device 9 Arithmetic device 12 Phosphate solution 81 Artificial pit 81a Micro hole 82a, 82b Voltmeter 83 Antimony electrode 84 Ag / AgCl electrode 85 Thermosetting resin body C ₂ (discharged from heat exchanger 1) Cooling water (test water to monitor)
E Detection value X Internal water (of artificial pit 81)

Claims (3)

An artificial pit provided with a simulated local corrosion structure is provided in a metal member of an aqueous plant, and an aqueous medium is continuously passed through the metal member provided with the artificial pit. The pH and chloride ion concentration of the internal water of the artificial pit The method of evaluating local corrosion characterized in that the occurrence or progress of local corrosion in the metal member is evaluated by measuring. The notched with a simulated structure of definitive local corrosion to the metal member of the aqueous plant provided, said notched continuously passed through an aqueous medium to the metal member provided is the internal water notched pH and chloride ion A method for suppressing local corrosion, comprising measuring concentration and suppressing occurrence or progress of local corrosion occurring in the metal member based on the measurement result.   The method for suppressing local corrosion according to claim 2, wherein a predetermined amount of phosphate is added to an aqueous plant according to the measurement result.

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