JP2824804B2 - Method and apparatus for measuring corrosion resistance of metal tubes - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for measuring the anticorrosion property of a coating in a thin tube such as a heat exchanger and an apparatus used for the method. More specifically, the present invention relates to a method for measuring corrosion resistance used for inspection of thin tubes at the time of periodic open inspection of equipment such as a heat exchanger and an apparatus used therefor.

(Prior art) Conventionally, in a heat exchanger using seawater, copper alloy thin tubes, carbon steel tubes, and tubes represented by stainless steel tubes have been detected for corrosion holes that have already been generated due to eddy current flaw detection. The evaluation of the anticorrosion property of the coating for corrosion prevention is merely a qualitative evaluation based on the color near the mouth of the thin tube. In addition, a method has been proposed for performing an electrochemical measurement of the vicinity of the capillary tube mouth and the tube sheet, but this method can be used only for the tube sheet and the capillary tube mouth. The characteristics of the corrosion of the capillary tube mouth and tube sheet that can be measured by this method are different from the heat transfer portion at the back of the capillary tube which is important for heat exchange for such a capillary tube. In other words, the corrosion prevention film is destroyed by the turbulence of the seawater flowing in the tube sheet and the thin tube mouth, and the film is generally thin or not present. Measurement at the thin tube mouth does not represent the entire thin tube. Was. In addition, even if small pits were found by eddy current testing, which was a conventional method for testing thin tubes,
In the future, there was no way to determine a practically important matter whether or not the corrosion hole grows.

(Problems to be Solved by the Invention) In a heat exchanger using seawater, it is safe to prevent leakage due to corrosion of the thin tube, which flows between seawater as cooling water and the medium to be cooled and hinders practical use. Or it is an economically important issue. However, as described above, a satisfactory method for quantitatively evaluating the anticorrosion property of the coating on the capillary surface for preventing corrosion has not yet been developed. Therefore, the present invention aims to solve the above problems,
In particular, it is an object of the present invention to provide a method for evaluating the anticorrosion property of a coating at an arbitrary position of a thin tube, a measuring method for achieving a quantitative evaluation, and an apparatus used for the method.

(Means for Solving the Problems) The above object has been achieved by the following measuring method and measuring device.

That is, the present invention provides: (1) a metal tube which is divided at regular intervals by a movable insulator so that a test solution can be introduced and discharged into the compartment, and both a reference electrode and a counter electrode are formed in the compartment by a metal; The compartment is filled with a test solution in an electrically insulating manner, and the compartment is filled with a test solution to form an electrochemical measurement cell. The measurement cell and the tube wall of the metal tube are placed outside the metal tube. A corrosion resistance measuring device for a metal pipe, which is electrically connected to a placed electrochemical measuring device.

(2) In the electrochemical measurement cell, an insulator consisting of a pair of disks each having a sealing means mounted on each side is opposed to each other and coaxially attached to a rod, and a counter electrode is provided inside one of the disks as an electrode. The corrosion resistance measuring apparatus according to claim 1, wherein a reference electrode is provided between both disks so as to be electrically insulated from the metal tube.

(3) The measuring device according to (1) or (2), further including a guide rod outside at least one of the insulators.

(4) The measuring device according to (1), (2) or (3), wherein the rod and the guide rod are integrated and penetrate one of the insulators.

(5) The distance between both insulators is adjustable (1),
The measuring device according to (2), (3) or (4).

(6) A method for measuring the corrosion resistance of a metal tube, wherein the electrochemical behavior is determined by the measuring device according to (1), (2), (3), (4) or (5).

(7) Use artificial seawater or seawater as the test solution (6)
Measurement method described in the section.

Is provided.

Next, one embodiment of the measuring device of the present invention will be described with reference to the drawings.

FIG. 1 is a cross-sectional view of a corrosion resistance measuring apparatus using the present invention, in which a metal thin tube is shown in a particularly enlarged manner, wherein 1 is a thin tube such as a heat exchanger whose corrosion resistance is to be measured, 2 is a tube sheet,
1a indicates the entrance of the capillary, that is, the port. 3, 3 are flanges having O-rings 4, 4;
Are connected and fixed at regular intervals, and the electrochemical measurement cell 6 is formed by the flanges 3 and 3, the reference electrode 7, and the counter electrode 8.
Is formed. 7 is a reference electrode attached to the flange 3,
8 is a counter electrode. Reference numeral 9 denotes an electrochemical measuring instrument. The reference electrode 7 is a reference electrode lead wire 10, and the counter electrode 8 is a counter electrode lead wire.
11 and connected to the thin tube 1 by a lead wire 12.

Next, reference numeral 13 denotes a nozzle for injecting a test liquid into the cell 6, and the nozzle is a pipe for feeding a test liquid from a test liquid tank 14 provided outside the pipe.
15 are connected. 16 is a communication hole 17 communicating with the cell 6
A return pipe for returning the test liquid connected to
Is a test solution stored in the test solution tank 14. 19 is a test liquid pump.

The electrode (counter electrode 8, reference electrode 7) attached to the flange 3 and the thin tube 1, which is the test electrode, should be electrically insulated from each other, so that the flange 3 and the O-ring 4 are made of an insulator such as resin. Can be For example, in order to insert into the thin tube 1 having an inner diameter of about 15 to 35 mm, the diameter of the flange 3 is about 10 to about
30 mm. Since the reference electrode on which the electrode is mounted on such a small flange needs to be small, for example, an electrode in which a silver wire is anodic-polarized in dilute hydrochloric acid to generate silver chloride on the surface or a metal zinc electrode is suitable.

The reference electrode only needs to be small and always exhibit a constant potential, and is not limited to the above. The counter electrode may be a metal such as a stainless steel wire, a copper wire, and a platinum wire that does not corrode violently with a test solution (such as seawater).

As the test liquid, in the case of a heat exchanger between seawater and another liquid, artificial seawater or salt water such as seawater is usually used, but is not limited thereto. Various liquids may be used depending on the liquid flowing through the metal tube.

Note that the nozzle 13 is not essential and may be a communication hole connected to the pipe 15. Further, each set including the electrodes and the lead wires only needs to be electrically insulated from other sets, and is not limited to the configuration shown in the drawing. Further, the protrusion of the tie rod to the outside of the cell may be eliminated and a guide rod may be used instead, and the interval between the two disks (flanges 3, 3) of the tie rod may be variable. In the present invention, the liquid may be filled in the cell 6 using a siphon without using the pump 19.

In the above description, an O-ring is used as a sealing means for the disks 3, 3, but this is, for example, a pneumatic O-ring. It is preferable that air is blown into the O-ring through a communication hole (not shown) in the tie rod 5 and the O-ring is brought into close contact with the thin tube by air pressure to keep the cell 6 airtight. By maintaining the airtightness of the flange with an O-ring by air pressure, airtightness can be maintained even if there are irregularities due to minute corrosion on the inner surface of the thin tube, and the test liquid is kept in the small electrochemical measurement cell without leaking. be able to.

 Next, the method for measuring corrosion resistance of the present invention will be described.

At the time of open inspection of a device to be inspected, for example, a heat exchanger, a small electrochemical measurement cell 6 shown in FIG. 1 is inserted using a tie rod 5 to a site to be measured in a thin tube. The tie rod also fixes the distance between the flanges 3,3.
Thereafter, the pump 19 causes the test liquid in the tank 14, for example, artificial seawater, to flow into a small chamber or cell 6 sandwiched between the two O-rings 4.

The test liquid does not leak out of the cell because the sealing is performed by the O-ring 4 as the sealing means. It was confirmed that the test liquid returned to the tank 14 from the pipe 16, but the electrochemical measurement device 9
Is started to measure the corrosion potential of the capillary. As a sealing means, an O-ring is typical for a circular pipe, but an appropriate sealing means is used according to the inner shape of the pipe.

After a certain period of time (about 5 to about 30 minutes) has elapsed after stopping the liquid filling and feeding, the potential of the thin tube portion corresponding to the cell portion as the test piece becomes constant. Thereafter, the electrochemical behavior of the tubule is measured by various electrochemical methods such as a micro-polarization method, a linear polarization method, an AC impedance method, and a claw-stat method.

For example, after confirming that the corrosion potential is constant and the inner surface of the thin tube has become stable, the measuring instrument 9 uses the wirings 11, 12
A small voltage is applied between the thin tube 1 and the counter electrode 8 by using the above method, and the voltage at this time is measured with respect to the reference electrode 7 connected to the measuring instrument by the wiring 10. At the same time, a current flows between the thin tube 1 and the counter electrode 8,
This is also measured. The polarization resistance is calculated from the obtained current-voltage ratio. Alternatively, so-called AC impedance measurement in which alternating currents of various frequencies of a minute voltage are applied to the thin tube 1 and the impedance at that time is measured can also be performed.

To evaluate the corrosion resistance of the coating according to the present invention, it is convenient to measure the polarization resistance. Generally, the smaller the measured polarization resistance, the lower the corrosion resistance. If the polarization resistance is smaller than about 50 kΩcm ² , it can be judged that the corrosion resistance of the coating is practically insufficient.
When the polarization resistance is larger than about 50 kΩcm ^2, the corrosion resistance of the film is good, and the larger the polarization resistance, the better the film. The specific boundary value of the polarization resistance is determined in each case by detailed conditions such as the alloy composition of the metal tube and the seawater composition.

In particular, the method of the present invention is suitable for measuring the corrosion resistance of a thin tube of a heat exchanger using a thin metal tube such as a copper alloy thin tube. Particularly as a heat exchanger to which the method of the present invention can be applied,
A typical example is a condenser of a power plant that uses seawater as cooling water.

(Examples) Next, the present invention will be described in more detail based on examples.

Used as a condenser tube of a power plant for many years and polarized by a small electrochemical measurement cell using a new tube made of the same material as a thin tube made of aluminum brass with a brown-colored coating that is considered to have good corrosion protection on the inner surface. The resistance was measured.

When measuring the polarization resistance, the test solution used was artificial clean seawater. After the seawater was poured into the measurement cell, the measurement of the polarization resistance was started after the corrosion potential was stabilized.
The polarization of the potential at that time is four conditions of ± 10 mV and ± 5 mV of the natural potential. Polarization resistance = applied voltage /
The polarization resistance was calculated from the current.

The polarization resistance of the new capillary was 3190Ω. The polarization resistance of the thin tube having a brownish film was 128000Ω. According to this measurement, the polarization resistance of the thin tube having no anticorrosion film formed thereon is small and easily corroded, while the polarization resistance of the thin tube having a brown-colored film which is considered to be rich in anticorrosion is very high. It was proved that the corrosion prevention of the film could be evaluated quantitatively.

In addition, it can be easily understood that other electrochemical measurements such as a linear polarization method and an AC impedance measurement method can be performed by the small electrochemical measurement cell based on the same principle as the polarization resistance performed here.

(Effect of the Invention) The measuring method and measuring device of the present invention have the following excellent effects.

In order to be able to install a small electrochemical measurement cell by inserting it into an arbitrary place in the thin tube without damaging the coating on the inner surface of the thin tube, 1) electrochemical measurement, that is, electricity, which is the essence of the corrosion reaction, in any place in the thin tube Know the chemical behavior.

2) In the case of copper alloy, the corrosion resistance is maintained by the film formed on the surface, but the corrosion resistance of the film can be quantitatively evaluated by electrochemical measurement.

3) When a small corrosion hole is found in the thin tube, the above measurement is performed at that portion, so that the growth rate of the future corrosion hole can be quantitatively evaluated, and the life can be predicted.

4) By evaluating the anticorrosiveness of the coating and the growth rate of the corrosive holes, it is possible to enhance the preventive measures against corrosion, which is a cause of leakage of the thin tube when using the heat exchanger.

5) Further, as is clear from the above, the corrosion resistance of the present invention can be measured more accurately by using seawater or the like having a composition in which a thin tube of an actual device is used directly in the thin tube, rather than modeling it. Has the advantage that it can be measured.

[Brief description of the drawings]

FIG. 1 is an enlarged sectional view of one embodiment of the metal pipe corrosion resistance measuring apparatus of the present invention. In the figure, 1 ... metal thin tube, 2 ... tube sheet, 3 ... flange,
4 ... O-ring, 5 ... Tie rod, 6 ... Cell, 7
…… Reference electrode, 8 …… Counter electrode, 9 …… Electrochemical measurement instrument, 10
…… Reference electrode lead wire, 11 …… Counter electrode lead wire, 12 …… Lead wire, 13 …… Test solution injection nozzle, 14 …… Test solution tank, 15 …… Test solution feed pipe, 16 …… Test solution return pipe , 17
…… Communication hole, 18 …… Test liquid, 19 …… Pump

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01N 27/26 351 G01N 17/02

Claims (7)

(57) [Claims] An inside of a metal tube is partitioned at regular intervals by a movable insulator, a test solution can be introduced and discharged into the compartment, and both a reference electrode and a counter electrode are provided in the compartment with respect to the metal tube. The compartment is filled with a test solution, and the compartment is filled with a test solution to form an electrochemical measurement cell. The measurement cell and the metal tube are placed outside the metal tube. An apparatus for measuring corrosion resistance of a metal tube, which is electrically connected to a chemical measuring apparatus. 2. In the electrochemical measurement cell, a pair of circular insulators having sealing means mounted on respective side circumferences are coaxially attached to a rod so as to face each other, and an electrode is provided inside one of the circular plates as an electrode. 2. The corrosion resistance measuring apparatus according to claim 1, wherein a counter electrode is provided between the two metal plates, and a reference electrode is provided between the two disks so as to be electrically insulated from the metal tube. 3. The measuring apparatus according to claim 1, further comprising a guide rod outside at least one of the insulators. 4. The measuring device according to claim 1, wherein the rod and the guide rod are integrated and penetrate one of the insulators. 5. The measuring device according to claim 1, wherein the distance between the two insulators is adjustable. 6. A method for measuring corrosion resistance of a metal tube, wherein the electrochemical behavior is determined by the measuring device according to claim 1, 2, 3, 4, or 5. 7. The method according to claim 6, wherein artificial seawater or seawater is used as the test solution.