JPH049653A - Method and instrument for measuring corrosion resistance of metallic pipe - Google Patents

Abstract

PURPOSE:To detect the corrosion reaction of an optional position by sectionalizing a portion to be measured by a measuring cell and filling the cell with a testing solution. CONSTITUTION:A compact electrochemical measuring cell 6 is inserted up to a measuring portion by a tie rod 5, a testing solution 18 is allowed to flow into the cell 6 held between O-rings 4, 4, and after confirming the return of the solution 18 from a pipe 16 to a tank 14, an electrochemical measuring apparatus 9 is started to measure the corrosion potential of capillary. At the time of fixing the potential of the capillary portion of the cell 6 after the lapse of a fixed time from the stop of liquid sending, a fine voltage is impressed between the capillary 1 and the counter electrode 8 by the apparatus 9 and a difference from a reference electrode 7 is measured. A current flowing between the capillary 1 and the electrode 8 is also measured to calculate a polarization resistance value. When the calculated value is <50kOMEGAcm<2>, the corrosion resistance of a film is decided as a practically lacking value and a larger value is decided as an effective value for corrosion resistance. Thus, an electrochemical reaction to be the original corrosion reaction on an optional portion in the capillary can be detected.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for measuring the corrosion resistance of a tube endothelial film of a thin tube such as a heat exchanger. More specifically, the present invention relates to a corrosion resistance measuring method used for inspecting thin tubes during periodic open inspections of equipment such as heat exchangers, and an apparatus used therefor.

(Conventional technology) Conventionally, copper alloy thin tubes and carbon steel tubes for heat exchangers using seawater,
Although corrosion holes that have already formed in pipes, typically stainless steel pipes, have been detected using eddy current flaw detection, the evaluation of the anticorrosive properties of coatings for preventing corrosion is only performed in the vicinity of the pipe opening of thin pipes. Only a qualitative evaluation was performed based on the color. Furthermore, a method has been proposed for performing electrochemical measurements near the tube opening and at the tube sheet, but this method can only be used at the tube sheet and tube opening. The corrosion characteristics of the capillary mouth and tube sheet that can be measured using this method are different from those of the heat transfer part deep inside the capillary, which is important for heat exchange in such a capillary. In other words, the anti-corrosion coating on the tube plate and tube opening is destroyed by the turbulence of the inflowing seawater, and the coating is generally thin or non-existent, and there is the problem that measurements at the tube opening do not represent the entire tube. Ta. Furthermore, even if small corrosion holes are discovered by eddy current testing, which is the conventional inspection method for thin tubes,
There was no way to determine whether or not the corrosion hole would grow in the future, which is an important matter in practice.

(Problem to be solved by the invention) In a heat exchanger that uses seawater, it is possible to prevent leakage due to corrosion of the thin tubes by circulating the seawater, which is the cooling water, and the medium to be cooled, which would hinder practical use. This is an important issue in terms of safety and economy. However, as mentioned above, a satisfactory method for quantitatively evaluating the anticorrosive properties of a film on the surface of a capillary tube for preventing corrosion has not yet been developed.

Therefore, the present invention aims to solve the problems described in 1 above, and in particular, 1) a method for evaluating the anti-corrosion properties of a film at any location on a capillary, and 2) a measurement for achieving quantitative evaluation. The purpose of this invention is to provide a method and a device used therefor.

(Means for Solving the Problems) The above objectives were achieved by the following measuring method and measuring device.

That is, the present invention provides the following features: (1) When evaluating the corrosion resistance of a metal pipe, an electrochemical measurement cell is inserted from the pipe opening to an arbitrary measurement site in the pipe, and then a test liquid is placed in the measurement cell that divides the inside of the pipe at that site. A method for measuring corrosion resistance of a metal tube (2), characterized in that the electrochemical behavior is determined by an electrochemical measuring device that is electrically connected to the measurement cell and the tube wall of the metal tube and is connected to the outside of the tube. ) Using artificial seawater or seawater as the test liquid (1)
Measurement method described in section (3) A pair of discs each equipped with a sealing means on the side periphery are mounted on the rod coaxially with facing each other, and a counter electrode is placed inside one of the discs as an electrode, and between the two discs. A reference electrode is provided so as to be electrically insulated from both discs, and one disc is provided with a test liquid feed hole into a small chamber between the two discs and a test liquid return hole from the small chamber. A corrosion resistance measuring device (4) having a guide rod on the outside of at least one of both disks. The measuring device described in (3) (5) The measuring device described in (4) in which the rod and the guide rod are integrated and pass through one of the disks, and (6) the distance between both disks is adjustable ( The present invention provides the measuring device described in item 3), (4), or (5).

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

FIG. 1 is a cross-sectional view of the corrosion resistance measuring device according to the present invention showing a particularly enlarged metal capillary tube, in which 1 is a capillary tube of a heat exchanger or the like whose corrosion resistance is to be measured, 2 is a tube plate, 1a
indicates the inlet or tube mouth of the capillary, 3.3 is the flange with O-ring 4.4, 5 is the flange 3.3
The flange 3.3, the reference electrode 7, and the counter electrode 8 connect and fix the electrochemical measurement cell 6 at regular intervals.
is formed. 7 is a reference electrode attached to flange 3;
8 is the opposite. 9 is an electrochemical measuring instrument, which is connected to the reference electrode 7 by a reference electrode lead wire JO, to the counter electrode 7 by a counter electrode lead wire 11, and to the thin tube 1 by a lead wire]2.
connected with.

Next, 13 is a test liquid injection nozzle into the cell 6, and this nozzle is connected to a test liquid feed pipe 15 from a test liquid tank 14 provided outside the tube. 16 is a return pipe for returning the test liquid connected to the communication hole 17 communicating with the inside of the cell 6 to the tank 14, and ]8 is the test liquid stored in the test liquid tank ]4. ] 9 is a test liquid pump.

Electrodes attached to the flange 3 (counter electrode 8, reference electrode 7
) and the thin tube which is the test electrode should be electrically insulated from each other, so the flange 3 and O-ring 4 are made of an insulating material such as resin. For example, the inner diameter is about 15...35m
The diameter of the flange 3 should be about 10 to about 30 mm in order to be inserted into the capillary tube 1 of m. Since the reference electrode that can be attached to such a small flange needs to be small, suitable electrodes include, for example, electrodes whose silver edges are anodically polarized in dilute hydrochloric acid to produce silver chloride on the surface, or metallic zinc electrodes.

The reference electrode is not limited to the one described above, as long as it is small and always exhibits a constant potential. The opposite pole is stainless steel L
Any metal that does not corrode severely in the test liquid (seawater, etc.) may be used, such as steel wire, copper wire, or platinum wire.

In the case of a heat exchanger between seawater and another liquid, seawater or salt water such as seawater is used as the test liquid, 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 communicating hole connected to the pipe 5. Further, each set of electrodes and lead wires may be electrically insulated from other sets, and is not limited to the configuration shown in the drawings. Further, the protruding portion of the tie rod to the outside of the cell may be eliminated and the tie rod may be used as a guide rod instead, and the spacing between the side discs (flanges 3.3) of the tie rod may be made variable.In addition, in this invention, the pump 19 The cell 6 may be filled with liquid using a siphon instead of using a siphon.

In the above, an O-ring was used as the sealing means for the disk 3.3, but this may be replaced by, for example, a pneumatic O-ring,
With the air removed from the O-ring, it is inserted into the thin tube, and after the insertion is complete, air is blown into the O-ring through the communication hole (not shown) in the tie rod 5.1. It is preferable to keep the inside of the cell 6 airtight by bringing the O-ring into close contact with the thin tube using air pressure. In this way, by keeping the flange airtight with a pneumatic O-ring, it is possible to maintain airtightness even if the inner surface of the capillary has minute irregularities due to corrosion, and prevents the test liquid from leaking into the small electrochemical measurement cell. It can be kept without any problems.

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

During an open inspection of a device to be inspected, such as a heat exchanger, a small electrochemical measurement cell 6 shown in FIG. 1 is inserted into the capillary to the measurement site using the tie rod 5. The tie rod also fixes the distance between the two flanges 3.3. Thereafter, the test liquid in the tank J4, such as artificial seawater, is caused to flow into the small chamber or cell G sandwiched between the two O-rings 4.4 by the pump 19.

Since sealing is performed using an O-ring 4 as a sealing means, the test liquid does not leak out of the cell.

After confirming that the test liquid has returned to the tank 14 from the pipe 16, the electrochemical measuring device 9 is activated to measure the corrosion potential of the capillary. A typical sealing means for a circular pipe is an O-ring, but an appropriate sealing means may be used depending on the internal shape of the pipe.

After a certain period of time (approximately 5 minutes to approximately 30 minutes) has elapsed after liquid filling and liquid feeding was stopped, the potential of the tube portion corresponding to the cell portion of the test piece becomes constant. Afterwards, the electrochemical behavior of the tubules will be measured using various electrochemical techniques such as the micropolarization method, linear polarization method, AC impedance method, and clostat method.

For example, after confirming that the corrosion potential is constant and the inner surface of the capillary is stable, the measuring device 9
2 is used to apply a minute voltage between the thin tube 1 and the counter electrode 8, and the voltage at this time is measured with respect to the reference electrode 7 connected to the measuring device through the wiring 10. At the same time, a current flows between the thin tube 1 and the counter electrode 8, and this is also measured. Polarization resistance is calculated from the ratio of current and voltage obtained. Alternatively, so-called alternating current impedance measurement can be carried out, in which a minute voltage of alternating current of various frequencies is applied to the capillary tube and the impedance at that time is measured.

Measuring polarization resistance is particularly convenient for evaluating the anticorrosion properties of the coating according to the present invention. In general, the smaller the measured polarization resistance, the smaller the corrosion protection (the polarization resistance is approximately 50kQc).
If it is smaller than vd, it can be determined that the corrosion protection of the film is insufficient for practical purposes. When the polarization resistance is approximately 50 Ωcnf, the anticorrosion properties of the coating are good, and the higher the polarization resistance, the better the coating. The specific boundary value of polarization resistance is determined in each case based on detailed conditions such as the alloy composition of the metal tube and the composition of seawater.

In particular, the method of the present invention is suitable for measuring the corrosion resistance of heat exchanger thin tubes using metal thin tubes such as copper alloy thin tubes.

In particular, a representative example of a heat exchanger to which the method of the present invention can be applied is a condenser in a power plant that uses seawater as cooling water.

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

Polarization was performed using a small electrochemical measurement cell using a new aluminum-brass tube made of the same material as the aluminum-brass tube, which has been used for many years as a condenser tube in power plants and has a brown coating on its inner surface that is thought to have good anti-corrosion properties. Measure the resistance 1:::.

When measuring polarization resistance 7, the test liquid used was artificial clean seawater, and after pouring seawater into the measurement cell 2, wait for the corrosion potential to stabilize - 1) Start measuring polarization resistance. did. The potential polarization at that time is 10 mV, which is the natural potential.
! ::: There are four conditions of 5 mV. Polarization resistance is calculated from the current flowing at this time; application chamber B: /W; polarization resistance is calculated from the current.

The polarization resistance of the new capillary was 3190Ω. The polarization resistance of a capillary with a brown film is 128,000Ω.)
is. This measurement shows that the polarization resistance of a tube without an anti-corrosion coating is small (and is easily corroded); It was demonstrated that the corrosion resistance of the coating can be evaluated quantitatively.

It is also easy to understand that other electrochemical measurements such as linear polarization method and alternating current impedance measurement method can also be performed using this compact electrified 7° measurement cell based on the same principle as the polarization resistance method performed here.

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

Because a small electrochemical measurement cell can be inserted and installed at any location within a capillary without damaging the film on the inner surface of the capillary, 1) Electrochemical measurements, that is, electricity, which is the essence of corrosion reactions, can be carried out at any location within the capillary. You can learn about chemical behavior.

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

3) If a small corrosion hole is found in a thin tube, by performing the above measurements on that part, the growth rate of the corrosion hole in the future can be quantitatively evaluated, and the life of the tube can be predicted.

4) By evaluating the anticorrosion properties of the coating and the growth rate of corrosion pores, preventive measures can be taken against corrosion, which is the cause of leakage in capillary tubes when heat exchangers are used.

5) Furthermore, as is clear from the above, the corrosion resistance of the present invention can be measured not by modeling but by using seawater or the like having a composition that is used directly in the capillary of an actual device, so that the corrosion resistance can be measured more accurately. It has the advantage that it can be measured.

[Brief explanation of drawings]

FIG. 1 is an enlarged sectional view of one embodiment of the metal pipe corrosion resistance measuring device of the present invention. In the figure, 1... Metal thin tube, 2... Tube plate, 3... Flange, 4... O-ring, 5... Tie rod, 6...
... Cell, 7... Reference electrode, 8... Counter electrode, 9...
Electrochemical measuring instrument, 10...Reference electrode lead wire, 1]・
...Counter electrode lead wire, 12...Lead wire, 13...Test liquid injection nozzle, 14...Test liquid tank, 15...
Test liquid feed piping, 16...Test liquid return piping, 17...
・Communication hole, J8...Test liquid, 19...Pump

Claims (6)

[Claims] (1) When evaluating the corrosion resistance of a metal pipe, insert an electrochemical measurement cell from the pipe opening to an arbitrary measurement site in the pipe, then fill the measurement cell that divides the pipe at that site with a test liquid, and perform the measurement. A method for measuring corrosion resistance of a metal tube, characterized by determining electrochemical behavior using an electrochemical measuring device that is electrically connected to a cell and the tube wall of the metal tube and placed outside the tube. (2) The measuring method according to claim (1), wherein artificial seawater or seawater is used as the test liquid. (3) A pair of discs, each equipped with a sealing means on the side periphery, are attached to the rod coaxially so as to face each other, and a counter electrode is placed inside one of the discs as an electrode, and a reference electrode is placed between the two discs. is located in an electrically insulating manner with respect to both discs, and one disc is provided with a test liquid feed hole to a small chamber between both discs and a test liquid return hole from the small chamber, and furthermore, the above-mentioned reference A corrosion resistance measuring device characterized by electrically connecting an electrode, a counter electrode, and a metal tube to be tested to an electrochemical measuring device. (4) The measuring device according to claim (3), further comprising a guide rod on the outside of at least one of both discs. (5) The measuring device according to claim (4), wherein the rod and the guide rod are integrated and pass through one of the discs. (6) Claim (3), wherein the distance between the two discs is adjustable;
The measuring device according to (4) or (5).