JP3257099B2 - How to monitor corrosion - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a corrosion monitoring method, and more particularly to a corrosion monitoring method for easily and accurately predicting and monitoring the progress of corrosion of a metal member in contact with a water system such as freshwater or seawater.

[0002]

2. Description of the Related Art If corrosion progresses in pipes, heat exchangers, or the like and leads to penetration, an unexpected situation such as a stoppage of plant operation may occur, and a technique for detecting corrosion in advance is required.

Conventionally, as a method of monitoring the pitting corrosion of a metal member in contact with an aqueous system, for example, a copper tube, a copper test piece is immersed in test water (aqueous system), and a change with time of a potential difference between the reference electrode and a test electrode is measured. The method is adopted.

However, a method of controlling the amount of a corrosion inhibitor to be added to prevent corrosion of a copper tube based on the measurement result of such a potential difference has been implemented. The amount of the corrosion inhibitor added is controlled so that the potential difference becomes 190 mV or less.

[0005] As described above, the monitoring of corrosion is extremely important not only for predicting and monitoring pitting corrosion, but also for controlling the amount of a corrosion inhibitor added for anticorrosion, and monitoring corrosion more reliably and easily. It is requested to do.

[0006]

However, the above-mentioned conventional monitoring method of immersing a test piece in test water and measuring a potential difference is a method for monitoring a potential rise of a metal member before pitting occurs. Is an excellent method, but requires long-term continuous measurement. In order to increase the potential response in the test water, it is necessary to devise the material and processing of the test piece. There was a problem such as.

The present invention solves the above-mentioned conventional problems, and eliminates the need for a special device for corrosion of a metal member in contact with a water system, without requiring special measures for the shape of a test piece, and for a short period of time. It is an object of the present invention to provide a method of monitoring corrosion, which can easily and surely monitor by measurement of corrosion.

[0008]

The present invention relates to a method for monitoring corrosion of a metal member in contact with an aqueous system, the method comprising immersing a test piece made of the same material as the metal member in the aqueous system, and then phosphating the test piece. The test piece is characterized in that the test piece is immersed in an acid-containing water, and after a certain period of time, the potential of the test piece is measured.

Hereinafter, the present invention will be described in detail. In the method of the present invention, there is no particular limitation on the shape and processing method of the test piece, and various types such as a tube, a half tube, a plate, and a line can be used.

The time during which the test piece is immersed in the water system is appropriately determined according to the degree of progress of corrosion depending on the relationship between the water system and the material of the test piece (the material of the metal member for monitoring corrosion). In this case, one to two weeks may be sufficient.

The test piece is immersed in an aqueous system for a certain period of time, and then immersed in phosphate-containing water. The phosphate-containing water is preferably a neutral or slightly alkaline phosphate buffer, and its phosphorus concentration is It is preferably at least 100 ppm.

The time for immersing the test piece in such phosphate-containing water is usually about 1 to 120 minutes, especially about 20 to 30 minutes.

The test piece was immersed in phosphate-containing water to increase the corrosion potential, and confirmed a clear increase in potential.
Corrosion can be reliably predicted.

When the test piece after being immersed in the phosphate-containing water is pulled up and returned to the original water system, the test piece follows the corrosion state of the water system in a relatively short time, and the surface state of the test piece is changed. There is no effect of immersion in phosphate-containing water. Thus, the method of the present invention can be performed using one test piece repeatedly.

For example, a water tank I containing water to be monitored and a water tank II containing water containing phosphate are prepared.
To perform semi-continuous monitoring by automatically switching the test piece so that it is always immersed in the water tank I, taken out of the water tank I only during measurement, immersed in the water tank II, and returned to the water tank I again. Can also.

As a spot-like method, a test piece can be immersed in a water tank filled with water to be monitored, and phosphate can be added at the time of measurement to adjust the water quality to a predetermined value.

[0017]

The corrosion potential (E _corr ) of a metal member in contact with an aqueous system, for example, a copper member, is determined by the following two reactions.

The anode reaction: Cu → Cu ²⁺ + 2e ^- cathodic _{^{reaction: 2e - + H 2 O +}} 1/2 O 2 → 2OH - microorganisms in the water, due to the adhesion of such scale components, these potential rise factor is accumulated on the surface of the metal member In this case, the potential increasing factor promotes the cathode reaction. The corrosion potential increases due to the promotion of the cathode reaction.

By the way, the phosphate-containing water has an action of suppressing the anodic reaction while maintaining such an aqueous cathodic reaction. For this reason, the corrosion potential increased by the promotion of the cathode reaction by the potential increasing factor in the aqueous system,
It is measured as a value amplified by the anodic reaction inhibiting action in the phosphate-containing water. For this reason, a rise in corrosion potential can be detected stably and reliably with high sensitivity.

The following is a description of the effect of increasing the corrosion potential increase according to the present invention in the case where the polarization curves of the anodic and cathodic reactions of a test piece immersed in an aqueous system are as shown in FIG. Will be described in detail. (Note that in FIG. 1, the polarization curve is schematically shown by a straight line.) In FIG. 1, φ _C is a cathode equilibrium potential, φ _A is an anode equilibrium potential, a straight line a is a cathode polarization curve, and a straight line b is It is an anodic polarization curve, and I _corr corresponding to the intersection of the polarization curves a and b is the corrosion current, φ
_corr is detected as corrosion potential.

When the scale and microorganisms adhere to the test piece and the cathodic reaction is accelerated, the cathodic polarization curve is shown by a broken line c, and the corrosion potential is φ _M1 corresponding to the intersection M ₁ between the broken line c and the straight line b. Becomes That is, the potential increases.

Conventionally, corrosion is predicted by detecting this potential φ _M1 .

On the other hand, in the present invention, by immersing the test piece in the phosphate-containing water, the anodic reaction is suppressed while the cathodic reaction is promoted (broken line c). When the anodic reaction is suppressed, the anodic polarization curve becomes the one shown by the one-dot chain line d, and the corrosion potential becomes φ _M2 corresponding to the intersection M ₂ between the dashed line c and the one-dot chain line d.
Becomes

As described above, the corrosion potential is _reduced from the conventional φ _M1 to φ _M2
As a result, the risk of corrosion or pitting can be predicted earlier and more reliably than before.

[0025]

The present invention will be described more specifically with reference to the following examples.

Example 1 Using a half-width copper tube as a test piece, two test pieces A and B were immersed in the following test waters A and B, respectively, for 8 days, and then immersed in 100 ml of the following phosphate buffer solution to cause corrosion. The potential was measured, and the change with time of the corrosion potential was shown in FIG.

Test water A: Heat storage water with pitting occurrence test water B: Heat storage water without pitting occurrence phosphate buffer: M / 15 Phosphate buffer (Sφrensen's buffer = M / 15) (Potassium dihydrogen phosphate + M / 15 sodium phosphate dibasic) (pH = 8.1, P = 4100 ppm) As is clear from FIG. 2, the potential of the test piece B immersed in the test water B almost increased. In contrast, test piece A immersed in test water A had a potential of about 30 minutes and a potential of 26.
It rose to 0 mV and corrosion could be clearly predicted.

[0028]

As described above in detail, according to the corrosion monitoring method of the present invention, no special device is required for a computer, a recorder and the like. The shape and processing method of the test piece are also arbitrary. An increase in corrosion potential can be detected by immersion for a short period of time. There is no need to measure potential changes continuously. It can be easily implemented on site. Since pitting potential can be detected early,
It is possible to add the corrosion inhibitor without losing the time of addition, and it is possible to perform reliable corrosion prevention. This prevents corrosion damage to metal members in contact with water systems and the occurrence of failures caused by such damages, enabling safe and stable operation of various plants and extending the life of metal device members. It is said.

[Brief description of the drawings]

FIG. 1 is a graph showing a schematic diagram of a polarization curve.

FIG. 2 is a graph showing the results of Example 1.

Continuation of the front page (56) References JP-A-56-155844 (JP, A) JP-A-1-121746 (JP, A) JP-A-4-93649 (JP, A) JP-B-45-5944 (JP) , B1) Nakauchi, H., Corrosion monitoring, Chemical Technology Research Institute News, Chemical Industry Data, Japan, 16/5, 118-124 (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 27 / 26 351 G01N 17/00 G01N 33/20 JICST file (JOIS)

Claims (1)

(57) [Claims] In a method for monitoring corrosion of a metal member in contact with an aqueous system, a test piece made of the same material as the metal member is immersed in the aqueous system, then immersed in phosphate-containing water, and after a certain period of time, A method for monitoring corrosion, comprising measuring a potential of the test piece.