JPH0598476A - Method for preventing corrosion of metal - Google Patents

Abstract

PURPOSE:To efficiently prevent the corrosion of a metal in a system in which the metal is kept in contact with water, especially in a system in which water is passed through a copper pipe and to make this method fit for a system using a heat transfer tube for a heat exchanger made of a copper pipe, copper piping for hot water supply or a copper pipe for an air conditioner. CONSTITUTION:In order to prevent the corrosion of a metal in a system in which the metal is kept in contact with water, the natural potential of the metal is monitored and a corrosion inhibitor is added to the system so that the potential is reduced to below pitting corrosion potential.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved method for preventing metal corrosion. More specifically, the present invention monitors the natural potential of a metal in a system in which a metal and water are in contact with each other, particularly a system in which water is passed through a copper tube, and the potential becomes less than the pitting potential. As described above, the present invention relates to a method for efficiently preventing metal corrosion by adding a corrosion inhibitor.

[0002]

2. Description of the Related Art Conventionally, copper has been widely used as a material for heat transfer pipes for heat exchangers and pipes for hot water supply, for example, because it has excellent heat transfer properties and corrosion resistance as compared with other metal materials. However, in a copper pipe used for such an application, pitting corrosion sometimes occurs and water leaks, which causes an unfavorable situation. Studies of such copper pitting phenomena have been conducted for a long time, especially in Europe, and it is known that when the natural potential of copper reaches a certain value or more, pitting corrosion occurs, and this potential is the pitting potential. It is called. For example, in a study using city water in Brussel, the pitting potential differs depending on the geometrical shape of the metal surface, and when the saturated sweet-potato electrode (SCE) is used as a reference, a copper tube having an inner diameter of 16 mm is 170 mV (SCE), Diameter 0.4 mm
150 mV for copper wire and 100 mV for 0.2 mm diameter copper wire
(SCE) has been reported to be "M. Pour.
baix, Corrosion, Vol. 25, pp. 267, 1969 ". It has also been reported that the pitting corrosion potential of a copper tube having an inner diameter of 11 mm in which hot water having a temperature of 60 ° C flows is 150 mV (SCE).
Vol. 113, 1981 ". Furthermore, by continuously adding phytic acid to the hot water, the natural potential of the copper pipe can be suppressed to 150 mV or less, the occurrence of pitting corrosion can be prevented, and the potential of the copper pipe where pitting corrosion has already occurred gradually. "Baba et al., Anticorrosion Technology, Volume 34, No. 1, Page 10, 1985" has also been reported to decline. However, although the pitting potential for copper is known as described above, in an actual system, the idea of preventing corrosion by controlling the addition of the corrosion inhibitor by measuring the spontaneous potential of the metal was proposed. Never happened.

[0003]

Under the above circumstances, the present invention efficiently corrodes metal in a system in which metal and water are in contact, especially in a system in which water is passed through a copper pipe. The purpose of the method is to provide a method for preventing it.

[0004]

Means for Solving the Problems As a result of intensive studies for achieving the above-mentioned object, the present inventors have found that a system in which a metal and water are in contact with each other, particularly a system in which water is passed through a copper pipe. In, while monitoring the self-potential of the metal, it was found that the object can be achieved by adding a corrosion inhibitor to the system so that the value becomes less than the pitting potential, and the present invention based on this finding. Has been completed. That is,
The present invention, in a system in which a metal and water are in contact with each other, adds a corrosion inhibitor to prevent corrosion of the metal, monitors the spontaneous potential of the metal, and makes the value less than the pitting potential. In addition, the present invention provides a method for preventing metal corrosion, which comprises adding a corrosion inhibitor to the system.

The present invention will be described in detail below. The pitting potential in the present invention is a spontaneous potential of a metal in which pitting corrosion starts to occur in a system in which a metal and water are in contact with each other, and the kind of the metal, the geometrical shape of the metal surface, It depends on the nature and temperature of the contacting water. Therefore, when applying the method of the present invention, it is necessary to previously determine the pitting potential in the system to which the method is applied. There is no particular limitation on the method for measuring the spontaneous potential of the metal for determining the pitting potential, a conventionally known method,
For example, a method using a saturated silver / silver chloride electrode (saturated sweet potato electrode) or a saturated calomel electrode can be used.

In the method of the present invention, the spontaneous potential of the metal in a system in which the metal and water are in contact is continuously or intermittently monitored, and the value is less than the pitting potential determined as described above. It is necessary to add a corrosion inhibitor so that The various electrodes described above can be used as a method for monitoring the natural potential. The addition of the corrosion inhibitor may be performed continuously or intermittently,
Alternatively, the self-potential may be detected and added by an automatic addition method. The type of the corrosion inhibitor is not particularly limited as long as it lowers the natural potential, and is appropriately selected depending on the type of metal. For example, when the metal to which the method of the present invention is applied is copper, hydrazine, sulfite, oxygen scavengers such as hydroquinone, L-ascorbic acid, and erythorbic acid, polyethylene polyamines such as phosphonic acids and pentaethylene hexamine, and thiosulfate. Chelating agents such as salts, propionamide, glycine, thiourea and thioglycolic acid are preferably used. As the system in which the method of the present invention is applied and the metal and water are in contact, a system in which water is passed through a copper pipe is particularly preferable, and examples of the copper pipe include a heat transfer pipe for a heat exchanger and a hot water supply pipe. Examples include piping and piping for air conditioning equipment.

[0007]

The present invention will be described in more detail by way of examples, which should not be construed as limiting the invention thereto. It is assumed that, out of 38 air handling units, the method of the present invention is applied to the site of the stored cold water system where pitting occurs in the heat exchanger copper coil of the unit about 3 years after installation in 29 units. And conducted the test. The copper coil of the heat exchanger in the field used the phosphorus deoxidized copper pipe of inner diameter 14.88 mm and wall thickness 0.5 mm, and the length was 502 m in the large unit.

Comparative Example 1 Using the test apparatus shown in the schematic view of FIG. 1, the relationship between the change in spontaneous potential of a copper pipe and the occurrence of pitting corrosion when no corrosion inhibitor was used was determined. A header is attached to the return pipe 5 for cold water (12 ° C.), a phosphorus deoxidized copper pipe 1 having an inner diameter of 14.88 mm, a wall thickness of 0.5 mm and a length of 2 m is connected in parallel, and a valve 4 is used to reduce the flow velocity in the pipe. Adjusting to 0.3 m / sec, 1 m / sec, 1.5 m / sec, dipping KCl saturated silver / silver chloride reference electrode 2 in heat storage tank 6, and measuring the natural potential of copper tube 1 by potentiometer 3 every day.
Measured twice. The test period is 56 days. The relationship between the test period and the natural potential of the copper tube at each flow rate is shown in FIG. As can be seen from FIG. 2, the spontaneous potential reached 190 mV (saturated silver / silver chloride electrode) or more at any flow rate, and pitting corrosion occurred. On the other hand, the copper pipe is divided into 5 cm, the maximum pitting depth of each section is obtained, and the data obtained in this way is processed by the extreme value statistical method. Life Prediction Primer ").
As a result, when the maximum pit depth was estimated for a copper pipe length of 502 m, it was 243 μm for 0.3 m / sec.
Therefore, it was estimated that pitting corrosion reaches 44% of the wall thickness in 56 days.

Example 1 Using the same test apparatus as in Comparative Example 1, the relationship between the change in spontaneous potential and the pitting depth when a corrosion inhibitor was used was determined in the same manner as in Comparative Example 1. In addition, hydrazine was used as a corrosion inhibitor, and the test period was 578 days. The relationship between the test period and the natural potential is shown in FIG. In FIG. 3, (a)
Shows the case where the flow velocity is 1 m / sec, and (b) shows the case where it is 0.3 m / sec. As can be seen from Fig. 3, at a flow rate of 1 m / sec, the spontaneous potential was suppressed to less than 190 mV (saturated silver / silver chloride electrode) by the addition of a corrosion inhibitor, and after the test for 578 days was completed, the copper tube was cut into half. Although the surface was observed for corrosion, it showed metallic luster and no pitting corrosion was observed. On the other hand, when the flow velocity is 0.3 m / sec, it is not enough to add the corrosion inhibitor once, and the spontaneous potential is 190 at the beginning of the test.
Pitting corrosion occurred above mv (saturated silver / silver chloride electrode). However, by the second addition, the spontaneous potential became less than 190 mV, and thereafter, the potential was around 100 mV (saturated silver / silver chloride electrode). Further, on the 104th day, the 263th day, and the 578th day, a part of the 2m copper pipe was cut out and the pitting depth was measured. The maximum depth of pitting in the 5 cm section was calculated and processed by the extreme value statistical method in the same manner as described above.
Estimating the maximum pit depth for 2m is 104
It was 35 μm by day, 43 μm by 263 days, and 66 μm by 578 days. Further, assuming that the pitting depth Y progresses in accordance with the equation Y = aT ^b (where a is 5 and b is 0.4) with respect to the elapsed days T, the thickness of 0.5 mm The estimated time required for the copper pipe to penetrate is over 30 years. From this result, it can be considered that the progress of pitting corrosion is extremely slow. It should be noted that the corrosion prevention method was applied to an actual system for about 3 years, but thereafter, leakage due to pitting corrosion of the copper coil did not recur. Therefore, by monitoring the self-potential, adding a corrosion inhibitor according to the behavior of the self-potential, by controlling the self-potential of the metal to less than the pitting potential, the method of the present invention for suppressing the progress of pitting corrosion, metal It can be seen that it is extremely effective as a method of preventing corrosion.

[0010]

INDUSTRIAL APPLICABILITY The method of the present invention monitors the self-potential of a metal in a system in which a metal and water are in contact with each other, particularly a system in which water is passed through a copper pipe, and the value is less than the pitting potential. It is a method of adding a corrosion inhibitor to the system so that the metal corrosion can be prevented very effectively. INDUSTRIAL APPLICABILITY The method of the present invention can be suitably applied to a system using, for example, a heat exchanger tube for a heat exchanger, a hot water supply tube made of a copper tube, and a copper tube for an air conditioner.

[0011]

[Brief description of drawings]

FIG. 1 is a schematic diagram of a test apparatus used in Examples and Comparative Examples of the present invention.

[0012]

FIG. 2 is a graph showing the relationship between the test period and the natural potential of the copper tube in Comparative Example 1.

[0013]

FIG. 3 is a graph showing the relationship between the test period and the natural potential of the copper tube in Example 1.

[0014]

[Explanation of symbols]

 1 Copper Tube 2 Electrode 3 Potentiometer 4 Valve 5 Cold Water Return Pipe 6 Heat Storage Tank

Claims (1)

[Claims] 1. In a system in which a metal and water are in contact with each other, when a corrosion inhibitor is added to prevent corrosion of the metal, the spontaneous potential of the metal is monitored and the value becomes less than the pitting potential. As described above, a method for preventing metal corrosion, which comprises adding a corrosion inhibitor to the system.