JPS6131897A - Heat exchange pipe with inner surface coating excellent in resistance to inflation of coating film - Google Patents

Abstract

PURPOSE:To increase cathodic protection effect and prevent inflation of film of corrosion- proofing coating, which is readily generate in cathodic protection, by a method wherein an electro-conductive metallic pipe is inserted into the end part of the heat exchanging pipe whose inner surface if coated with the film of corrosion-proofing coating to conduct the metallic pipe electrically to the heat exchanging pipe. CONSTITUTION:The conductive metallic insert member 2 is fitted into the inner surface at the tip end side of the heat exchanging pipe 1 coated with the corrosion-proofing film P and a part or the total length of the insert member 2 is conducted electrically to the pipe 1. Accordingly, corrosion-proofing electric current, generated by the electric protection, flows at first through the insert member 2 and flows to the direction of the heat exchanging pipe 1 through the conducting section E. The conductivity of the insert member 2 is much higher than the same of the corrosion-proofing film P, therefore, most of the corrosion-proofing current, flowing through the heat exchanging pipe 1, flows from the insert member 2 to the direction of the pipe 1. As a result, reactions of formulas ( I ), (II) in the diagram which are generated at the boundary surface of coating of the corrosion-proofing coating film P, may be restrained remarkably and the inflation of the coating film P may be prevented as much as possible. The most common insert member 2 is copper, iron, aluminum or alloys containing these metals.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an internally coated heat exchange tube whose inner surface is coated with an anticorrosive coating, and in particular to prevent blistering of the anticorrosive coating that is likely to occur when cathodic protection is applied. The present invention relates to a heat exchanger tube that prevents this and significantly extends its life.

[Conventional technology]

For various types of heat exchange pipes, including double water tanks in thermal or nuclear power generation equipment, the inner surface should be coated with anti-corrosion coating (various resin paints such as epoxy and polyester) in order to improve corrosion resistance and durability. There are many.

On the other hand, even when such inner-coated heat exchange tubes are used, external power supply methods or zinc or A cathodic protection method using sacrificial anodes using iron, etc. is often adopted. However, it has been confirmed that when the above-mentioned cathodic protection method is applied to equipment using internally coated heat exchange tubes, blistering occurs in the coating film (the blistering problem almost never occurs unless cathodic protection is applied). The reason for this occurrence is explained as follows.

That is, at the metal interface at the bottom of the coating film, the following CI) formula and ([[)
A reduction reaction as shown in the formula is occurring. 0! +2H20+4e 140H(I)2H++2
e-#H2↑ (II) The hydroxide ions generated by formula (I) make the water that has passed through the paint film alkaline, so in paint films with low alkali resistance, adhesion to metals decreases and blisters occur. It becomes easier. Moreover, since the hydrogen gas produced by the reaction of formula (II) acts as an internal pressure pushing up the coating film, the coating film becomes even more likely to swell. (I), ([[)
The equilibrium potential of the reaction in the formula depends on the pH as shown in Figure 2, but the pH of seawater widely used in double water pipes in power generation equipment is 8.0 to 8.5, and the polarization potential is is -75
0 mV.

S, C, E and below "t'6rukoto2>'multiple (Dte,
It is thought that both reactions of formulas (I) and (II) occur, resulting in noticeable blistering of the coating film.

Measures to deal with these problems include: ■ Keeping the cathode potential as noble as possible (for example, using an iron sacrificial anode to prevent the so-called over-corrosion state due to a drop in potential more than necessary), or ■ The coating film can be removed by applying current in a concentrated manner to the unpainted part (for example, as disclosed in Utility Model Application Publication No. 57-145187, the vicinity of the pipe end is left unpainted and the current is concentrated in this part), etc. A method of suppressing the reduction reaction in the lower part) has been proposed.

[Problem that the invention seeks to solve]

However, the above method (■) not only has insufficient corrosion prevention effect when exposed to severe usage conditions, but also imposes a heavy economic burden when applied to large heat exchangers such as double water tanks. Moreover, although the swelling can be reduced to some extent, it cannot be completely prevented. In addition, in the method (2) above where the area near the pipe end is unpainted, sufficient effect cannot be obtained if the length of the unpainted part is short. It needs to be 300mm or more. Therefore, depending on the usage conditions, the effect of cathodic protection may reach all of the unpainted parts, and the corrosion resistance may actually decrease.

Therefore, there is a strong demand for the development of a technique that can reliably prevent blistering of anticorrosive coatings without causing the above-mentioned problems.

[Means for solving problems]

The present invention provides an inner-coated heat exchange tube with excellent coating blistering resistance that can satisfy the above-mentioned requirements. The main feature is that a conductive metal tube is inserted into the limb canal so as to be electrically connected to the limb canal.

[Effect]

As explained earlier, the cause of blistering in the anticorrosive coating is that the anticorrosive current generated by the five corrosions at the cathode causes the above ((
This is due to the reactions of formulas I) and (II) occurring, and as long as an anticorrosion current does not flow through the painted area, blistering of the υ coating film cannot occur. However, with the method described above in which a non-coated area is provided at the tube end and the anticorrosive current is concentrated on that area, the potential distribution generated on the tube plate surface is uneven, and when the current density is high, the coating film still swells. On the other hand, when the current density is low, corrosion of the non-coated parts progresses rapidly. Furthermore, even with constant potential polarization, the potential may change depending on the conditions, and in such cases, blistering of the coating film and corrosion of non-coated areas occur alternately.

In contrast, in the present invention, for example, FIG.
As shown in the figure, a conductive metal insert material (hereinafter sometimes simply referred to as insert material) 2 is fitted into the inner surface of the front end side of the heat exchange tube 1 coated with the anticorrosive coating P, and a part or the entire length thereof is inserted into the tube 1. It is electrically connected. 3 in the figure indicates a tube plate. Therefore, the anticorrosion current generated by cathodic protection first flows through the insert material 2, and then flows toward the heat exchanger tube 1 through the conductive portion E (the distal end surface of the tube 1 in the figure). In other words, since the insert material 2, which is made of a conductive metal, has a much higher conductivity than the anticorrosive coating, most of the anticorrosion current flowing inside the heat exchange tube 1 is directed from the insert material 2 toward the heat exchange tube 1. flows. As a result, the above-mentioned (1) and (
The reaction of formula II) is significantly suppressed, and blistering of the coating film P can be prevented as much as possible.

In addition, with this method, the insert material 2 is simply inserted from the tip of the heat exchange tube 1 which has an anti-corrosion coating formed on the inner surface, so the anti-corrosion coating P on the inner peripheral surface of the end of the heat exchange tube 1 is considerably removed. Compared to the conventional method, which involves peeling over a long length, the work is easier and faster. Moreover, the anti-corrosion coating on the inner surface P
Since the insert material 2 for anti-corrosion current conduction is placed while leaving it essentially as it is, there is no risk of corrosion progressing intensively in the non-painted parts as in the conventional example, and it is quite severe. Sufficient anticorrosion effect can be obtained even when used under certain conditions.

In Fig. 1, as a specific measure to electrically conduct the insert material 2 and the heat exchange pipe 1, a 7 flange portion 2a is formed at the leading edge of the insert material 2, and this is press-fitted to the end face of the heat exchange pipe. Although an example has been shown in which electrical conduction is achieved, the present invention is not limited to this, for example, by peeling off the anticorrosive coating P on the leading edge side,
It is also possible to bring the outer circumferential surface of the insert material 2 into direct contact with the inner surface of the heat exchange tube 1 for electrical conduction, or to form several protrusions on the outer circumferential surface of the insert material 2 so that the heat exchange When fitting into the tube 1, it is also possible to peel off a part of the anticorrosive coating P with the protrusions so that the top surface of the protrusions communicates with the inner surface of the heat exchange tube 1. In addition, since the insert material 2 is originally cathodic-protected to ensure sufficient corrosion protection, it does not matter whether the potential to the heat exchange tube 1 or the tube sheet 3 is noble or base; Any metal may be used as long as it has general strength and seawater resistance. However, the most common are copper, iron, aluminum, or alloys containing these. Furthermore, there are no particular restrictions on the dimensions of the insert material 2, but the length is suitably in the range of 50 to 1000 mm, and the outer diameter can be sealed to the inner surface of the heat exchange tube 1. It is better to use dimensions. In addition, as a method for fixing the insert material 2 to the inner circumferential surface of the end of the heat exchange tube 1, there are two methods: 1. When fixing the limb tube 1 to the tube plate 3 using a tube expander, ■Method of bonding the two through conductive adhesive;■Method of joining using brazing filler metal;■Method of changing the outer diameter of the insert material 2 to the inner diameter of the heat exchange tube 1. A method of fitting the insert material 2 into the limb pipe 1 by making it slightly larger, (2) providing a female thread at the end of the heat exchange pipe 1 and providing a male thread on the outer peripheral surface of the insert material 2;
A method of screwing the two together, etc. can be adopted.

〔Example〕

Example 1 Length: 3,000 mm, inner diameter: 25.4 mm, wall thickness: 1.
A 245 mm copper heat exchange tube with an epoxy resin anti-corrosion coating applied to its inner surface was subjected to the end face treatments shown in Table 1, and each was subjected to cathodic protection tests using the external power source method under the conditions shown in Table 2. I did this. The results are shown in Table 3.

Example 2 Using the same anti-corrosion coated pipe as that used in Example 1, a plurality of anti-corrosion treated pipes with copper alloy inserts of different lengths attached to the tube ends were manufactured, and each was treated with cathodic protection. The swelling status of the anticorrosion coating was investigated when the applied potential was changed. The fluid in the pipe and the test period were as shown in Table 2.

As is clear from Tables 1 to 3, the heat exchange tube according to the present invention (No. 1) has extremely little swelling of the coating film compared to other heat exchange tubes (No. 2 to 4), and has excellent corrosion resistance. It can be seen that it has.

In addition, the results in Table 4 show that when the added potential for cathodic protection is small, it is possible to sufficiently prevent blistering of the paint film by simply installing a relatively short insert material, but as the added potential increases, Unless the insert material is made longer, a sufficient blistering prevention effect cannot be obtained. However, this length is 1
By setting the thickness to about 000 yn, a sufficient effect can be obtained even when the additional current is large.

〔Effect of the invention〕

The present invention is constructed as described above, but the key point is that a conductive metal tube is inserted into the inner surface of the end of the heat exchange tube whose inner surface is coated with an anticorrosion coating film, and the conductive metal tube is electrically connected to the heat exchange tube. In addition to enhancing the cathodic protection effect, it is possible to prevent the blistering of the anticorrosive coating film that tends to occur with cathodic protection, and the life of the heat exchange tube can be significantly extended.

[Brief explanation of the drawing]

Fig. 1 is a partial longitudinal sectional view showing an embodiment of the present invention, and Fig. 2 shows the equilibrium potential and pH of the reduction reaction caused by cathodic protection.
It is a graph showing the relationship.

Claims (1)

[Claims] An internally coated heat exchange tube whose inner surface is coated with an anticorrosion coating film, and a conductive metal tube is provided on the inner surface of the tube end side so as to be in close contact with the coating film and to be electrically conductive with the tube. An inner-coated heat exchange tube with excellent coating blistering resistance, characterized by the fact that it is inserted into the tube.