JPH0438958B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical field) The present invention relates to a copper alloy tube with an inner surface anticorrosive coating for heat exchangers, and in particular improves the electrical corrosion resistance of the coating formed on the inner surface of the tube without impairing its properties as an anticorrosive coating. The present invention also relates to a copper alloy tube as a heat transfer tube, which has improved heat transfer performance. (Prior art and its problems) Traditionally, heat transfer tubes made of brass, aluminum, arsenic, silicon, etc. have been used in condensers and other heat exchangers for thermal power plants, scientific factories, ships, etc. So-called special brass tubes with additives and copper alloy tubes made of copper, nickel, and iron, such as so-called Cypronickel tubes, have been widely used, but in these heat exchangers, seawater or river seawater is used as cooling water. Due to usage, various types of corrosion occur on the inner surfaces of these heat transfer tubes. When such corrosion occurs, deposits containing corrosion products adhere to the inner surfaces of the heat exchanger tubes, reducing the heat transfer coefficient of the heat exchanger. For this reason, in order to use such copper alloy tubes as heat transfer tubes, various methods have been proposed to prevent corrosion on the inner surface of the tubes. , Japanese Patent Publication No. 1983-
As disclosed in Publication No. 166271, etc., the method of forming an anticorrosive coating film on the inner surface of a pipe with a predetermined thickness using a predetermined resin paint is superior to other methods in terms of corrosion resistance and workability. It is superior in comparison and has already been put into practical use today. By the way, corrosion of a heat exchanger is not limited to the copper alloy tube that is the heat exchanger tube (heat transfer tube), but also to other materials that make up the water chamber, such as the water chamber wall, valves,
It is also necessary to consider the tube plate, etc.
In addition, even if a predetermined anticorrosion coating film is formed on the inner surface of the heat exchanger tube, the end portion of the tube may be damaged due to damage caused by inflow foreign matter mixed into the heat transfer fluid (cooling fluid) flowing through the heat exchanger tube. If peeling of the paint film cannot be completely prevented, and if such peeling occurs, erosion, so-called inlet attack, will occur in the peeled area, so these problems can be solved. To do this, a cathodic protection device is generally installed in the water chamber of the heat exchanger. However, when a copper alloy pipe with a predetermined inner surface anti-corrosion coating is placed in such a cathodic protection environment, depending on the conditions of the cathodic protection, the inner surface corrosion protection coating may Due to the alkali (OH ^- ) produced by the action,
Paint film blistering, in other words, alkali deterioration is caused,
This has the inherent problem of accelerating the peeling of the paint film, and also increases the heat transfer resistance of the paint film, which has been experienced as reducing the heat transfer coefficient by around 10%. be. Such alkaline deterioration is caused by cathodic protection caused by H ₂ O and O ₂ present in seawater, etc. as a cooling fluid that penetrates the paint film and reaches the metal surface under the paint film. This is a paint film deterioration phenomenon that is caused by reacting with electrons supplied to the metal surface from inside the metal and generating alkali (OH ^- ), but with conventional paint films, such deterioration phenomenon cannot be prevented. It was difficult. (Structure of the Invention) Here, as a countermeasure against the above-mentioned alkali deterioration, the present inventors focused on the permeability of molecules, ions, etc. within the coating film, and by improving it, the above-mentioned reaction could be prevented. It is desirable to remove the generated OH ^- from the paint film to the cooling fluid (cooling water such as seawater) as quickly as possible.To do this, it is necessary to increase defects in the paint film as much as possible, contrary to conventional paint film design. We thought that this would be effective, and as a means to do so, we increased the pigment content in the resin paint that forms the coating film.As a result of various studies, we decided to increase the pigment content beyond that contained in ordinary resin paints. The inventors discovered the fact that by increasing the amount of heat transfer, the alkali deterioration of the coating film can be effectively suppressed, and at the same time, the heat transfer performance can be improved, leading to the completion of the present invention. That is, the present invention improves the electrical corrosion resistance and heat transfer performance of the internal coating film of a copper alloy tube with an internal anticorrosive coating for a heat exchanger, without impairing its properties as an anticorrosive coating. For this purpose, in a copper alloy tube with an inner surface anti-corrosion coating as a heat transfer tube, which is installed in a heat exchanger and allows a prescribed cooling fluid to flow inside the tube, such inner surface is The anticorrosion coating film is formed using a synthetic resin paint in which the ratio (P/B) of the pigment content (P) to the resin solid content (B) in the paint is within the range of 3 to 10 by weight. It is characterized by: As described above, in the case of the copper alloy tube with the inner surface anticorrosion coating film according to the present invention, instead of the conventional resin paint having a ratio of pigment content to resin solid content (P/B) of about 2 to 2.5, P/B with increased pigment content = 3~
A coating film is formed on the inner surface of such a copper alloy tube using synthetic resin paint No. 10, which effectively discharges alkali (OH ^- ) generated on the metal surface of the tube to the cooling fluid side. The resulting coating film was advantageously formed, thereby suppressing alkali deterioration and effectively improving the heat transfer coefficient. (Specific explanation of the structure) By the way, as the copper alloy tube material used in the present invention, any tube material conventionally used for heat exchanger tubes is applicable, but the present invention particularly In some cases, aluminum brass tubes, such as JIS-H-3300-C-6870, 6871,
Aluminum brass materials such as 6872, JIS-H-
Tubes made of Cypronickel materials such as 3300-C-7060- and 7150 are preferably used. And such copper alloy tubes, for example, have an inner diameter of 10~40
It will be used as a long thin tube with a length of about mm, especially about 15 to 25 mm, and a length of about 4 to 40 m, especially about 5 to 25 m. Then, on such a long copper alloy tube with a small hole diameter, a prescribed resin paint is uniformly applied to the inner surface of the tube by a painting method such as spray painting, and a film of a prescribed thickness is applied thereto. A thick anti-corrosion coating film is formed, but in the present invention, such an anti-corrosion coating film is formed by a coating operation using a synthetic resin paint with an increased content of pigments. It is. In other words, as the synthetic resin paint used in such anticorrosive coating operations, an air-drying type (normal temperature drying type) is generally suitably used, and in such a paint, an alkyd resin is used as a film-forming element. , one or two organic polymer resins (including modified products) consisting of vinyl resins (vinyl chloride, vinyl acetate, etc.), polyurethane resins, epoxy resins, polyester resins, and acrylic resins (acrylic esters, etc.) More than one species is used. Such an organic polymer resin may be treated with a suitable solvent (for example, alcohol, ester, ether, ketone resin, aromatic hydrocarbon, etc.)
Further, as pigments, rust preventive pigments such as zinc-based and chromium oxide-based pigments, coloring pigments such as iron oxide, etc.
Extender pigments such as CaCO ₃ , Al ₂ O ₃ , SiO _{2 ,} Cu ₂ O, Cu
A film-forming liquid composition (paint) that dries at room temperature is prepared by blending antifouling pigments such as A (synthetic) resin paint having a ratio (P/B) of (P) to resin solid content (B) in the range of 3 to 10 by weight was used. In short, the present invention provides a resin coating that is applied to the inner surface of a copper alloy tube to form the desired anticorrosion coating, and the ratio (P/B) of the pre-pigment content to the resin solid content is 3 to 3 in terms of weight ratio. 10, by using a synthetic resin paint with an increased pigment content, the corrosion resistance of the paint film formed on the inner surface of the copper alloy pipe can be improved without impairing its properties as an anti-corrosion film. In order to improve cathodic corrosion protection (alkali deterioration),
Furthermore, when the heat transfer property can be advantageously improved and the value of P/B is smaller than 3,
It is not possible to sufficiently improve the cathodic protection and heat transfer properties of such a coating film, and if the P/B value is greater than 10, the pigment dispersion becomes uneven and the formation of a coating film becomes difficult. This results in problems such as poor properties and powdery coating, resulting in poor abrasion resistance. The thickness of the anticorrosive coating film formed on the inner surface of the copper alloy pipe by normal coating operations using such a prescribed resin coating is generally a thickness that does not give a large heat transfer resistance value. In this case, it is determined appropriately depending on the type of resin paint,
For example, at a maximum coating thickness of 50 to 300 μm or less, particularly at a coating thickness of 30 μm or less, an inner surface anticorrosion coating film having lower heat transfer resistance will be formed. (Examples) Below, some examples of the present invention will be shown to clarify the present invention more specifically, but the present invention will not be limited in any way by the description of such examples. Needless to say, this is not something that can be accepted. In addition to the following examples and the above-mentioned specific description, the present invention includes various changes, modifications, and changes based on the knowledge of those skilled in the art, as long as they do not depart from the spirit of the present invention. It should be understood that improvements and the like may be made. Note that all ratios and percentages in the following examples are expressed on a weight basis unless otherwise specified. First, outer diameter: 25.4mm, wall thickness: 1.25mm, length:
2500mm aluminum brass tube (JIS-H-3300-
C-6871) as a test tube, and (a) high temperature
The inner surface was treated with either DX gas (12% CO ₂ + N ₂ ) annealing, (b) sandblasting (alumina particles: 150 μm), or (c) chromate treatment (immersed in 2% NaCr ₂ O ₇ for 15 minutes). did. Next, using 22 types of paints based on phthalic acid-based polyester resin paints or epoxy-modified alkyd resin paints listed in Tables 1 and 2 below (resin solid content was 50% in each case), the above method was applied. Painting was applied to the inner surface of the treated test tube using an air spray coating machine, aiming for a film thickness of 20 μm, and the coating was formed while forcibly removing the volatile solvent from the tube. . In addition, both paints
It uses a room-temperature curing resin that hardens at room temperature. In addition, in Table 1 showing phthalic acid-based polyester resin paint systems, Samples Nos. 1 to 3 are commercially available paint compositions with a regular blending ratio, and mainly include color pigments. The sample was prepared by increasing the amount of Red Garla, which is a
Paints No. 4 to 13 were prepared. The paint thus prepared has a ratio of total pigment content (P) to resin solid content (B) of 2.2 to 11.5. P/B is 5.8
When it becomes larger than , it starts to have a tendency to powder,
11.5 was deemed unsuitable as an organic resin film. Furthermore, in Table 2 showing epoxy-modified alkyd resin paint systems, Samples No. 14 to 16 are commercially available paint compositions, to which a coloring pigment (Red Garla) and an antifouling pigment (Cu) are added. increase the amount,
By increasing the P/B value to around 3.5 to 3.7,
Nos. 17 to 22 were prepared. The test tube surface finish symbols in Tables 1 and 2 have the following meanings. D: DX gas annealed surface S: Sandblast surface C: Chromate surface

【table】

[Table] Then, we evaluated the performance of the test tubes on which the inner surface anticorrosive coating was formed using these 22 types of paints, examined their effectiveness as anticorrosive coatings for the inner surface, and summarized the results in the table below. It is shown in Table 3. In addition, each performance evaluation was performed according to the following method. (a) Polarization resistance value (R) Seawater is passed through the inner surface of each test tube at a flow rate of 2 m/sec, and in this state, the sample is lowered from the natural potential to -
200 mV of cathode polarization is applied, and the polarization resistance value is calculated from the cathode polarization current (I _p ) required at that time according to the following formula. R=200/I _p × [2π ² a ³ /ρ] However, a: radius of the test tube, ρ: specific resistance value of seawater (b) Contaminated seawater corrosion resistance Seawater containing sulfur ions at a concentration of 0.1 ppm was : While the flow was made to flow continuously over the inner surface of the test tube for 3 months at 2 m/sec, a sponge ball with a diameter 3 mm larger than the inner diameter of the tube was
Passed 250 times. (c) Sponge ball passage durability Seawater was flowed into the test tube at a flow rate of 2 m/sec, and at that time, a sponge ball with a diameter 3 mm larger than the inner diameter of the tube similar to the above was inserted.
While replacing the ball with a new one when it wears down to mm,
It was passed through 100,000 times and the amount of loss of the coating film was measured. After the test, the sample is cut in half, its surface is dried, adhesive tape is pasted on top of it, and by peeling it off vigorously, a coating film adheres to the tape surface of the adhesive tape. I observed whether or not. (d) Electrolytic corrosion resistance The test tube is used as a cathode, and the tube end potential is -650, -800, -
Negatively polarize using the constant potential method to 950mV (calomel electrode reference). The flow velocity in the pipe was 2 m/sec. After testing for one month at each potential, washing with water, drying, and cutting the test tube vertically, 100mm from the end of the tube.
After repeating the operation of attaching an adhesive tape and peeling it off vigorously three times at the same location, the peeled area was determined using an image analysis method for an area of 5 cm x 1 cm. (E) Heat transfer resistance value of coating film (r) Using a heat transfer coefficient measuring device, measure the heat transfer coefficient of the unpainted aluminum brass tube and the sample tube at the same time.
(k ₀ , k ₁ ), the heat transfer resistance value (r) of the coating film is calculated using the following formula. r=1/k ₁ -1/k ₀ The symbols used in the evaluation of "coating film adhesion to tape surface" in Table 3 have the following meanings. ○: No paint film adhering to the tape surface 〓: Minor paint film adhesion is observed on the tape surface ●: Paint film adhering entirely to the tape surface

【table】

[Table] As is clear from the results in Table 3, the comparative materials (No. 1 to 3 and No. 14 to 16; P/B = 2.2, 2.3)
No matter which resin paint is used, there is no problem in terms of corrosion resistance and durability, but it is recognized that the electrolytic corrosion protection is inferior. Moreover, the difference is large depending on the surface finishing method of the test tube: the sandblasted surface is significantly inferior, and the chromate finished surface is superior, but such a chromate finish causes a significant increase in cost and is not practical. . On the other hand, in sample No. 4 (P/B = 2.6), although the improvement effect of cathodic protection and heat conductivity is recognized, it cannot be said that it is so remarkable, but in sample No. 5 to 13, the P/B When the value is 3 or more, it is recognized that the improvement effect on both characteristics is significant. Furthermore, in the samples (No. 6, 7; 9, 10) with P/B = 3.8, 5.8, improvements in both properties were observed even on the sandblasted surface. However, as the P/B value increases, the adhesion (durability) of the paint film tends to deteriorate, and the P/B value tends to deteriorate.
= 5.8, some signs of deterioration occurred, P/B = 11.5
(No. 13), its reliability becomes a problem in practical use. In addition, regarding the polarization resistance value and corrosion under the coating film, no significant difference was observed in the range of P/B = 2.2 to 11.5. In addition, in sample Nos. 17 to 22, in which the P/B value was kept around 3.6 and the pigment content was changed, all performances were extremely excellent as anticorrosion coatings.
A significant performance improvement is observed compared to conventional products. Therefore, it is considered that the type of pigment selected does not matter as long as a coating film can be formed. In addition, the Cu powder added in samples No. 19 to 22 has an antifouling effect and suppresses the adhesion of marine organisms to the inner surface of the tube, so it maintains the heat transfer performance at a high value after long-term use. Demonstrate the characteristics you gain. Furthermore, the effect of the P/B value described above can be observed in other types of resin paints that are commonly put into practical use, other than the two types shown in the examples above. And, due to the increase in P/B value as mentioned above,
In particular, the reason for the improved cathodic protection is that defects in the coating film increase, and as a result, OH ^- generated under the coating film during cathodic protection is eliminated from the system through the defects in the coating film. This is thought to be due to the fact that the high PH level of the seawater under the coating film is suppressed. (Effects of the Invention) As is clear from the above description, according to the present invention, alkaline deterioration of organic resin coatings under cathodic protection, which has been considered inevitable for coated objects, can be significantly reduced. At the same time, the deterioration in heat transfer performance, which had been a drawback of copper alloy tubes with internal anticorrosion coatings, was improved, and the secondary effect was that this was improved. Moreover, the copper alloy tube with the inner surface anti-corrosion coating according to the present invention is exactly the same as conventional products in terms of cost and productivity, and there are no problems.
In conventional painting systems, the surface treatment of the material is extremely important, and in some cases, painting is performed after chromate treatment.Furthermore, when targeting existing pipes, the sandblasting surface is In many cases, it was not possible to obtain good adhesion, but in the case of the present invention, it is less affected by the underlying surface and has the feature that it can be applied to sandblasted surfaces. It is.

Claims (1)

[Claims] 1. A body alloy tube with an inner surface anti-corrosion coating as a heat exchanger tube, which is attached to a heat exchanger and allows a prescribed cooling fluid to flow through the tube, and the inner surface anti-corrosion coating has a pigment content (P) in the paint. A heat exchanger with an anti-corrosion paint film on the inner surface, characterized in that it is formed using a synthetic resin paint having a ratio (P/B) of 3 to 10 by weight. Copper alloy tube.