JP2801032B2 - Corrosion prevention method for heat exchanger tubes for heat exchangers - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a heat exchanger tube for preventing the corrosion of a heat exchanger tube for a heat exchanger through which cooling water such as seawater flows by the cooling water by forming a protective film. The present invention relates to a method for preventing corrosion of heat tubes.

[Related Art] Copper or copper alloy heat transfer tubes are frequently used in heat exchangers in which seawater, river seawater, or freshwater flows as cooling water. In some cases, a heat exchanger tube for a heat exchanger having a protective film formed on the inner surface of the tube in advance is used. However, the heat transfer tube made of copper or copper alloy may be corroded by cooling water depending on its use conditions. For example, the vicinity of the pipe end on the inlet side of the cooling water may be subject to local erosion called inlet attack due to strong turbulence. Conventionally, in order to prevent this inlet attack, a cathodic protection method has been applied. If only cathodic protection is not sufficient, a resin coating is applied only near the pipe end.

[Problems to be Solved by the Invention] However, when a cathodic protection treatment is performed after the formation of the resin coating, there is a drawback that the resin coating swells depending on the cathodic protection conditions. Further, when the resin film is made thicker to enhance the anticorrosion effect, there is a disadvantage that the heat transfer performance is reduced and the production cost is increased.

Furthermore, with the technology of forming a protective film on the inner surface of the tube in advance, when the heat transfer tube is mounted on the heat exchanger, the work is performed by expanding the tube onto the tube plate, so that a part of the previously formed film is damaged. Was received.

In addition, a heat transfer tube without an anticorrosion film formed on the inner surface is installed in the heat exchanger, and ferrous ions are injected into cooling water during operation of the heat exchanger to form a protective film on the inner surface of the heat transfer tube. Methods have also been adopted. There is an advantage that a good protective film can be formed by the implantation of ferrous ions, and the effect of cathodic protection is easily obtained. However,
This method has the disadvantage that it takes a long time to form a protective film and is not effective in preventing corrosion at the beginning of water passage. For this reason, under severe corrosive conditions such as receiving an inlet attack, the former method, which has an excellent anticorrosion effect even in the initial stage of water passage, must be employed. Therefore, there is a demand for the development of an anticorrosion method capable of sufficiently preventing corrosion of a pipe end by improving the former method of forming a protective film in advance.

The present invention has been made in view of such a problem, and a low-cost corrosion-resistant protective film that does not cause swelling of the film due to the electrolytic protection process and that has a very small decrease in heat transfer performance due to the formation of the film is provided. It is an object of the present invention to provide a method for preventing corrosion of a heat exchanger tube for a heat exchanger, which can be formed by using a heat exchanger.

[Means for Solving the Problems] The method for preventing corrosion of a heat exchanger tube for a heat exchanger according to the present invention is a method for preventing corrosion of a heat exchanger tube for a heat exchanger that allows cooling water to flow through the inside of the tube. After applying the iron powder suspension to the inner surface of the tube in the above region, by passing an oxidizing gas to oxidize iron in the iron powder suspension, iron hydroxide or iron oxide is used as a main component. With magnetic permeability of 1.00 to 1.15
Is formed in a range of 100 mm or more from the end of the tube.

[Action] In the present invention, since the main component of the film formed on the inner surface of the tube end is iron hydroxide or iron oxide, sufficient corrosion resistance is obtained and the film swells regardless of the cathodic protection conditions. Does not occur. And, in addition to the film itself containing iron hydroxide or iron oxide as a main component, which contributes to the improvement of corrosion resistance, by forming this film uniformly,
When the cathodic protection is performed, the inner surface of the tube easily reaches the anticorrosion potential. For this reason, this film also has the effect of enhancing the effect of cathodic protection.

The formation area of the protective film is at least 100 mm or more from the pipe end, and at most 2000 mm or less. The area that receives the so-called inlet attack is often within 100 mm from the pipe end,
By forming a protective film at least in a region 100 mm or more from the end of the tube including this region, the inlet attack can be reliably prevented even in an environment where the end of the tube is susceptible to corrosion. Further, when the formation area of the protective film is at most 2,000 mm from the pipe end, a decrease in heat transfer performance can be minimized.

The magnetic permeability of the film is 1.00 to 1.15. If the permeability of the coating exceeds 1.15, the progress of corrosion on the inner surface of the pipe is monitored nondestructively by eddy current inspection during the use of the heat exchanger after the formation of the protective coating. This is because the inspection accuracy of the flaw detection is reduced.

Further, the composition of the film needs to contain 10% or more of iron, and it is necessary that 50% or more of the iron is present as iron hydroxide or iron oxide. Such a film can be formed thinly and uniformly on the inner surface of the tube, and also has excellent corrosion resistance. For this reason, the effect of the cathodic protection can be enhanced without deteriorating the heat transfer performance, and the inlet attack can be effectively prevented.

Next, a method for forming a film having the above-described characteristics will be specifically described. First, the heat transfer tube is attached to the tube plate of the heat exchanger. Then, after applying an iron powder suspension to the inner surface of the heat transfer tube, an oxidizing gas is passed through the inner surface of the tube to oxidize the iron powder applied to the inner surface of the tube, thereby causing iron hydroxide or oxidizing on the inner surface of the tube. Form a film mainly composed of iron. By the way, if the inner surface of the heat transfer tube is coated with a film before attaching the heat transfer tube to the heat exchanger tube sheet, if the heat transfer tube is expanded to attach to the heat exchanger tube sheet, The film peels off. However, in the above-described method of passing an oxidizing gas after the application of the iron powder suspension, after the heat transfer tube is attached to the heat exchanger, that is, after expanding the pipe, and before passing the cooling water, the corrosion resistance is reduced. An excellent protective film can be formed.

As the oxidizing gas, it is preferable to use air containing moisture because the processing cost is low. In this case, if the partial pressure of water vapor in the air is less than 1.9 mmHg, it takes a long time to oxidize the iron, and the workability is poor. Decrease.
On the other hand, when air having a water vapor partial pressure exceeding 32.0 mmHg is passed, a film having poor adhesion is obtained. For this reason, air containing water having a partial pressure of water vapor of 1.9 to 32.0 mmHg is used as the oxidizing gas.

[Examples] Next, examples of the present invention will be described in comparison with comparative examples.

First, a preliminary test regarding the conditions for forming the protective film was performed using a single tube. That is, the outer diameter is 25.4mm and the wall thickness is 1.245m
After applying an iron powder suspension to the inner surface of an aluminum brass tube (JISH3300C6872T) of m length, air of various partial pressures of steam is blown to the inner surface of the tube, so that the color tone, adhesion, and transparency of the formed film The magnetic susceptibility and the associated eddy current inspection were tested for reliability.

In addition, as the iron powder suspension, 10 1.6N hydrochloric acid and 8
Iron powder with an average particle size of 4 μm
The one added with only Kg was used. The evaluation of the adhesion and the anticorrosion effect of the film was performed by a jet test using seawater. That is, the tubular sample was halved in the longitudinal direction, and the test was performed for 45 days after irradiating a seawater jet stream at a flow rate of 12.0 mm / sec from a nozzle having a diameter of 2 mm to the sample for 45 days.
The state of peeling of the film and the state of occurrence of corrosion were investigated. Furthermore,
A drill hole (artificial flaw) is artificially added to a depth of 20% and 50% of the pipe wall thickness in advance, and the measurement result of the thinning rate obtained by the eddy current inspection and the actual defect depth are compared. By comparison, the reliability of the eddy current flaw detection was evaluated.

 The test results are shown in Table 1 below.

As shown in Table 1, Comparative Example 1 having a low water vapor partial pressure
In the case of 2, most of the iron in the film was present as metallic iron, so that the magnetic permeability of the film was high, and as a result, the reliability of the eddy current inspection was reduced. That is, when comparing the estimation results of the wall thinning rate in Table 1 with the depth of the artificial flaw (20% or 50%), the results in each example are almost the same, while those in the comparative example are almost the same. The estimated wall loss was measured much deeper than the actual depth. In the case of Comparative Example 1, the anticorrosion effect was insufficient because the iron content in the film itself was small. Furthermore, in the case of Comparative Example 3 having a high water vapor partial pressure, the adhesion of the film was insufficient, and a sufficient anticorrosion effect was not obtained. From these results, when forming a film after mounting the heat transfer tube on the heat exchanger, it is necessary to keep the water vapor partial pressure of the air to be blown from 1 mmHg or less or 33.0 mmHg or more.

Next, in a heat exchanger equipped with 1750 aluminum brass tubes (JISH3300 C6872T) having an outer diameter of 19.0 mm, a wall thickness of 1.2 mm, and a length of 6150 mm, 300 heat transfer tubes were replaced with new heat transfer tubes at the same time as the periodic inspection. Was replaced. Some of the new pipes are attached to the tube sheet by pipe expansion, then an iron powder suspension is applied to the inner surface of the pipe, and then air containing steam is blown to make iron hydroxide as the main component. A film was formed. However,
For iron powder suspension, 10 kg of iron powder with an average particle size of 4 μm is 10 1.6 N
It is a mixture of hydrochloric acid and 8 ethanol. The range of application of this iron powder suspension in the tube is 50 mm (Comparative Example 1), 120 mm (Example 6), 350 mm (Example 7) or 1000 mm from the end of the tube.
mm (Example 8). For comparison, an epoxy resin film was formed in a region up to 1000 mm from the end of the tube in the same manner as before (Comparative Example 5), and an epoxy resin film was used on the inner surface of the tube without performing a film forming process (Comparative Example). Example 6) was prepared.

With respect to the heat exchanger adjusted under such conditions, the sacrificial anode method using zinc as an anode was employed for the cathodic protection of the end of the tube, and the tube was used for 11 months while passing natural seawater. In evaluating the heat exchanger after use, first, the state of corrosion occurrence of the entire pipe was grasped by eddy current inspection. The results are shown in Table 2 below.

Next, from among the heat transfer tubes of Examples 6 to 8 and Comparative Examples 4 to 6 in Table 2, a tube showing a typical eddy current flaw detection signal was removed, and the state of the film and corrosion was inspected. The evaluation of the film and the state of corrosion is based on the remaining state of the film formed on the inner surface of the pipe before water flow, the occurrence and depth of corrosion, the comparison between the eddy current inspection results and the actual depth of wall thinning, etc. went. The results are shown in Table 3 below.

As is clear from Tables 2 and 3, in the case of a tube in which a film was not formed in the vicinity of the end of the tube (Comparative Example 6), an inlet attack was relatively large, and the maximum corrosion depth was 0.40 mm.
Met. Tubes having a film formation range of up to 50 mm from the end of the tube (Comparative Example 4) are also susceptible to inlet attack. In addition, as in the conventional case, the pipe coated with the epoxy resin (Comparative Example 5) showed little corrosion after 11 months of use, but peeling and swelling occurred in the coating, and the subsequent corrosion was expected to progress. . On the other hand, the corrosion of the pipes (Examples 6 to 8) in which the coating was formed to a range of 120 mm or more from the pipe end was extremely slight.

[Effects of the Invention] According to the present invention, corrosion can be prevented from the initial stage of water passage, swelling does not occur in the film even by the cathodic protection treatment, and the decrease in heat transfer performance due to the film is extremely small. In addition, this film can be formed at low cost. Therefore, the heat exchanger tube for a heat exchanger is extremely effective as a heat exchanger tube for a heat exchanger used in a severe corrosive environment such as one subject to an inlet attack.

────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code FI F28F 19/06 F28F 19/06 C (56) References JP-A-57-134566 (JP, A) JP-A-57-134567 ( JP, A) JP 6-15959 (JP, B2) (58) Field surveyed (Int. Cl. ⁶ , DB name) C23F 11/00 F28F 19/00-19/06

Claims (2)

(57) [Claims] In a method for preventing corrosion of a heat exchanger tube for a heat exchanger in which cooling water is passed through a tube, an iron powder suspension is applied to an inner surface of a region of the heat exchanger tube at least 100 mm from an end of the tube and then oxidized. Oxidizing iron in the iron powder suspension by passing a conductive gas to form a coating having iron hydroxide or iron oxide as a main component and a magnetic permeability of 1.00 to 1.15 within a range of 100 mm or more from the pipe end. A method for preventing corrosion of a heat exchanger tube for a heat exchanger, wherein 2. The oxidizing gas has a water vapor partial pressure of 1.9 to 32.
The method for preventing corrosion of a heat exchanger tube for a heat exchanger according to claim 1, wherein the air is 0 mmHg air.