JP7439790B2 - Resin-coated steel pipe and its manufacturing method - Google Patents

Description

The present invention relates to a resin-coated steel pipe mainly used as a gas pipe and a method for manufacturing the same.

Polyethylene-coated steel pipes, which are made by extruding a polyethylene coating layer with an adhesive or adhesive after subjecting the outer surface of the steel pipes to surface preparation such as pickling or blasting, are widely used as steel pipes for gas pipelines such as city gas. It is used.
The inner surface of this polyethylene-coated steel pipe may be unpainted or may be coated with an epoxy resin paint, an inorganic zinc-rich paint, or the like.

When polyethylene-coated steel pipes with internal coatings are applied to gas pipes, thermal decomposition products (hereinafter referred to as "mist") of the internal coating film are generated due to the heat during welding, and this mist causes electromagnetic interference in the piping system. There is a problem in that it adheres to valves and filters, causing problems such as malfunctioning solenoid valves and filter clogging.
To address these problems, Patent Document 1 discloses a technique for coating the inner surface of a polyethylene-coated steel pipe with an inorganic zinc-rich paint containing an alkyl silicate as a binder component. Polyethylene-coated steel pipes have been rapidly adopted in recent years because they suppress the generation of mist due to heat during welding of gas pipes.

Japanese Patent Application Publication No. 2013-173340

Polyethylene-coated steel pipes whose inner surfaces are coated with an inorganic zinc-rich paint containing alkyl silicate as a binder component can suppress the generation of mist due to heat during welding, but the present inventors have investigated According to the research, it was found that there were the following issues. In other words, when gas pipes are internally coated with an inorganic zinc-rich coating, the smoothness of the coating decreases due to surface irregularities caused by zinc particles (zinc dust) contained in the coating, and the zinc particles are exposed on the coating surface. It was found that these conditions caused several problems.

First, there is a problem in that the odorant contained in the gas flowing into the pipe is adsorbed by the coating film, and the odor of the gas is initially reduced. The causes of this are (i) surface irregularities caused by zinc particles (zinc dust) contained in the paint film, which increases the surface area of the paint film and makes it easier for the odorant to be adsorbed to the paint film surface; (ii) The zinc grains present (exposed) on the membrane surface have a high affinity for the sulfur content of the odorant and serve as adsorption sites for the odorant. This problem of reduced gas odor can be resolved by constantly flowing gas, but when gas is flowed after construction of gas pipes and odor confirmation work is carried out, the odor cannot be detected easily, so it is necessary to flow a large amount of gas. This reduces the efficiency of odor checking work and causes problems such as anxiety for the installer.
In addition, welding of gas pipes generates dirt such as spatter and iron powder, so cleaning is performed using pigs after welding, but the smoothness of the internal coating film is low, and in addition, there is no zinc on the coating surface. When grains are present (exposed), dirt tends to adhere to the coating surface, resulting in poor cleaning performance.

Therefore, an object of the present invention is to solve the problems of the prior art as described above, to suppress the generation of mist due to heat during welding, and (i) when applied to a gas pipe, to reduce the amount of mist contained in the gas flowing into the pipe. (ii) dirt such as spatter and iron powder caused by welding is difficult to adhere to, and cleaning after welding is easy. An object of the present invention is to provide a resin-coated steel pipe and a method for manufacturing the same.

As a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention discovered that the coating film on the inner surface of the tube is a lower coating film containing an alkyl silicate polycondensation reaction product and zinc dust (with alkyl silicate as the binder component). On top of the coating (based on inorganic zinc-rich paint), an upper coating film containing an alkyl silicate polycondensation reaction product as the main component is formed to create a smooth coating surface, and the zinc particles of the lower coating film are applied to the coating surface. We have discovered that the above problems can be solved by preventing exposure to

The present invention was made based on such knowledge and has the following gist.
[1] A resin-coated steel pipe having a resin coating layer or coating film on the outer surface of the pipe and a coating film on the inner surface of the pipe,
The coating film on the inner surface of the tube contains a lower coating film (A1) containing 5% by mass or more of an alkylsilicate polycondensation reaction product and 30% by mass or more of zinc dust, and 50% by mass or more of an alkylsilicate polycondensation reaction product, A resin-coated steel pipe characterized by having an upper coating film (A2) that does not contain zinc dust or contains zinc powder in a smaller amount than the lower coating film (A1).
[2] The resin-coated steel pipe according to [1] above, characterized in that the coating film on the inner surface of the tube has a surface roughness Rz of 18 μm or less.

[3] The resin-coated steel pipe according to [1] or [2] above, wherein the upper coating film (A2) does not contain powder having an average particle size of 5 μm or more.
[4] In the resin-coated steel pipes according to any of [1] to [3] above, the lower coating film (A1) and the upper coating film (A2) are inorganic coating films that do not contain organic compounds. Features resin-coated steel pipes.
[5] In the resin-coated steel pipe according to any of [1] to [4] above, the thickness of the lower coating film (A1) and the upper coating film (A2) are each 5 to 80 μm. A resin-coated steel pipe featuring:
[6] The resin-coated steel pipe according to any one of [1] to [5] above, which has a polyethylene coating layer on the outer surface of the pipe.

[7] A method for manufacturing a resin-coated steel pipe having a resin coating layer or coating film on the outer surface of the pipe and a coating film on the inner surface of the pipe, comprising:
When painting the inner surface of a pipe, a paint (a1) for the lower layer coating containing 5% by mass or more of alkyl silicate and/or alkyl silicate oligomer and 30% by mass or more of zinc dust in the solid content of the paint. Then, a paint (a1) for the lower layer coating that contains 50% by mass or more of alkyl silicate and/or alkyl silicate oligomer in the solid content of the paint and does not contain zinc dust or contains zinc dust. A coating film having a lower layer coating film (A1) and an upper layer coating film (A2) is formed on the inner surface of the pipe by applying a coating material (A2) for the upper layer coating film containing a content smaller than that of the upper coating film (A2). A method for manufacturing resin-coated steel pipes.

[8] In the manufacturing method of [7] above, the resin coating is characterized in that a coating film having a lower coating film (A1) and an upper coating film (A2) and having a surface roughness Rz of 18 μm or less is formed on the inner surface of the tube. Method for manufacturing covered steel pipes.
[9] In the manufacturing method of [7] or [8] above, the resin-coated steel pipe is characterized in that the paint (a2) for the upper layer coating does not contain powder with an average particle size of 5 μm or more. Production method.
[10] In any of the manufacturing methods described in [7] to [9] above, the paint for the lower layer coating (a1) and the paint for the upper layer coating (a2) contain organic compounds as the solid content of the coating that becomes the coating film. 1. A method for manufacturing a resin-coated steel pipe, characterized by using an inorganic paint that does not contain.
[11] In any of the manufacturing methods of [7] to [10] above, the dry film thickness of the lower layer coating film (A1) and the dry film thickness of the upper layer coating film (A2) are each 5 to 80 μm. Characteristic manufacturing method for resin-coated steel pipes.
[12] A method for producing a resin-coated steel pipe according to any one of [7] to [11] above, characterized in that a polyethylene coating layer is formed on the outer surface of the pipe.

The resin-coated steel pipe of the present invention suppresses the generation of mist due to heat during welding work, and (i) odorants contained in the gas flowing inside the pipe are less likely to be adsorbed by the coating film; (ii) The effect is that dirt such as spatter and iron powder caused by welding is difficult to adhere to. Therefore, when resin-coated steel pipes are used as gas pipes, problems such as solenoid valve malfunction and filter clogging due to adhesion of mist generated by the heat during welding work can be alleviated, and the problems described in (i) above can be improved. As a result, the odor of the odorant can be immediately confirmed when gas is introduced after gas work, which improves work efficiency.In addition, as a result of the effect (ii) above, cleaning of the inner surface of pipes after welding work ( Cleaning using a pig, etc.) becomes easier, and cleaning work efficiency can be improved.

In one embodiment of the resin-coated steel pipe of the present invention, a cross section in the thickness direction of the inner surface coating film is schematically shown, and FIG. FIG. 1(a) is an explanatory diagram showing a state in which an upper layer coating film is formed on the lower layer coating film.

The resin-coated steel pipe of the present invention has a resin coating layer or coating film on the outside surface of the pipe and a coating film on the inside surface of the pipe, and can be used for various purposes, particularly for gas pipes and the like. This is a resin-coated steel pipe suitable for gas pipes. Typical examples include polyethylene-coated steel pipes, hard vinyl chloride-coated steel pipes, etc. that have a resin coating layer on the outer surface of the pipe, and epoxy-coated steel pipes that have a coating film on the outer surface of the pipe, but are limited to these. It is not something that will be done.
The coating film on the inner surface of the tube (inner surface coating) consists of a lower coating film A1 (first layer) containing 5% by mass or more of an alkyl silicate polycondensation reaction product and 30% by mass or more of zinc dust; It is composed of a coating film that contains 50% by mass or more and has an upper coating film A2 (second layer) that does not contain zinc dust or contains zinc dust in a smaller amount than the lower coating film A1.

The alkyl silicate polycondensation reaction product (compound having a siloxane bond) contained in the coating film on the inner surface of the tube (lower coating film A1 and upper coating film A2) is combined with the alkyl silicate (including oligomers) contained in the paint. The alkyl silicate coated on the inner surface of the tube reacts with moisture in the air, hydrolyzes, and further polycondenses to form siloxane bonds, forming a strong polymer compound with siloxane bonds, i.e. It becomes an alkyl silicate polycondensation reaction product. The coating film containing this alkyl silicate polycondensation reaction product suppresses the generation of mist (a product of thermal decomposition of the coating film) due to heat during welding, and this mist can cause malfunctions in solenoid valves in gas pipes and damage to filters. Problems such as clogging can be prevented.

If the content of the alkyl silicate polycondensation reaction product in the lower coating film A1 is less than 5% by mass, the film forming properties of the coating film will be poor and the necessary coating strength cannot be ensured. In relation to the content of zinc dust, the practical upper limit of the content of the alkyl silicate polycondensation reaction product is 70% by mass, but the content of the alkyl silicate polycondensation reaction product is in the range of 30 to 70 mass%. It is preferable because it has excellent film forming properties, corrosion resistance, and the effect of reducing mist during welding.In particular, the range of 40 to 60% by mass has the best film forming property, corrosion resistance, and the effect of reducing mist during welding, and also reduces the generation of zinc fume. This is particularly preferable since it can be suppressed.

In addition, the zinc powder contained in the lower coating film A1 is effective in ensuring rust prevention after the inner surface is coated and until piping construction. That is, the sacrificial anti-corrosion effect of the zinc dust suppresses the occurrence of rust on the inner surface of the steel pipe, and even if it does occur, it has the effect of preventing the occurrence of large rust that would cause problems such as malfunction or clogging of the gas pipe.
In addition to metallic zinc powder, the zinc powder may be a powder of an alloy containing zinc as a main component (for example, an alloy with one or more of aluminum, magnesium, tin, etc.).
If the content of zinc dust in the lower coating film A1 is less than 30% by mass, the sacrificial anticorrosion effect of the coating film will be insufficient, and rust will easily occur during storage. In relation to the content of the alkyl silicate polycondensation reaction product, the practical upper limit of the content of zinc dust is 95% by mass, but the content of zinc dust in the range of 30 to 70% by mass improves film-forming properties and corrosion resistance. is preferable because it has an excellent effect of reducing mist during welding, and in particular, a range of 40 to 60% by mass has the best film forming properties, corrosion resistance, and the effect of reducing mist during welding. This is particularly preferable since the generation of zinc fume (which is caused by this) can also be suppressed.

The lower coating film A1 contains components other than the alkyl silicate polycondensation reaction product and zinc powder, such as colloidal silica (water-dispersed colloidal silica, etc.), amorphous silica, silica gel, talc, calcium carbonate, titanium oxide, alumina white, It may contain one or more types of inorganic fine particles (pigments) such as basic magnesium carbonate. These inorganic fine particles (pigments) have the effect of increasing the strength of the coating film, and water-dispersed colloidal silica is particularly preferred because it is particularly effective in increasing the strength of the coating film and also has the effect of reducing welding defects. In order to obtain such an effect, the content of the inorganic fine particles is preferably 1% by mass or more. However, the upper limit of the total content of components other than the alkyl silicate polycondensation reaction product and zinc dust is preferably about 10% by mass.
The lower coating film A1 is preferably an inorganic coating film that does not contain an organic compound such as an organic resin in order to suppress the generation of mist during welding. Further, even when an organic compound is included, the upper limit is preferably about 5% by mass.

The upper coating film A2 is formed to smooth the coating surface on the inner surface of the pipe and to cover the lower coating film A1 so that the zinc particles of the lower coating film A1 are not exposed to the coating surface on the inner surface of the pipe. In order to suppress the generation of mist during welding, ensure the necessary coating strength, and smooth the coating surface, the content of the alkyl silicate polycondensation reaction product should be 50% by mass or more. . Further, from the above viewpoint, the preferable content of the alkyl silicate polycondensation reaction product in the upper coating film A2 is 75% by mass or more, and the more preferable content is 85% by mass or more. Furthermore, the upper coating film A2 does not contain zinc powder, from the viewpoint of not exposing zinc particles to the coating surface on the inner surface of the pipe, or reducing the amount of zinc particles even if exposed to the coating surface. , or even if it contains zinc powder, the content should be lower than that of the lower coating film A1. In this case, the types of zinc dust that can be used are the same as those described above with respect to the lower coating film A1.
By forming such an upper coating film A2, the coating surface on the inner surface of the tube is smoothed compared to the case of forming a single layer coating film containing zinc dust (prior art), and the coating surface of the inner surface of the pipe is smoothed, and the lower coating film A1 Since the zinc particles are not exposed on the coating surface of the inner surface of the pipe, when used as a gas pipe, (i) the odorant contained in the gas flowing into the pipe is less likely to be adsorbed by the coating, and (ii) welding. The effect is that dirt such as spatter and iron powder caused by construction is difficult to adhere to.

Further, the upper coating film A2 preferably does not contain powder (including powder particles) having an average particle size of 5 μm or more in order to smooth the coating surface on the inner surface of the tube. Furthermore, even when powder with an average particle size of 5 μm or more is included, the upper limit is preferably about 5% by mass. Examples of this powder include metal powder such as zinc powder, inorganic particles other than metal powder (eg, extender pigment), and organic particles. Here, the average particle size is a volume-based median diameter calculated from the measured value of particle size distribution by laser diffraction/scattering method, and is obtained using a commercially available laser analysis/scattering type particle size distribution measuring device.
Furthermore, the upper coating film A2 is also preferably an inorganic coating film that does not contain organic compounds such as organic resins in order to suppress the generation of mist during welding.

The upper coating film A2 contains one or more types of inorganic fine particles (pigments) such as colloidal silica (water-dispersed colloidal silica, etc.), amorphous silica, and silica gel as components other than the alkyl silicate polycondensation reaction product. It's okay. As mentioned earlier, these inorganic fine particles (pigments) have the effect of increasing the strength of the coating film, but water-dispersed colloidal silica is particularly effective in increasing the strength of the coating film and reducing welding defects. It is preferable because there is also In order to obtain such an effect, the content of the inorganic fine particles is preferably 1% by mass or more.

In addition, in the present invention, in order to further improve the smoothness of the coating film on the inner surface of the tube (the coating film having the lower coating film A1 and the upper coating film A2), the surface roughness Rz (maximum height Rz ) is preferably 18 μm or less, and therefore, the upper coating film A2 is preferably formed so as to obtain this surface roughness Rz. By setting the surface roughness Rz of the coating film on the inner surface of the pipe to 18 μm or less, the smoothness of the coating film is further increased, so when used as a gas pipe, (i) the odorant contained in the gas flowing into the pipe (ii) stains such as spatter and iron powder caused by welding work are less likely to adhere to the coating.

As for the coating thickness on the inner surface of the tube, it is preferable that the lower coating film A1 has a thickness of 5 to 80 μm, and the upper coating film A2 has a thickness of 5 to 80 μm.
If the film thickness of the lower layer coating A1 is less than 5 μm, the rust prevention effect will decrease, while if the film thickness exceeds 80 μm, the internal stress of the paint film will increase and it will be easy to peel off, and the influence of zinc fume generation during welding. also becomes larger. Further, a particularly preferable film thickness is 10 to 30 μm, which is optimal from the viewpoint of rust prevention, suppression of zinc fume generation during welding, and suppression of mist generation during welding.
If the thickness of the upper coating film A2 is less than 5 μm, the effect of covering the roughness of the lower coating film A1 will be reduced. On the other hand, when the film thickness exceeds 80 μm, the internal stress of the coating film becomes large and it becomes easy to peel off. Further, a particularly preferable film thickness is 10 to 30 μm, which is optimal from the viewpoint of smoothing the coating film and suppressing adsorption of the odorant.
In addition, if the total thickness of the coating film on the inner surface of the tube (lower coating film + upper coating film) exceeds 100 μm, problems such as easy cracking due to the internal stress of the coating may occur, so the total coating thickness should be The thickness is preferably 100 μm or less.

The alkyl silicate (including the case where it is an oligomer), which is the source of the alkyl silicate polycondensation reaction product contained in the lower coating film A1 and the upper coating film A2, has an alkyl group such as an ethyl group, a methyl group, a propyl group, etc. For example, ethyl silicate, methyl silicate, tetraethyl orthosilicate, tetramethyl orthosilicate, tetra-n-propylorthosilicate, tetra-i-propylorthosilicate, tetra-n-butyl orthosilicate, tetra-sec-butyl Examples include orthosilicates, and one or more of these can be used, but among them, those in which the alkyl group is an ethyl group are optimal in terms of film-forming properties.

FIG. 1 is an explanatory diagram schematically showing a cross section in the thickness direction of the inner surface coating film in an embodiment of the resin-coated steel pipe of the present invention, and FIG. 1(A) shows a case where only the lower coating film A1 is formed. FIG. 1A shows a state in which an upper coating film A2 is formed on the lower coating film A1.
The state in which only the lower coating film A1 is formed as shown in FIG. The surface roughness Rz of the coated film surface is large due to surface irregularities due to zinc particles (powder) contained in the coated film, and the zinc particles are exposed on the coated film surface. In this state, the surface area of the paint film increases due to surface irregularities caused by zinc particles (zinc dust) contained in the paint film, and the odorant is easily adsorbed to the paint film surface, and the odorant is present (exposed) on the paint film surface. Zinc particles have a high affinity with the sulfur content of the odorant, and act as adsorption sites for the odorant, making it easy for the odorant contained in the gas flowing into the pipe to be adsorbed to the paint film. In addition, stains such as spatter and iron powder generated during welding of gas pipes tend to adhere to the coating surface, resulting in poor cleaning performance.

In addition, even if the surface roughness Rz of the coating surface is small (for example, surface roughness Rz: 18 μm or less), if the zinc particles are exposed on the coating surface as shown in Figure 1 (A), the coating will deteriorate. Zinc particles present (exposed) on the film surface serve as adsorption sites for the odorant, so the odorant is easily adsorbed to the paint film, and stains such as spatter and iron powder tend to adhere to the paint film surface.
On the other hand, as shown in FIG. 1(a), by forming the upper coating film A2 on the lower coating film A1, the coating film surface is smoothed, and the lower coating film A1 is formed on top of the upper coating film A1. Covered with A2, the zinc particles of the lower coating film A1 can be prevented from being exposed to the coating surface, and as a result (i) the odorant contained in the gas flowing into the pipe is adsorbed to the coating film. (ii) It is difficult for stains such as spatter and iron powder generated during welding to adhere.

As mentioned earlier, a typical example of the resin-coated steel pipe targeted by the present invention is a polyethylene-coated steel pipe. It is particularly suitable for PE2S of JIS G3477-2 and P2S of JIS G3469, which are often used for medium-pressure conduits mainly connected by welding. However, it is not limited to these.

In the present invention, the film thickness and surface roughness Rz of the coating film on the inner surface of the tube are measured as follows.
First, regarding the film thickness of the lower coating film A1, when the lower coating film A1 is applied, an electromagnetic film thickness gauge is used to measure the inner coating thickness at four points in the circumferential direction near both ends of the pipe (four points equidistantly spaced in the circumferential direction). Measure the film thickness, and let the average value be the film thickness of the lower coating film A1.
Regarding the film thickness of the upper coating film A2, the inner coating film (coating film having the lower coating film A1 and the upper coating film A2) was measured using an electromagnetic film thickness meter at four points in the circumferential direction (equal in the circumferential direction) near both ends of the pipe. The coating thickness is measured at 4 points at intervals, the average value thereof is determined, and the value obtained by subtracting the thickness of the lower coating film A1 from this average value is determined as the thickness of the upper coating film A2.
In addition, the surface roughness Rz of the inner surface coating film (the coating film having the lower coating film A1 and the upper coating film A2) was measured using a small surface roughness meter at four points in the circumferential direction near both ends of the pipe (in the circumferential direction). Measure the surface roughness Rz (maximum height Rz) of the inner coating film at four equally spaced points in a range of about 10 mm along the tube axis direction, and take the average value as the surface roughness Rz of the coating film. .

Next, the method for manufacturing a resin-coated steel pipe of the present invention will be explained using a polyethylene-coated steel pipe for gas pipes as an example. In addition, in the manufacture of other resin-coated steel pipes, a method similar to the following explanation is adopted for the inner surface coating.
The type of steel pipe (original pipe) used is not particularly limited as long as it is a steel pipe used for gas pipes. Generally, SGP defined in JIS G3452 and STPG defined in JIS G3454 are most often used. In order to perform welding and joining, it is preferable that both tube ends are beveled in advance.
Before the steel pipe (original pipe) is coated with inner and outer resin coatings, it is pickled or blasted (shot blasting, grit blasting, etc.) for the purpose of conditioning the surface to remove rust and dirt from the inner and outer surfaces of the steel pipe. Remove mill scale.

When painting the inner surface of a steel pipe, the inner surface of the pipe is coated with paint A1 for the lower layer coating, which contains at least 5% by mass of alkyl silicate and/or alkyl silicate oligomer and at least 30% by mass of zinc powder, based on the solid content of the paint. coating), and then contains at least 50% by mass of alkyl silicate and/or alkyl silicate oligomer in the solid content of the paint, and does not contain zinc dust, or contains zinc dust more than paint a1 for the lower layer coating. By painting (applying) paint a2 for the upper layer coating film containing a small amount, a coating film having a lower coating film A1 and an upper coating film A2 (preferably with a surface roughness Rz of 18 μm or less) is formed on the inner surface of the tube. form a coating film).
Each component (solid content) is mixed into the paints a1 and a2 so as to form a coating film having the component composition described above. Furthermore, although alkyl silicate may be used alone in paints a1 and a2, it may also be used in the form of an oligomer in which alkyl silicate is condensed to some extent. The effect is the same in either form.

The inner surface of steel pipes is generally painted using airless spraying. Paint A1 for the lower layer coating is painted (coated) on the inner surface (base) of the steel pipe, and after drying to a certain extent, Paint A2 for the upper layer coating is applied. coating) and drying to form an inner coating film. In internal surface painting using airless spraying, an arm with a nozzle at the tip is inserted into the pipe, paint is sprayed from the nozzle tip while the steel pipe is rotated, and the entire length of the inner surface of the steel pipe is coated as the arm is gradually pulled out. Both the lower coating film A1 and the upper coating film A2 are made to have the desired thickness by adjusting the pulling speed of the arm. After coating, a paint containing an alkyl silicate and/or an alkyl silicate oligomer hardens and forms a film when it comes into contact with moisture in the air, and after this film formation, it may be heated in a drying oven.

The outer surface of the steel pipe is subjected to chemical conversion treatment, such as chromate treatment or non-chromate treatment (for example, phosphate treatment, phosphoric acid treatment), as required, and then undercoated with an asphalt-based adhesive or the like.
Thereafter, the outer surface of the steel pipe is coated with polyethylene via a crosshead die (round die) or T die of an extrusion molding machine. In addition, the P2S and PE2S types are coated with polyethylene for anticorrosion, and then coated with polyethylene as an additional protective layer.
It is also possible to apply a resin coating such as epoxy resin coating to the outer surface of the steel pipe (i.e., make it a resin-coated steel pipe such as an epoxy-coated steel pipe). In this case, the steel pipe is placed on a turning roller, etc. While rotating, apply resin coating such as epoxy resin coating to the entire outer surface of the steel pipe using an airless sprayer.
The resin-coated steel pipe of the present invention is manufactured by the manufacturing method as described above.

A polyethylene-coated steel pipe P2S (200A) as defined in JIS G3469 was manufactured using SGP (steel pipe with beveled ends on both ends) as defined in JIS G3452 as a master pipe.
Grit blasting (Sa2.5) was applied to the inner and outer surfaces of the steel pipe to remove rust, dirt, and mill scale from the surface of the steel pipe. This steel pipe was coated with an inner surface and a resin coated outer surface using the following method to obtain a polyethylene-coated steel pipe.
Comparative examples of polyethylene-coated steel pipes include one with a single-layer coating film (corresponding to the lower coating film of the present invention) formed on the inner surface of the tube, one with an epoxy resin coating on the inner surface of the tube, and one with no coating on the inner surface of the tube. The product was manufactured as follows.

(a) Internal coating Ethyl silicate was used as the alkyl silicate (including the case where it is an oligomer) in both the paint for the lower layer coating film and the paint for the upper layer coating film. In addition, water-dispersed colloidal silica was added as another additive to both the paint for the lower layer coating and the paint for the upper layer coating.
The inner surface coating was performed by airless spraying for both the lower layer coating film and the upper layer coating film. The inner surface of the steel pipe (base material) was coated with a paint for the lower layer coating, and after drying to a certain extent, the paint for the upper layer coating was applied and allowed to air dry to form the inner surface coating. For this inner surface painting, an arm with a nozzle at the tip was inserted into the inner surface of the tube, paint was jetted from the nozzle tip while the steel tube was rotated, and the entire length of the inner surface of the steel tube was coated as the arm was gradually pulled out. Both the lower layer coating film and the upper layer coating film were made to have the desired film thickness by adjusting the pull-out speed of the arm.

(b) External resin coating The external surface of the pipe was undercoated with an asphalt adhesive. The outer surface of the tube coated with this undercoat is coated with anticorrosive polyethylene using a crosshead die (round die) of an extrusion molding machine, and then polyethylene as a protective layer resin is coated on top of that, forming a resin coating layer. did.

The film thickness of the lower layer coating film and the upper layer coating film of the inner surface coating of the polyethylene coated steel pipe, and the surface roughness Rz of the coating film were measured by the method described above.
The following characteristics were evaluated for the obtained polyethylene-coated steel pipes of the present invention examples and comparative examples.
(1) Welding jointability (presence of mist generation during welding, presence of solenoid valve sticking due to mist)
To install the gas conduit, the ends of polyethylene-coated steel pipes were circumferentially welded together using arc welding, and the presence and extent of mist (thermal decomposition products of the paint film) generated during this process was investigated. In addition, a solenoid valve was attached to the gas pipe and used for half a year, and the presence and extent of malfunction caused by mist adhering to the solenoid valve was investigated.
◎: There is no mist generated during welding, and therefore there is no malfunction of the solenoid valve due to mist adhesion.
○: Almost no mist is generated during welding, and therefore there is no malfunction of the solenoid valve due to mist adhesion.
△: A slight amount of mist was generated during welding, and as a result, some malfunction of the electromagnetic valve occurred due to adhesion of the mist.
×: A large amount of mist was generated during welding, resulting in frequent malfunctions of the electromagnetic valve due to adhesion of the mist.

(2) Odor of odorant (non-adsorption of odorant to coating film)
City gas was flowed at 3 Nm ³ /hr through a 12 m pipe made of polyethylene-coated steel pipe, and the odor of the odorant was confirmed.
◎: An odor comes out immediately after the gas starts flowing.
○: There is no odor within 10 seconds after starting to flow the gas, but after that the odor begins.
△: There is no odor for more than 10 seconds and less than 1 minute after the gas starts flowing, but after that, an odor starts.
×: The odor does not come out until more than 1 minute has passed since the gas started flowing.

(3) Cleaning workability (cleanability of slag, spatter, etc. generated during welding)
After the ends of polyethylene-coated steel pipes were circumferentially welded and joined using arc welding, the welded area was cleaned with a sponge pig, and the cleaning performance of slag, spatter, etc. generated during welding (including the running performance of the pig) was investigated. .
◎: Can be cleaned with a sponge pig without any problems.
○: There is some catching with the sponge pig.
△: Dirt cannot be removed thoroughly with a sponge pig.
×: The sponge pig gets caught and the pig's running is hindered.

A1 Lower layer coating
A2 Upper coating film
a1, a2 paint

Claims (10)

A resin-coated steel pipe having a resin coating layer or coating film on the outer surface of the pipe and a coating film on the inner surface of the pipe,
The coating film on the inner surface of the tube consists of an alkyl silicate polycondensation reaction product of 5% by mass or more and 70% by mass or less , 30% by mass or more and 95% by mass or less of zinc dust , and 10% by mass or less (however, 0% by mass is included). a lower coating film (A1) consisting of colloidal silica , an alkyl silicate polycondensation reaction product of 50% by mass or more and 100% by mass or less , and 50% by mass or less (however, including 0% by mass) of colloidal silica, A resin-coated steel pipe characterized by having an upper coating film (A2) that does not contain zinc dust. The resin-coated steel pipe according to claim 1, wherein the coating film on the inner surface of the pipe has a surface roughness Rz of 18 μm or less. The resin-coated steel pipe according to claim 1 or 2, wherein the upper coating film (A2) does not contain powder having an average particle size of 5 μm or more. The resin-coated steel pipe according to any one of claims 1 to 3 , wherein the lower coating film (A1) and the upper coating film (A2) each have a thickness of 5 to 80 μm. The resin-coated steel pipe according to any one of claims 1 to 4 , which has a polyethylene coating layer on the outer surface of the pipe. A method for producing a resin-coated steel pipe having a resin coating layer or coating film on the outer surface of the pipe and a coating film on the inner surface of the pipe, the method comprising:
When painting the inner surface of the tube, the inner surface of the tube is coated with alkyl silicate and/or alkyl silicate oligomer with a coating solid content of 5% by mass or more and 70% by mass or less, zinc powder of 30% by mass or more and 95% by mass or less , and 10% by mass or less. (However, 0% by mass is included.) A coating material (a1) for the lower coating film consisting of colloidal silica is applied, and then an alkyl silicate and/or alkyl silicate having a solid content of 50% by mass or more and 100% by mass or less is applied. By applying a paint for the upper layer coating (A2 ) consisting of oligomers and 50% by mass or less (including 0% by mass) of colloidal silica and containing no zinc dust, a lower coating film is formed on the inner surface of the pipe. A method for manufacturing a resin-coated steel pipe, comprising forming a coating film having (A1) and an upper coating film (A2). The method for producing a resin-coated steel pipe according to claim 6, characterized in that a coating film having a lower coating film (A1) and an upper coating film (A2) and having a surface roughness Rz of 18 μm or less is formed on the inner surface of the pipe. . The method for manufacturing a resin-coated steel pipe according to claim 6 or 7 , wherein the paint (a2) for the upper layer coating film does not contain powder having an average particle size of 5 μm or more. The resin-coated steel pipe according to any one of claims 6 to 8 , wherein the dry thickness of the lower coating film (A1) and the dry thickness of the upper coating film (A2) are each 5 to 80 μm. manufacturing method. The method for manufacturing a resin-coated steel pipe according to any one of claims 6 to 9 , characterized in that a polyethylene coating layer is formed on the outer surface of the pipe.

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