JP6954788B2 - Manufacturing method of polyethylene-coated steel pipe for gas conduit and polyethylene-coated steel pipe for gas conduit - Google Patents

Description

The present invention relates to a method for producing a polyethylene-coated steel pipe and a polyethylene-coated steel pipe, and more particularly to a method for producing a polyethylene-coated steel pipe for a gas conduit and a polyethylene-coated steel pipe for a gas conduit.

The role of city gas, water pipelines and various buried pipes is becoming more important. As these piping materials, polyethylene-coated steel pipes are used. A polyethylene-coated steel pipe is a steel pipe in which the outer surface of the steel pipe is coated with a polyethylene resin to improve durability such as corrosion resistance and chemical resistance. Polyethylene coated steel pipes are defined in, for example, JIS G3469 (2010).

On the inner surface of the polyethylene-coated steel pipe, a coating film may not be formed or a coating film may be formed. If a coating film is not formed, rust is likely to occur from the inner surface of the steel pipe during the storage period from the production of the steel pipe to the joining at the site. Further, if the generated rust is peeled off after joining the steel pipe, it may diffuse into the steel pipe and solidify. That is, it may be necessary to remove rust after joining the steel pipes. Therefore, in many cases, a coating film is formed on the inner surface of the polyethylene-coated steel pipe for the purpose of improving corrosion resistance. The coating film is often formed after the inner surface of the polyethylene-coated steel pipe is blasted. This is because the blasting treatment exposes the active surface and exerts an anchor effect, thereby enhancing the adhesion of the coating film.

As a coating film formed on the inner surface of a polyethylene-coated steel pipe, an epoxy resin coating film has been widely used in the past. The epoxy resin coating film physically blocks contact between moisture and oxygen that cause corrosion and the inner surface of the polyethylene-coated steel pipe (inner surface of the original pipe), and provides high corrosion resistance.

Generally, a polyethylene-coated steel pipe for a gas conduit is blasted for the purpose of an anchor effect on the inner surface of the original pipe, and then the epoxy resin coating composition is applied. The above-mentioned epoxy resin coating film is obtained by curing the epoxy resin coating composition. As a result, high corrosion resistance can be obtained in polyethylene-coated steel pipes for gas conduits in which an epoxy resin coating film is formed on the inner surface.

By the way, welding is generally used for joining polyethylene-coated steel pipes for gas conduits. Welding is performed from the outside of the polyethylene-coated steel pipe for gas conduit by partially peeling off the polyethylene on the outer surface of the polyethylene-coated steel pipe for gas conduit to prevent combustion. On the other hand, since it is difficult to remove the coating film on the inner surface of the polyethylene-coated steel pipe for gas conduits, welding is usually performed with the coating film on the inner surface remaining.

The coating film on the inner surface of the polyethylene-coated steel pipe for gas conduits preferably does not easily burn or thermally decompose during welding. However, when, for example, a conventionally widely used epoxy resin coating film is formed on the inner surface of a polyethylene-coated steel tube, an organic component such as an epoxy resin in the epoxy resin coating film burns during welding, resulting in pyrolysis gas and It produces pyrolysis and can adversely affect the health of welders. Further, the pyrolyzed product such as epoxy resin may diffuse into the gas conduit and solidify inside the gas conduit.

For the above reasons, a coating film suitable for the inner surface of a polyethylene-coated steel pipe for a gas conduit has been studied. For example, Patent Documents 1 to 3 form a coating film different from the epoxy resin on the inner surface of a polyethylene-coated steel pipe for a gas conduit.

The polyethylene-coated steel pipe described in JP2013-173340 (Patent Document 1) is a polyethylene-coated steel pipe having a polyethylene resin coating layer on the outer surface, and the polyethylene-coated steel pipe has a coating film on the inner surface. The coating film is a cured coating film of a coating material containing 80 parts by weight or more and 95 parts by weight or less of alkyl silicate with respect to 100 parts by weight of the solid content of the coating material. It is described in Patent Document 1 that this makes it possible to reduce the amount of thermal decomposition products generated from the inner coating portion during welding, which causes clogging of filters and valves.

The polyethylene-coated steel pipe described in JP-A-2015-168117 (Patent Document 2) is a polyethylene-coated steel pipe having a coating layer made of polyethylene resin on the outer surface, and contains 40 to 70% by mass of zinc powder and a molybdic acid compound. It has a coating film containing 1 to 10% by mass, 10 to 20% by mass of solvent glue lacquer silica, and 10 to 40% by mass of alkyl silicate on the inner surface. It is described in Patent Document 2 that this makes it possible to provide a polyethylene-coated steel pipe in which the amount of mist or the like generated from the inner surface is extremely small even if welding or the like with a high amount of heat input is performed on the outer surface.

The gas coated steel pipe described in JP-A-2016-175315 (Patent Document 3) contains 5 to 50% by mass of alkyl silicate and / or modified alkyl silicate and 3 to 40 mass% of collodyl silica as the paint solid content. It has a cured coating film containing 30 to 70% by mass of zinc powder on the inner surface. It is described in Patent Document 3 that this makes it possible to perform good welding without removing the inner coating film, suppress the generation of mist due to welding heat, and obtain a coated steel pipe for gas having good corrosion resistance. ing.

Japanese Unexamined Patent Publication No. 2013-173340 Japanese Unexamined Patent Publication No. 2015-168117 Japanese Unexamined Patent Publication No. 2016-175315

Patent Documents 1 to 3 describe that a coating composition containing a siloxane-based binder such as alkyl silicate and zinc powder is applied onto the inner surface of a blasted steel pipe. On the other hand, immediately after the blasting treatment, corrosion progresses on the inner surface of the polyethylene-coated steel pipe. Therefore, in order to improve the adhesion of the coating film by the blasting treatment, the next surface treatment or the formation of the coating film must be performed within several hours after the blasting treatment. In this case, there are restrictions on the work contents at the time of manufacturing. As a result, the production efficiency may decrease. Therefore, improvement of production efficiency has been required.

By the way, a polyethylene-coated steel pipe for a gas conduit needs to stably transport a large amount of gas. Therefore, high gas transport efficiency is required for polyethylene-coated steel pipes for gas conduits. The pressure of the gas flowing inside the polyethylene-coated steel pipe for the gas conduit is lost due to various factors. Factors of gas pressure loss are, for example, the cross-sectional area of the flow path, the surface area of the pipe wall, the roughness of the pipe wall, the flow velocity, the flow method (turbulent flow or laminar flow), and the like.

Until now, there have been few developments of polyethylene-coated steel pipes for gas conduits for the purpose of achieving both corrosion resistance suitable for gas transportation, suppression of thermal decomposition products generated during welding, and improvement of gas transportation efficiency. Therefore, for example, even if the above-mentioned technique is used, a polyethylene-coated steel pipe for a gas conduit having high gas transport efficiency may not be obtained.

An object of the present invention is to provide a polyethylene-coated steel pipe for a gas conduit which has corrosion resistance suitable for gas transportation, suppresses the generation of thermal decomposition products during welding, and further enhances gas transportation efficiency. The present invention provides a method for producing a polyethylene-coated steel pipe for a gas conduit with improved production efficiency.

The polyethylene-coated steel pipe for a gas conduit of the present embodiment includes an original pipe, a phosphate coating film, a coating film, and a polyethylene resin coating film. The phosphate coating is placed on the inner surface of the original tube. The coating is arranged on the phosphate coating. The coating film contains a siloxane-based binder and zinc powder. The surface roughness Rz of the coating film is less than 30.0 μm. The polyethylene resin coating is placed on the outer surface of the original pipe.

The method for producing a polyethylene-coated steel pipe for a gas conduit according to the present embodiment includes a preparation step, a chemical conversion treatment step, a coating composition coating step, a coating composition curing step, and a polyethylene coating step. In the preparation process, the original pipe is prepared. In the chemical conversion treatment step, the inner surface of the original pipe is subjected to chemical conversion treatment to form a phosphate film on the inner surface of the original pipe. In the coating composition coating step, a coating composition containing an alkyl silicate condensate and zinc powder is coated on the phosphate film. In the coating composition curing step, the coated coating composition is cured. In the polyethylene coating step, a film containing polyethylene resin is formed on the outer surface of the original pipe.

The polyethylene-coated steel pipe for a gas conduit according to the present embodiment has corrosion resistance suitable for gas transportation, suppresses the generation of thermal decomposition products during welding, and is excellent in gas transportation efficiency. Further, the method for producing a polyethylene-coated steel pipe for a gas conduit according to the present embodiment is excellent in production efficiency.

FIG. 1 is a cross-sectional view of an example of a polyethylene-coated steel pipe for a gas conduit according to the present embodiment.

Hereinafter, the present embodiment will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are designated by the same reference numerals, and the description thereof will not be repeated.

The present inventors have conducted various studies on polyethylene-coated steel pipes for gas conduits. As a result, the following findings were obtained.

The present inventors first examined the corrosion resistance of polyethylene-coated steel pipes for gas conduits. In many conventional polyethylene-coated steel pipes for gas conduits, an epoxy resin coating film is formed on the inner surface. As a result, high corrosion resistance was obtained even after the piping was completed.

However, due to recent improvements in gas quality, the water content of the gas has been reduced, and the gas has been reduced to a reducing property. That is, in the usage environment after the piping is completed, the degree of corrosion resistance required for the inner surface of the polyethylene-coated steel pipe for gas conduit is lower than that of, for example, a polyethylene-coated steel pipe for water supply. On the other hand, corrosion resistance is not required for the inner surface of polyethylene-coated steel pipes for gas conduits. Corrosion resistance is required to the extent that clear corrosion such as red rust does not occur from the time of manufacture to the completion of piping through transportation and storage.

In order to obtain such corrosion resistance, the sacrificial anticorrosion effect of zinc can be obtained by forming a coating film containing zinc powder on the inner surface of the original pipe of the polyethylene-coated steel pipe for gas conduit. This enhances the corrosion resistance of the inner surface of the polyethylene-coated steel pipe for gas conduits.

Next, the present inventors examined the effect of suppressing thermal decomposition products during welding of polyethylene-coated steel pipes for gas conduits. Welding is generally used as a method for joining polyethylene-coated steel pipes for gas conduits. Therefore, it is preferable that the coating film formed on the inner surface of the original pipe of the polyethylene-coated steel pipe for gas conduit is hard to be burned and thermally decomposed at the time of welding. The alkyl silicate condensate is hydrolyzed and condensed by the moisture in the air to form a coating film. This coating film has a siloxane-based binder. A coating film having a siloxane-based binder is less likely to burn and thermally decompose during welding as compared with a conventional epoxy resin coating film. Therefore, if the coating film formed on the inner surface of the polyethylene-coated steel pipe is a coating film containing a siloxane-based binder, the effect of suppressing thermal decomposition products is enhanced. Therefore, a coating film containing a siloxane-based binder and zinc powder is formed on the inner surface of a polyethylene-coated steel pipe for a gas conduit. In the present specification, the siloxane-based binder is a compound having a siloxane bond (Si—O), and specific examples thereof include compounds obtained by hydrolyzing and condensing an alkyl silicate condensate. It is preferable that the above-mentioned coating film substantially does not contain an organic component that generates a thermal decomposition product due to welding. Here, the term "substantially free" means, for example, 3% by mass or less.

The present inventors further investigated the gas transport efficiency of polyethylene-coated steel pipes for gas conduits. As a result, it was found that when the blasting treatment is performed to form the blasted surface and the coating film is formed on the blasting surface in order to obtain the anchor effect as in the conventional case, the gas transport efficiency may decrease. Therefore, as a result of investigating the cause, the present inventors obtained the following findings.

The blast surface formed by the blast treatment for the purpose of the anchor effect has a large surface roughness. If the inner surface of the original pipe is blasted for the purpose of anchoring effect, the roughness of the inner surface of the original pipe becomes large. If the inner surface roughness of the original pipe is large, the surface roughness of the coating film formed on the original pipe is large. If the surface roughness of the coating film is large, the roughness of the tube wall is large. If the roughness of the pipe wall is large and the surface area of the pipe wall is large, the pressure loss of the gas becomes large. As a result, the pressure loss of the gas flowing inside the polyethylene-coated steel pipe for the gas conduit becomes large, and the gas transport efficiency decreases.

Therefore, the roughness of the inner surface of the polyethylene-coated steel pipe for gas conduits before forming the coating film is reduced. As a result, the surface roughness of the coating film formed on the coating film is smoothed, and the gas transport efficiency is improved.

However, in order to improve the gas transport efficiency, unlike the conventional method, when the roughness of the inner surface of the original pipe is suppressed, it becomes difficult to obtain the anchor effect, so that the adhesion of the coating film to the inner surface of the original pipe is lowered. .. Therefore, a coating film for enhancing the adhesion of the coating film is further formed between the inner surface of the original pipe of the polyethylene-coated steel pipe for gas conduit and the coating film. Specifically, a phosphate film is formed by chemical conversion treatment, and a coating film is formed on the phosphate film. This enhances the adhesion of the coating film. In addition, the phosphate coating has a higher effect of suppressing thermal decomposition products than the coating of organic substances. As a result, the adhesion of the coating film is enhanced and the effect of suppressing thermal decomposition products is enhanced.

Here, even if the coating film is formed on the phosphate coating film, the corrosion resistance may still be lower than that in the case of forming the conventional epoxy resin coating film on the blast surface. However, in the case of polyethylene-coated steel pipes for water supply, high corrosion resistance is required continuously during use after the piping is completed, whereas in the case of polyethylene-coated steel pipes for gas conduits, the piping is completed and the water content is high. High corrosion resistance is not required if low gas flows. Therefore, in the case of a polyethylene-coated steel pipe for a gas conduit, if a coating film is formed on the phosphate coating, the required corrosion resistance can be sufficiently ensured. Further, by forming a phosphate film under the coating film, the blasting treatment for the purpose of the anchor effect can be omitted, and the roughness of the inner surface can be reduced. It was found that this can improve the efficiency of gas transportation, which is important for gas pipelines.

Further, if a phosphate film is formed by chemical conversion treatment, corrosion on the surface of the original pipe can be further suppressed. Therefore, unlike the blast treatment, the time limit from the chemical conversion treatment to the next surface treatment or the formation of the coating film is reduced. As a result, production efficiency can be improved.

The polyethylene-coated steel pipe for gas conduit of the present embodiment completed based on the above findings includes an original pipe, a phosphate coating film, a coating film, and a polyethylene resin coating film. The phosphate coating is placed on the inner surface of the original tube. The coating is arranged on the phosphate coating. The coating film contains a siloxane-based binder and zinc powder. The surface roughness Rz of the coating film is less than 30.0 μm. The polyethylene resin coating is placed on the outer surface of the original pipe.

The polyethylene-coated steel pipe for a gas conduit of the present embodiment has corrosion resistance suitable for gas transportation, suppresses the generation of thermal decomposition products during welding, and is excellent in gas transportation efficiency.

Preferably, the inner surface of the original pipe is not a blast surface.

Preferably, the zinc powder contains scaly zinc powder. In this case, the corrosion resistance of the polyethylene-coated steel pipe for gas conduits is further enhanced.

Preferably, the phosphate coating is a zinc phosphate calcium coating.

Preferably, the thickness of the coating film is 25 μm or less. In this case, the generation of thermal decomposition products due to welding can be further suppressed.

The method for producing a polyethylene-coated steel pipe for a gas conduit according to the present embodiment includes a preparation step, a chemical conversion treatment step, a coating composition coating step, a coating composition curing step, and a polyethylene coating step. In the preparation process, the original pipe is prepared. In the chemical conversion treatment step, the inner surface of the original pipe is subjected to chemical conversion treatment to form a phosphate film on the inner surface of the original pipe. In the coating composition coating step, a coating composition containing an alkyl silicate condensate and zinc powder is coated on the phosphate film. In the coating composition curing step, the coated coating composition is cured. In the polyethylene coating step, a film containing a polyethylene resin is formed on the outer surface of the original pipe.

The method for manufacturing a polyethylene-coated steel pipe for a gas conduit of the present embodiment has few restrictions on the work content at the time of manufacturing. Therefore, the production efficiency is excellent.

Preferably, in the above manufacturing method, the inner surface of the original pipe is not blasted. In this case, the production efficiency is further increased.

The polyethylene-coated steel pipe for gas conduits of the present embodiment will be described in detail below.

[Polyethylene coated steel pipe for gas conduit]
FIG. 1 is a cross-sectional view of an example of a polyethylene-coated steel pipe for a gas conduit according to the present embodiment. With reference to FIG. 1, the polyethylene-coated steel pipe 100 for a gas conduit includes an original pipe 1, a polyethylene resin coating film 10, a phosphate coating film 11, and a coating film 12. The original pipe 1 has an outer surface 2 and an inner surface 3. The polyethylene resin coating 10 is arranged on the outer surface 2 of the original pipe 1. The phosphate coating 11 is arranged on the inner surface 3 of the original pipe 1. The coating film 12 is arranged on the phosphate coating film 11.

A well-known steel pipe can be used as the original pipe 1. The steel pipe is, for example, a steel pipe composed of one selected from the group consisting of carbon steel, alloy steel or stainless steel. The steel pipe is, for example, a steel pipe specified in JIS G3452 (2014), JIS G3454 (2012), and the like.

As used herein, the blasted surface refers to a blasted surface. The blasted surface also includes a surface that has been subjected to, for example, degreasing, pickling, washing, etc. after the blasting treatment. The inner surface 3 of the original pipe 1 may be a blast surface. For example, when the scale is thickly formed on the inner surface 3 of the original pipe 1 due to the pipe manufacturing conditions, a blast treatment may be performed. However, preferably, the inner surface 3 of the original pipe 1 is not a blast surface. That is, preferably, the inner surface 3 of the original pipe 1 is the inner surface 3 that has not been blasted. In this case, the roughness of the inner surface 3 of the original pipe 1 can be stably reduced.

[Phosphate coating]
The phosphate coating 11 is arranged on the inner surface 3 of the original pipe 1. The phosphate coating 11 is a well-known phosphate coating. The phosphate coating 11 contains, for example, one or more selected from the group consisting of zinc phosphate, zinc phosphate calcium, iron phosphate and manganese phosphate. Preferably, the phosphate coating 11 is a zinc phosphate calcium coating. Zinc calcium phosphate coating, Scholl Zeit _{(CaZn 2 (PO 4) 2} · 2H 2 O), hopeite _{(Zn 3 (PO 4) 2} · 4H 2 O) and phosphophyllite _(Zn 2 Fe _(PO 4) is a complex of ₂ · _{4H 2} O). Impurities may be contained in the phosphate coating 11. Here, the impurity is a substance other than phosphate, which is contained in the phosphate coating 11 during the production of the polyethylene-coated steel tube 100 for gas exchange, and is contained in a content within a range that does not affect the effect of the present invention. Contains substances that are

[Method for identifying phosphate coating]
For example, the phosphate coating 11 is identified by the following method. A polyethylene-coated steel pipe 100 for a gas conduit is cut vertically in the axial direction. X-ray diffraction measurement is performed on the cross section of the phosphate coating 11 exposed by cutting. X-ray diffraction measurement and identification of the phosphate coating 11 are performed by a method according to JIS K3151 (1996).

The lower limit of the film weight of the phosphate film 11 is not particularly limited. However, when the film weight of the phosphate film 11 is 3 g / m ² or more, the adhesion of the coating film 12 is stably increased. Therefore, the lower limit of the coating weight of the preferable phosphate coating 11 is 3 g / m ² . The upper limit of the film weight of the phosphate film 11 is not particularly limited. However, if the film weight of the phosphate film 11 ^{exceeds 6 g / m 2} , the adhesion of the coating film 12 may be impaired. Therefore, the upper limit of the coating weight of the preferable phosphate coating 11 is 6 g / m ² , and more preferably 5 g / m ² .

[Measuring method of coating weight of phosphate coating]
For example, the film weight of the phosphate film 11 is measured by the following method. First, a polyethylene-coated steel pipe 100 for a gas conduit is cut to prepare a test piece (width 2 cm × length 5 cm) containing the inner surface 3 of the original pipe 1. The test piece is immersed in a 50 g / L chromium trioxide aqueous solution at 75 ± 2 ° C. This dissolves the phosphate film 11. Weigh the test piece before and after melting. Repeat melting until the weight of the test piece before and after melting does not change. Next, the coating film 12 peeled off from the inner surface 3 of the original pipe 1 is recovered. The weight of the coating film 12 is measured. The weight of the test piece after dissolution and the weight of the measured coating film 12 are subtracted from the weight of the test piece before dissolution. Thereby, the weight of the phosphate coating 11 is calculated. The weight of the obtained phosphate coating 11 is divided by the area corresponding to the inner surface 3 of the test piece. The obtained value is taken as the coating weight (g / m ² ) of the phosphate coating 11. If a steel pipe before forming the coating film is available, the weight of the phosphate coating may be measured on the steel pipe.

[Coating film]
The coating film 12 is arranged on the phosphate coating film 11. The coating film 12 contains a siloxane-based binder and zinc powder. The siloxane-based binder is a compound having a siloxane bond, and specific examples thereof include a compound obtained by hydrolyzing and condensing an alkyl silicate condensate. Since the coating film 12 contains a siloxane-based binder, it has an excellent effect of suppressing thermal decomposition products during welding. Further, if the coating film 12 contains zinc powder, the corrosion resistance of the polyethylene-coated steel pipe 100 for a gas conduit is enhanced by sacrificial corrosion protection. The content of zinc powder in the coating film 12 is preferably 30 to 90% by mass, more preferably 40 to 80% by mass.

[Zinc powder]
The zinc powder is a powder of metallic zinc or a powder of an alloy containing zinc as a main component (eg, an alloy of zinc with at least one selected from aluminum, magnesium and tin).

[Shape of zinc powder]
The shape of the zinc powder may be any shape and is not limited. The shape of the zinc powder is, for example, spherical or scaly. The zinc powder preferably contains scaly zinc powder (scaly zinc powder). The scaly zinc powder refers to zinc powder having an aspect ratio (average length / average thickness) of 10 to 150, which is represented by the ratio of the average length to the average thickness. The aspect ratio of zinc powder is preferably 20 to 100. Since the scaly zinc powder has a larger specific surface area than the spherical zinc powder (spherical zinc powder), the sacrificial anticorrosion effect is high. Further, the coating film 12 containing the scaly zinc powder can keep the zinc powders in close contact with each other. Therefore, the coating film 12 can be easily thinned, and the sacrificial anticorrosion effect can be easily obtained. Further, the coating film 12 containing scaly zinc powder can easily obtain a shielding effect of water and oxygen. That is, if the zinc powder contains scaly zinc powder, the corrosion resistance of the polyethylene-coated steel pipe 100 for a gas conduit is further enhanced. When the zinc powder contains scaly zinc powder, the scaly zinc powder is preferably 15 to 90% by mass, preferably 15 to 70% by mass, based on the total content of 100% by mass of the zinc powder. Is more preferable. The zinc powder does not have to contain spherical zinc powder.

[Other ingredients]
The coating film 12 may appropriately contain a siloxane-based binder and other components other than zinc powder as long as the object and effect of the present invention are not impaired. Examples of other components include conductive pigments, rust preventive pigments (excluding zinc powder), molybdenum compounds, and sedimentation inhibitors. The conductive pigment is, for example, one or two selected from the group consisting of zinc oxide and carbon powder. The rust preventive pigment is, for example, one or more selected from the group consisting of zinc aluminum phosphate compounds, zinc calcium phosphite compounds, zinc strontium phosphite compounds, aluminum tripolyphosphate compounds and zinc cyanamide compounds. .. The molybdenum compound is, for example, one or more selected from the group consisting of metal molybdenum, molybdenum oxide, and metal salts of molybdenum acid. The sedimentation inhibitor is, for example, one or more selected from the group consisting of an organic bentonite-based sedimentation inhibitor, a polyethylene oxide-based sedimentation inhibitor, a fumed silica-based sedimentation inhibitor, and an amide-based sedimentation inhibitor.

[Method for identifying coating film]
[Identification of siloxane bond]
For example, the siloxane bond in the coating film 12 is identified by the following method. Infrared spectroscopy is measured on the coating film 12. A Fourier transform infrared spectrophotometer (FT-IR) IRTracker-100 (trade name) manufactured by Shimadzu Corporation is used for the measurement. From the measurement chart, the peak of ^{1010 to 1090 cm -1} derived from the Si—O—Si skeletal vibration is identified. This identifies that a compound having a siloxane bond is contained.

[Identification of zinc powder]
For example, the zinc powder in the coating film 12 is identified by the following method. A part of the polyethylene-coated steel pipe 100 for a gas conduit including the coating film 12 is cut and used as a test piece. The surface or cross section of the coating film 12 of the test piece is surface-observed using a scanning electron microscope (SEM) (TM-3000) manufactured by Hitachi High-Technologies Corporation. Then, elemental analysis is performed by energy dispersive X-ray spectroscopy (EDX) (Quantax70) manufactured by Hitachi High-Technologies Corporation, and it is identified that zinc-containing particles are contained in the coating film 12. Further, the Zn content with respect to Si, O, C, H and Zn is calculated from the result of elemental analysis. The obtained Zn content is converted into mass% to obtain the zinc powder content (mass%) in the coating film 12.

[Measurement method of aspect ratio of scaly zinc powder]
For example, the aspect ratio of scaly zinc powder is determined by the following method. The scaly zinc powder is observed using a scanning electron microscope (SEM) (TM-3000) manufactured by Hitachi High-Technologies Corporation. For the observation magnification, select a magnification according to the average length of the scaly zinc powder. Observe 50 scaly zinc powders. In one scaly zinc powder, the surface surrounded by the outer circumference is the main surface, and the maximum diameter observed from the direction perpendicular to the main surface is measured. The average value of the maximum diameters of 50 scaly zinc powders is taken as the average length. Further, the thickness is the shortest distance passing through the center of gravity connecting two points on the outer circumference when observed from the horizontal direction with respect to the main surface. The average value of the thicknesses of 50 scaly zinc powders is taken as the average thickness. The aspect ratio (average length / average thickness) is calculated from the average length and average thickness of scaly zinc powder. When the coating composition is available, the zinc powder is separated from the coating composition and the aspect ratio of the scaly zinc powder is measured.

[Thickness of coating film (dry film thickness)]
The thickness of the coating film 12 is 25 μm or less. The coating film 12 generates less thermal decomposition products during welding as compared with a conventional epoxy resin coating film or the like. In addition, if the coating film 12 is thin, the generation of thermal decomposition products due to welding can be further suppressed. Therefore, the coating film 12 is preferably thin. Preferably, the thickness of the coating film 12 is 20 μm or less, and more preferably 15 μm or less. In particular, when the coating film 12 contains scaly zinc powder, the corrosion resistance of the polyethylene-coated steel tube 100 for gas conduit can be stably improved even if the thickness (dry film thickness) of the coating film 12 is 10 μm or less. can. The lower limit of the thickness of the coating film 12 may be such that corrosion resistance is ensured. The lower limit of the thickness of the coating film 12 is, for example, 5 μm.

[Measuring method of coating film thickness]
For example, the thickness (dry film thickness) of the coating film 12 is measured by the following method. The surface of the coating film 12 is measured using an eddy current phase film thickness meter. As the film thickness meter, PHASCOPE ™ PMP10 manufactured by Fisher Instruments Co., Ltd. is used. The measurement is performed at four points at a 90 ° pitch with respect to the inner circumference of the steel pipe. If the measured values vary widely or a thin value (for example, 10 μm or less) can be obtained as the thickness, the cross section of the coating film 12 may be observed under a microscope for measurement.

[Surface roughness of coating film]
The surface roughness Rz of the coating film 12 is less than 30.0 μm. In order to reduce the surface roughness of the coating film 12, the surface roughness of the phosphate coating film 11 may be reduced in the manufacturing process. As will be described in the production method described later, the surface roughness of the phosphate film 11 is small. Therefore, the surface roughness after coating the coating film 12 on the phosphate coating film 11 is also reduced. That is, the surface roughness of the coating film 12 becomes small. As a result, the pressure loss of the gas is reduced, and the gas transport efficiency of the polyethylene-coated steel pipe 100 for the gas conduit is increased. The smaller the surface roughness of the coating film 12, the more preferable it is. Therefore, the upper limit of the surface roughness Rz of the coating film 12 is preferably 25 μm, more preferably 20 μm. The lower limit of the surface roughness Rz of the coating film 12 is not particularly limited. The lower limit of the surface roughness Rz of the coating film 12 is, for example, 1 μm.

[Measuring method of coating film surface roughness]
For example, the surface roughness of the coating film 12 is measured by the following method. The maximum height roughness Rz is measured on the surface of the coating film 12 by a method according to JIS B0601 (2013). As the measuring device, a surface roughness shape measuring device Handy Surf E-35A (manufactured by Tokyo Seimitsu Co., Ltd.) is used. The cutoff value is 2.5 mm, the evaluation length is 12.5 mm, and the measurement range is 80 μm. The measurement is performed at four points at a 90 ° pitch with respect to the inner circumference of the steel pipe. Each measurement is performed along the axial direction of the steel pipe. The average value of the measurement results at the four locations is defined as the surface roughness Rz of the coating film 12.

[Polyethylene resin coating]
The polyethylene resin coating 10 is arranged on the outer surface 2 of the original pipe 1. For example, the polyethylene resin coating 10 may be directly arranged on the outer surface 2 of the original pipe 1, or for example, the outer surface 2 of the original pipe 1 is subjected to a chemical conversion treatment such as a phosphate treatment to form a chemical conversion treatment coating. It may be formed and formed on the chemical conversion coating. The polyethylene resin coating 10 is a well-known polyethylene resin coating. The polyethylene resin coating 10 may contain a lubricant, an antioxidant, a pigment and the like in addition to the ethylene-based polymer. The polyethylene resin coating 10 is, for example, the polyethylene resin coating described in JIS G3469 (2010). The lower limit of the thickness of the polyethylene resin coating 10 is, for example, 0.4 mm. The upper limit of the thickness of the polyethylene resin coating 10 is, for example, 5.0 mm.

[Production method]
The method for producing the polyethylene-coated steel pipe 100 for a gas conduit according to the present embodiment includes a preparation step, a chemical conversion treatment step, a coating composition coating step, a coating composition curing step, and a polyethylene coating step.

[Preparation process]
In the preparation process, the original pipe 1 is prepared. When there is a possibility that the scale is formed excessively thick on the inner surface 3 of the original pipe 1, the inner surface 3 may be blasted for the purpose of removing the scale in the preparatory step. In the preparatory step, the inner surface 3 and / or the outer surface 2 may be pickled, degreased, washed, or the like.

[Chemical conversion process]
In the chemical conversion treatment step, the inner surface 3 of the original pipe 1 is subjected to chemical conversion treatment to form a phosphate coating 11 on the inner surface 3 of the original pipe 1. At this time, not only the inner surface 3 of the original pipe 1 but also the outer surface 2 of the original pipe 1 may be subjected to chemical conversion treatment at the same time (in that case, a phosphate film is formed on the outer surface 2 of the original pipe 1). Will be). The chemical conversion treatment is carried out by a well-known method using a commercially available chemical conversion treatment liquid or the like. After the chemical conversion treatment, washing with water or the like may be carried out.

[Surface roughness after chemical conversion treatment process (surface roughness of phosphate coating)]
After the chemical conversion treatment step, the surface roughness Rz of the phosphate coating film 11 is preferably less than 30 μm. If the roughness of the surface 4 of the phosphate film 11 is small, the surface roughness of the coating film 12 formed on the phosphate film 11 is small. Therefore, the roughness of the pipe wall inside the polyethylene-coated steel pipe 100 for gas conduit becomes small. As a result, the pressure loss of the gas flowing inside the polyethylene-coated steel pipe 100 for the gas conduit is reduced, and the gas transport efficiency is increased. The smaller the surface roughness Rz of the phosphate film 11, the more preferable. Therefore, the surface roughness Rz of the phosphate film 11 is preferably 25 μm or less, and more preferably 20 μm or less. The lower limit of the surface roughness Rz of the phosphate film 11 is not particularly limited, but is, for example, 1 μm. In the present specification, the surface roughness of the phosphate coating 11 is the roughness of the inner surface of the steel pipe after the phosphate coating 11 is formed.

[Measurement method of surface roughness Rz of phosphate film]
For example, the surface roughness Rz of the phosphate film 11 is measured by the following method. First, a test piece containing the surface of the phosphate coating 11 is prepared. Next, the maximum height roughness Rz is measured on the surface of the phosphate coating 11 of this test piece by a method according to JIS B0601 (2013). As the measuring device, a surface roughness shape measuring device Handy Surf E-35A (manufactured by Tokyo Seimitsu Co., Ltd.) is used. The cutoff value is 2.5 mm, the evaluation length is 12.5 mm, and the measurement range is 80 μm. The measurement is performed at four points at a 90 ° pitch with respect to the inner circumference of the steel pipe. Each measurement is performed along the axial direction of the steel pipe. The average value of the measurement results at the four locations is defined as the surface roughness Rz of the phosphate coating 11.

In the production method of the present embodiment, the phosphate film 11 is formed instead of the blasting treatment for the purpose of anchoring effect, and the adhesion of the coating film 12 is enhanced. Therefore, the surface roughness of the phosphate coating 11 is small.

Further, in the case of the blasting treatment, the inner surface 3 of the original pipe 1 is corroded immediately after the blasting treatment. Therefore, it was necessary to perform the next surface treatment within several hours after the blast treatment. On the other hand, in the production method of the present embodiment, the phosphate coating 11 is formed by chemical conversion treatment. Therefore, the corrosion resistance of the inner surface 3 of the original pipe 1 is enhanced. The next surface treatment may be performed within a few days after the chemical conversion treatment. As a result, in the case of chemical conversion treatment, restrictions on the work content at the time of manufacturing are reduced as compared with the case of blast treatment. As a result, production efficiency is increased.

Preferably, in the production method of the present embodiment, the inner surface 3 of the original pipe 1 is not blasted before the chemical conversion treatment step. It is preferable that the blasting treatment is not performed regardless of whether the blasting treatment is for the purpose of imparting an anchor effect or for the purpose of removing scale. As a result, the roughness of the inner surface 3 of the original pipe 1 can be stably reduced. In the production method of the present embodiment, it is preferable that the inner surface 3 of the original pipe 1 is not blasted at all. In this case, the production efficiency is further increased.

[Paint composition coating process]
In the coating composition coating step, the coating composition is coated on the phosphate coating 11. First, the paint composition is prepared. The coating composition contains an alkyl silicate condensate and zinc powder.

The alkyl silicate condensate is a general term for substances produced by partially hydrolyzing and condensing an alkyl silicate. For example, the alkyl silicate condensate can be obtained by mixing the alkyl silicate with an acid such as hydrochloric acid and water. Alkyl silicates are, for example, tetramethyl orthosilicate, tetraethyl orthosilicate, tetra-n-propyl orthosilicate, tetra-i-propyl orthosilicate, tetra-n-butyl orthosilicate, tetra-sec-butyl orthosilicate, methylpolysilicate. And one or more selected from the group consisting of ethyl polysilicate.

As the zinc powder, the above-mentioned zinc powder can be used without limitation. In addition, commercially available zinc powder can be used. The scaly zinc powder is, for example, manufactured by ECKART GmbH, STANDART ™ Zinc flake AT, STANDART ™ Zinc flake GTT, STANDART ™ Zinc flake TV, STANDART ™ Zinc Flake (trademark) Zinc flake , And STAPA ™ 4 ZnSn30 and the like. Examples of the spherical zinc powder include F-2000 (trade name) manufactured by Honjo Chemical Co., Ltd. The zinc powder content in the coating composition may be appropriately adjusted so that the desired zinc powder content can be obtained in the coating film after the coating composition is cured. The content of zinc powder in the coating composition is, for example, 10 to 90 parts by mass with respect to 100 parts by mass of the coating composition.

The coating composition may contain other components in addition to the alkyl silicate condensate and zinc powder. The other component is, for example, another component contained in the above-mentioned coating film 12. The coating composition may further contain an organic solvent. The organic solvent is, for example, one or more selected from the group consisting of alcohol solvents, ester solvents, ketone solvents, aromatic solvents and glycol solvents.

The alkyl silicate condensate and zinc powder are mixed to obtain a coating composition. Next, the obtained coating composition is coated on the phosphate coating 11. The painting method can be set as appropriate. Examples of the painting method include spraying and brush painting. In the case of spray painting, a rod with an injection port at the tip is charged inside the steel pipe. Then, the coating composition is injected into the steel pipe from the injection port. At this time, it is preferable to inject while rotating the steel pipe or the rod having the injection port. The steel pipe and the rod with the injection port may be rotated in the opposite direction. The rotation speed of the steel pipe or the rod with the injection port and the injection amount of the coating composition can be appropriately adjusted according to the thickness (dry film thickness) of the coating film 12.

[Paint composition curing process]
In the coating composition curing step, the coated coating composition is cured by drying. Drying is performed by holding the coated coating composition in an air atmosphere. The coating composition is cured by evaporation of an organic solvent or the like in the coating composition and further hydrolysis condensation of the alkyl silicate condensate. The drying temperature is usually 5 to 40 ° C., preferably 10 to 30 ° C., and the drying time is, for example, 5 hours or less. As a result, the coating film 12 is obtained.

[Polyethylene coating process]
In the polyethylene coating step, the polyethylene resin coating 10 is formed on the outer surface 2 of the original pipe 1. The polyethylene resin coating 10 can be formed by a well-known method. For example, the outer surface 2 of the original pipe 1 is preheated and an adhesive or an adhesive is applied. Subsequently, the outer surface 2 of the original pipe 1 is covered with the heated or melted polyethylene by an extruder. If necessary, the polyethylene resin coating 10 may have two or more layers.

The polyethylene coating step may be carried out after forming the phosphate coating 11 on the outer surface 2 of the original pipe 1. In this case, it is carried out within the range where rust does not occur on the outer surface 2 of the original pipe 1. Further, it may be carried out within several hours after the blasting treatment on the outer surface 2 of the original pipe 1. The polyethylene coating step may be carried out, for example, before the coating composition coating step. The polyethylene coating step may be carried out, for example, after the coating composition curing step.

By the above steps, the polyethylene-coated steel pipe 100 for a gas conduit of the present embodiment can be manufactured.

Hereinafter, examples will be described.

[Preparation process]
A steel plate and a steel pipe were prepared as materials corresponding to the original pipe (the present invention is intended for a steel pipe, but the above-mentioned effects of roughness and corrosion resistance can be understood even in an evaluation test using a steel plate as the material. Therefore, for convenience of experiment, some steel plates were used). The steel plate was a general structural rolled steel material SS400 specified in JIS G3101 (2015). The steel plate was a 70 mm × 150 mm × 2.3 mm steel plate. The steel pipe was a carbon steel pipe SGP for piping specified in JIS G3452 (2014). The steel pipe was a steel pipe having a wall thickness of 5 mm and a diameter of 165.2 mm. As shown in Table 1, blasting or chemical conversion treatment was carried out on the surface or inner surface of the steel plate or steel pipe of each test number. In test number 1, blasting was performed. In test numbers 2-5, no blasting was performed. In Test Nos. 2 to 5, chemical conversion treatment was carried out after removing the scale by pickling.

[Chemical conversion process]
Chemical conversion treatment was carried out on the steel plates or steel pipes of test numbers 2 to 5. As the chemical conversion treatment liquid, a commercially available zinc phosphate calcium phosphate treatment liquid (manufactured by Nihon Parkerizing Co., Ltd.) was used. The steel plate or steel pipe was immersed in the chemical conversion treatment liquid for 5 minutes. The conditions of the chemical conversion treatment liquid (for example, temperature and acidity) were adjusted with reference to the conditions recommended by the manufacturer so that a sound and predetermined amount of adhesion could be obtained. The obtained zinc phosphate-calcium-treated coating was a dense coating without so-called scale or unevenness, and the coating weight was about 5 g / m ² . In Test No. 1, no chemical conversion treatment was carried out.

[Measurement of surface roughness Rz]
After blasting or chemical conversion treatment was applied to the surface or inner surface of the steel plate or steel pipe of each test number, the surface roughness Rz (inner surface roughness Rz for the steel pipe) was measured. The surface roughness was measured in the same manner as the above-mentioned method for measuring the surface roughness Rz of the coating film. Further, after forming the coating film on the steel plate or the steel pipe of each test number, the surface roughness (inner surface roughness for the steel pipe) was measured in the same manner to obtain the surface roughness Rz of the coating film. The results are shown in Table 1.

[Paint composition coating process]
An alcohol solution of the alkyl silicate condensate and a zinc powder paste were prepared by the following procedure, respectively. In the following description, "parts" means "parts by mass" unless otherwise specified.

[Preparation Example 1] Preparation of alcohol solution of alkyl silicate condensate Ethyl silicate 40 (manufactured by Corcote Co., Ltd.) 31.5 parts, industrial ethanol 10.4 parts, deionized water 5 parts, and 35 mass% hydrochloric acid 0.1 After charging the parts into a container and stirring at 50 ° C. for 3 hours, 53 parts of isopropyl alcohol was added to prepare an alcohol solution of the alkyl silicate condensate.

[Preparation Example 2-1] Preparation of Zinc Powder Paste 1 1.3 parts of TIXOGEL MPZ (trade name: manufactured by BYK Adaptives GmbH) as a settling inhibitor and 6.3 parts of xylene as an organic solvent, 3.1 parts Butyl acetate and 4.1 parts of isobutyl alcohol were charged in a polyethylene container, glass beads were added, and the mixture was shaken with a paint shaker for 3 hours. Next, add 10 parts of STANDART ™ Zinc flake GTT as scaly zinc powder and 10 parts of F-2000 (trade name; manufactured by Honjo Chemical Co., Ltd.) as spherical zinc powder, and shake for another 5 minutes to zinc. The ends were dispersed. Then, the glass beads were removed using an 80 mesh net to prepare a zinc powder paste 1.

[Preparation Example 2-2] Preparation of Zinc Powder Paste 2 2.1 parts of TIXOGEL MPZ as an anti-settling agent, 10.6 parts of xylene as an organic solvent, 5.3 parts of butyl acetate and 7.0 parts of isobutyl Alcohol was charged in a polyethylene container, glass beads were added, and the mixture was shaken with a paint shaker for 3 hours. Next, 34 parts of F-2000 was added as spherical zinc powder, and the mixture was further shaken for 5 minutes to disperse the zinc powder. Then, the glass beads were removed using an 80 mesh net to prepare a zinc powder paste 2.

A coating composition 1 was prepared by mixing 100 parts by mass of an alcohol solution of an alkyl silicate condensate and 34.8 parts by mass of a zinc powder paste 1. A coating composition 2 was prepared by mixing 100 parts by mass of an alcohol solution of an alkyl silicate condensate and 59 parts by mass of a zinc powder paste 2.

The paint composition 1 or the paint composition 2 was coated on the steel plates or steel pipes of test numbers 1 to 5. The steel pipe was spray-painted by the method described above. The steel sheet was also spray-painted under the same conditions. For test number 1, the coating composition was applied within 4 hours after blasting. For test numbers 2-5, the coating composition was coated 3 days after the chemical conversion treatment.

[Paint composition curing process]
The steel plate or steel pipe of each test number was held at room temperature for 7 days. As a result, the coating composition was dried and cured to form a coating film.

[Measurement of coating film thickness (dry film thickness)]
For the steel plate or steel pipe of each test number, the thickness of the coating film (dry film thickness) was measured by the above-mentioned method. The results are shown in Table 1.

[Corrosion resistance test]
Corrosion resistance tests were carried out on the steel plates and steel pipes of each test number. Corrosion resistance was evaluated by an outdoor exposure test. In Test Nos. 1 and 2, the treated surface of the steel sheet was exposed outdoors for 6 months with the surface tilted upward by 45 ° toward the south. Then, the appearance defined in ASTM D610 (2012) was evaluated. In test numbers 3-5, the steel pipe was exposed outdoors for 6 months. After that, the appearance was evaluated in the same manner. The results are shown in Table 1. If it is "◯" in Table 1, it has corrosion resistance that does not cause any problem in practical use as a polyethylene-coated steel pipe for a gas conduit. In addition, "○" and "×" in Table 1 were judged according to the following criteria, respectively.
◯: The evaluation result according to the evaluation standard of ASTM D610 (2012) is 8 to 10.
X: The evaluation result according to the evaluation standard of ASTM D610 (2012) is 7 or less.

[Evaluation results]
With reference to Table 1, Test Nos. 2-5 provided a phosphate coating on the surface of the steel sheet or the inner surface of the steel pipe. Test numbers 2 to 5 did not undergo blasting for the purpose of anchoring effect. Therefore, the surface roughness Rz of the coating film is less than 30.0 μm, and it can be said that the gas transport efficiency is excellent. Further, since the coating film contains a siloxane-based binder, the generation of thermal decomposition products during welding is suppressed as compared with an epoxy resin coating film containing an organic substance as a main component. Further, in Test Nos. 2 to 5, since the coating film was provided, rust was not observed after the outdoor exposure test, and the corrosion resistance suitable for gas transportation was shown.

In Test Nos. 2 to 5, the coating composition coating step was further carried out 3 days after the chemical conversion treatment, and excellent production efficiency was exhibited.

On the other hand, in Test No. 1, a blasting treatment was carried out for the purpose of an anchor effect in order to improve the adhesion of the coating film. Therefore, the roughness Rz of the inner surface of the steel pipe was 62.7 μm. As a result, the surface roughness Rz of the coating film is 68.7 μm, and it can be said that the surface is rough and the gas transport efficiency is low.

The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented within a range that does not deviate from the gist thereof.

100 Polyethylene-coated steel pipe for gas conduit 1 Original pipe 2 Outer surface of original pipe 3 Inner surface of original pipe 10 Polyethylene resin coating 11 Phosphate coating 4 Surface of phosphate coating (Inner surface of steel pipe after formation of phosphate coating) )
12 coating film

Claims (7)

Polyethylene coated steel pipe for gas conduits
The original pipe and
With the phosphate coating placed on the inner surface of the original pipe,
A coating film arranged on the phosphate coating film, containing a siloxane-based binder and zinc powder, and having a surface roughness Rz of less than 30.0 μm.
A polyethylene-coated steel pipe for a gas conduit, comprising a polyethylene resin coating arranged on the outer surface of the original pipe. The polyethylene-coated steel pipe for a gas conduit according to claim 1.
A polyethylene-coated steel pipe for a gas conduit whose inner surface is not a blast surface. The polyethylene-coated steel pipe for a gas conduit according to claim 1 or 2.
The zinc powder is a polyethylene-coated steel pipe for a gas conduit containing scaly zinc powder. The polyethylene-coated steel pipe for a gas conduit according to any one of claims 1 to 3.
The phosphate coating is a polyethylene-coated steel pipe for a gas conduit, which is a zinc phosphate calcium coating. The polyethylene-coated steel pipe for a gas conduit according to any one of claims 1 to 4.
A polyethylene-coated steel pipe for a gas conduit having a coating film having a thickness of 25 μm or less. The method for manufacturing a polyethylene-coated steel pipe for a gas conduit according to any one of claims 1 to 5.
The process of preparing the original pipe and
A step of chemical conversion treatment of the inner surface of the original pipe to form a phosphate film on the inner surface of the original pipe, and a step of forming a phosphate film.
A step of coating a coating composition containing an alkyl silicate condensate and zinc powder on the phosphate coating, and
The step of curing the coated coating composition and
A method for producing a polyethylene-coated steel pipe for a gas conduit, which comprises a step of forming a coating film containing a polyethylene resin on the outer surface of the original pipe. The method for manufacturing a polyethylene-coated steel pipe for a gas conduit according to claim 6.
A method for producing a polyethylene-coated steel pipe for a gas conduit, wherein the inner surface of the original pipe is not blasted.

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