JP2019044850A - Polyethylene coated steel pipe for gas piping and manufacturing method for polyethylene coated steel pipe for gas piping - Google Patents

Abstract

To provide a polyethylene coated steed pipe for gas piping having anticorrosion suitable for gas transport, capable of suppressing occurrence of heat decomposition objects in welding, and further capable of enhancing gas transport efficiency.MEANS FOR SOLVING THE PROBLEM: A polyethylene coated steel pipe (100) for gas piping comprises an original pipe (1), a phosphate coat (11), a coating film (12), and a polyethylene resin coat (10). The phosphate coat (11) is arranged on an internal surface (3) of the original pipe (1). The coating film (12) is arranged on the phosphate coat (11). The coating film (12) contains siloxane-based binder and zinc powder. The surface roughness Rz of the coating film (12) is less than 30.0 μm. The polyethylene resin coat (10) is arranged on an external surface (2) of the original pipe (1).SELECTED DRAWING: Figure 1

Description

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

  The role of city gas and water pipelines and buried pipes is becoming increasingly important. A polyethylene-coated steel pipe is used as these piping materials. 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 the durability such as corrosion resistance and chemical resistance. Polyethylene-coated steel pipes are defined, for example, in JIS G3469 (2010).

  On the inner surface of a polyethylene-coated steel pipe, there are cases where a coating is not formed and cases where a coating is formed. If a coating film is not formed, rust is likely to be generated from the inner surface of the steel pipe during the storage period until it is joined on site after steel pipe production. Moreover, when the generated rust peels off after joining of the steel pipe, it may diffuse into the steel pipe and solidify. That is, removal of rust may be required after joining of a steel pipe. For this reason, in many cases, a coating film is formed on the inner surface of the polyethylene-coated steel pipe for the purpose of improving the corrosion resistance. The coating is often formed after blasting the inner surface of the polyethylene-coated steel pipe. By blasting, an active surface is exposed and an anchor effect is exhibited, and adhesion of the coating film is enhanced.

  Conventionally, an epoxy resin coating film has been widely used as a coating film formed on the inner surface of a polyethylene-coated steel pipe. The epoxy resin coating film physically blocks the contact between the moisture and oxygen causing corrosion and the inner surface of the polyethylene-coated steel pipe (the inner surface of the original pipe) to provide high corrosion resistance.

  In general, an epoxy resin coating composition is applied to a polyethylene-coated steel pipe for gas conduit after blasting the inner surface of the raw pipe for the purpose of anchoring. By curing the epoxy resin coating composition, the above-mentioned epoxy resin coating film is obtained. Thereby, high corrosion resistance is obtained in the polyethylene-coated steel pipe for gas conduits in which the epoxy resin coating film is formed on the inner surface.

  By the way, welding is generally used to join polyethylene-coated steel pipes for gas conduits. The 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 in order to prevent combustion. On the other hand, since it is difficult to peel off the coating on the inner surface of the polyethylene pipe for gas conduits, welding is usually performed with the coating on the inner surface remaining.

  It is preferable that the coating film on the inner surface of the polyethylene pipe for gas conduit be resistant to combustion and thermal decomposition at the time of welding. However, when an epoxy resin coating film widely used in the past is formed on the inner surface of a polyethylene-coated steel pipe, for example, an organic component such as epoxy resin in the epoxy resin coating film burns during welding. It may generate pyrolyzate and adversely affect the welder's health. Furthermore, thermal decomposition products such as epoxy resin may also diffuse into the gas conduit and solidify therein.

  From the above reasons, a coating film suitable for the inner surface of a polyethylene-coated steel pipe for gas conduit has been studied. For example, patent documents 1-3 form a coating film different from an epoxy resin in the inner surface of polyethylene-coated steel pipe for gas conduits.

  The polyethylene-coated steel pipe described in JP2013-173340A (patent document 1) is a polyethylene-coated steel pipe having a coating layer of polyethylene resin on the outer surface, and the polyethylene-coated steel pipe has a coating film on the inner surface, The coating film is characterized in that it 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 relative to 100 parts by weight in the solid content of the coating. It is described in patent document 1 that the generation amount of the thermal decomposition product which is generated from the part of inner surface coating at the time of welding and causes clogging of a filter, a valve, etc. can be reduced by this.

  The polyethylene-coated steel pipe described in JP-A-2015-168117 (patent document 2) is a polyethylene-coated steel pipe having a covering layer made of polyethylene resin on the outer surface, and 40 to 70% by mass of zinc powder, molybdic acid compound And 1 to 10% by mass, 10 to 20% by mass of solvent-bonded lacquer silica, and 10 to 40% by mass of alkyl silicate on the inner surface. It is described in patent document 2 that a polyethylene-coated steel pipe can be provided in which the amount of generation of mist or the like generated from the inner surface is extremely small even if welding or the like having a high heat input is applied to the outer surface.

  The coated steel pipe for gas described in Japanese Patent Application Laid-Open No. 2016-175315 (Patent Document 3) contains 5 to 50% by mass of alkyl silicate and / or modified alkyl silicate as paint solids and 3 to 40 of corodyl silica. It has a cured coating of a paint containing 30% by mass and 30% by mass of zinc powder on the inner surface. It is described in patent document 3 that it is possible to weld well without removing the inner surface coating film, to suppress generation of mist due to welding heat, and to obtain a coated steel pipe for gas having good corrosion resistance. ing.

JP, 2013-173340, A JP, 2015-168117, A JP, 2016-175315, A

  The patent documents 1 to 3 describe that a coating composition containing a siloxane-based binder such as an alkyl silicate and zinc powder is applied on the inner surface of a blast treated steel pipe. On the other hand, corrosion progresses on the inner surface of the polyethylene-coated steel pipe immediately after blasting. Therefore, in order to improve the adhesion of the coating film by blasting, it was necessary to perform the next surface treatment or formation of a coating within several hours after the blasting treatment. In this case, the work content at the time of manufacture is restricted. As a result, production efficiency may be reduced. Therefore, improvement in production efficiency has been required.

  By the way, it is necessary to transport a large amount of gas stably for the polyethylene pipe for gas conduits. Therefore, high gas transport efficiency is required for polyethylene-coated steel pipes for gas conduits. The pressure of the gas flowing inside the polyethylene pipe for gas conduits is lost due to various factors. The factor of the pressure loss of gas is, 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 direction (turbulence 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 transport, suppression of pyrolysis products generated during welding, and improvement of gas transport efficiency. Therefore, even if it uses the above-mentioned technique, for example, there was a case where polyethylene coated steel pipe for gas conduits with high gas transport efficiency could not be obtained.

  An object of the present invention is to provide a polyethylene-coated steel pipe for gas conduits, which has corrosion resistance suitable for gas transportation, suppresses the generation of thermal decomposition products during welding, and further improves the gas transportation efficiency, It is an object of the present invention to provide a method for producing polyethylene-coated steel pipe for gas conduits, which has enhanced production efficiency.

  The polyethylene-coated steel pipe for gas conduit of this embodiment comprises a raw pipe, a phosphate coating, a coating, and a polyethylene resin coating. The phosphate coating is disposed on the inner surface of the raw tube. The coating is disposed on the phosphate coating. The coating contains a siloxane binder and zinc powder. The surface roughness Rz of the coating is less than 30.0 μm. The polyethylene resin coating is disposed on the outer surface of the raw pipe.

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

  The polyethylene-coated steel pipe for gas conduits according to the present embodiment has corrosion resistance suitable for gas transport, generation of a thermal decomposition product at the time of welding is suppressed, and gas transport efficiency is excellent. Moreover, the manufacturing method of the polyethylene-coated steel pipe for gas conduits by this 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 have the same reference characters allotted and description thereof will not be repeated.

  The present inventors have variously studied polyethylene-coated steel pipes for gas conduits. As a result, the following findings were obtained.

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

  However, recent improvements in gas quality have reduced the water content of the gas and further reformed the gas to reducibility. That is, in the use environment after the piping is completed, the degree of corrosion resistance required for the inner surface of the polyethylene pipe for gas conduit is lower than, for example, polyethylene pipe for water supply. On the other hand, corrosion resistance is not necessarily required for the inner surface of a polyethylene-coated steel pipe for gas conduit. Corrosion resistance is required to such an extent that clear corrosion such as red rust does not occur between transport and storage and completion of piping after production.

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

  The present inventors next examined the suppression effect of the thermal decomposition product at the time of welding of the polyethylene-coated steel pipe for gas conduits. Welding is a general method of joining polyethylene-coated steel pipes for gas conduits. Therefore, it is preferable that the coating film formed on the inner surface of the raw pipe of the polyethylene pipe for gas conduits is less likely to be burned and pyrolyzed at the time of welding. The alkyl silicate condensate is hydrolyzed and condensed by moisture in the air to form a coating. This coating has a siloxane-based binder. Coatings having a siloxane-based binder are less susceptible to combustion and thermal decomposition during welding as compared to conventional epoxy resin coatings. 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 containing a siloxane-based binder and zinc powder is formed on the inner surface of a polyethylene-coated steel pipe for gas conduit. In the present specification, a siloxane-based binder is a compound having a siloxane bond (Si-O), and specific examples thereof include a compound obtained by hydrolytic condensation of an alkyl silicate condensate. It is preferable that the above-mentioned coating film does not substantially contain an organic component that generates a thermal decomposition product by welding. Here, not containing substantially means, for example, 3% by mass or less.

  The inventors further examined the gas transport efficiency of polyethylene-coated steel pipe for gas conduit. As a result, it has been found that, as in the prior art, in the case where a blast surface is formed by blasting to obtain an anchor effect and a coating film is formed thereon, the transport efficiency of gas may be reduced. Then, as a result of investigating about this cause, the present inventors acquired the following knowledge.

  The blasted surface formed by blasting for the purpose of anchoring effect has a large surface roughness. If the inner surface of the raw pipe is blasted for the purpose of anchoring, the roughness of the inner surface of the raw pipe is increased. If the roughness of the inner surface of the raw pipe is large, the surface roughness of the coating formed thereon will be large. The greater the surface roughness of the coating, the greater the roughness of the tube wall. The greater the tube wall roughness and the greater the tube wall surface area, the greater the pressure loss of the gas. As a result, the pressure loss of the gas flowing inside the polyethylene-coated steel pipe for gas conduits increases, and the gas transport efficiency decreases.

  Therefore, the roughness of the inner surface of the polyethylene-coated steel pipe for gas conduit prior to film formation is reduced. Thereby, the surface roughness of the coating film formed thereon is smoothed, and the transport efficiency of the gas is enhanced.

  However, unlike the conventional case where the roughness of the inner surface of the original pipe is suppressed to improve the transport efficiency of gas, it is difficult to obtain the anchor effect, and the adhesion of the coating film to the inner surface of the original pipe decreases. . Therefore, a coating for enhancing the adhesion of the coating is further formed between the inner surface of the raw pipe of the polyethylene-coated steel pipe for gas conduit and the coating. Specifically, a phosphate coating is formed by a chemical conversion treatment to form a coating on the phosphate coating. This improves the adhesion of the coating. In addition, the phosphate coating has a high effect of suppressing thermal decomposition products as compared with the coating of an organic substance. Thereby, while improving the adhesiveness of a coating film, the inhibitory effect of a thermal decomposition product is heightened.

  Here, even if the coating film is formed on the phosphate film, the corrosion resistance may still be lower than the case where the conventional epoxy resin film is formed on the blasted surface. However, in the case of polyethylene-coated steel pipes for water supply, high corrosion resistance is required continuously during use after the completion of piping, whereas in the case of polyethylene-coated steel pipes for gas conduits, the piping is completed and the water content is If low gas flows, high corrosion resistance is not required. Therefore, in the case of a polyethylene-coated steel pipe for gas conduit, if a coating film is formed on a phosphate coating, the required corrosion resistance can be sufficiently ensured. Furthermore, by forming a phosphate coating under the coating, blasting for the purpose of anchoring can be omitted, and the roughness of the inner surface can be reduced. It turned out that the transport efficiency of gas which is important for gas conduits can be raised by this.

  Moreover, if the phosphate coating is formed by chemical conversion treatment, the corrosion of the surface of the raw pipe can be further suppressed. Therefore, unlike blasting, the time restriction until the next surface treatment or formation of a coating is performed after chemical conversion is alleviated. As a result, production efficiency can be improved.

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

  The polyethylene-coated steel pipe for gas conduit of the present embodiment has corrosion resistance suitable for gas transportation, generation of a thermal decomposition product at the time of welding is suppressed, and gas transport efficiency is excellent.

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

  Preferably, the zinc powder contains scaly zinc powder. In this case, the corrosion resistance of the polyethylene pipe for gas conduit further increases.

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

  Preferably, the thickness of the coating is 25 μm or less. In this case, the generation of a thermal decomposition product by welding can be further suppressed.

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

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

  Preferably, in the above manufacturing method, the blasting is not performed on the inner surface of the raw pipe. In this case, the production efficiency is further enhanced.

  Hereinafter, the polyethylene-coated steel pipe for gas conduit of the present embodiment will be described in detail.

[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 of the present embodiment. Referring to FIG. 1, a polyethylene-coated steel pipe 100 for a gas conduit includes a raw pipe 1, a polyethylene resin coating 10, a phosphate coating 11, and a coating 12. The raw pipe 1 has an outer surface 2 and an inner surface 3. The polyethylene resin film 10 is disposed on the outer surface 2 of the raw pipe 1. The phosphate coating 11 is disposed on the inner surface 3 of the raw pipe 1. The coating 12 is disposed on the phosphate coating 11.

  The raw pipe 1 can use a well-known steel pipe. The steel pipe is, for example, a steel pipe made of one kind selected from the group consisting of carbon steel, alloy steel or stainless steel. The steel pipe is, for example, a steel pipe defined in JIS G3452 (2014) and JIS G3454 (2012).

  As used herein, a blasted surface refers to a blasted surface. For example, after blasting, the blasted surface also includes a surface subjected to treatments such as degreasing, pickling and washing. The inner surface 3 of the raw pipe 1 may be a blast surface. For example, in the case where the scale is formed thick on the inner surface 3 of the raw pipe 1 due to the pipe forming conditions, the blasting may be performed. However, preferably, the inner surface 3 of the raw pipe 1 is not a blast surface. That is, preferably, the inner surface 3 of the raw pipe 1 is the non-blasted inner surface 3. In this case, the roughness of the inner surface 3 of the raw pipe 1 can be stably reduced.

[Phosphate coating]
The phosphate coating 11 is disposed on the inner surface 3 of the raw pipe 1. The phosphate coating 11 is a known phosphate coating. The phosphate coating 11 contains, for example, one or more selected from the group consisting of zinc phosphate, zinc calcium phosphate, iron phosphate and manganese phosphate. Preferably, the phosphate coating 11 is a zinc calcium phosphate 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) It is a complex of ₂ · 4H ₂ O). The phosphate coating 11 may contain impurities. Here, the impurities are substances other than phosphate, which are contained in the phosphate coating 11 during the production of the polyethylene-coated steel pipe 100 for gas exchange, and are contained in a content which does not affect the effects of the present invention Containing substances.

[Method of identifying phosphate film]
For example, the phosphate coating 11 is identified by the following method. The polyethylene pipe for gas conduit 100 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. The X-ray diffraction measurement and the identification of the phosphate coating 11 are performed by a method in accordance with JIS K3151 (1996).

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

[Method of measuring the coating weight of phosphate coating]
For example, the coating weight of the phosphate coating 11 is measured by the following method. First, a polyethylene pipe coated with a gas conduit 100 is cut to prepare a test piece (2 cm wide × 5 cm long) including the inner surface 3 of the raw pipe 1. The test piece is immersed in a 50 g / L chromium trioxide aqueous solution at 75 ± 2 ° C. Thereby, the phosphate coating 11 is dissolved. The weight of the test piece before and after dissolution is measured. Repeat dissolution until the weight of the test piece before and after dissolution does not change. Next, the coating film 12 peeled off from the inner surface 3 of the raw pipe 1 is recovered. The weight of the coating 12 is measured. From the weight of the test piece before dissolution, the weight of the test piece after dissolution and the weight of the measured coating 12 are subtracted. Thus, 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. In addition, what is necessary is just to measure the weight of a phosphate film by the steel pipe when the steel pipe before coating-film formation is available.

[Coating]
The coating 12 is disposed on the phosphate coating 11. The coating 12 contains a siloxane binder and zinc powder. The siloxane-based binder is a compound having a siloxane bond, and specifically, a compound obtained by hydrolytic condensation of an alkyl silicate condensate is mentioned. The coating film 12 is excellent in the suppression effect of the thermal decomposition product at the time of welding since it contains a siloxane-based binder. In addition, if the coating film 12 contains zinc powder, the corrosion resistance of the polyethylene pipe for a gas conduit 100 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 a zinc-based alloy (eg, an alloy of zinc and 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 scale-like zinc powder (scale-like zinc powder). Scale-like zinc powder refers to zinc powder having an aspect ratio (average length / average thickness) represented by the ratio of average length to average thickness of 10 to 150. The aspect ratio of zinc powder is preferably 20 to 100. Since scaly zinc powder has a large specific surface area as compared with spherical zinc powder (spherical zinc powder), the effect of sacrificial corrosion protection is high. Furthermore, the coating film 12 containing scaly zinc powder can keep the zinc powder in close contact with each other. Therefore, it is easy to thin the coating film 12, and it is easy to obtain the effect of sacrificial corrosion prevention. Moreover, the coating film 12 containing scale-like zinc powder is easy to obtain the shielding effect of water and oxygen. That is, when the zinc powder contains scaly zinc powder, the corrosion resistance of the polyethylene pipe for a gas conduit 100 is further enhanced. When the zinc powder contains scaly zinc powder, the scaly zinc powder is preferably 15 to 90% by mass, and 15 to 70% by mass, with respect to 100% by mass in total of the zinc powder content. Is more preferred. The zinc powder may not contain spherical zinc powder.

[Other ingredients]
The coating film 12 may contain other components other than the siloxane-based binder and the zinc powder as appropriate as long as the purpose and effect of the present invention are not impaired. Examples of the other components include conductive pigments, rust preventive pigments (except zinc powder), molybdenum compounds and anti-settling agents. 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 phosphite strontium phosphite compounds, aluminum tripolyphosphate compounds and cyanamide zinc compounds . The molybdenum compound is, for example, one or more selected from the group consisting of metal molybdenum, molybdenum oxide and metal salts of molybdic acid. The anti-settling agent is, for example, one or more selected from the group consisting of an organic bentonite-based anti-settling agent, an oxidized polyethylene-based anti-settling agent, a fumed silica-based anti-settling agent and an amide-based anti-settling agent.

[Method of identifying coating film]
[Identification of siloxane bond]
For example, siloxane bonds in the coating 12 are identified in the following manner. The infrared spectrum of the coating 12 is measured. For measurement, a Fourier transform infrared spectrophotometer (FT-IR) IRTracer-100 (trade name) manufactured by Shimadzu Corporation is used. From the measurement chart, a peak of 1010 to 1090 cm ⁻¹ derived from Si—O—Si framework vibration is identified. This identifies that the compound which has a siloxane bond is contained.

[Identification of zinc powder]
For example, zinc powder in the coating 12 is identified by the following method. A part of the polyethylene-coated steel pipe 100 for gas conduit containing the coating 12 is cut into test pieces. Surface observation is performed on the surface or cross section of the coating 12 of the test piece using a scanning electron microscope (SEM) (TM-3000) manufactured by Hitachi High-Technologies Corporation. Thereafter, elemental analysis is performed by energy dispersive X-ray spectroscopy (EDX) (Quantax 70) manufactured by Hitachi High-Technologies Corporation to identify that the coating 12 contains particles containing zinc. In addition, the content of Zn relative to Si, O, C, H and Zn is calculated from the result of elemental analysis. The obtained Zn content is converted to% by mass to obtain the content (% by mass) of zinc powder in the coating film 12.

[Method of measuring aspect ratio of scale-like zinc powder]
For example, the aspect ratio of scale-like 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. The observation magnification is selected in accordance with the average length of the scaly zinc powder. Observe 50 scaly zinc dust. In one scaly zinc powder, the surface with the largest area surrounded by the outer periphery is the main surface, and the maximum diameter observed from the direction perpendicular to the main surface is measured. Let the average value of the maximum diameter of 50 scaly zinc powder be the average length. In addition, when observing from the horizontal direction with respect to the main surface, a thickness is the shortest distance passing through the center of gravity by connecting two points on the outer periphery. Let the average value of the thickness of 50 scaly zinc powder be the average thickness. The aspect ratio (average length / average thickness) is determined from the average length and the average thickness of the flaky zinc powder. When the paint composition is available, the zinc powder is separated from the paint composition and the aspect ratio of the scaly zinc powder is measured.

[Thick film thickness (dry film thickness)]
The thickness of the coating film 12 is 25 μm or less. The coating film 12 generates less thermal decomposition products generated at the time of welding as compared with the conventional epoxy resin coating film and the like. In addition, if the coating film 12 is thin, generation of thermal decomposition products by 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, more preferably 15 μm or less. In particular, when the coating film 12 contains scale-like zinc powder, the corrosion resistance of the polyethylene pipe 100 for a gas conduit can be stably enhanced even if the thickness (dry film thickness) of the coating film 12 is 10 μm or less. it can. The lower limit of the thickness of the coating film 12 may be such that corrosion resistance can be ensured. The lower limit of the thickness of the coating 12 is, for example, 5 μm.

[Method of measuring thickness of coating film]
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 a film thickness meter, PHASECOPE (registered trademark) PMP10 manufactured by Fisher Instruments Inc. is used. The measurement is performed at four points at a pitch of 90 ° to the inner circumference of the steel pipe. In addition, when the variation of the measured value is large or a thin value (for example, 10 μm or less) is obtained as the thickness, the cross section of the coating film 12 may be observed by a microscope.

[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 12, the surface roughness of the phosphate coating 11 may be reduced in the manufacturing process. The surface roughness of the phosphate coating 11 is small as described in the manufacturing method described later. Therefore, the surface roughness after coating the coating 12 on the phosphate coating 11 also decreases. That is, the surface roughness of the coating film 12 is reduced. As a result, the pressure loss of gas is reduced, and the gas transport efficiency of the polyethylene-coated steel pipe 100 for gas conduit is increased. The surface roughness of the coating film 12 is preferably as small as possible. Therefore, the upper limit of the surface roughness Rz of the preferable coating film 12 is 25 μm, and 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.

[Method of measuring surface roughness of coating film]
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 a measuring instrument, a surface roughness shape measuring instrument Handysurf 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 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 four locations is taken as the surface roughness Rz of the coating film 12.

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

[Production method]
The method for producing the polyethylene pipe for gas conduit 100 of 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 step, the raw pipe 1 is prepared. If there is a possibility that the scale is formed excessively thick on the inner surface 3 of the raw pipe 1, the inner surface 3 may be subjected to blasting for the purpose of removing the scale in the preparation step. In the preparation step, the inner surface 3 and / or the outer surface 2 may be treated such as pickling, degreasing and washing.

[Chemical conversion treatment process]
In the chemical conversion treatment step, the inner surface 3 of the raw pipe 1 is subjected to a chemical conversion treatment to form a phosphate coating 11 on the inner surface 3 of the raw pipe 1. At this time, chemical conversion may be applied not only to the inner surface 3 of the raw pipe 1 but also to the outer surface 2 of the raw pipe 1 (in this case, a phosphate coating is formed on the outer surface 2 of the raw pipe 1) Will be The chemical conversion treatment is carried out according to a known method using a commercially available chemical conversion solution or the like. After the chemical conversion treatment, water washing or the like may be performed.

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

[Method of measuring surface roughness Rz of phosphate coating]
For example, the surface roughness Rz of the phosphate coating 11 is measured by the following method. First, a test piece including the surface of the phosphate coating 11 is prepared. Next, with respect to the surface of the phosphate coating 11 of this test piece, the maximum height roughness Rz is measured by a method in accordance with JIS B0601 (2013). As a measuring instrument, a surface roughness shape measuring instrument Handysurf 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 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 four locations is taken as the surface roughness Rz of the phosphate coating 11.

  In the manufacturing method of this embodiment, the phosphate coating 11 is formed instead of the blasting for the purpose of the anchor effect to enhance the adhesion of the coating 12. Therefore, the surface roughness of the phosphate coating 11 is small.

  Furthermore, in the case of blasting, corrosion of the inner surface 3 of the raw pipe 1 proceeds immediately after blasting. Therefore, it was necessary to carry out the next surface treatment within several hours after blasting. On the other hand, in the manufacturing method of this embodiment, the phosphate film 11 is formed by chemical conversion treatment. Therefore, the corrosion resistance of the inner surface 3 of the raw pipe 1 is enhanced. After the chemical conversion treatment, the next surface treatment may be performed within several days. Thereby, in the case of the chemical conversion treatment, compared with the case of the blast treatment, the restriction of the work content at the time of manufacture is alleviated. As a result, production efficiency is enhanced.

  Preferably, in the manufacturing method of the present embodiment, the blast treatment is not performed on the inner surface 3 of the raw pipe 1 before the chemical conversion treatment step. The blast treatment is preferably not performed even if it is a blast treatment for the purpose of imparting an anchor effect or a blast treatment for the purpose of removing scale. Thereby, the roughness of the inner surface 3 of the raw pipe 1 can be stably reduced. In the manufacturing method of the present embodiment, it is preferable that no blasting is performed on the inner surface 3 of the raw pipe 1 at all. In this case, the production efficiency is further enhanced.

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

  The alkyl silicate condensate is a general term for substances formed by partial hydrolysis and condensation of alkyl silicate. For example, an alkyl silicate condensate can be obtained by mixing an alkyl silicate, an acid such as hydrochloric acid, and water. Examples of the alkyl silicate include tetramethyl orthosilicate, tetraethyl ortho silicate, tetra-n-propyl ortho silicate, tetra-i-propyl ortho silicate, tetra-n-butyl ortho silicate, tetra-sec-butyl ortho silicate, and methyl polysilicate And one or more selected from the group consisting of ethyl polysilicate and

  Zinc powder can use the above-mentioned zinc powder without restriction. Moreover, a zinc powder can use a commercially available thing. For example, scaly zinc powder is manufactured by ECKART GmbH, STANDART (registered trademark) Zinc flake AT, STANDART (registered trademark) Zinc flake GTT, STANDART (registered trademark) Zinc flake TV, STANDART (registered trademark) Zinc flake G, STAPA (registered trademark) 4 ZnAl7 And STAPA (trademark) 4 ZnSn30 etc. are mentioned. Examples of spherical zinc powder include F-2000 (trade name) manufactured by Honso Chemical Co., Ltd., and the like. The content of zinc powder in the coating composition may be appropriately adjusted so that the desired content of zinc powder can be obtained in the coating 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 the zinc powder. The other components are, for example, other components 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 paint composition. Next, the obtained paint composition is coated on the phosphate coating 11. The method of painting can be set appropriately. Examples of the method of painting include spray, brushing and the like. In the case of spray coating, insert a rod with a jet at the tip into the inside of the steel pipe. Then, the paint composition is injected into the inside of the steel pipe from the injection port. At this time, it is preferable to spray while rotating a steel pipe or a rod with a jet port. The steel pipe and the rod with the injection port may be rotated in the reverse direction. The number of rotations 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.

[Coating composition curing process]
In the coating composition curing step, the coated coating composition is cured by drying. Drying is carried out by maintaining the painted coating composition in an air atmosphere. The evaporation of organic solvents and the like in the coating composition and the further hydrolytic condensation of the alkyl silicate condensate cure the coating composition. 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. Thereby, the coating film 12 is obtained.

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

  The polyethylene coating step may be performed after forming the phosphate coating 11 on the outer surface 2 of the raw pipe 1. In this case, it carries out in the range which rust does not generate in outer surface 2 of original pipe 1. Moreover, you may implement within several hours after blasting the outer surface 2 top of the raw pipe 1. FIG. The polyethylene coating process may, for example, be performed before the coating composition coating process. The polyethylene coating step may, for example, be performed after the coating composition curing step.

  The polyethylene-coated steel pipe 100 for gas conduit of the present embodiment can be manufactured by the above steps.

  Examples will be described below.

[Preparation process]
Steel plates and steel pipes were prepared as raw materials corresponding to raw pipes. (Note that although the present invention is directed to steel pipes, the effects of the above-described roughness and corrosion resistance can be understood even in evaluation tests using steel sheets. (Some steel plates were used for the convenience of the experiment). The steel plate was a rolled steel SS400 for general structure defined 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 for piping SGP specified in JIS G3452 (2014). The steel pipe was a steel pipe having a thickness of 5 mm and a diameter of 165.2 mm. As shown in Table 1, blasting or conversion treatment was performed on the surface or inner surface of the steel plate or steel pipe of each test number. In Test No. 1, blasting was performed. In the test numbers 2 to 5, no blasting treatment was performed. In the test numbers 2 to 5, after removing the scale by pickling, conversion treatment was performed.

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

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

[Coating composition coating process]
An alcohol solution of an alkyl silicate condensate and a zinc dust paste were respectively prepared by the following procedure. In the following description, “parts” indicates “parts by mass” unless otherwise stated.

Preparation Example 1 Preparation of Alcohol Solution of Alkyl Silicate Condensate 31.5 parts of Ethyl Silicate 40 (manufactured by Colcoat Co., Ltd.), 10.4 parts of industrial ethanol, 5 parts of deionized water, and 35 mass% hydrochloric acid The part was charged in a vessel and stirred at 50 ° C. for 3 hours, and then 53 parts of isopropyl alcohol was added to prepare an alcohol solution of alkyl silicate condensate.

Preparation Example 2-1 Preparation of Zinc Powder Paste 1 1.3 parts of TIXOGEL MPZ (trade name; BYK Additives GmbH) as an anti-settling agent, 6.3 parts of xylene as an organic solvent, 3.1 parts of Butyl acetate and 4.1 parts of isobutyl alcohol were charged in a polyethylene container, glass beads were added, and the mixture was shaken for 3 hours with a paint shaker. Then, add 10 parts of STANDART (registered trademark) Zinc flake GTT as scaly zinc powder and 10 parts of F-2000 (trade name; manufactured by Honso Chemical Co., Ltd.) as spherical zinc powder, and shake for 5 minutes as well. I dispersed the end. Thereafter, the glass beads were removed using an 80 mesh net to prepare a zinc dust paste 1.

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

  Coating composition 1 was prepared by mixing 100 parts by weight of an alcohol solution of an alkyl silicate condensate and 34.8 parts by weight of zinc powder paste 1. 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 zinc powder paste 2.

  The paint composition 1 or the paint composition 2 was applied to steel plates or steel pipes of test numbers 1 to 5. The steel pipe was spray-painted by the method described above. It spray-coated on the steel plate also on the same conditions. For Test No. 1, the paint composition was applied within 4 hours after blasting. About test number 2-5, the coating composition was applied 3 days after chemical conversion treatment.

[Coating composition curing process]
The steel plate or steel pipe of each test number was kept at normal temperature for 7 days. Thereby, the coating composition was cured by drying to form a coating film.

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

[Corrosion resistance test]
The corrosion resistance test was implemented with respect to the steel plate and steel pipe of each test number. Corrosion resistance was evaluated by an outdoor exposure test. In the test numbers 1 and 2, the coated side of the steel plate was exposed to the outside for 6 months while being inclined 45 ° upward to the south. Thereafter, the appearance as defined in ASTM D610 (2012) was evaluated. In test numbers 3-5, the steel pipe was exposed outdoors for 6 months. Thereafter, the appearance was similarly evaluated. The results are shown in Table 1. If it is "(circle)" in Table 1, it will have corrosion resistance without a problem practically as a polyethylene-coated steel pipe for gas conduits. In addition, in Table 1, "(circle)" and "x" each judged based on the following references | standards.
○: The evaluation result according to the evaluation criteria of ASTM D610 (2012) is 8-10.
X: The evaluation result by the evaluation criteria of ASTM D610 (2012) is 7 or less.

[Evaluation results]
Referring to Table 1, test numbers 2 to 5 were provided with a phosphate coating on the steel plate surface or the inner surface of the steel pipe. The test numbers 2 to 5 did not carry out 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 transport efficiency of the gas is excellent. In addition, since the coating film contains a siloxane-based binder, generation of a thermal decomposition product at the time of welding is suppressed as compared with an epoxy resin coating film or the like containing an organic substance as a main component. Furthermore, in the test numbers 2 to 5, no rust was observed after the outdoor exposure test because of the coating film, and the corrosion resistance suitable for gas transport was exhibited.

  In the test numbers 2 to 5, the coating composition coating step was further performed 3 days after the chemical conversion treatment, and showed excellent production efficiency.

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

  The embodiment of the present invention has been described above. However, the embodiments described above are merely examples for implementing 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 without departing from the scope of the invention.

100 Polyethylene coated steel pipe for gas conduit 1 Raw pipe 2 Outer surface of raw pipe 3 Inner surface of raw pipe 10 Polyethylene resin coating 11 Phosphate coating 4 Surface of phosphate coating (inner surface of steel pipe after phosphate coating formation )
12 Coating

Claims (7)

Polyethylene coated steel pipe for gas conduit
With the pipe
A phosphate coating disposed on the inner surface of the raw pipe;
A coating disposed on the phosphate coating, containing a siloxane 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 film disposed on the outer surface of the raw pipe. It is a polyethylene-coated steel pipe for gas conduits according to claim 1,
The polyethylene-coated steel pipe for gas conduits, wherein the inner surface of the raw pipe is not a blast surface. It is a polyethylene-coated steel pipe for gas conduits according to claim 1 or claim 2, wherein
The said zinc powder contains scale-like zinc powder, The polyethylene-coated steel pipe for gas conduits. It is a polyethylene-coated steel pipe for gas conduits according to any one of claims 1 to 3,
The polyethylene-coated steel pipe for gas conduits, wherein the phosphate coating is a zinc calcium phosphate coating. It is a polyethylene-coated steel pipe for gas conduits according to any one of claims 1 to 4,
The polyethylene-coated steel pipe for gas conduits, wherein the thickness of the coating is 25 μm or less. A method for producing a polyethylene-coated steel pipe for gas conduit according to any one of claims 1 to 5, wherein
A process of preparing a pipe;
Forming a phosphate coating on the inner surface of the raw pipe by subjecting the inner surface of the raw pipe to chemical conversion treatment;
Applying a coating composition containing an alkyl silicate condensate and zinc powder onto the phosphate coating;
Curing the painted coating composition.
Forming a coating containing a polyethylene resin on the outer surface of the raw pipe. A method of manufacturing polyethylene-coated steel pipe for gas conduit according to claim 6, wherein
The manufacturing method of the polyethylene coating steel pipe for gas conduits which does not carry out blasting to the said inner surface of the said original pipe.

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