JP2018001546A - Steel pipe coated with three layers of polyolefin resin having end shape excellent in peel resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve problems of a steel pipe coated with three layers of polyolefin that, when an on-site anti-corrosion treatment is performed on a pipe portion where the pipe end coating is removed, adhesion of the junction between the portion where the on-site anti-corrosion treatment was performed and the three layered polyolefin is insufficient and that, when the pipe is stored on a yard for a long time, the pipe end where the coating is removed becomes corroded and peeling occurs at the end portion of the polyolefin coating part.SOLUTION: A steel pipe coated with three layers of polyolefin having an end shape of the present invention has a structure where, when fabricating the coated end portion, a primer, formed of an epoxy resin having excellent anti-corrosiveness and adhesion and coated in a thick film of 150 to 600 μm, is allowed to remain and extend. By employing this structure, sealability in an on-site anti-corrosion treatment is improved. Further, even when corrosion occurs due to outdoor exposure, little peeling is caused because the polyolefin resin having high stress does not become the end portion.SELECTED DRAWING: Figure 3

Description

  A polyolefin resin-coated steel pipe is usually used for the outer surface coating for corrosion protection of line pipes used for energy transportation of oil and gas, etc., but the present invention is a three-layer polyolefin resin having an end shape excellent in this peel resistance. It relates to a coated steel pipe.

  In the outer surface polyolefin resin-coated steel pipe used for the line pipe, a three-layer coated steel pipe is used when long-term corrosion resistance and weathering resistance during construction are required. The structure is a three-layer structure composed of an epoxy resin primer layer, an adhesive layer, and a polyolefin resin layer having high anticorrosion properties. Polyolefin resins to be coated include polyethylene resins and polypropylene resins. However, since they are inexpensive and highly reliable, in general, polyethylene resins are used, and polypropylene resins are used when high temperature and weather resistance are required. used. These three-layer coated steel pipes are produced at the coating plant and welded on site.

  Normally, the outer surface coating is shipped with the steel material exposed in order to remove the coating at the end of the tube in consideration of the deterioration of the coating due to the thermal effect during welding at the site. FIG. 1 shows the shape of a steel pipe end portion applied to a general three-layer polyolefin resin coating by a line pipe. The weakest part of the steel pipe coating is the end, and moisture may enter from there to cause peeling of the coating.

Therefore, in order to reduce the stress at the end, it is common practice to make the coated end at an oblique shape as shown in FIG. In Patent Document 1, a processing apparatus for forming this oblique shape is shown, and in Patent Document 2, a curved surface shape is shown for forming an oblique shape with a wire brush.
This slanted shape improves the familiarity between the sleeve and the steel pipe coating end when the exposed steel part is corroded with a cross-linked polyethylene sleeve (shrinkable sleeve) after on-site welding joining, and the underground water intrusion after embedding There is a function to prevent. This relationship is shown in FIG.

  However, even with the above measures, it is not possible to completely prevent water from entering when buried for a long time. Therefore, Patent Document 3 proposes that a polyolefin primer is applied to the steel pipe covering end as a method for further improving the adhesion between the sleeve and the steel pipe covering end to prevent water from entering. However, this method has a problem that the number of processes in the field increases.

Japanese Utility Model Publication No. 3-7415 Japanese Patent Laid-Open No. 3-121770 JP-A-2-255293

The weakest part of the steel pipe coating is the end face, and when water penetrates into the end face and the steel surface is corroded, the coating peels off. Therefore, there is a means to completely prevent the intrusion of water, but this problem can be solved. However, in an infrastructure that has been used for a long period of several decades, such as a line pipe, the water molecule is small. It is difficult to completely prevent the intrusion.
Accordingly, there is a demand for the development of a technique that minimizes the penetration of water and does not lead to peeling of the coating.

That is, the gist of the present invention is as follows.
(1) In a polyolefin-coated steel pipe in which an epoxy resin primer layer, a polyolefin adhesive layer, and a polyolefin layer are sequentially laminated from the surface of the steel pipe, the polyolefin adhesive layer and the polyolefin layer at the coated end are formed at an angle of 45 degrees or less with the steel pipe surface. A polyolefin-coated steel pipe having an end shape excellent in peel resistance, wherein only 5 mm or more of the epoxy resin primer layer is left uncoated from the end portion to the outside.
(2) The polyolefin coating having an end shape excellent in peel resistance according to the above (1), wherein the epoxy resin primer layer is formed using a powder coating material and has a thickness of 150 to 600 μm Steel pipe.
(3) After the polyolefin coating is formed, the end of the polyolefin adhesive layer is first formed with a cutting blade and the polyolefin layer is formed at an angle of 45 degrees or less with the steel pipe surface, and then the epoxy resin primer layer is 5 mm with a steel brush. The method for producing a polyolefin-coated steel pipe having an end shape excellent in peel resistance according to the above (1) or (2), characterized in that it is arranged so that it is left uncoated.

Next, the contents of the gist of the present invention will be described.
As a result of examining the shape of the pipe end of the three-layer polyolefin-coated steel pipe, it was found that it is the simplest and most effective to extend the primer part beyond the end of the polyolefin coating. The outline is shown in FIG. Even if water penetrates into the steel surface as a result of the end portion of the coating so-called steel surface and the end portion of the coating moving to the end of the primer portion by protruding the primer portion and making it 5 mm longer (extend) Only the primer part is peeled off, and the coating is not peeled off, and a three-layer polyolefin coating having long-term durability can be obtained.
In addition, since the extended primer portion does not have an upper polyolefin coating layer, the contraction stress of the layer does not work, and peeling does not easily occur.
Furthermore, as a secondary effect, the welded part is covered with a shrink sleeve for the purpose of corrosion protection after the polyolefin-coated steel pipe is welded on site, but the adhesion between the sleeve and the extended primer part is improved, and the steel pipe in service It is possible to further prevent water from entering the coating. (Fig. 4)
Although it seems that the same effect can be obtained if the primer is applied on site, the primer layer applied at the time of polyolefin coating and the primer layer applied at the site are not completely integrated for many years. There is a possibility that peeling of the coating may occur when both layers are peeled off and water enters here.

High corrosion resistance and processing resistance are required for the portion of the primer that extends only, so inorganic pigment is added at 20% by weight or more, and the resin component is adjusted so that the glass transition temperature after heat curing is 95 ° C or higher. The powdered epoxy resin is applied and heat-cured so as to have a film thickness of 150 to 600 μm.
When an amorphous solid (for example, a polyolefin resin) is heated, a solid that is hard (high rigidity) and has no fluidity at low temperatures will rapidly decrease in rigidity and viscosity within a narrow range. Increases nature. Such a temperature is called a glass transition temperature. Therefore, a polyolefin resin having a higher glass transition temperature becomes a harder resin at a low temperature (normal temperature).
Thus, since the coating film hardness is ensured by increasing the temperature of the inorganic pigment and the glass transition temperature, it becomes easy to remove only the upper polyolefin coating. Further, by setting the film thickness to 150 μm or more, it is possible to leave a film thickness that does not generate pinholes even after processing, which is effective for ensuring corrosion resistance.

  Next, a method for forming the end of the coated steel pipe of the present invention will be described. Usually, in order to avoid the thermal influence of welding, the coated end portion of the resin-coated steel pipe is cut by 100 mm or more from the pipe end portion, and a steel exposed portion is provided so that the influence of the welding heat does not reach the coating. At this time, an oblique shape (tapering) process is performed so as to exist at the tube end covering end.

  In the three-layer coated steel pipe of the present invention, the coating of the pipe end is cut or brushed, and a 45 ° or less taper-shaped polyolefin coating and only an epoxy resin primer layer of 5 to 50 mm is extended to the taper end. Processing is performed by leaving the steel material exposed from the primer layer to the end of the steel pipe. In order to perform the process of leaving only one primer layer among the three layers, the lowermost epoxy resin primer needs to have thickness and resistance to cutting and brushing. For this reason, the powder epoxy resin which can ensure the thickness which can be easily processed is applied. Moreover, as a coating material, the powder epoxy resin which contains a pigment 20weight% or more and whose glass transition temperature after hardening is 95 degreeC or more is used. The thickness of the primer layer at this time shall be 150-600 micrometers.

  In the three-layer polyolefin-coated steel pipe using the powder epoxy resin of the present invention, by performing a process of performing a predetermined taper process and a process of leaving only the primer layer on both ends of the pipes of the steel pipe, a local weld joint part is obtained. In addition to improving the anti-corrosion property (FIG. 4), it is possible to prevent peeling from the coated end during storage in the storage area.

FIG. 1 is a sectional view showing an end structure of a general three-layer polyolefin resin-coated steel pipe. FIG. 2 is an example of a cross-sectional view when a shrink sleeve with an adhesive is used for corrosion protection after field welding in the case of using a coated end structure of a general three-layer polyolefin resin-coated steel pipe. FIG. 3 is a sectional view showing the end structure of the three-layer polyolefin resin-coated steel pipe of the present invention. FIG. 4 is an example of a cross-sectional view when a shrink sleeve with an adhesive is used for anticorrosion after field welding when the coated end structure of the three-layer polyolefin resin-coated steel pipe of the present invention is used. FIG. 5 is a process schematic diagram of the method of forming the coated end shape according to the present invention, in which a coating other than the primer is removed using a cutting tool and a taper process is performed. FIG. 6 is a schematic view of a method of removing using a rotating wire brush in order to create a steel pipe exposed portion.

Hereinafter, the present invention will be described in detail.
The coating of the three-layer polyolefin resin-coated steel pipe body portion of the present invention is formed in three layers in the order of the primer layer 5, the adhesive layer 3, and the polyolefin resin layer 4 on the outer surface of the steel pipe 1 by an existing method.
The steel pipe used for the coating is not particularly limited, and any steel type such as ordinary steel or high alloy steel can be applied. In addition, size and thickness restrictions are only attributable to the equipment. If there is adhesion of oil or the like to the steel pipe, it is removed in advance and then the rust and dirt on the surface of the steel pipe are removed and blasting is performed to ensure the roughness required for bonding. As a polishing material used for blasting, steel grid shot grains are generally used. Further, when a clean surface is required, a ceramic material such as alumina may be used. When dirt such as iron powder adheres to the surface after the blast treatment, treatments such as brushing, suction, and washing with liquid can be performed.

  Next, a cleaning process or a chemical conversion process is performed. For the chemical conversion treatment, phosphoric acid or other treatments can be used. However, when particularly high anticorrosion performance is required, for example, a coating type chromate chemical conversion treatment is performed using, for example, PALCROME 100 manufactured by Nihon Parker Rising Co., Ltd. Also good.

  Next, the epoxy resin primer layer will be described. The primer layer 5 is required to have a film thickness of 150 μm or more necessary for obtaining the tube end covering structure of the present invention and cutting resistance and brush resistance to processing. In the liquid type used conventionally, when a large amount of inorganic pigment is added in order to improve cutting resistance and brush resistance, sedimentation and viscosity problems occur. Further, since thick film coating is difficult, a powder epoxy resin capable of thick film coating is formed by electrostatic powder coating on a heated steel pipe.

  Powder epoxy resin paints are bisphenol A type epoxy resin and bisphenol F type epoxy resin, or mixed with polyfunctional phenol novolac type epoxy resin or halogenated epoxy resin. A compound prepared by adding a curing agent such as imidazole compound or dicyandiamide is used. The characteristic required in the present invention is the durability of the film during cutting or brushing. Therefore, 20-50% by weight of an inorganic pigment is blended as the powder epoxy resin powder coating.

  As the inorganic pigment, a pigment such as silica, titanium oxide, wollastonite, mica, talc, kaolin, chromium oxide, zinc borate, and zinc phosphate, metal powder such as zinc and Al, ceramic powder, and the like can be used as appropriate. . Also, since the hardness of the film is important for cutting resistance and brushing resistance, a material having a high glass transition temperature is used to increase the hardness of the coating film. Specifically, the glass transition temperature is preferably 95 ° C. or higher.

  The powder epoxy resin paint is available from Nippon Paint Co., Ltd. or Kansai Paint Co., Ltd. in Japan, and the one corresponding to the present invention is selected and used. The thickness of the primer layer should be 150 μm or more, but if the thickness is too thick, the internal stress of the coating will increase, and if stress is applied by repeated cooling and heating due to day and night or changing seasons, it will peel off especially when exposed outdoors. Since it becomes easy, 150-600 micrometers is suitable.

  After forming the powder epoxy resin primer layer 5, the polyolefin resin layer 4 is laminated via the modified polyolefin resin adhesive layer 3. As the modified polyolefin resin adhesive, a polyolefin resin modified with maleic anhydride, a copolymer of polyolefin and maleic anhydride, or a copolymer of polyolefin, acrylic ester and maleic anhydride can be used. .

  The modified polyolefin resin adhesive is supplied as powder or pellets. Examples of the adhesive used for the polyethylene coating include NE060, NE065, NE080 manufactured by Mitsui Chemicals, Borcoat ME0420 manufactured by BOREALIS, Lucalen G3710E manufactured by Lyondell Basell, and the like. In the case of pellets, the outer surface of the steel pipe after primer application is coated with a resin that has been heated and melted using an adhesive extruder using a T die. For small and medium diameter steel pipes, round dies may be used. As another method, there is a method in which the modified polyolefin resin adhesive is pulverized and powdered, and this powder is applied. By these methods, an adhesive layer of 0.1 to 0.4 mm is formed.

  As the polyolefin resin to be coated on the modified polyolefin adhesive layer 3, those commercially available for coating a steel pipe can be used. A typical polyolefin is polyethylene, and in addition, polypropylene is used for high temperatures. As the polyethylene resin, a brand used for coating a steel pipe can be used. For example, NOVATEC ER002S manufactured by Japan Polyethylene Co., Ltd., Borcoat HE3450 manufactured by BOREALIS Co., Ltd. Can be used.

  Polypropylene resins generally use copolymers with better low-temperature properties than homopolymers, and include color pigments, filling enhancers, antioxidants, UV absorbers, and hindered amine-based weather resistance as measures against heat resistance and weather resistance. It is preferable that an agent or the like is added, and has both low temperature toughness and oxidation deterioration resistance at high temperature use. When carbon black is used as a color pigment in a polypropylene resin, the effect of the antioxidant disappears at a high temperature, so that it is not used in a high performance coated steel pipe product. For this reason, the color of the color pigment is generally white. As the polypropylene resin, for example, TX1843B of Japanese polypropylene in Japan, Borcoat BB108E-1199 manufactured by BOREALIS, and Moplen Coat EP60R / BIANCO manufactured by LyondellBasell can be used overseas.

  The polyolefin resin layer 4 is usually coated with 2 mm or more in order to suppress wrinkling during handling. The thicker the polyolefin resin layer, the better the weather resistance and corrosion resistance. However, the thicker the film, the greater the internal stress, so 6 mm or less is desirable.

  An example of a method for forming the tube end structure having the tapered shape of the present invention shown in FIG. 3 and the tube end structure having the primer layer 5 as an extension thereof will be described below. After manufacturing a three-layer polyolefin resin-coated steel pipe, the interface between the primer and adhesive can be cut while the steel pipe is rotated as shown in FIG. In order to achieve this, the cutting blade 8 having a shape parallel to the steel pipe is pushed into the inside from the surface to perform cutting. Thereby, it is possible to form a tapered cross section having a constant angle of 45 degrees or less in the polyolefin resin coating portion and leave only the primer.

  Next, as shown in FIG. 6, the coating is completely scraped off by the rotating wire brush 9 to form a steel pipe exposed portion (cutback) of 100 mm or more from the pipe end. In the case of a general cutback, 150 mm is secured. Finally, if adhesive remains on the primer layer of the extension formed in Fig. 3, the adhesion with the local anticorrosion is inhibited, so the primer surface is polished with a wire brush or the like as necessary. Remove the adhesive and finish.

  The primer single layer portion needs to have a distance that prevents water from entering the polyolefin coating portion during service of the pipeline, and it is necessary to extend it by at least 5 mm. There is no particular restriction on the maximum length, but if it is too long, processing is difficult, and depending on the relationship with the cutback distance, if the steel exposed part at the coated end is standard 150 mm, the primer should be 50 mm or more When it is extended, it is within 100 mm (distance from the welded portion) that is easily affected by the heat of welding during field welding.

[Examples 1 to 5]
Hereinafter, Examples 1 to 5 in which polyethylene is used for the polyolefin of the present invention and the taper angle and the primer coating thickness are changed will be specifically described.
As the steel pipe, a 5.5A length of 200A JIS G3452 piping carbon steel pipe was used. The outer surface of the steel pipe was subjected to IGD's TGD-70 grid blasting treatment to remove the rust. Thereafter, the steel pipe was subjected to a surface cleaning treatment to remove dirt, iron powder, and the like.
After heating the steel pipe to 220 ° C. with a heating device, a powder epoxy resin primer (BASEPOX PE50-1081, manufactured by Arsonsisi, pigment addition amount 27%, glass transition temperature 102 ° C.) is a film thickness range of 150 to 150 of the present invention. After carrying out electrostatic powder coating at 600 μm, NE065 pellets manufactured by Mitsui Chemicals Co., Ltd. as a polyethylene adhesive were formed into a sheet-like semi-molten state using an extruder and a T-die, and coated by wrapping. Subsequently, NOVATEC ER002S pellets manufactured by Nippon Polyethylene Co., Ltd. were formed into a sheet-like semi-molten state by using an extruder and a T die, and the polyethylene coating was wound and coated. The adhesive film thickness was adjusted to 0.2 mm, and the polyethylene resin coating was adjusted to 3 mm. After coating, water cooling was performed to produce a three-layer polyethylene resin-coated steel pipe.

  The produced three-layer polyethylene resin-coated steel pipe is rotated and tapered by a cutting blade having an angled blade and a blade parallel to the steel pipe, and cutting of adhesive and polyethylene resin coating to form an extended primer layer And went at the same time. The angles of the taper processing were selected to be 30 ° and 45 ° within the range of the present invention. At this time, the coated end of the polyethylene resin was adjusted to be 150 mm from the end of the steel pipe. Thereafter, the coating was scraped off from the end of the steel pipe using a wire brush to expose the steel surface, and the primer single layer portion was left with a length of 20 mm. About the primer single layer part, only the surface was further ground with a wire brush and finished so that no adhesive remained on the surface, and Examples 1 to 5 were produced.

[Examples 6 to 8]
Examples 6 to 8 in the case where polypropylene is used for the polyolefin of the present invention and the length of the primer extension portion is changed will be specifically described below.
As the steel pipe, a 5.5A length of 200A JIS G3452 piping carbon steel pipe was used. The outer surface of the steel pipe was subjected to IGD's TGD-70 grid blasting treatment to remove the rust. Thereafter, the steel pipe was subjected to a surface cleaning treatment to remove dirt, iron powder, and the like.
After heating the steel tube to 220 ° C. with a heating device, a powder epoxy resin primer (Scotchkote 226N +, 3M, 44% pigment addition, 106 ° C. glass transition temperature) is electrostatically applied at a film thickness range of 300 μm of the present invention. After powder coating, pellets of Mitsubishi Chemical AP-P501 were formed into a sheet-like semi-molten state using an extruder and a T-die as an adhesive, and were wound and covered. Next, for the polypropylene coating, pellets of Japanese polypropylene TX1843B were formed into a sheet-like semi-molten state using an extruder and a T-die, and then wound and coated. The adhesive film thickness was adjusted to 0.2 mm, and the polypropylene resin coating was adjusted to 3 mm. After coating, water cooling was performed to produce a three-layer polypropylene resin-coated steel pipe.

  The produced three-layer polypropylene resin-coated steel pipe is rotated and tapered by a cutting blade having an angled blade and a blade parallel to the steel pipe, and an adhesive and polyethylene resin-coated cutting for forming an extended primer layer And went at the same time. The taper processing angle was selected to be 45 degrees within the range of the present invention. At this time, the coated end of the polypropylene resin was adjusted to be 150 mm from the end of the steel pipe. Thereafter, the coating was scraped off from the end of the steel pipe using a wire brush to expose the steel surface, and the primer single layer portion was adjusted to remain at a length of 5 to 50 mm which is the range of the present invention. About the primer single layer part, only the surface was further ground with a wire brush and finished so that no adhesive remained on the surface, and Examples 6 to 8 were produced.

[Comparative Examples 1-3]
Hereinafter, the case where polyethylene is used for the polyolefin of the present invention and there is no taper will be specifically described as Comparative Example 1, and the case where the primer single layer portion is absent or small is referred to as Comparative Examples 2 and 3.
As the steel pipe, a 5.5A length of 200A JIS G3452 piping carbon steel pipe was used. The outer surface of the steel pipe was subjected to IGD's TGD-70 grid blasting treatment to remove the rust. after that. The surface of the steel pipe was cleaned to remove dirt and iron powder.
After heating the steel tube to 220 ° C. with a heating device, a powder epoxy resin primer (BASEPOX PE50-1081, manufactured by Arsonsisi, 27% pigment addition, glass transition temperature 102 ° C.) was used in the film thickness range of 150 μm of the present invention. After performing electrostatic powder coating, pellets of NE065 manufactured by Mitsui Chemicals Co., Ltd. as a polyethylene adhesive were formed into a sheet-like semi-molten state using an extruder and a T-die, and were wound and coated. Subsequently, NOVATEC ER002S pellets manufactured by Nippon Polyethylene Co., Ltd. were formed into a sheet-like semi-molten state by using an extruder and a T die, and the polyethylene coating was wound and coated. The adhesive film thickness was adjusted to 0.2 mm, and the polyethylene resin coating was adjusted to 3 mm. After coating, water cooling was performed to produce a three-layer polyethylene resin-coated steel pipe.

[Comparative Example 1]
The produced three-layer polyethylene resin-coated steel pipe is rotated and processed with a cutting blade having a blade having a 90-degree angle and a blade parallel to the steel pipe, and an adhesive and polyethylene resin-coated for forming an extended primer layer. Cutting was performed at the same time. At this time, the coated end of the polyethylene resin was adjusted to be 150 mm from the end of the steel pipe. Thereafter, the coating was scraped off from the end of the steel pipe using a wire brush to expose the steel surface, and the primer single layer portion was left with a length of 5 mm. About the primer single layer part, only the surface was further ground with a wire brush and finished so that no adhesive remained on the surface, and Comparative Example 1 was produced.

[Comparative Example 2]
The produced three-layer polyethylene resin-coated steel pipe is rotated and tapered with a cutting blade having a 45-degree angle blade and a blade parallel to the steel pipe, and an adhesive and a polyethylene resin coating for forming an extended primer layer The cutting process was performed simultaneously. The taper processing angle was selected to be 45 degrees within the range of the present invention. At this time, the coated end of the polyethylene resin was adjusted to be 150 mm from the end of the steel pipe. Thereafter, the coating was scraped off from the end of the steel pipe using a wire brush to expose the steel surface, and Comparative Example 2 was produced so as not to leave the primer single layer portion (0 mm).

[Comparative Example 3]
The produced three-layer polyethylene resin-coated steel pipe is rotated and tapered with a cutting blade having a 45-degree angle blade and a blade parallel to the steel pipe, and an adhesive and a polyethylene resin coating for forming an extended primer layer The cutting process was performed simultaneously. At this time, the coated end of the polyethylene resin was adjusted to be 150 mm from the end of the steel pipe. Thereafter, the coating was scraped from the end of the steel pipe using a wire brush to expose the steel surface, and the primer single layer portion was left with a length of 2 mm to produce Comparative Example 3.

[Comparative Example 4]
Hereinafter, the case where polyethylene is used for the polyolefin of the present invention and the primer portions have different thicknesses will be specifically described.
As the steel pipe, a 5.5A length of 200A JIS G3452 piping carbon steel pipe was used. The outer surface of the steel pipe was subjected to IGD's TGD-70 grid blasting treatment to remove the rust. Thereafter, the steel pipe was subjected to a surface cleaning treatment to remove dirt, iron powder, and the like.
After heating the steel tube to 220 ° C. with a heating device, a powder epoxy resin primer (BASEPOX PE50-1081, manufactured by Arsonsisi, 27% pigment addition, glass transition temperature 102 ° C.) is 100 μm outside the film thickness range of the present invention. After performing electrostatic powder coating, pellets of NE065 manufactured by Mitsui Chemicals Co., Ltd. as a polyethylene adhesive were formed into a sheet-like semi-molten state using an extruder and a T-die, and were wound and coated. Subsequently, NOVATEC ER002S pellets manufactured by Nippon Polyethylene Co., Ltd. were formed into a sheet-like semi-molten state by using an extruder and a T die, and the polyethylene coating was wound and coated. The adhesive film thickness was adjusted to 0.2 mm, and the polyethylene resin coating was adjusted to 3 mm. After coating, water cooling was performed to produce a three-layer polyethylene resin-coated steel pipe.

  The produced three-layer polyethylene resin-coated steel pipe is rotated and tapered with a cutting blade having a 45-degree angle blade and a blade parallel to the steel pipe, and an adhesive and a polyethylene resin coating for forming an extended primer layer The cutting process was performed simultaneously. At this time, the coated end portion of the polyolefin resin was adjusted to be 150 mm from the end portion of the steel pipe. Thereafter, the coating was scraped off from the end of the steel pipe using a wire brush to expose the steel surface, and the primer single layer portion was left with a length of 4 mm to produce Comparative Example 4.

[Comparative Example 5]
Hereinafter, the case where polyethylene is used for the polyolefin of the present invention and the glass transition temperature of the primer portion is insufficient will be specifically described.
As the steel pipe, a 5.5A length of 200A JIS G3452 piping carbon steel pipe was used. The outer surface of the steel pipe was subjected to IGD's TGD-70 grid blasting treatment to remove the rust. Thereafter, the steel pipe was subjected to a surface cleaning treatment to remove dirt, iron powder, and the like.
After heating the steel tube to 220 ° C with a heating device, apply the powder epoxy resin primer (domestic prototype, pigment addition amount 25%, glass transition temperature 91 ° C) with electrostatic powder coating at 150 µm in the film thickness range of the present invention. After implementation, NE065 pellets manufactured by Mitsui Chemicals, Inc. as polyethylene adhesive were formed into a sheet-like semi-molten state using an extruder and a T-die, and were wound and covered. Subsequently, NOVATEC ER002S pellets manufactured by Nippon Polyethylene Co., Ltd. were formed into a sheet-like semi-molten state by using an extruder and a T die, and the polyethylene coating was wound and coated. The adhesive film thickness was adjusted to 0.2 mm, and the polyethylene resin coating was adjusted to 3 mm. After coating, water cooling was performed to produce a three-layer polyethylene resin-coated steel pipe.

  The produced three-layer polyethylene resin-coated steel pipe is rotated and tapered with a cutting blade having a 45-degree angle blade and a blade parallel to the steel pipe, and an adhesive and a polyethylene resin coating for forming an extended primer layer The cutting process was performed simultaneously. At this time, the coated end of the polyethylene resin was adjusted to be 150 mm from the end of the steel pipe. Thereafter, the coating was scraped off from the end of the steel pipe using a wire brush to expose the steel surface, and the primer single layer portion was left with a length of 4 mm to produce Comparative Example 5.

[Comparative Example 6]
Hereinafter, the case where polyethylene is used for the polyolefin of the present invention and the addition of the inorganic pigment in the primer portion is insufficient will be specifically described.
As the steel pipe, a 5.5A length of 200A JIS G3452 piping carbon steel pipe was used. The outer surface of the steel pipe was subjected to IGD's TGD-70 grid blasting treatment to remove the rust. Thereafter, the steel pipe was subjected to a surface cleaning treatment to remove dirt, iron powder, and the like.
After heating the steel tube to 220 ° C with a heating device, apply the powder epoxy resin primer (domestic prototype, pigment addition amount 11%, glass transition temperature 101 ° C) with electrostatic powder coating at the film thickness range of 150μm of the present invention. After implementation, NE065 pellets manufactured by Mitsui Chemicals, Inc. as polyethylene adhesive were formed into a sheet-like semi-molten state using an extruder and a T-die, and were wound and covered. Subsequently, NOVATEC ER002S pellets manufactured by Nippon Polyethylene Co., Ltd. were formed into a sheet-like semi-molten state by using an extruder and a T die, and the polyethylene coating was wound and coated. The adhesive film thickness was adjusted to 0.2 mm, and the polyethylene resin coating was adjusted to 3 mm. After coating, water cooling was performed to produce a three-layer polyethylene resin-coated steel pipe.

  The produced three-layer polyethylene resin-coated steel pipe is rotated and tapered with a cutting blade having a 45-degree angle blade and a blade parallel to the steel pipe, and an adhesive and a polyethylene resin coating for forming an extended primer layer The cutting process was performed simultaneously. At this time, the coated end portion of the polyolefin resin was adjusted to be 150 mm from the end portion of the steel pipe. Thereafter, the coating was scraped off from the end of the steel pipe using a wire brush to expose the steel surface, and the primer single layer portion was left with a length of 4 mm to produce Comparative Example 6.

[Performance test results of Examples and Comparative Examples]
In order to confirm the relationship between the difference in the end specification of the steel pipe and the peeling resistance due to corrosion in the examples and comparative examples manufactured by the above method, the pipe end was cut at a length of 500 mm and exposed outdoors for one year. Thereafter, the coating was peeled from the polyolefin coating end, and the peeling length at 8 points on the circumference was measured to obtain the average peeling length.
Moreover, in order to evaluate the anticorrosion property in the local anticorrosion coating, the steel pipes were similarly cut to a length of 500 mm from the pipe end portions, and these steel pipes were circumferentially welded and joined. Thereafter, a local anticorrosion coating was carried out using a polyethylene resin shrink sleeve comprising an inner layer 6 of the local anticorrosion shrink sleeve and an outer layer 7 of the local anticorrosion shrink sleeve, and a simulated local joint portion for corrosion evaluation was produced. As for the corrosion resistance of the local joint part, after immersing the specimen for 90 days in salt water at 80 ° C. as an accelerated test, the local anticorrosion coating on the joint part is removed and the coating is applied from the polyolefin coating end as described above. The average peel length was determined by measuring the peel length at 8 points on the circumference.

About the said test result, the Example and the comparative example were put together in Table 1.
As is apparent from the results of Examples of the present invention shown in Table 1, in the present invention in which the polyolefin coating is tapered and only the primer layer is left at the tip, the peeling stress at the end of the coating is small. Moreover, since corrosion peeling is also suppressed by the remaining primer, peeling does not occur even if exposed to the outdoors for one year. Furthermore, even if the local anticorrosion is a two-layer coating, the three-layered part can be formed at both ends of the circumference by the primer remaining, so that water and ions, which are corrosion factors, are bonded to the steel material and the local coating. As a result, it was difficult to reach the end of the polyolefin coating even in the accelerated test.

On the other hand, in the case of Comparative Example 1 in which the polyolefin-coated end portion does not have a taper shape, there is a problem in the anticorrosion property because the peeling stress is concentrated on the end portion and the local anticorrosion is not sufficiently adapted to the corner portion. Arise. As in Comparative Examples 2 and 3, when the remaining primer length necessary for the present invention is insufficient, corrosion peeling occurs when exposed outdoors, and the pipe end sealing effect is insufficient when exposed to local corrosion, resulting in peeling.
In addition, if the film thickness required for the primer and the cutting resistance and brushing resistance (glass transition temperature and added amount of inorganic pigment) due to processing are insufficient as in Comparative Examples 4 to 6, the corrosion resistance of the remaining primer portion decreases. As a result, end peeling of the polyolefin coating occurs.

  As is clear from the above results, in the existing three-layer polyolefin resin-coated steel pipe, the end cover specifications using only the cutback and taper were not sufficient for exposure to storage and anticorrosion after field coating. . On the other hand, in the end specification of the coating that performs the processing to leave the primer portion of the present invention, the peeling stress due to repeated cooling and heating, which is a problem in outdoor exposure, is small, and the primer portion having high anticorrosion properties is extendedly coated. Therefore, the peeling of the polyolefin coating end does not start due to exposure, and the anticorrosion property of the local anticorrosion part can be greatly improved because the sealing effect is high in the local anticorrosion.

DESCRIPTION OF SYMBOLS 1 Steel pipe 2 Primer layer 3 Adhesive layer 4 Polyolefin resin layer 5 Primer layer by powder epoxy resin coating with extended coating 6 Inner layer of local anticorrosion shrink sleeve 7 Outer layer of local anticorrosion shrink sleeve 8 Taper processing of coated end and primer remaining Cutting blade 9 shaped to remove the coating on the part 9 Rotating wire brush

Claims (3)

  In a polyolefin-coated steel pipe in which an epoxy resin primer layer, a polyolefin adhesive layer, and a polyolefin layer are sequentially laminated from the steel pipe surface, the polyolefin adhesive layer and the polyolefin layer at the coated end are formed at an angle of 45 degrees or less with respect to the steel pipe surface. A polyolefin-coated steel pipe having an end shape excellent in peeling resistance, wherein only 5 mm or more of an epoxy resin primer layer is left uncoated from the end to the outside.   2. The polyolefin-coated steel pipe having an end shape excellent in peel resistance according to claim 1, wherein the epoxy resin primer layer is formed using a powder coating material and has a thickness of 150 to 600 [mu] m.   After the polyolefin coating is formed, the end of the polyolefin adhesive layer is first formed with a cutting blade, and the polyolefin layer is formed at an angle of 45 degrees or less with the steel pipe surface, and then an epoxy resin primer layer is applied by 5 mm or more with a steel brush. The method for producing a polyolefin-coated steel pipe having an end shape excellent in peeling resistance according to claim 1, wherein the steel pipe is arranged as described above.