JP7339877B2 - Internally coated steel pipe - Google Patents

Description

TECHNICAL FIELD The present invention relates to an internally coated steel pipe used for gas pipes and the like.

In some cases, the inner surface of steel pipes used for gas pipes and the like is coated with a paint containing a large amount of zinc dust (inorganic zinc paint) for the purpose of anticorrosion. As a paint containing a large amount of zinc dust and a steel pipe having an inner surface coated with this paint, those described in Patent Documents 1 to 4, for example, have been proposed.

Patent Document 1 discloses an original tube, a phosphate coating placed on the inner surface of the original tube, a phosphate coating placed on the phosphate coating, containing a siloxane-based binder and zinc dust, and having a surface roughness Rz is less than 30.0 μm, and a polyethylene resin coating disposed on the outer surface of the original pipe.

Patent document 2 contains a siloxane-based binder, zinc dust _, and _{predetermined} amounts of a ferric oxide source and a molybdenum compound. Primary antirust coating compositions with ratios from 1/0.1 to 1/2.5 are described.

Patent Document 3 discloses that the outer surface has a coating layer of polyethylene resin and the inner surface has a coating film, and the coating film contains 5 parts by weight or more of alkyl silicate and zinc dust per 100 parts by weight of the solid content of the paint. A polyethylene coated steel pipe for gas with excellent weldability is described, which is a cured coating film of a coating containing 30 parts by weight or more of.

In Patent Document 4, a gas coating having a cured coating film of a coating containing 5 to 50% by mass of modified alkyl silicate, 3 to 40% by mass of colloidal silica, and 30 to 70% by mass of zinc dust as the solid content of the coating is provided on the inner surface. Covered steel pipes (excluding the case where the cured coating film of paint contains 1 to 10% by mass of a molybdate compound) are described.

JP 2019-044850 A Japanese Patent No. 3844369 Japanese Patent No. 5516649 Japanese Patent No. 6295987

By the way, when the outer surface of a steel pipe that has been internally coated is fused or welded (hereinafter also referred to as “welding, etc.”), the components of the inner coating film (especially the zinc component) are thermally decomposed, damaging the coating film and causing mist. In some cases, dust-like or string-like solids are generated (the mist, dust-like or string-like solids generated by thermal decomposition are hereinafter also referred to as "mist, etc."). If such mist or the like is released into the steel pipe, there is a possibility that the filter in the gas pipe will become clogged, or the devices and equipment connected to the gas pipe will malfunction.
In addition, if the outer surface of the steel pipe is welded at a low speed, the amount of heat input is high, so there is a possibility that the amount of generated mist or the like increases.

In general, in welding or the like, it is desirable to remove the coating film before performing the treatment in order to prevent generation of mist or the like. However, it is often difficult to remove the coating film on the inner surface of the steel pipe before welding or the like due to its structure.
Therefore, there is a demand for an internally coated steel pipe in which the coating film is not easily affected by heat even if welding or the like is performed without removing the coating film, and the corrosion resistance is maintained.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an internally coated steel pipe that produces a small amount of mist or the like generated from the inner surface even when welding or the like with a high heat input is performed on the outer surface, and that maintains a high degree of corrosion resistance.

In order to solve the above problems, the present inventors have made intensive studies and found that 35 to 75% by mass of zinc powder, 10 to 40% by mass of an alkylsilicate polycondensation reaction product, and 3 inorganic pigments having a melting point of over 2000 ° C. Even if the inner surface coated steel pipe has a coating film containing up to 15% by mass on the inner surface, even if the outer surface is welded with a high heat input, the amount of mist generated from the inner surface is small and the corrosion resistance is maintained at a high level. We found that and completed the present invention.

That is, the present invention is the following (1) to (5).
(1) An inner surface having a coating film containing 35 to 75% by mass of zinc dust, 10 to 40% by mass of an alkylsilicate polycondensation product, and 3 to 15% by mass of an inorganic pigment having a melting point of over 2000°C. painted steel pipe.
(2) The internally coated steel pipe according to (1), wherein the inorganic pigment contains aluminum oxide and/or calcium oxide.
(3) The internally coated steel pipe according to (2), wherein the mass ratio of the aluminum oxide to the calcium oxide in the inorganic pigment is 1:0.2-5.
(4) The inner surface coated steel pipe according to any one of (1) to (3), which has a coating layer made of polyolefin on the outer surface.
(5) The inner coated steel pipe according to any one of (1) to (4), which is used as a gas pipe.

ADVANTAGE OF THE INVENTION According to the present invention, it is possible to provide an internally coated steel pipe that produces a small amount of mist or the like generated from the inner surface even when welding or the like with a high heat input is applied to the outer surface, and that maintains a high degree of corrosion resistance.

FIG. 4 is a schematic perspective view for explaining a method of forming a weld bead on the outer surface of the inner surface-coated steel pipe;

The present invention will be described.
The present invention has a coating film containing 35 to 75% by mass of zinc dust, 10 to 40% by mass of an alkylsilicate polycondensation reaction product, and 3 to 15% by mass of an inorganic pigment having a melting point of over 2000° C. on the inner surface. It is an inner surface coated steel pipe.
Hereinafter, this is also referred to as "the steel pipe of the present invention". Further, the coating film that the steel pipe of the present invention has on the inner surface is hereinafter also referred to as "the coating film of the present invention".

In the present invention, the "inner surface" of the steel pipe is the surface that is in contact with the cavity inside the steel pipe. The "outer surface" of the steel pipe is a surface provided on the outer side of the steel pipe and substantially facing the above-described inner surface.

First, each component contained in the coating film of the present invention will be described in detail.

First, the coating of the present invention contains zinc dust. Zinc dust is a component that acts as a sacrificial anode to add corrosion resistance to steel pipes. The zinc dust is a powder made of metallic zinc, and the average particle size thereof is preferably about 1 to 20 μm, more preferably 2 to 15 μm, and more preferably 3.5 to 10 μm. More preferred.
Here, the average particle size is defined by taking a photograph of zinc powder at a magnification of 5000 times using a scanning electron microscope (SEM), arbitrarily selecting 500 pieces from the obtained photograph, and using a vernier caliper to measure the projected area of each. The equivalent circle diameter is measured to determine the cumulative particle size distribution (volume basis), and the average particle size (median size) is calculated from this to obtain the value.

The content of zinc dust in the coating film of the present invention is 35 to 75% by mass, preferably 45 to 65% by mass, more preferably 50 to 60% by mass. When the content of zinc dust is within such a range, a coating film having high anticorrosion properties can be obtained. In addition, even when the outer surface of the coating film of the present invention is welded or the like, it is difficult for the coating film of the present invention to generate mist or the like derived from zinc components such as zinc oxide, even though the content of zinc dust is as described above.

Additionally, the coating of the present invention comprises an alkylsilicate polycondensation reactant. This alkylsilicate polycondensation reaction product is obtained by applying a paint containing alkylsilicate to the inner surface of a steel pipe, then using the moisture in the air to hydrolyze the alkylsilicate to generate silanol groups, and then undergoing a polycondensation reaction. A strong siloxane bond is formed by This increases the strength of the coating film. The alkyl silicate is preferably ethyl silicate (tetraalkoxysilane).

The content of the alkylsilicate polycondensation reaction product in the coating film of the present invention is 10 to 40% by mass, preferably 12 to 37% by mass, more preferably 15 to 35% by mass. When the content of the alkylsilicate polycondensation product is within this range, the strength of the coating film of the present invention is further increased.

Furthermore, the coating film of the present invention contains an inorganic pigment having a melting point of more than 2000°C. The inorganic pigment having a melting point of more than 2000° C. improves the heat resistance of the coating film of the present invention and further reduces the heat effect when the outer surface of the steel pipe of the present invention is welded or the like. The inorganic pigment having a melting point of more than 2000° C. preferably contains aluminum oxide or calcium oxide, and more preferably contains both aluminum oxide and calcium oxide.

In addition, when the inorganic pigment having a melting point of more than 2000° C. contains both aluminum oxide and calcium oxide, the balance between heat resistance and corrosion resistance is more favorable. is preferably 1:0.2-5, more preferably 1:0.5-2, even more preferably 1:0.7-1.5.

The content of the inorganic pigment having a melting point of over 2000° C. in the coating film of the present invention is 3 to 15% by mass, preferably 4 to 12% by mass. When the inorganic pigment contains both aluminum oxide and calcium oxide, the content of aluminum oxide and calcium oxide in the coating film of the present invention is preferably 2 to 10% by mass, and preferably 3 to 6% by mass. is more preferable. When the content of the inorganic pigment having a melting point of over 2000°C is within this range, it does not significantly affect the properties of the coating film, and when the outer surface of the steel pipe of the present invention is subjected to welding or the like, The coating film of the present invention becomes difficult to thermally decompose, suppresses generation of mist and the like, and retains its anti-corrosion properties at a high level even after welding or the like.

Furthermore, the coating film of the present invention may contain components other than those described above, for example, additives contained in the paint used to form the coating film of the present invention (inorganic pigments with a melting point of 2000 ° C. or less ( zinc chloride, etc.), molybdenum compounds (phosphomolybdic acid, etc.), carbon black, anti-settling agents (crystalline silica, etc.), leveling agents, sagging agents, etc.).
Components derived from these additives, that is, components other than zinc powder, alkylsilicate polycondensation reaction products, and inorganic pigments having a melting point of more than 2000° C. are 10% by mass or less in the coating film of the present invention. is preferable, and it is more preferably 7% by mass or less.

Here, regarding the content rate measurement of the components contained in the coating film of the present invention, the content rate of the organic component is measured by combining infrared spectroscopy and pyrolysis gas chromatography/mass spectrometry, and the content of the inorganic component is The content is measured by fluorescent X-ray analysis.

The steel pipe of the present invention has the coating film of the present invention as described above on its inner surface. Although the thickness of the coating film of the present invention is not particularly limited, it is preferably 5 to 100 μm, more preferably 10 to 40 μm, even more preferably 15 to 30 μm. In addition, the thickness of this coating film is measured using an electromagnetic film thickness meter.

Furthermore, the steel pipe of the present invention preferably has a coating layer made of polyolefin on its outer surface. And the coating layer which consists of this polyolefin is more preferable in it being a polyethylene coating layer. The coating layer made of polyolefin can be formed, for example, by coating the outer surface of the steel pipe with a polyolefin resin from an extruder through a round die or a T-die.
Alternatively, the coating film of the present invention is applied to the inner surface of a steel pipe having a coating layer made of polyolefin on the outer surface in advance, such as a steel pipe defined in JIS G3477 polyethylene-coated steel pipe "PE2S", "PE1H", "P3X-M", etc. may be formed.

The steel pipe of the present invention having such a structure exerts favorable effects particularly when used as a gas pipe (transport pipe for gases such as gas fuel, air, nitrogen, oxygen, and hydrogen). If the inner surface of a gas pipe is damaged by welding or the like on the outer surface, gas flows in from the damaged portion and corrosion easily progresses. Damage to the inner surface is less likely to occur, and therefore corrosion as described above is less likely to occur.

Next, the method for manufacturing the steel pipe of the present invention will be described in detail.

The steel pipe of the present invention is produced by coating the inner surface of the steel pipe with a paint in which zinc powder, an alkyl silicate, and an inorganic pigment having a melting point of over 2000°C are added to the solvent in the ratio described above, and then drying to separate and remove the solvent. can be manufactured by

A solvent can be used as this solvent. The solvent refers to components such as water and organic solvents that volatilize when the paint after coating is dried. Specific examples of organic solvents include ethylbenzene, xylene, butyl acetate, isobutanol, isopropanol, tert-butanol, methanol, ethanol, and methyl ethyl ketone. These solvent components may contain only one type, but may contain a plurality of types.

The content of coating film components (zinc powder, alkyl silicate, inorganic pigment having a melting point of over 2000° C., etc.) contained in the paint is preferably 30 to 85% by mass, more preferably 40 to 75% by mass. , 45 to 65% by mass.

Moreover, the method of coating the inner surface of the steel pipe with the paint as described above is not particularly limited, and a conventionally known method can be applied.
For example, it is preferable to apply an airless spray method, and in order to form a uniform coating film, it is more preferable to use a device that rotates the steel pipe in a fixed direction (for example, a turning roll) for coating.
The method of coating the inner surface of a steel pipe will be described below using the airless spray method using a turning roll as an example.

For painting, an airless sprayer with a nozzle at the tip of the arm is used. It is preferable to use a nozzle tip having a pattern angle of 20 to 60° and a tip diameter of 0.2 to 0.5 mm.
First, a steel pipe is placed on a pair of turning rolls and rotated at a constant speed around the pipe axis of the steel pipe. From one pipe end of this rotating steel pipe, the arm of the airless spray device is inserted into the cavity in the steel pipe, the arm is moved to the other pipe end, and then the tip of the nozzle is directed downward. Then, while moving the arm from the other pipe end to the one pipe end at a constant speed, the paint is sprayed onto the inner surface of the steel pipe at a pressure of 5 to 15 MPa and a discharge rate of 300 to 1500 g/min to completely coat the inner surface of the steel pipe. and apply so that the coating film is uniform. At this time, the distance (spraying distance) between the inner surface of the steel pipe and the tip of the nozzle when spraying the paint is preferably 150 to 600 mm.

After painting, the paint applied to the inner surface of the steel pipe is dried to form a coating film. Although the drying method is not particularly limited, it is preferable to cure the coated steel pipe in a well-ventilated room. Also, the curing period is preferably 2 to 7 days.

In addition, it is preferable to surface-treat the inner surface of the steel pipe before coating the inner surface of the steel pipe. This is to remove oil stains, scales, and the like adhering to the inner surface. A conventionally known method can be used for the surface treatment, and examples thereof include grit blasting and pickling.

The steel pipe of the present invention can be obtained by such a manufacturing method.

The steel pipe of the present invention thus obtained is characterized in that even if the outer surface is melt-cut or welded, it is difficult for mist or the like to be generated from the inner surface. In particular, even if fusion cutting or welding with a high heat input is performed, it is difficult to generate mist or the like.

Arc welding is preferable as a welding method applicable to the steel pipe of the present invention. Examples of arc welding include shielded arc welding, gas shielded arc welding (MIG welding, MAG welding), and TIG welding.

Examples of the present invention will be described below, but the present invention is not limited to the following examples, and various modifications are possible within the technical concept of the present invention.

Eighteen types of paint containing zinc powder (average particle size 4 μm), alkyl silicate (ethyl silicate), inorganic pigment, etc. at different contents were prepared as coating film components (components forming the coating film). Then, each of them was applied to the inner surface of the steel pipe by the method described later to form a coating film, and the inner surface coating according to Examples 1 to 15 and Comparative Examples 1 to 3 having the coating film having the composition shown in Table 1 below on the inner surface Got a steel pipe.
These paints used a mixture of two or more organic solvents such as xylene, ethylbenzene, butyl acetate, isobutanol, isopropanol, ethanol, methanol, methyl ethyl ketone, ethylbenzene and tert-butanol. Also, the coating film component in the paint was 50 to 55% by mass.
Other additives in these paints and coating films are crystalline silica, carbon black, a molybdenum compound (phosphomolybdic acid), and zinc chloride.

The method of coating the inner surface coated steel pipes according to Examples 1 to 15 and Comparative Examples 1 to 3 was carried out as follows.
First, a steel pipe having a polyethylene coating layer on the outer surface was prepared using a steel pipe for piping (manufactured by Nippon Steel Corporation, SGP200A×1 m) specified in the JIS G 3452 standard.
Next, steel grit (particle size: 0.7 mm) was sprayed onto the inner surface of the steel pipe at a pressure of about 0.5 MPa to perform grit blasting. By this treatment, grease, dirt, scale, etc. adhering to the inner surface of the steel pipe were removed.

Next, the inner surface of the steel pipe was coated by the airless spray method using the turning roll described above. As the coating device, an airless spray device (manufactured by Asahi Sunac Co., Ltd., model: Ecoponzinc) having a nozzle at the tip of an arm was used. A nozzle tip with a pattern angle of 30° and a tip diameter of 0.3 mm was used. The conditions of the coating method are shown below.
Rotational speed of turning roll: 140 m/min
Airless spray pressure: 11 MPa
Airless spray discharge rate: 600g/min
Spraying distance of airless spray: 200mm

After the coating as described above, the steel pipe was aged for 7 days in a well-ventilated room. Then, the inner coated steel pipes of Examples 1 to 15 and Comparative Examples 1 to 3 having inner coating thicknesses of 15 to 30 μm thus obtained were evaluated by the following method.

<Weld bead forming method>
A weld bead was formed on the outer surface of each of the internally coated steel pipes of Examples 1 to 15 and Comparative Examples 1 to 3 obtained as described above. A method for forming this weld bead will be described with reference to FIG.

First, the inner surface of the steel pipe 1 was preheated to a temperature not exceeding 95° C. by a jet heater, and a cutting line penetrating the polyethylene coating layer on the outer surface was cut with a cutter. Then, the polyethylene coating layer was peeled off with a chisel, a jet chisel, and a metal spatula, and residual resin on the steel surface was removed with a belt sander and a wire brush.
Next, the welding rod 7 is directed to the center of the outer surface of the steel pipe 1 in the longitudinal direction, and the welding machine 5 is operated to perform covered arc welding (manual welding) at a welding speed of 0.06 m/min. A weld bead 9 having a diameter (outer diameter) of 60.5 mm and a width of 15 mm was formed. The welding current at this time was 120A. In addition, the welding speed was controlled by adjusting the time required for forming the circular weld bead 9 once by using a stopwatch.
As the welding rod 7, a welding rod (LB-52U manufactured by Kobe Steel, Ltd., rod diameter 3.2 mm, rod length 400 mm) corresponding to the low-hydrogen welding rod E4316 specified in JIS Z 3211 was used. . Also, before forming the weld bead, the welding rod 7 was heated at 350° C. for 60 minutes in order to re-dry the coating.

Then, the following method was used to evaluate the effect of the welding process by forming the weld bead described above on the coating film on the inner surface of the steel pipe.

<Confirmation test for mist during welding>
When the weld bead was formed, the inside of the steel pipe was visually observed from the pipe end of the steel pipe, and the presence or absence of white or light yellow smoke (mist) was evaluated as "yes" or "no".

In addition, the presence or absence of dust-like solid matter or thread-like solid matter generated by thermal decomposition of the coating film was evaluated by the following method. Here, the dust-like solids refer to fine dust-like thermal decomposition products splashed from the coating film by heating. Further, the filamentous solid refers to a thermally decomposed product in which coating film components are melted by heating, drooping, and solidified into a filament. The presence or absence of the dust-like solids and thread-like solids was visually checked during formation of the weld bead.

Furthermore, when these solids were produced, they were carefully scraped off from the inner surface of the steel pipe with a scoop, collected on a piece of paper, and weighed using a precision balance. Based on this result, if the mass of these solids is 0.03 g or less, it is evaluated as "○", if it is more than 0.03 g and less than 0.13 g, it is evaluated as "△", and if it is 0.13 g or more, it is evaluated as "×". did. Then, the total mass (g) of dust-like solids and thread-like solids was also calculated.
These results are shown in Table 1 below.

<Outdoor exposure test>
The inner coated steel pipes of Examples 1 to 15 and Comparative Examples 1 to 3 with weld beads formed were exposed to the outdoors, and the appearance of the weld bead backside coating on the inner surface of the steel pipe after 30 days, 90 days, 180 days, and 365 days. was visually confirmed, and the case where no rust was generated was evaluated as ◯, and the case where rust was generated was evaluated as ×.
The results are also shown in Table 1 below.

From the above results, the inner surface coated steel pipes of Examples 1 to 15 having, on the inner surface, a coating film containing zinc powder, an alkylsilicate polycondensation reaction product, and aluminum oxide and/or calcium oxide, which are inorganic pigments having a melting point of over 2000°C. (Steel pipe of the present invention) is an extremely excellent inner coated steel pipe that does not generate mist from the inner surface even when the outer surface is welded with a high heat input, and maintains a high degree of corrosion resistance. .

In particular, the inner surface-coated steel pipes of Examples 1 to 7, which have a coating film containing both aluminum oxide and calcium oxide as inorganic pigments with a melting point of over 2000 ° C. on the inner surface, even if welding with a high heat input is performed on the outer surface. It was found that the inner coated steel pipe produced no mist, produced less dust-like solids and thread-like solids, and maintained a high degree of corrosion resistance.

On the other hand, the inner surface-coated steel pipe of Comparative Example 1, which has a coating film containing zinc dust and an alkylsilicate polycondensation reaction product on the inner surface but does not contain an inorganic pigment, has a considerably reduced corrosion resistance and generates mist due to welding. It was an internally coated steel pipe that generated a very large amount of dust-like solids and thread-like solids.
In addition, the inner surface coated steel pipes of Comparative Examples 2 and 3, which have a coating film containing zinc dust, an alkylsilicate polycondensation reaction product, and an inorganic pigment having a melting point of less than 2000 ° C. on the inner surface, have a reduced corrosion resistance and In the steel pipe with inner surface coating, dust-like solids and thread-like solids were generated a little more by welding.

1 inner coated steel pipe 5 welding machine 7 welding rod 9 welding bead

Claims (3)

A coating film containing 35 to 75% by mass of zinc dust, 10 to 40% by mass of an alkyl silicate polycondensation reaction product, and 3 to 15% by mass of an inorganic pigment having a melting point of over 2000 ° C. on the inner surface ,
the inorganic pigment comprises aluminum oxide and calcium oxide;
The mass ratio of the aluminum oxide and the calcium oxide in the inorganic pigment is 1:0.2 to 5.
Internally coated steel pipe. The inner surface coated steel pipe according to claim 1 , having a coating layer made of polyolefin on the outer surface. The internally coated steel pipe according to claim 1 or 2 , which is used as a gas pipe.

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