JP2016175315A - Coated steel pipe for gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a coated steel pipe for gas that can be welded favorably without removing an inner surface coating film, can suppress mist generation from welding heat, and has excellent corrosion resistance.SOLUTION: A coated steel pipe for gas has, on its inner surface, a cured coating film of a paint comprising alkyl silicate and/or modified alkyl silicate 5-50 mass%, colloidal silica 3-40 mass%, and zinc dust 30-70 mass% in terms of paint solid content.SELECTED DRAWING: None

Description

  The present invention relates to a gas-coated steel pipe used for a gas conduit or the like.

  BACKGROUND ART Coated steel pipes for gas in which the outer surface of the steel pipe is coated with polyethylene or painted are widely used as steel pipes for gas conduits such as city gas.

  As for the inner surface of the steel pipe, there are known a non-painted case and a case coated with an epoxy resin paint or the like.

  In the case of an unpainted case on the inner surface, rust is likely to occur from the inner surface of the steel pipe during the storage period after the production of the steel pipe and before joining on site. Moreover, if rust peels after joining steel pipes, it may clog piping members such as filters and solenoid valves, causing malfunction. That is, it may be necessary to remove rust after joining the steel pipes.

  As a countermeasure for such a problem, there is a case where the inner surface of the steel pipe is coated. The purpose of the inner surface coating is to prevent rust during the period from the production of the gas-coated steel pipe to the completion of the gas conduit by joining the steel pipes on site.

  When welding a branch pipe or joint to an existing steel pipe with an inner surface coating film, it is difficult to remove the inner surface coating film before welding, so welding is usually performed with the inner surface coating film. . In that case, the pyrolyzate (mist) of the inner surface coating film generated by the heat of welding may clog the piping members such as filters and solenoid valves, causing malfunction. As a countermeasure against this mist, the technique of Patent Document 1 is disclosed. Patent Document 1 is a polyethylene-coated steel pipe having a polyethylene resin coating layer on the outer surface, and the polyethylene-coated steel pipe has a coating film on the inner surface, and the coating film is alkylated with respect to 100 parts by weight in the solid content of the paint. Disclosed is a polyethylene-coated steel pipe excellent in weldability, which is a cured coating film of a paint containing silicate in an amount of 80 parts by weight to 95 parts by weight.

  On the other hand, when joining steel pipes, such as when laying a gas conduit, the outer surface coating and the inner surface coating film at the end of the pipe are mostly partially peeled off and welded together in most cases. In addition to the purpose of preventing the generation of mist, this also has the purpose of preventing various welding defects such as pits and blowholes caused by the inner surface coating film during welding. However, peeling off the inner surface coating film is an extremely complicated operation and has a heavy work load, and a solution to this problem is desired.

JP 2013-173340 A

  There is a demand to suppress mist generated from the inner surface coating film at the time of welding and to make welding possible without removing the inner surface coating film when joining steel pipes, and to reduce the load on construction.

  On the other hand, there is also a demand to reduce the load such as rust removal measures due to corrosion while retaining the primary rust prevention function on the inner surface until the pipe construction. However, no coated steel pipe for gas that has good weldability, good suppression of mist generation, and good corrosion resistance has been conventionally provided.

  Therefore, it is an object to be solved to provide a gas-coated steel pipe that can be welded satisfactorily without removing the inner surface coating film, can suppress generation of mist due to welding heat, and has good corrosion resistance.

  As a result of intensive studies, the present inventors have come to obtain knowledge that the solid content composition of the paint to be coated on the inner surface of the steel pipe is important. Furthermore, the above problems can be solved by adjusting the content of alkyl silicate and / or modified alkyl silicate, colloidal silica, and zinc dust in the paint to an appropriate range and forming a cured coating film of the paint on the inner surface of the steel pipe. I got the knowledge.

  The present invention has been completed based on the above findings. The gist of the present invention is as follows.

  [1] Gas having, on the inner surface, a cured coating film of a paint containing 5 to 50% by mass of alkyl silicate and / or 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 paint Coated steel pipe.

  [2] The gas-coated steel pipe according to [1], wherein the colloidal silica is aqueous colloidal silica.

  [3] The gas-coated steel pipe according to [1] or [2], wherein the modified alkyl silicate is an initial condensate of an alkyl silicate.

  The gas-coated steel pipe of the present invention can be welded satisfactorily without removing the inner surface coating film, can suppress generation of mist due to welding heat, and has good corrosion resistance.

  The gas-coated steel pipe for gas according to the present invention has a primary rust preventive action until laying of a gas conduit, and suppresses the generation of mist from the cured coating film when welding joints or the like between steel pipes or steel pipes. Moreover, since the gas-coated steel pipe of the present invention can be welded satisfactorily, additional work such as removal of a cured coating film is not required when welding.

  Hereinafter, the present invention will be described in detail. In addition, this invention is not limited to the following embodiment.

(paint)
The coating material used in the present invention contains alkyl silicate and / or modified alkyl silicate, colloidal silica, and zinc powder in a specific ratio. The paint is applied to the inner surface of the steel pipe and cured to obtain a cured coating film (inner surface coating film).

  The means for curing the paint is not particularly limited. For example, it can be cured and formed into a film by contact with moisture in the air. Moreover, you may heat in a drying furnace after film forming.

  In the present invention, the solid content of the paint, except for a solvent such as 2-propanol (isopropanol) and 1-butanol, which volatilizes and diffuses into the atmosphere when a cured coating film is formed, The component for comprising a cured coating film is meant.

Alkyl silicates and modified alkyl silicates containing 5 to 50% by mass of alkyl silicate and / or modified alkyl silicate as paint solids mainly contribute to the suppression of mist generation due to welding heat and the formation of a cured coating film. Alkyl silicate is preferred. When the content is less than 5% by mass, the curability of the coating film and the coating film strength are lowered. Moreover, a cured coating film may not be formed depending on circumstances, and good corrosion resistance cannot be obtained unless a cured coating film is formed. If the content exceeds 50% by mass, welding defects (pits, blowholes, etc.) are likely to occur during welding. Moreover, good corrosion resistance cannot be obtained due to a decrease in the zinc content. From the above, the content of alkyl silicate and / or modified alkyl silicate contained in the paint is 5 to 50% by mass. The content is preferably 10 to 45% by mass.

  The number of carbon atoms of the alkyl group of the alkyl silicate is not particularly limited. The carbon number is preferably 1 to 3. The smaller the carbon number of the alkyl group, the faster the hydrolysis reaction and the faster the film forming speed. More preferably, the carbon number is 2 (ethyl group). An ethyl group is more preferable from the viewpoint of easy handling as a paint (having an appropriate drying / film-forming speed), durability improvement of the coating film, and corrosion resistance. In the case of a methyl group, the reaction is too fast, whereas in the case of an alkyl group having a molecular weight larger than that of a propyl group, the reaction rate is slow and handling may be inferior.

  The modified alkyl silicate is a compound in which the properties of the alkyl silicate are changed by causing a reaction such as a condensation reaction with other alkyl silicates or other compounds. As the modified alkyl silicate, preferably, the alkyl silicate is modified as an initial condensate by partial condensation. Modified alkyl silicates are preferably used because they have better drying properties than alkyl silicates.

Colloidal silica containing 3 to 40% by mass of colloidal silica as a coating solid content mainly contributes to reduction of welding defects during welding. If the content is less than 3% by mass, welding defects increase. When content exceeds 40 mass%, the sclerosis | hardenability of a coating material and the cured coating film strength will fall, and corrosion resistance will fall. As mentioned above, content of colloidal silica contained in a coating material shall be 3-40 mass%. The content is preferably 5 to 30% by mass.

  Examples of colloidal silica include aqueous (water-dispersed) colloidal silica and solvent-based colloidal silica. From the viewpoint of reducing welding defects, aqueous colloidal silica is preferred. In order to improve the dispersibility in the coating material, even the aqueous colloidal silica is preferably acidic aqueous colloidal silica dispersed in an acidic aqueous solution.

  The particle size of colloidal silica is not particularly limited. In order to improve the dispersibility in the paint, the particle size is preferably 4 to 100 nm, and more preferably 10 to 30 nm. In the present invention, the particle size of colloidal silica is determined by a method such as a specific surface area measurement value (JIS Z8830) by a BET adsorption method.

Zinc powder containing 30 to 70% by mass of zinc powder as a coating solid content is powdery zinc. Zinc dust is dispersed in a solvent in the paint. Zinc powder mainly contributes to improving the corrosion resistance of the coating film. When content is less than 30 mass%, the corrosion resistance of a cured coating film will be inferior. If the content exceeds 70%, welding defects such as blow holes and pits are likely to occur unless the inner surface coating of the end face is peeled off. As mentioned above, content of the zinc dust contained in a coating material shall be 30-70 mass%. The content is preferably 40 to 60% by mass.

Optional Components In the present invention, the coating solid content includes alkyl silicate and / or modified alkyl silicate, colloidal silica, and zinc dust. In addition to these components, as optional components (other components), various paint additives and pigments such as dispersants, leveling agents, and anti-sagging agents for improving cured coating film performance are also included in the paint as paint solids. May be included. In addition, inorganic powders such as rutile powder, synthetic rutile powder, chromium powder and zircon powder can be used as optional components. These powders are preferably anhydrous having no water of crystallization to prevent welding defects.

  The solvent for the paint is not particularly limited. For example, water, paint solvents (2-propanol, 1-butanol, xylene, ethylbenzene, methanol, etc. can be used.

  The concentration of the solid content of the paint in the paint can be set as appropriate. If a preferable example is given dare, the amount of paint solids in a paint is 40-90 mass%.

  The film thickness of the cured coating film formed using a paint is not particularly limited. The thickness of the cured coating film is preferably 10 to 30 μm from the viewpoint of corrosion resistance and coating film durability.

(Steel pipe)
A known steel pipe that can be used for transporting a gas such as a gas conduit can be appropriately used in the present invention. For example, polyethylene-coated steel pipes for gas, particularly polyethylene-coated steel pipes defined in JIS G3469, JIS G3477-1, 3477-2, and 3477-3, and gas-coated pipes that have been subjected to room temperature drying paint or baking coating on the outer surface Etc. can be used. Moreover, JISG3452 (SGP) and JISG3454 (STPG) can also be used.

  In order to perform welding joint, it is preferable that both pipe ends of the steel pipe are beveled in advance.

(Preferred production method)
Hereinafter, the preferable manufacturing method of the coated steel pipe for gas of this invention is demonstrated.

Substrate adjustment step This is a step of removing rust, dirt, and mill scale on the inner and outer surfaces of the steel pipe by performing pickling, blasting (shot blasting, grit blasting, etc.) on the surface of the steel pipe.

Inner surface painting process The paint which has a component composition of this invention is painted on the inner surface of a steel pipe. The coating method is not particularly limited. For example, it is preferable to paint the inner surface of the steel pipe by airless spray. In this case, the painting method is to insert an arm with a nozzle at the tip inside the steel pipe, spray the paint from the nozzle tip while rotating the steel pipe, and paint the entire length of the steel pipe while gradually pulling out the arm. It is common. The drawing speed is adjusted in advance according to the outer diameter of the steel pipe to obtain an appropriate film thickness. The film thickness can be measured with an electromagnetic film thickness meter. After coating, the paint hardens and forms a film by contact with moisture in the air. You may heat in a drying furnace after film forming.

Chemical conversion treatment such as chromate treatment, non-chromate treatment, phosphate treatment, and phosphoric acid treatment is performed on the outer surface of the steel pipe as required.

  In the case of a polyethylene-coated steel pipe, undercoating with an asphalt-based adhesive material, primer coating with an epoxy resin adhesive, powder primer coating with a powder epoxy resin, or the like is performed. Thereafter, the steel pipe surface is coated with polyethylene from the extruder through a round die or a T die. In adhesive-type exterior coating, coating may be performed by coextrusion to form a modified polyethylene layer between the polyethylene and the primer. In addition, for P2S type outer surface coating, etc., after coating with anticorrosion polyethylene, polyethylene as a protective layer is further coated thereon.

  When manufacturing a coated tube, a known outer surface coating such as airless spraying is performed after the substrate preparation step and the inner surface coating step. The types of paint are various primary rust prevention treatments (wash primer, inorganic zinc rich primer, organic zinc rich primer), various paints (synthetic resin blend paint, rust preventive paint, phthalic acid resin paint, vinyl chloride resin paint, acrylic resin paint, General-purpose paints such as epoxy resin paint, urethane resin paint, synthetic resin emulsion) are used.

  Examples of the present invention will be described below. The technical scope of the present invention is not limited to the following examples.

  A paint having the composition of each invention example and comparative example shown in Table 1 “Coating for inner surface coating” (excluding Comparative Example 12; Comparative Example 12 was not subjected to inner surface coating described later) was prepared. The particle diameter of colloidal silica was 10 to 30 nm. As the aqueous colloidal silica, acidic aqueous colloidal silica was used. As the modified alkyl silicate, an alkyl silicate precondensate was used. As the solvent, 2-propanol and 1-butanol were used, and the solid content of the paint in the paint was 60% by mass.

  After pickling and blasting the inner and outer surfaces of a steel pipe (JIS G3452), insert an arm with a nozzle at the tip inside the steel pipe, spray the paint from the nozzle tip while rotating the steel pipe, and gradually pull out the arm. The paint was applied to the entire length of the steel pipe. Furthermore, the paint was hardened by rotating the steel pipe for a certain period of time and then allowing it to stand. The film thickness of the cured coating film formed on the inner surface of the steel pipe was in the range of 10 to 25 μm.

  The following test was performed on the coated steel pipe for gas produced by the above process, and the results are shown in Table 1.

(Welding test)
The ends of the steel pipes were welded without peeling off the cured coating film (inner surface coating film), and the presence or absence of welding defects was determined by a radiation transmission test. The welding method was TIG welding.

  Judgment was made according to JIS Z3104, and according to the classification of scratches, Class 1 was ◎, Class 2 was ◯, Class 3 was △ 4 was x, Circle was pass, and ◎ was better. The results are shown in Table 1 “Welding Quality”.

  In addition, even when the arc welding was replaced with the TIG welding, almost the same result was obtained.

(Mist generation suppression test)
The steel pipe ends are welded together by TIG welding without removing the cured coating film (inner surface coating film), and the mist generated from the inner surface coating film is collected, and the amount of mist is measured with a dust meter (digital dust meter LD-5D). Measured with The measurement conditions were that a fan was set on the inner surface of the steel pipe, the generated mist was introduced into a 1 m ³ box-shaped chamber, and the mist in the chamber was measured with a dust meter.

Judgment was evaluated by increasing or decreasing the count number of the dust meter before and after welding, and the evaluation criteria were as follows. And ◯ was passed and ◎ was better. The results are shown in Table 1 “Mist test”.
◎: Increase in count after welding is 5 or less ○: Increase in count after welding is 6 or more and 10 or less Δ: Increase in count after welding is 11 or more and 100 or less ×: Increase in count after welding is 101 or more (corrosion resistance test)
The manufactured gas-coated steel pipe was visually inspected for rust condition when exposed to the outdoors for one year. The evaluation criteria were as follows. And ◯ was passed and ◎ was better. The results are shown in Table 1 “Corrosion resistance”.
◎ There is no rust on the inner surface of the tube including the inner surface end of the tube ○ There are some dotted rust on the inner surface of the tube, but there is no rust on other parts of the inner surface of the tube △ Red rust is generated on the inner surface of the tube × Inside the tube There is generation of rust that peels off in layered or granular form at the edge of the surface

  In Table 1, a remarks column is provided. Although zinc fume is different from the mist to be suppressed in the present invention, zinc fume itself may have toxicity such as fume heat, and is preferably less.

Claims (3)

  Gas coating with a cured coating film of paint containing 5 to 50% by weight of alkyl silicate and / or modified alkyl silicate as solid content of paint, 3 to 40% by weight of colloidal silica, and 30 to 70% by weight of zinc dust on the inner surface Steel pipe.   The gas-coated steel pipe according to claim 1, wherein the colloidal silica is aqueous colloidal silica.   The coated steel pipe for gas according to claim 1 or 2, wherein the modified alkyl silicate is an initial condensate of an alkyl silicate.