JP3405621B2 - Polyethylene coated steel pipe - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material in which a steel pipe is coated with polyethylene on its surface, and more particularly, to both excellent impact resistance at extremely low temperatures and excellent scratch resistance at high temperatures. The present invention relates to a polyethylene-coated steel pipe having excellent corrosion resistance. [0002] As a line pipe for petroleum or natural gas, a polyethylene-coated steel pipe coated with polyethylene and protected from corrosion is sometimes used. [0003] As a polyethylene-coated steel pipe, a steel pipe obtained by laminating polyethylene via a modified polyethylene on the outer surface of a steel pipe subjected to a base treatment is frequently used. [0004] In recent years, energy development in the polar regions has progressed,
The laying of pipelines has been started at cryogenic temperatures of 60 ° C. or lower. The coating may be damaged due to contact between the pipe and heavy equipment during construction and collision between the pipes. [0005] At such extremely low temperatures, it is important to improve the low-temperature impact resistance of the polyethylene coating. In addition, the crude oil to be collected has gradually become heavy oil,
There may be a need for a pipeline that transports at high temperatures of ° C. [0006] Pipelines are often used buried in soil, and when transporting a high-temperature fluid, the polyethylene coating on the outer surface is softened, which may cause penetration damage due to pebbles or the like. For polyethylene coated steel pipes used at such high temperatures, it is necessary to improve the flaw resistance at high temperatures. [0007] Therefore, in a pipeline that is operated at a high temperature in an extremely cold region, it is important to ensure the flaw resistance at a high temperature in addition to the low temperature impact resistance of the polyethylene coating. In general, physical properties of polyethylene vary depending on the density. Low-density polyethylene with low density is soft and has good brittleness resistance at low temperatures, but has poor scratch resistance at high temperatures. High-density polyethylene with high density is hard,
The brittleness resistance at low temperatures is poor, but the flaw resistance at high temperatures is good. Conventionally, these polyethylenes having different densities and physical properties have been selected depending on the application, and the surface of a steel material subjected to a base treatment has been coated with a single layer via a modified polyethylene layer. For example, when low-density polyethylene is used for cold regions, the impact resistance up to -45 ° C. is excellent. However, -6
The impact at an extremely low temperature of 0 ° C. or less cracks the coating, resulting in insufficient impact resistance. Moreover, the scratch resistance at 80 ° C. is poor. When a steel pipe coated with high-density polyethylene alone is used for a pipeline for transporting crude oil at a high temperature, the flaw resistance up to 80 ° C. is excellent. However, if an impact is applied at a low temperature of -60 ° C or lower, the coating is cracked and the impact resistance is insufficient. In order to improve the above-mentioned problems, for example, in the case of coating the outer surface of a steel pipe, as disclosed in JP-B-59-49179, a low-density polyethylene is coated on the lower surface of the steel pipe via a modified polyethylene layer. By laminating and coating high-density polyethylene on the upper layer, impact resistance during construction and flaw resistance during embedding are improved, and impact resistance at low temperatures up to -45 ° C is improved. [0014] However, simply applying a low-density polyethylene and a high-density polyethylene in a layered manner provided insufficient impact resistance at an extremely low temperature of -60 ° C. [0015] That is, in the prior art, it was difficult to realize an inexpensive polyethylene-coated steel pipe excellent in both impact resistance at -60 ° C and scratch resistance at 80 ° C. In order to solve the above-mentioned problems, the present inventors have made a study on the constitution of the upper and lower plastic layers of the laminated coating disclosed in Japanese Patent Publication No. 59-49179. We studied diligently. As a result, as shown in FIG. 1, a composition comprising a linear low-density polyethylene, a linear low-density polyethylene and a low-density polyethylene, was provided on the surface of a steel material subjected to a base treatment via a modified polyethylene layer. Alternatively, only when a colored high-density polyethylene is laminated and coated on a composition comprising a linear low-density polyethylene and a high-density polyethylene, the impact resistance at an extremely low temperature of −60 ° C. and the indentation resistance at 80 ° C. The present inventors have found that a polyethylene-coated steel pipe excellent in corrosion resistance, inexpensive, and excellent in anticorrosion properties can be obtained, and have reached the present invention. Hereinafter, the present invention will be described. The steel pipe used in the present invention is a steel pipe made of alloy steel such as carbon steel or stainless steel. In addition, stainless steel, titanium, aluminum, nickel,
A clad steel material in which a metal such as copper or an alloy steel such as a nickel-chromium-molybdenum alloy or a nickel-chromium-molybdenum-tungsten alloy is laminated can also be used. The inner surface, outer surface or both surfaces of the carbon steel pipe may be plated with a metal such as zinc, aluminum, or chromium.
It is also possible to use alloy plating of aluminum, zinc-nickel, zinc-nickel-chromium, or the like, or a plated steel pipe obtained by subjecting these metal plating or alloy plating to dispersion plating in which a dispersant such as silica or titanium oxide is dispersed. Before the steel pipe used in the present invention is coated with a modified polyethylene layer, as shown in FIG. 1, if a base treatment comprising a chromate coating (2) and an epoxy primer coating (3) is performed, a larger size is obtained. Adhesive strength and more excellent anticorrosion properties are obtained, which is preferable. First, a chromate treating agent is applied on a surface of a steel material from which scale has been removed by blasting, pickling, degreasing, etc., with a roll, a brush, or the like, and heated and baked to form a chromate film. The chromate treating agent is, for example, an aqueous solution of chromic anhydride to which an organic reducing agent or the like is added and heated to partially reduce the hexavalent chromium in the aqueous solution to trivalent chromium. A reduced aqueous solution obtained by adding an organic reducing agent or the like to a mixture obtained by adding and dispersing or a mixed aqueous solution of chromic anhydride and phosphoric acid and heating to partially reduce hexavalent chromium in the aqueous solution to trivalent chromium, A mixture in which silica fine particles are added and dispersed is used. An epoxy primer coating (3) is formed by coating epoxy on a chromate coating (2) on the surface of a steel pipe (1).
A primer composed of a mixture of a curing agent and a pigment is applied by a method such as spray coating or ironing, and is cured by heating. As the epoxy, a diglycidyl ether of bisphenol A or bisphenol F alone or a mixture is preferable. As the curing agent, alicyclic amines, aliphatic amines, dicyandiamide, modified imidazole and the like are used. As the pigment, an inorganic pigment such as silica or titanium oxide is used. Good results are obtained when the chromate film has a thickness of ²⁰ mg / m ² to 1000 mg / m ^{2 in} terms of the total amount of chromium deposited after heating and baking. If the amount is less than 20 mg / m ² , the corrosion resistance is not sufficient, and if it exceeds 1000 mg / m ² , the adhesion between the steel material and the epoxy primer decreases. The epoxy primer layer has a thickness of 10 μm to 200 μm.
Good results are obtained with a thickness of μm. If it is less than 10 μm, the corrosion resistance is insufficient, and if it exceeds 200 μm, the low-temperature impact resistance is reduced. The modified polyethylene (4) includes ethylene homopolymer or ethylene and 1-butene, propylene,
Adhesion to steel when using ethylene-α-olefin copolymer alone or α-olefin copolymer such as 1-hexene or 1-octene, or maleic anhydride-modified polyethylene obtained by grafting maleic anhydride to a mixture of these. Excellent in properties and preferred. With respect to the modification of maleic anhydride, the addition amount of maleic anhydride to 1 g of polyethylene, ethylene-α-olefin copolymer, or a mixture thereof is 1%.
It should be in the range of × 10 ^-6 to 1 × 10 ^-4 mol. When the added amount is less than 1 × 10 ^-6 mol and 1 × 1
If it exceeds ^0-4 mol, the adhesion between the colorless polyethylene layer and the epoxy primer layer will be poor, and the impact resistance will be reduced. In the modified polyethylene (4), even in the case of a composition comprising the above-mentioned maleic anhydride-modified polyethylene and unmodified polyethylene, the content of maleic anhydride per 1 g of the composition is from 1 × 10 ⁻⁶ to 1 × 10 ⁻⁶ . It can be used in a range of 1 × 10 ^-4 mol. The modified polyethylene layer has a thickness of 80 to 400.
Good results are obtained with a thickness of μm. If it is less than 80 μm, the adhesive strength is insufficient. The colorless polyethylene (5) of the present invention includes a linear low-density polyethylene, a composition comprising linear low-density polyethylene and low-density polyethylene, or a composition comprising linear low-density polyethylene and high-density polyethylene. In the case of a composition, a composition containing 30% by weight or more of linear low-density polyethylene can be used. When the colorless polyethylene (5) is only a low-density polyethylene other than a linear low-density polyethylene, only a high-density polyethylene, and a composition having a linear low-density polyethylene content of less than 30% by weight, Cracks occur in the coating upon impact at -60 ° C. Good results can be obtained when the thickness of the colorless polyethylene layer is 0.5 mm or more. When the thickness is 0.5 mm or less, the coating is cracked by the impact of -60 ° C. The linear low-density polyethylene referred to herein means ethylene and an α-olefin such as 1-butene, propylene, 1-hexene or 1-octene in the presence of a chromium or Ziegler catalyst. It is a low-density polyethylene that can be produced by copolymerization at a medium to low pressure by a gas phase method or a liquid phase method. The colored polyethylene (6) of the present invention includes a high-density polyethylene having a density in the range of 0.940 to 0.960 g / cm ³ and a pigment in the range of 0.1 to 7% by weight, or high-density polyethylene. The composition of high-density polyethylene and linear low-density polyethylene also has a density of 0.940 to 0.960 g /
It can be used if it is in the range of cm ³ . Pigments to be mixed with the colored polyethylene include carbon black (black), phthalocyanine green (green), phthalocyanine blue (blue), isoindolinone yellow (yellow), quinacrine red (red), perinone red (red) and the like. Organic pigments, inorganic pigments such as titanium oxide (white), chromium oxide (green), iron oxide (red, yellow), titanium yellow (yellow), and cobalt oxide (blue, purple), and the like. A mixture of is available. The kneading amount of these pigments is adjusted to be in the range of 0.1 to 7% by weight in polyethylene. -60 ° C when the mixing amount of the pigment is less than 0.1% by weight or more than 7% by weight
Cracks occur in the coating due to the impact of. The thickness of the colored polyethylene layer is 0.2
Good results are obtained when the thickness is in the range of ３3 mm. If it is less than 0.2 mm, the flaw resistance at 80 ° C. is not sufficient, and if it exceeds 3 mm, the coating is cracked by an impact of −60 ° C. The colored polyethylene (6) of the present invention may contain, in addition to the pigment, an antioxidant, an ultraviolet absorber, a flame retardant, an antistatic agent and the like according to the intended use. Hereinafter, the present invention will be described in detail with reference to examples. EXAMPLE A steel pipe (SGP200A × 5500 mm length ×
The outer surface of 5.8 mm thick is grid blasted to remove rust, and a chromate treatment agent (3 to all chromium in aqueous solution)
The weight ratio of valent chromium is 0.4, the weight ratio of silica is 2.0,
A phosphoric acid (weight ratio of 1.0) was applied with a brush and dried. The total amount of chromium deposited on the chromate film was 550.
mg / m ² . An epoxy primer (a mixture of 100 parts by weight of diglycidyl ether of bisphenol A, 3 parts by weight of particulate silica and 50 parts by weight of an alicyclic amine-based curing agent) is spray-coated on the outer surface of the chromated steel pipe, and the steel pipe is subjected to high-frequency induction heating. And cured so that the surface temperature becomes 220 ° C. The thickness of the epoxy primer coating is 50 μm
Met. On the surface thereof, two layers of a modified polyethylene (a modified polyethylene obtained by modifying a homopolymer of ethylene with maleic anhydride and the amount of maleic anhydride added to 1 g of the modified polyethylene is 1 × 10 ^-5 mol) and a colorless polyethylene are combined in a T-die. Extrusion coated. The thickness of the modified polyethylene is 250 μm,
The thickness of the colorless polyethylene was 1.5 mm. Next, the surface was extruded and coated with colored polyethylene from another T-die and cooled to produce a polyethylene-coated steel pipe. The thickness of the colored polyethylene was 1.0 mm. The colorless polyethylenes having different compositions and densities were compared using A to C in Table 1. The density was measured according to ASTM D 1505. As the types of the colored polyethylene, a and b in Table 2 were used, and those having different densities were compared. Table 3 shows the components of the colorless polyethylene and the colored polyethylene. These coated steel pipes were subjected to an impact test in accordance with the provisions of ASTM G14, and the occurrence of coating cracks was observed. The impact test was performed at -60C. Further, the penetration depth of these coated steel pipes was measured at 80 ° C. according to DIN 30670. As a comparison, a polyethylene-coated steel pipe coated with a colorless polyethylene and a colored polyethylene whose composition and density are out of the range of the present invention was produced, and the same impact test and measurement of the indentation depth were performed. The results are shown in Table 4. From Table 4, it can be seen that a linear low-density polyethylene, a composition comprising a linear low-density polyethylene and a low-density polyethylene or a linear A colored polyethylene having a density in the range of 0.935 to 0.960 g / cm ³ and a pigment in the range of 0.1 to 7% by weight is coated on the composition comprising the low-density polyethylene and the high-density polyethylene in the form of a pigment. Only in the case of laminating coating, the coating does not crack due to the impact of -60 ° C, and the indentation depth at 80 ° C is reduced. [Table 1] [Table 2] [Table 3] [Table 4] As is clear from the examples, the polyethylene-coated steel pipe of the present invention exhibits excellent impact resistance even at an extremely low temperature of −60 ° C. when used for laying pipelines. Further, even when the pipeline is operated at a high temperature, it has excellent flaw resistance, so that it exhibits excellent anticorrosion which has never been achieved before.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a partial cross section of a polyethylene-coated steel pipe of the present invention. [Description of Signs] 1 Steel pipe 2 Chromate coating 3 Primer coating 4 Modified polyethylene layer 5 From linear low-density polyethylene, linear low-density polyethylene containing 30% by weight or more of linear low-density polyethylene, and low-density polyethylene Or a composition comprising a linear low-density polyethylene containing at least 30% by weight of a linear low-density polyethylene and a high-density polyethylene, having a density in the range of 0.935 to 0.960 g / cm ³ and a pigment. Colored polyethylene containing 0.1 to 7% by weight

Continuing from the front page (72) Inventor Nobuyoshi Yoshizaki 1 Kimitsu, Kimitsu City Nippon Steel Corporation Kimitsu Works (72) Inventor Hiroyuki Mimura 1 Kimitsu City Kimitsu City Nippon Steel Corporation Kimitsu Works (56 References JP-A-59-49179 (JP, A) JP-A-6-340026 (JP, A) JP-A-6-99544 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB Name) B32B 1/00-35/00

Claims (1)

(57) [Claims] [Claim 1] A modified polish is applied to the surface of a steel pipe which has been subjected to a base treatment.
Through the ethylene layer, the density and the colorless polyethylene
0.935 to 0.960 g / cm ³ and the pigment is 0.1
Colored polyethylene containing up to 7% by weight
A polyethylene coated steel pipe, wherein the colorless polyethylene layer is
Linear low-density polyethylene, a composition comprising linear low-density polyethylene and low-density polyethylene or a composition comprising linear low-density polyethylene and high-density polyethylene, in the case of the composition, features and to Lupo Riechiren coated steel pipe that the Jo low density polyethylene is a composition containing 30 wt% or more.