JP3168904B2 - Exterior polyethylene coated steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an outer polyethylene-coated steel pipe (hereinafter simply referred to as polyethylene-coated steel pipe) which can be efficiently manufactured with high productivity and which can be used in a wide temperature range from -60 ° C to 60 ° C. It is.

[0002]

2. Description of the Related Art An outer polyolefin-coated steel pipe in which the outer surface of a steel pipe is coated with a thick film of a polyolefin resin such as polyethylene or polypropylene to prevent corrosion of steel (hereinafter, simply referred to as a polyolefin-coated steel pipe) has excellent anticorrosion performance. Therefore, it is used as a line pipe for transporting crude oil and natural gas.

[0003] With the increase in energy demand in recent years, as the development of oil fields under severe environments such as polar regions and deserts increases, the use environment of line pipes is becoming increasingly severe. In particular, the guaranteed use temperature of line pipes in extremely cold regions such as Siberia was conventionally up to -45 ° C, but recently the demand for line pipes in extremely low temperature environments up to -60 ° C has been increasing.

Under these circumstances, there is a growing demand for a polyolefin-coated steel pipe which can be used in a wide temperature range from a low temperature of -60 ° C. to a high temperature of + 60 ° C. However, in the conventional polyolefin-coated steel pipe, either low temperature or high temperature is required. However, the mechanical properties and corrosion resistance were not sufficient, and it was not possible to meet such demands. Especially for cryogenic temperatures of -45 to -60 ° C, there was not much demand before,
At present, it is not clear what kind of material is suitable.

[0005] Polyolefin resin is a non-polar polymer material and therefore has excellent chemical stability and mechanical properties, but has a drawback that adhesion to a polar surface such as a steel pipe is poor. Lack of adhesion of the polyolefin coating to the steel pipe reduces the corrosion resistance and mechanical properties of the polyolefin-coated steel pipe. For this reason, a modified polyolefin resin obtained by copolymerizing olefin with a small amount of unsaturated carboxylic acid (eg, maleic acid) or its anhydride is interposed between the polyolefin coating and the steel pipe as an adhesive layer. It is conventional to coat a polyolefin resin on the layer.

[0006] By interposing a modified polyolefin resin as an adhesive layer, the initial adhesion of the polyolefin resin layer to the surface of the steel pipe is remarkably improved. When subjected to a long-term cooling / heating cycle due to the difference in temperature between the two, the adhesion decreases. As a countermeasure, the surface of a steel pipe is first provided with a phosphate-based or chromic acid-based chemical conversion treatment layer, or an epoxy-based primer layer, or both, and then the adhesive layer and the coating layer are formed. ing.

Accordingly, a polyolefin-coated steel pipe used in a wide temperature range has one or both of a base chemical conversion layer and an epoxy-based primer layer on the outer surface of the steel pipe in order from the bottom.
It has an adhesive layer of a modified polyolefin resin and a coating layer of a polyolefin resin.

As an example of such a polyolefin-coated steel pipe, JP-A-57-113881 discloses a method in which a chromic acid-based or phosphate-based chemical conversion treatment layer, an epoxy resin or a modified epoxy resin is formed on the outer surface of a steel pipe. A polyethylene-coated steel pipe having a thermosetting primer layer, a modified polyolefin-based adhesive layer, and a polyethylene resin coating layer is disclosed. The primer layer is, for example, bisphenol A
Apply a mixture of a type epoxy resin and an amine-based curing agent,
It is formed by heating to cure the coating. This polyethylene-coated steel pipe has excellent corrosion resistance at room temperature and high temperature (evaluated by cathodic electrolytic peeling resistance), but has low adhesive strength at room temperature and impact resistance at cryogenic temperatures below -45 ° C. Inferior.

[0009] Japanese Patent Application Laid-Open No. 19041/1992 discloses a method in which a chromic acid-based chemical conversion layer and / or an epoxy-based primer layer is formed on the outer surface of a steel pipe, and a crosslinked polyethylene resin is formed thereon via an adhesive layer of a modified polyethylene resin. A polyethylene-coated steel pipe having laminated layers is disclosed. However, although this polyethylene-coated steel pipe has excellent durability at high temperatures, the coating layer of the cross-linked polyethylene resin becomes brittle at extremely low temperatures of -60 ° C, and it becomes coated when subjected to a drop impact test by the ASTM method. Cracks occur in the layer.

JP-A-60-4054 discloses a high Tg type epoxy resin having a glass transition (Tg) of 80 ° C. or more as a primer layer and a maleic anhydride having a low temperature embrittlement temperature of −20 ° C. or less as an adhesive layer. Modified polypropylene resin, using a resin having a low-temperature embrittlement temperature of 0 ° C. or lower containing a crystalline ethylene-propylene block copolymer as a main component as a coating layer,
A polypropylene coated steel tube is described. This coated steel pipe also has excellent mechanical properties and corrosion resistance at high temperatures,
In a low temperature range of 30 ° C. or lower, the coating layer is embrittled at a low temperature and cannot be used.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to provide a polyolefin-coated steel pipe which has excellent productivity and can be used in a wide temperature range from -60 ° C to 60 ° C. More specifically, a polyolefin resin having a relatively large melt extrusion amount is used for the coating layer so that the melt extrusion coating can be performed efficiently, and the coating layer exhibits excellent adhesion and corrosion resistance over the entire temperature range, and has a long term. The coating layer does not peel off even after the cooling and heating cycle, and the coating layer does not become brittle even at an extremely low temperature of −60 ° C.
It is to provide a polyolefin-coated steel pipe.

[0012]

Means for Solving the Problems The inventors of the present invention have a temperature range of -60.degree.
We have been working on the development of polyolefin-coated steel tubes that can be used in a wide temperature range of 60 ° C. As a result, a primer having a Tg of 60 to 80 ° C. formed from an epoxy composition containing a bisphenol-type epoxy resin, phenylglycidyl ether, an amine-based curing agent and a reaction accelerator, and may further contain a novolak-type epoxy resin. It has been found that a polyolefin-coated steel pipe in which a modified polyolefin resin having a low-temperature embrittlement temperature of -50 ° C and a modification rate of 0.2% or more is formed as an adhesive layer on the layer can be used in the above temperature range. This polyolefin-coated steel tube withstands an impact of 30 J at a very low temperature of −60 ° C. in a drop impact test conforming to the ASTM method. However, assuming line pipe handling and transport operations in cold regions, more severe impact tests (eg,
It is desirable to endure the flap.

In subsequent studies, the coating layer was coated with 100 parts by weight of linear low density polyethylene and 10 parts by weight of high pressure low density polyethylene.
When composed of a polyethylene resin composition comprising 50 parts by weight and optionally 7 parts by weight or less of an ethylene-vinyl acetate copolymer, it has excellent low-temperature impact resistance that can withstand drips at −60 ° C. It has been found that a polyethylene coated steel pipe can be obtained. However, since this polyethylene resin composition has a high melt viscosity and a low melt extrusion amount per unit time, there is a problem that productivity is low when a polyethylene-coated steel pipe is manufactured by a melt extrusion coating method using the same. is there.

As a result of further studies to solve this problem, it has been found that when high-pressure low-density polyethylene is used as the polyethylene, the low-temperature impact resistance greatly changes depending on the average molecular weight. That is, in the high-pressure low-density polyethylene, when the average molecular weight is very large, the low-temperature impact resistance at extremely low temperatures is remarkably improved, and even at a high molecular weight, the melt extrudability during coating is superior to that of the linear low-density polyethylene. There was found. Therefore, when the coating layer is composed of such a high-molecular-weight high-pressure low-density polyethylene,
A polyethylene-coated steel pipe that can be used in the above temperature range, withstands a severe impact test at −60 ° C., and can be manufactured with high productivity can be obtained. The present invention has been completed based on this finding.

Here, the gist of the present invention is that an outer surface of a steel pipe is
In order from the bottom, a base chemical conversion layer, an epoxy primer layer,
In an outer polyethylene-coated steel pipe having an adhesive layer made of a modified polyolefin resin and a coating layer made of a polyethylene resin, the thickness of the adhesive layer is 0.1 to 1.0 mm, the total thickness of the adhesive layer and the coating layer is 2 to 6 mm, and The coating layer is made of a polyethylene resin containing 90% by weight or more of a high-pressure low-density polyethylene having a weight average molecular weight of 120,000 to 200,000, or a mixture of 100 parts by weight of the polyethylene resin and 7 parts by weight or less of an ethylene-vinyl acetate copolymer. An outer surface polyethylene coated steel pipe characterized by the following.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The polyethylene-coated steel pipe of the present invention comprises, in order from the bottom, a chemical conversion treatment layer, an epoxy-based primer layer, an adhesive layer made of a modified polyolefin resin, and a coating layer made of a polyethylene resin on the outer surface of the steel pipe. 4 coatings.

The type and dimensions of the steel pipe to be coated are not particularly limited. The steel pipe may be made of an alloy steel such as stainless steel, but a carbon steel pipe is usually used for a line pipe for transporting crude oil or natural gas. Also, a plated steel pipe can be used. Before applying the coating, it is preferable to remove the mill scale from the outer surface of the steel pipe and clean the surface by a conventional means such as alkali degreasing, blasting with a grid or a shot.

The underlying chemical conversion treatment layer is made of a chromic acid-based treatment solution.
(Chromate treatment solution) or a phosphate-based treatment solution and can be formed by an ordinary method. The chromate treatment liquid is preferably a coating-type partial reduction chromate treatment liquid containing silica fine powder and hexavalent and trivalent chromium ions from the viewpoint of anticorrosion properties and productivity, but a reaction type or electrolytic type chromate treatment liquid is also used. it can. The coating type chromate treatment liquid is
It may further contain additional components such as phosphoric acid. The total amount of chromate treatment (including phosphoric acid, etc.)
The amount of Cr attached is preferably in the range of 50 to 1000 mg / m ² , particularly preferably in the range of 50 to 500 mg / m ² . The phosphate chemical treatment is preferably performed using a reactive phosphating solution containing zinc phosphate or iron phosphate, but may be an electrolytic phosphating treatment.

An epoxy primer layer is formed on the chemical conversion treatment layer. The epoxy-based primer layer is formed by applying a primer composition containing an epoxy resin and a curing agent and, if necessary, a reaction accelerator (catalyst) by a known method such as spray coating, roller coating, brush coating, ironing, and flow coating. It can be formed by coating and then heating by means such as high-frequency induction heating, far-infrared heating, gas heating and the like to cure the coating. The thickness of the epoxy-based primer layer is preferably in the range of 5 to 100 μm, particularly preferably 10 to 50 μm.

As the epoxy resin, one or more selected from bisphenol-type epoxy resins, novolak-type epoxy resins, urethane-modified epoxy resins, and other various glycidyl ether-type or glycidyl ester-type epoxy resins can be used. Preferred epoxy resins are those based on bisphenol A type epoxy resins. As the curing agent, an amine-based curing agent is preferable.

In a preferred embodiment, the primer layer is made of a bisphenol type epoxy resin containing a bisphenol A type epoxy resin as a main component, phenyl glycidyl ether,
It is formed from an epoxy-based primer composition containing an amine-based curing agent and a reaction accelerator, and may further contain a novolak-type epoxy resin. The glass transfer temperature (Tg) of the primer layer is 60 to 80 ° C. By forming a primer layer from this primer composition, the corrosion resistance and low-temperature impact resistance of the polyethylene-coated steel pipe are improved.

The bisphenol A type epoxy resin used in the epoxy primer composition has an epoxy equivalent of 1
Those having about 70 to 3000 are preferable. The bisphenol-type epoxy resin is preferably composed of only a bisphenol A-type epoxy resin, but other bisphenol-type epoxy resins (eg, hydrogenated bisphenol A-type epoxy resin,
Bisphenol F type epoxy resin, etc.) in a small amount (ie, 30% by weight or less of bisphenol A type epoxy resin)
Can be used together.

The novolak type epoxy resin may be either a cresol type or a phenol novolak type. By using a bifunctional bisphenol-type epoxy resin and a polyfunctional novolak-type epoxy resin in combination, the primer can be rapidly cured at a relatively low temperature. Therefore, when the primer is thermoset at a relatively low temperature, it is preferable to add a novolak type epoxy resin.

The phenylglycidyl ether is added for adjusting the viscosity of the primer composition. Examples of the amine-based curing agent include modified heterocyclic diamines, modified aliphatic polyamines, modified aromatic polyamines, modified polyamidoamines, and the like.From the viewpoint of low-temperature impact resistance and corrosion resistance,
Modified heterocyclic diamines are preferred.

As the reaction accelerator, for example, an imidazole-based reaction accelerator can be used. The mixing ratio of each component of the epoxy primer composition is 40 to 80 parts by weight of a bisphenol-type epoxy resin, 0 to 30 parts by weight of a novolak-type epoxy resin, and 15 to phenylglycidyl ether.
35 parts by weight, amine-based curing agent 10-45 parts by weight, reaction accelerator
1.5 to 4.5 parts by weight. It is easy to prepare this epoxy-based primer composition as a two-part composition in which a curing agent and a reaction accelerator are separated from other components, as in the case of conventional products. To form a one-part composition.

A coating layer made of a polyethylene resin is formed on the epoxy-based primer layer via an adhesive layer made of a modified polyolefin resin. Both layers can be formed by melt-extrusion coating a resin on a preheated steel pipe.
Melt extrusion coating of steel pipes is generally performed using a round die method (coating a tubular molten resin film extruded from a round die onto a steel pipe), or a T-die method (using a molten resin film extruded from a T die onto a steel pipe in a spiral shape). Wrapped around). As the coating sequence, a lamination method in which the resin of the adhesive layer and the resin of the coating layer are sequentially melt-extruded and coated, and a co-extrusion method in which the resin of the adhesive layer and the resin of the coating layer are simultaneously melt-extruded and coated, are possible. In the case of the laminating method, the coating of the adhesive layer can be performed by a powder coating method in which a powder resin is electrostatically coated.

The adhesive layer is formed from a modified polyolefin resin obtained by modifying a polyolefin with a small amount of an unsaturated carboxylic acid such as maleic acid, fumaric acid, itaconic acid, crotonic acid, acrylic acid, methacrylic acid or an anhydride thereof. I do. Since the coating layer is a polyethylene-based resin, it is preferable that the polyolefin of the adhesive layer is also polyethylene. The thickness of the adhesive layer is 0.1 to 1.0 mm. If the thickness of the adhesive layer is thinner than 0.1 mm, the adhesion of the coating layer to the steel pipe decreases. Since the modified polyolefin resin used for the adhesive layer is expensive, if the thickness of the adhesive layer is more than 1.0 mm, the production cost is significantly increased. The preferred thickness of the adhesive layer is 0.2-0.
7 mm.

Preferred as the adhesive layer of the polyethylene-coated steel pipe of the present invention is maleic anhydride-modified polyethylene having a low-temperature embrittlement temperature of -50 ° C. or less and a modification rate of 0.2% or more. It is a linear low-density polyethylene modified with maleic anhydride having an embrittlement temperature of -60 ° C or less and a modification rate of 0.25% or more. Here, the modification rate is the content of the modified material (eg, maleic anhydride) in the resin.
(Copolymerization ratio, weight%).

Linear low-density polyethylene is obtained by converting α into ethylene.
-A low-density polyethylene resin produced by copolymerizing an olefin by a medium / low-pressure method, unlike a high-pressure low-density polyethylene having many short chains and long chains in a molecule, It has a molecular structure close to that of high-density polyethylene in which a small amount of short branched chains whose length and number are controlled by copolymerized α-olefins are bonded to polyethylene linear chains.

The polyethylene resin constituting the coating layer of the polyethylene-coated steel pipe of the present invention has a weight average molecular weight (Mw) of 1
It contains at least 90% by weight (preferably at least 95% by weight, more preferably 100% by weight) of 20,000 to 200,000, preferably 125,000 to 180,000 high-pressure low-density polyethylene. High-pressure low-density polyethylene uses
This is a polyethylene obtained by polymerizing under high temperature and high pressure such as 0 ° C and 100-300 MPa, and has many long and short branched chains in the molecule and a density of 0.910-
Because of its low value of 0.929, it is called high-pressure low-density polyethylene.

Since the molecular weight of the high-pressure low-density polyethylene is usually about 45,000 to 120,000, the high-pressure low-density polyethylene used in the present invention has a very high molecular weight. The use of low-density polyethylene for the coating layer has been known, but the molecular weight has not been specifically considered so far. In the present invention, the average molecular weight of the high-pressure low-density polyethylene greatly affects the low-temperature impact resistance, and when a high-molecular-weight Mw of 120,000 or more than conventional general products is used, a high impact resistance at an extremely low temperature of −60 ° C. Because of the discovery of a novel finding that has not been known until now, the high-pressure low-density polyethylene having such a high molecular weight is used.

When the weight-average molecular weight of the high-pressure low-density polyethylene constituting the coating layer is less than 120,000, the low-temperature impact resistance of the polyethylene-coated steel pipe is reduced, and it is difficult to withstand a severe impact test such as driving at -60 ° C. become unable. On the other hand, the weight average molecular weight of this polyethylene is 200,
If it exceeds 000, the melt viscosity of the resin becomes too high, the melt extrusion amount decreases, and the productivity of the polyethylene-coated steel pipe by melt extrusion coating deteriorates.

The high-pressure low-density polyethylene constituting the coating layer is in a small amount (10% by weight or less, preferably 5% by weight or less).
If so, other polyethylene (e.g., high-density or linear low-density polyethylene produced by a medium-low pressure method, high-pressure low-density polyethylene having a molecular weight outside the above range, crosslinked polyethylene, various copolymerized polyethylenes, etc.) It may be contained. However, when the content of such other polyethylene exceeds 10% by weight of the entire polyethylene, the impact resistance at cryogenic temperatures is reduced.

When the polyethylene of the coating layer is mainly made of high-density polyethylene, the low-temperature toughness is low, so that the low-temperature impact resistance is poor. In addition, when the polyethylene of the coating layer is mainly linear low-density polyethylene, the low-temperature impact resistance is high, but the extrusion amount during the melt extrusion coating is small as described above,
The productivity of the polyethylene coated steel pipe decreases.

The coating layer comprises 100 parts by weight of a polyethylene resin containing 90% by weight or more of a high-pressure low-density polyethylene having a weight average molecular weight within the above range and 7 parts by weight or less of an ethylene-vinyl acetate copolymer (preferably 5 parts by weight or less). By blending a small amount of the ethylene-vinyl acetate copolymer, the impact resistance of the coating layer at -60 ° C can be further improved. However, when the compounding amount exceeds 7 parts by weight, the Vicat softening point of the polyethylene resin becomes too low, and the high-temperature impact resistance of the polyethylene-coated steel pipe decreases.

According to the present invention, the total thickness of the modified polyolefin resin adhesive layer and the polyethylene resin coating layer is 2
It should be within the range of 6 mm. The total thickness of the adhesive layer and the resin layer is 2
If it is less than mm, the high-temperature impact resistance at 60 ° C decreases. On the other hand, if the total thickness exceeds 6 mm, the low-temperature impact resistance at -60 ° C and the thermal cycling resistance at -60 ° C decrease due to an increase in residual stress, which is disadvantageous in cost. The preferred total thickness of the two layers is 2.5 to 5 mm.

As described above, the thickness of the adhesive layer is 0.1 to 1.
Since it is 0 mm, the thickness of the coating layer may be 1.0 to 5.9 mm in order to make the total thickness 2 to 6 mm. However, in order to ensure sufficient corrosion resistance, the thickness of the coating layer is preferably 1.5 mm or more, more preferably in the range of 2 to 4 mm.

The polyethylene-coated steel pipe of the present invention uses a high-pressure low-density polyethylene of high molecular weight excellent in low-temperature toughness as a coating layer, and uses a chemical conversion treatment and an epoxy-based primer coating on the base to provide a low-temperature resistance. Improves impact resistance and withstands severe impact test at -60 ° C. In addition, the thermal cycling resistance, the high temperature impact resistance and the high temperature corrosion resistance are also good. Further, the high-pressure low-density polyethylene described above,
Unlike the linear low-density polyethylene, since the melt extrusion amount is large, the productivity of the polyethylene-coated steel pipe is not hindered. As a result, the polyethylene coated steel pipe of the present invention has a
It can be used in a wide temperature range from ℃ to 60 ℃ and can be manufactured with high productivity.

[0039]

Example: After the outer surface of a carbon steel pipe (outside diameter 24 inches x wall thickness 15 mm x length 10 m) was shot blasted, a silica-containing coating-type partially reduced chromate treatment solution (Cr ⁶⁺ / total Cr ratio =
(0.59 to 0.65, containing phosphoric acid)
The mixture was heated and baked for 1 minute to form a chromate layer having a total Cr adhesion of 250 mg / m ² .

On this chromate layer, an epoxy-based primer composition was applied by air spray so as to have a dry film thickness of 20 μm, and was heated to 130 ° C. for 1 minute to cure the coating. An epoxy-based primer layer was formed. The epoxy primer composition used was composed of 60 parts by weight of bisphenol A diglycidyl ether, 20 parts by weight of phenylglycidyl ether, 35 parts by weight of a modified heterocyclic diamine curing agent, and 3 parts by weight of an imidazole-based reaction accelerator. Was.

Thereafter, the steel pipe was preheated to 125-135 ° C., and immediately, the modified polyethylene resin constituting the adhesive layer and the polyethylene resin shown in Table 1 constituting the coating layer were respectively extrusion-coated by the T-die method. After cooling with water, a polyethylene-coated steel pipe was obtained. The modified polyethylene resin used was a linear low-density polyethylene modified with maleic anhydride, and had a low-temperature embrittlement temperature of −60 ° C. and a modification rate of 0.22%.

The ethylene-vinyl acetate copolymer (EVA) added to a part of the coating layer has a vinyl acetate content of 3% by weight.
It was. Table 1 shows the thickness of the adhesive layer, the total thickness of the adhesive layer and the coating layer, the resin composition of the coating layer, and the T-die extrusion amount (measured at 135 ° C.) of the polyethylene resin or the composition used for the coating layer.

The resulting polyethylene-coated steel pipe was tested for low-temperature impact resistance, high-temperature impact resistance, high-temperature corrosion resistance, and thermal cyclability by the following methods. The test results are also shown in Table 1.

(Low-Temperature Impact Test) A 300 × 300 mm test piece cut out of a polyethylene-coated steel pipe was cooled in a low-temperature constant-temperature and constant-humidity chamber and kept at −60 ° C. for 30 minutes. After the cooling, a test piece (commercially available, blade length: 22 mm) was hammered into the coated surface of the test piece with human power, and the presence or absence of cracks in the coating of the test piece was visually determined.

(High-Temperature Impact Test) A coated surface of a 300 × 300 mm test piece cut from a polyethylene-coated steel pipe was subjected to a drop impact test in accordance with the ASTM G-14 method in a constant temperature room at 60 ° C.
The pinhole tester was used to determine the presence or absence of pinholes on the test piece at a voltage of 10 kV. The results are shown by the maximum impact value at which no pinhole is generated.

(High Temperature Corrosion Resistance Test) The high temperature corrosion resistance was evaluated by the cathodic electrolytic peeling resistance. A 9 mm diameter drill hole was drilled in the coated surface of a 200 × 200 mm test piece cut from a polyethylene coated steel pipe until the steel surface substrate was exposed.
Immerse in 3% saline at constant temperature,
A voltage of 1.5 V was applied to the test piece. After allowing to stand for 30 days under these conditions, the polyethylene layer at the drill hole was cut off, and the coating peeling diameter around the hole was measured.

(Cooling / Heat Cycle Test) A 150 × 70 mm test piece cut from a polyethylene-coated steel pipe was subjected to a series of heat cycles consisting of −60 ° C. × 1 hour → heating 3 hours → 70 ° C. × 1 hour → temperature lowering 7 hours. 20 times. Thereafter, the presence or absence of peeling of the polyethylene film on the end face of the test piece was visually observed.

[0048]

[Table 1]

[0049]

As can be seen from the results in Table 1, the polyethylene-coated steel pipe of the present invention has excellent mechanical properties and corrosion resistance in a wide temperature range from -60 ° C to 60 ° C, Good cycleability. In particular, it withstands a severe impact at -60 [deg.] C., and has a significantly improved low-temperature impact resistance. Further, the melt extrusion amount by the T-die is large, and the productivity of the polyethylene coated steel pipe is also good. Therefore, the polyethylene-coated steel pipe of the present invention can be used at -60 ° C to 60 ° C.
It can be used in a wide temperature range of ° C and can be produced efficiently.

On the other hand, when the weight average molecular weight of the high-pressure low-density polyethylene of the coating layer is less than 120,000, the low-temperature impact resistance at -60 ° C. is reduced. Is reduced. On the other hand, if the total thickness of the resin layer exceeds 6 mm, the low-temperature impact resistance at -60 ° C. and the thermal cycling resistance decrease, and if it is less than 2 mm, the high-temperature impact resistance decreases. Further, when the coating layer is mainly composed of linear low-density polyethylene, the extrusion amount of the coating layer is remarkably reduced.
The drop in low-temperature impact resistance at ℃ is remarkable.

Claims (2)

(57) [Claims] 1. An outer polyethylene-coated steel pipe having a base chemical conversion layer, an epoxy-based primer layer, an adhesive layer made of a modified polyolefin resin, and a coating layer made of a polyethylene resin on the outer surface of the steel pipe in order from the bottom. The thickness is 0.1 to 1.0 mm, and the total thickness of the adhesive layer and the coating layer is 2
~ 6 mm, and the coating layer has a weight average molecular weight of 120,000 ~
An outer surface polyethylene-coated steel pipe comprising a polyethylene resin containing 200,000 or more of high-pressure low-density polyethylene by 90% by weight or more. 2. An outer polyethylene-coated steel pipe having a base chemical conversion layer, an epoxy primer layer, an adhesive layer made of a modified polyolefin resin, and a coating layer made of a polyethylene resin on the outer surface of the steel pipe in order from the bottom. The thickness is 0.1 to 1.0 mm, and the total thickness of the adhesive layer and the coating layer is 2
~ 6 mm, and the coating layer has a weight average molecular weight of 120,000 ~
An outer surface polyethylene coated steel pipe comprising a mixture of 100 parts by weight of a polyethylene resin containing at least 90% by weight of 200,000 high-pressure low-density polyethylene and 7 parts by weight or less of an ethylene-vinyl acetate copolymer.