JP6610685B2 - Polyethylene-coated steel pipe and method for producing the same - Google Patents

Description

  The present invention relates to a polyethylene-coated steel pipe excellent in the resistance to cathodic peeling of a polyethylene coating and a method for producing the same. In particular, the present invention relates to a polyethylene-coated steel pipe suitable for use in gas pipes, water pipes, cable protection pipes, line pipes and the like.

  A polyethylene-coated steel pipe having a steel pipe surface coated with polyethylene as an anticorrosion layer has excellent corrosion resistance and is used for various pipes. In particular, polyethylene-coated steel pipes are increasingly used for laying on the seabed or underground, and in that case, anticorrosion is often used in combination. Although the anticorrosion of the polyethylene-coated steel pipe is further enhanced by the electrocorrosion protection, there is a problem that the polyethylene coating easily peels from the outer surface of the steel pipe, and this problem is known as cathodic peeling.

  It is known that chromate treatment is effective as a method for suppressing such cathode peeling. For example, Patent Document 1 discloses a resin-coated heavy-duty anticorrosive steel material that is a chromate-coated steel material having a chromate layer on the surface of the steel material, in which an epoxy primer layer, a maleic anhydride-modified polyolefin layer, and a polyolefin layer are sequentially laminated. Patent Document 2 discloses that a specific epoxy primer is applied to the surface of a steel material and that chromate treatment is performed as a base treatment.

  However, in recent years, there has been a demand for an organic coated steel material excellent in corrosion resistance that is not subjected to chromate treatment, that is, non-chromate treatment, from the viewpoint of reducing environmental load. In response to this, Patent Document 3 discloses an organic coated steel sheet that is non-chromated and has excellent corrosion resistance.

JP-A-2005-35061 JP 2000-190422 A JP 2011-111638 A

  However, the methods described in Patent Documents 1 and 2 have a problem of requiring chromate treatment with a high environmental load. The method described in Patent Document 3 improves the corrosion resistance by treating a plated steel sheet, and does not suppress cathode peeling.

  SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described problems of the prior art, and to provide a polyethylene-coated steel pipe that can be manufactured without performing chromate treatment, and a method for manufacturing the same, as well as being excellent in the resistance to cathode peeling of a polyethylene coating. There is.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, it was found that the amount of dust contained between the outer surface of the steel pipe and the primer layer has an influence on the resistance to cathode peeling. And it discovered that cathode peeling can be suppressed by reducing the amount of dust which exists between a steel pipe outer surface and a primer layer. Furthermore, it has been found that the cathode layer can be further suppressed by containing a predetermined amount of one or more compounds selected from vanadium compounds and aluminum phosphates, and further zinc oxide in the primer layer.

  Therefore, a polyethylene-coated steel pipe is obtained by applying a primer containing one or more compounds selected from vanadium compounds and aluminum phosphate, and further a zinc oxide-containing primer to a plurality of steel pipes having dust adhered to the outer surface of the steel pipe. The present invention was completed by conducting a detailed study of the relationship between the dust adhesion amount, the vanadium compound and aluminum phosphate content, and the resistance to cathode peeling.

  The gist of the present invention is as follows.

[1] A polyethylene-coated steel pipe having a primer layer, an adhesive polyethylene layer, and a polyethylene layer in order from the side in contact with the outer surface of the steel pipe on the outer surface of the steel pipe,
When the surface of the primer layer in contact with the outer surface of the steel pipe is mapped with Fe by EPMA, the area ratio of Fe is 20% or less,
And the polyethylene-coated steel pipe which contains 5-50 mass% in total in the said primer layer the 1 type, or 2 or more types of compound chosen from the vanadium compound and the aluminum phosphate.

  [2] The polyethylene-coated steel pipe according to the above [1], wherein the primer layer further contains 2 to 20% by mass of zinc oxide.

  [3] The polyethylene-coated steel pipe according to [1] or [2], which is used under cathodic protection.

  [4] The polyethylene-coated steel pipe according to any one of [1] to [3], which is used for laying on the seabed or underground.

[5] A method for producing a polyethylene-coated steel pipe according to any one of [1] to [4],
After blasting the outer surface of the steel pipe, an adhesive medium is applied to the outer surface of the steel pipe by a method defined in JIS Z0313 (2004), then the adhesive medium is attached to white paper, and 256 of the adhesive medium is analyzed by image analysis. A method for producing a polyethylene-coated steel pipe, wherein the brightness at a gradation is measured and a steel pipe having an average value of 220 or more is coated with polyethylene.

  [6] The method for producing a polyethylene-coated steel pipe according to [5], wherein the polyethylene coating is a polyethylene coating having a primer layer, an adhesive polyethylene layer, and a polyethylene layer in order from the side in contact with the outer surface of the steel pipe.

  [7] The method for producing a polyethylene-coated steel pipe according to [5] or [6], wherein a silane coupling agent is applied to the outer surface of the steel pipe and dried, and then the polyethylene coating is performed.

  [8] The polyethylene according to any one of [5] to [7], wherein the primer is applied to the steel pipe and dried by any one of infrared heating, induction heating, hot air heating, or a combination thereof. Manufacturing method of coated steel pipe.

  In the present invention, the phrase “excellent in cathode peel resistance” means that the average value of the cathode peel distance is 15 mm or less in the cathode peel test described in Examples described later. A coating having a primer layer, an adhesive polyethylene layer, and a polyethylene layer is referred to as a polyethylene coating, and a steel pipe having a polyethylene coating is referred to as a polyethylene-coated steel pipe.

  According to the present invention, a polyethylene-coated steel pipe excellent in resistance to cathodic peeling can be obtained. In the polyethylene-coated steel pipe of the present invention, when galvanic protection is used in combination, the cathode peeling of the polyethylene coating is effectively suppressed, and excellent corrosion resistance is exhibited. Thus, since the polyethylene-coated steel pipe of the present invention has excellent cathode peeling resistance, it is particularly suitable for use for laying on the seabed or underground.

  The configuration of the present invention will be described below. The polyethylene-coated steel pipe of the present invention is one or two selected from the amount of dust contained between the outer surface of the steel pipe and the primer layer, and the vanadium compound and aluminum phosphate in the primer layer in order to obtain excellent cathode peeling resistance. It is necessary to define the content of the above compounds. The dust is mainly composed of Fe, and usually contains blast-treated particles, steel pipe fragments peeled off by blasting, and scales remaining on the outer surface of the steel pipe.

1. About the amount of dust contained between the outer surface of the steel pipe and the primer layer Cathodic peeling occurs between the outer surface of the steel pipe and the primer layer. If a large amount of dust generated by blasting or the like adheres to the outer surface of the steel pipe, the adhesion between the outer surface of the steel pipe and the primer layer at that portion is not sufficient, so that the cathode is likely to peel off. Therefore, excellent cathode peel resistance can be obtained by controlling the amount of dust contained between the outer surface of the steel pipe and the primer layer as described below.

  For example, the polyethylene coating including the primer layer of the polyethylene-coated steel pipe is forcibly removed, and the surface of the separated polyethylene coating that is in contact with the outer surface of the steel pipe (the surface of the peeled primer layer that is in contact with the outer surface of the steel pipe) is EPMA (electronic The Fe area ratio (the ratio of the area where Fe is detected to the measured area) is set to 20% or less by mapping Fe with a line microanalyzer). By setting the area ratio of Fe to 20% or less, the adhesion between the primer layer and the outer surface of the steel pipe is improved, and the cathode peeling hardly occurs. When the area ratio of Fe exceeds 20%, the adhesion between the primer layer and the outer surface of the steel pipe is lowered, and the cathode peeling is likely to occur. The area ratio of Fe is preferably 15% or less, more preferably 10% or less.

  The method for forcibly removing the polyethylene coating from the outer surface of the steel pipe is not particularly limited. For example, a 20 mm × 50 mm portion of a polyethylene-coated steel pipe is cut out to form a test specimen. After the test specimen is immersed in liquid nitrogen, it is taken out and the polyethylene coating is struck with a plastic hammer. The containing polyethylene coating separates from the outer surface of the steel pipe.

  The surface of the primer layer that is in contact with the outer surface of the separated polyethylene-coated steel pipe is subjected to Fe mapping with EPMA under the following analysis conditions. In addition, when mapping Fe by EPMA, it is preferable to prepare a plurality of measurement specimens, perform Fe mapping on each measurement specimen, and set the average value to the area ratio of Fe.

  In addition, as described in the manufacturing method described later, for example, by performing dust removal processing such as air blow on the outer surface of the steel pipe, the amount of dust attached to the outer surface of the steel pipe can be reduced.

-EPMA analysis conditions-
EPMA can be measured under various conditions. In the present invention, using a scanning electron microscope, the acceleration voltage is 5 to 10 kV, the beam diameter is 10 μm, and the 10 μm step is 0.1 second per point. Can be measured.

2. About steel pipe There is no restriction | limiting in particular in the kind, dimension, etc. of the steel pipe (base material steel pipe) used by this invention, For example, an electric-resistance-welded steel pipe, a spiral steel pipe, a UOE steel pipe, a press bend steel pipe etc. are mentioned. However, it is not limited to these. The base steel pipe is preferably JIS standard 200A to 600A (pipe outer diameter 216.3 to 609.6 mm). The base steel pipe is preferably an electric resistance steel pipe from the viewpoint of strength and economy.

3. About primer layer A general material can be used for a primer. For example, an epoxy primer (JIS G3477-1: 2012) can be used as a primer for forming the primer layer. In the present invention, when the primer layer contains a specific amount of one or two or more compounds selected from vanadium compounds and aluminum phosphates, cathode peeling hardly occurs. The total content of one or more compounds selected from vanadium compounds and aluminum phosphates is 5% by mass or more. When the amount is less than 5% by mass, the effect of suppressing cathode peeling cannot be obtained. Moreover, even if it contains 50 mass% or more, an effect does not change and it is uneconomical. Therefore, the content of one or more compounds selected from vanadium compounds and aluminum phosphates is 5 to 50% by mass in total, preferably 5 to 30% by mass. The vanadium compound and aluminum phosphate are not particularly limited. Examples of the vanadium compound include vanadium pentoxide, calcium vanadate, magnesium vanadate, and the like. As aluminum phosphate, in the present invention, aluminum phosphate is defined as a general term for substances in which aluminum is bonded to the structure of phosphoric acid. In addition to phosphoric acid, the structure of polyphosphoric acid is also included. Examples of the aluminum phosphate include aluminum phosphate (AlPO ₄ ), aluminum dihydrogen tripolyphosphate or derivatives thereof. In addition, the above-mentioned content is a ratio with respect to the total amount of primers.

  In the present invention, in addition to one or two or more compounds selected from a vanadium compound and aluminum phosphate, the primer layer contains zinc oxide, thereby further preventing the cathode peeling. Zinc oxide is preferably 2% by mass or more. When the amount is less than 2% by mass, the effect of suppressing cathode peeling cannot be obtained. On the other hand, even if it contains more than 20 mass% in total, the effect does not change and it is uneconomical. Therefore, when it contains zinc oxide, it is set to 2 to 20% by mass. In addition, content of zinc oxide is a ratio with respect to the total amount of a primer.

  After applying a silane coupling agent to the outer surface of the steel pipe and drying it to form a surface treatment layer, a primer layer can be further formed on the surface treatment layer. Although a silane coupling agent is not specifically limited, What has functional groups, such as an epoxy group, a mercapto group, and an amino group, can be used. Examples of the silane coupling agent having an epoxy group include 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltriethoxysilane, and 3-glycidoxypropyl. Examples include methyldimethoxysilane. Examples of the silane coupling agent having a mercapto group include 3-mercaptopropylmethyldimethoxysilane and 3-mercaptopropyltrimethoxysilane. Examples of the silane coupling agent having an amino group include 3-aminopropyltrimethoxysilane and 3-aminopropyltriethoxysilane.

4). Polyethylene coating The polyethylene-coated steel pipe of the present invention is a polyethylene-coated steel pipe having a primer layer, an adhesive polyethylene layer, and a polyethylene layer in this order from the side in contact with the outer surface of the steel pipe. The polyethylene coating can be produced by forming a primer layer, an adhesive polyethylene layer, and a polyethylene layer in this order from the side in contact with the outer surface of the steel pipe. The polyethylene coating of the present invention does not include a surface treatment layer formed by chromate treatment.

  The primer layer is as described above.

  Examples of the adhesive polyethylene resin for forming the adhesive polyethylene layer include maleic anhydride-modified polyethylene and unsaturated carboxylic acid-modified polyethylene.

As the polyethylene resin for forming the polyethylene layer, any of low density, medium density, and high density polyethylene resins may be used. Particularly, the high density polyethylene resin (density: 0.94 to 0.97 g / cm ³ ). Is desirable from the viewpoint of corrosion resistance. If necessary, additives such as antioxidants, ultraviolet absorbers, flame retardants, pigments, fillers, lubricants, antistatic agents, and colorants can be added to the polyethylene resin.

  The polyethylene coating (the total of the primer layer, the adhesive polyethylene layer, and the polyethylene layer) preferably has a thickness of about 1 mm to 6 mm from the viewpoint of corrosion resistance and economy. From the standpoint of corrosion resistance, a more preferred lower limit is 2.5 mm.

5. About a manufacturing method The manufacturing method of the polyethylene covering steel pipe of this invention is demonstrated below.

  First, the outer surface of the steel pipe is blasted. Then, dust on the outer surface of the steel pipe is removed by dust removal treatment such as air blow. At this stage, the amount of dust adhering to the outer surface of the steel pipe is examined. Specifically, dust on the outer surface of the steel pipe after air blowing is attached to an adhesive medium (for example, cellophane adhesive tape (JIS Z1522: 2009)) by a method defined in JIS Z0313 (2004). The adhesive medium to which dust is adhered is attached to white paper, and the luminance of the adhesive medium at 256 gradations is measured by image analysis. As a result of the measurement, it is confirmed that the average luminance value is 220 or more. Further, the dust removal status may be confirmed by comparing with a sample sample that has been confirmed to have a luminance of 220 or more in advance. When the amount of dust attached is large, the average value of luminance is low. When polyethylene coating is performed in a state where the average value of luminance is 220 or less and the amount of dust is small, the adhesion between the outer surface of the steel pipe and the primer layer is improved, and the cathode peeling hardly occurs. When the average value of the luminance is less than 220, the adhesion between the primer layer and the outer surface of the steel pipe is lowered, and the cathode peeling tends to occur. The average value of luminance is preferably 230 or more, and more preferably 240 or more.

  The adhesive medium is a transparent thin film having an adhesive layer having an adhesive strength of 1.8 N / 10 mm or more.

  The white paper has a whiteness of 90% to 95% (also referred to as white plain paper in the examples described later). Usable white paper includes, for example, PPC paper and inkjet paper shown in JIS P0001: 1998.

  The luminance is measured with 256 gradations. More specifically, the luminance is measured by using, for example, a multifunction machine, scanning the above-mentioned adhesive medium (cellophane adhesive tape) affixed on white paper using an electronic reading device such as a scanner, and commercially available images. Using the analysis software, an arbitrary position of the image in the adhesive medium can be specified, and the average value of the luminance in the range can be measured by image analysis.

  The size of the image analysis is not particularly limited, but is preferably a 20 mm × 20 mm rectangle from the viewpoint of simplicity. Further, in obtaining the average value of luminance, the number of measurements is two or more. The number of measurements is preferably 2 to 5 times.

  For steel pipes with an average value of less than 220 for the entire luminance, dust attached to the outer surface of the steel pipe is removed again by dust removal processing such as air blow, and the luminance is measured again by the above method to determine pass / fail. If the average value is 220 or more, it is acceptable.

  Next, a polyethylene pipe is coated on the steel pipe having an average value of 220 or more in the above brightness.

  The method for forming the polyethylene coating is not particularly limited. For example, a total of 5 to 50% by mass of one or more compounds selected from vanadium compounds and aluminum phosphates are mixed in a steel pipe whose outer surface dust is removed and the average value of the brightness is 220 or more. A primer such as an epoxy primer is applied, heated to a predetermined temperature (for example, 130 to 170 ° C.) with infrared rays, a high-frequency heating device, a hot air furnace, or a combination thereof, and an adhesive polyethylene layer having polarity on the outer surface of the steel pipe A polyethylene layer is sequentially extruded from a round die, a T die, or the like to form a coating layer. Then it is cooled.

  In addition, 5 to 50% by mass in total of one or two or more compounds selected from vanadium compounds and aluminum phosphates were mixed in a steel pipe from which dust on the outer surface was removed and the average value of the brightness was 220 or more. Examples include a method in which a primer such as an epoxy primer is applied and heated, and then the adhesive polyethylene layer and the polyethylene layer are simultaneously extruded and coated with a round die, a T die, or the like. In this case, two coating layers are formed at a time. In this case, in particular, the adhesion of the polyethylene coating layer after the polyethylene-coated steel pipe is excellent.

  Moreover, as an epoxy primer, the epoxy primer which contains 5-50 mass% of 1 type or 2 or more types of compounds chosen from the vanadium compound and aluminum phosphate, and also contains 2-20 mass% of zinc oxide is apply | coated. You can also.

  Further, after a silane coupling agent is applied to the outer surface of the steel pipe and dried to form a surface treatment layer, a polyethylene coating can be further formed on the surface treatment layer by the above method.

  In addition, when apply | coating and drying a primer to a steel pipe, it is preferable to carry out by any one of infrared heating, induction heating, hot air heating, or these combination.

  As described above, the polyethylene-coated steel pipe of the present invention is obtained. The polyethylene-coated steel pipe of the present invention is used under cathodic protection, that is, used with cathodic protection. Further, it can be suitably used as a polyethylene-coated steel pipe for laying on the seabed or underground.

  Examples of the present invention will be described below. In this example, the following methods differ in the amount of dust attached between the outer surface of the steel pipe and the primer layer, and the content of one or more compounds selected from the vanadium compound and aluminum phosphate in the primer are different. . 1-15 polyethylene coated steel tubes were tested. The technical scope of the present invention is not limited to the following examples.

(Measurement of brightness)
For the blasted JIS SGP 200A steel pipe, dust adhered to the outer surface was removed by air blowing. Next, dust on the outer surface of the steel pipe was adhered to a cellophane adhesive tape (size: 24 mm × 200 mm) by a method prescribed in JIS Z0313 (2004), and the cellophane adhesive tape was attached to white plain paper. The cellophane adhesive tape affixed to the white plain paper was then converted into electronic data by a scanner, and the brightness of the cellophane adhesive tape was determined by image analysis using image analysis software (Photoshop CS6 manufactured by Adobe). In addition, three pieces of cellophane adhesive tapes for image analysis were prepared for each steel pipe by the above method, the luminance was measured once for each piece, and the average value of the total three times of luminance was set as the luminance shown in Table 1. .

(Polyethylene coating)
First, a primer layer was formed on the outer surface of the steel tube subjected to the air blow treatment. Epoxy in which commercially available liquid epoxy resin ("Basic liquid type 828" manufactured by Mitsubishi Chemical Corporation) and curing agent ("Modified aliphatic amine grade T" manufactured by Mitsubishi Chemical Corporation) are mixed with the additives shown in Table 1 The primer was spray-coated so that the film thickness was 40 μm, and was heated to a surface temperature of 150 ° C. by induction heating and cured. The vanadium compound or aluminum phosphate was adjusted to a predetermined mass% with respect to the liquid epoxy resin. As shown in Table 1, vanadium pentoxide or magnesium vanadate was used as the vanadium compound, and aluminum dihydrogen phosphate (“K-WHITE # 105” manufactured by Teika Co., Ltd.) was used as the aluminum phosphate.

  Then, a commercially available adhesive polyethylene resin (“Admer NE065” manufactured by Mitsui Chemicals, Inc.), polyethylene resin (“HI-ZEX5100E” manufactured by Prime Polymer Co., Ltd., which is a high-density polyethylene resin) was coated by extrusion coating, A polyethylene-coated steel pipe having a polyethylene coating layer thickness of 5 mm was produced.

(Surface treatment layer formation)
Some steel pipes were coated with a silane coupling agent after the air blow treatment. As a silane coupling agent, 3-glycidoxypropyltrimethoxysilane having an epoxy group (“KBM-403” manufactured by Shin-Etsu Chemical Co., Ltd.) is used, and pure water is used so that the silane coupling agent is 1% by mass. The treatment liquid diluted in (1) was applied to the outer surface of the steel pipe by spraying, and then heated by an induction heating device so that the treatment liquid was completely dried to form a surface treatment layer. Subsequently, said polyethylene coating was performed. Those having a surface treatment layer are described as “epoxysilane” in the “surface treatment” column of Table 1.

(Fe mapping in EPMA)
An end portion of 20 mm × 50 mm was cut out from the steel pipe coated with polyethylene obtained as described above and used as a measurement sample. This measurement sample was immersed in liquid nitrogen and then taken out, and the polyethylene coating was hit with a plastic hammer to peel the polyethylene coating from the outer surface of the steel pipe. The five areas on the outer surface side of the peeled polyethylene-coated steel pipe were each subjected to mapping of Fe by EPMA using the method described above with a measurement area of 10 mm × 10 mm, and the area ratio of Fe (the area where Fe was detected relative to the measurement area) The average value of the area ratios of Fe at five locations was defined as the area ratio of Fe shown in Table 1. The case where the area ratio of Fe shown in Table 1 was 20% or less was regarded as acceptable.

(Reference example: No. 16)
As a reference example, a polyethylene-coated steel pipe using a steel pipe subjected to chromate treatment was produced. Chromate treatment solution (“Cosmer 100” manufactured by Kansai Paint Co., Ltd.) diluted with pure water to 1/5 (mass ratio) is used, so that the Cr equivalent adhesion amount is 300 mg / m ^{2 on the} outer surface of the steel pipe. The chromate layer was formed by spray coating and drying by heating so that the steel tube surface temperature reached 100 ° C. In the primer layer, a silane coupling agent, a vanadium compound, and aluminum phosphate were not contained. Other than that was carried out similarly to the other Example, and produced the polyethylene covering steel pipe. The amount of Cr equivalent deposited is 1 m based on the production of a dummy plate having a chromate layer, dissolving a chromate film of a predetermined area with 10% by mass NaOH, and measuring the Cr amount in the solution by absorptiometry. ^It calculated | required by calculating the Cr conversion adhesion amount per ^2. FIG.

(Cathode peeling test)
In order to investigate the cathodic peel resistance when a polyethylene-coated steel pipe was used together with the anticorrosion, the cathodic peel distance was measured and evaluated by the following method.

  First, a test sample of an appropriate size was collected from the polyethylene-coated steel pipe produced by the above method. A circular artificial defect with a diameter of 6 mmφ was formed in the center of the sample, and the outer surface of the steel pipe was exposed. An acrylic cylinder having an inner diameter of 70 mmφ centered on the artificial defect portion was placed vertically on the polyethylene coating, fixed to the polyethylene coating with a sealing material, and the inside of the cylinder was filled with a 3% by mass NaCl aqueous solution to form a cell.

  Platinum was used as the counter electrode, and the potential of the steel pipe at the artificial defect portion was maintained at -1.5 V vs SCE using a potentiostat. The sample was left still in a constant temperature bath at 80 ° C., and the potential was maintained for 28 days.

  Next, after collecting the sample, the acrylic cylinder was removed, and the polyethylene coating was removed from the periphery of the artificial defect using a chisel and a cutter. In the part where the polyethylene coating was peeled off from the outer surface of the steel pipe, the surface of the steel pipe around the artificial defect was discolored. The discolored part is regarded as the part from which the polyethylene coating is peeled off, and the discolored part in four directions from the center of the artificial defect part (one of the tube axis directions is 12 o'clock, 12 o'clock, 3 o'clock, 6 o'clock, 9 o'clock) The length from the end of the artificial defect was measured, and the average value was defined as the cathode separation distance (mm). The smaller the value of the cathode peeling distance, the better, and “15 mm or less” was accepted. The accepted polyethylene-coated steel pipe is suitable for laying on the seabed or underground. Table 1 shows the test results.

  Invention Example No. In Nos. 1 to 8 and 15, the cathode peeling distance satisfies a specified value of 15 mm or less, and exhibits excellent cathode peeling resistance. In addition, this invention example No. 1 to 8 and 15, since the effect of excellent cathode peeling resistance is shown using a JIS standard 200A steel pipe, even a steel pipe having a larger diameter, for example, a steel pipe of JIS standard 600A, is naturally excellent in cathode peeling resistance. It can be said that.

  No. 9-14 are comparative examples. No. Nos. 9 and 10 have a large amount of dust on the outer surface of the steel pipe, the area ratio of Fe exceeds 20%, the average value of luminance is less than 220, and the cathode peeling distance does not satisfy the regulation.

  No. Nos. 11 and 12 have a total amount of one or more compounds selected from the vanadium compound and aluminum phosphate in the primer of less than 5% by mass, and the cathode peeling distance is long and does not satisfy the regulations.

  No. Nos. 13 and 14 have an area ratio of Fe of 20% or less, but the primer layer contains no vanadium compound or aluminum phosphate. Compared with 1 to 8 and 15, the cathode separation distance is long and does not satisfy the regulation.

  No. Reference numeral 16 denotes a chromate treatment material of a reference example. The chromate treatment material has a large amount of dust adhering to the outer surface of the steel pipe, and the iron area by EPMA exceeds 20%, but the cathode peeling distance satisfies the regulation. However, since it contains chromium, it has a high environmental impact and is not desirable.

  A test was conducted on a polyethylene-coated steel pipe containing zinc oxide in the primer layer. Specifically, the amount of dust adhering between the outer surface of the steel pipe and the primer layer, the content of one or more compounds selected from vanadium compounds and aluminum phosphate in the primer, and the content of zinc oxide are different. No. For the polyethylene-coated steel pipes 17 to 22, the same luminance measurement as in Example 1, polyethylene coating, surface treatment layer formation, and Fe mapping with EPMA were performed, and the cathode peeling distance was measured. Zinc oxide (manufactured by Taika Co., Ltd.) was added to the primer in the same manner as the vanadium compound and aluminum phosphate, and the mixture was well mixed and dispersed in the primer.

  Table 2 shows the test results.

  Invention Example No. In Nos. 17 to 22, the cathode stripping distance satisfies the specified 15 mm or less, and further shows a cathode stripping property superior to that in the case of not containing zinc oxide (Example 1).

Claims (6)

A polyethylene-coated steel pipe having a primer layer, an adhesive polyethylene layer, and a polyethylene layer in order from the side in contact with the outer surface of the steel pipe on the outer surface of the steel pipe,
When the surface of the primer layer in contact with the outer surface of the steel pipe is mapped with Fe by EPMA, the area ratio of Fe is 20% or less,
And, on the primer layer, a vanadium compound with containing 5 to 50 mass%, further contains zinc oxide 2-20 wt% on said primer layer, polyethylene-coated steel pipe. Is used under cathodic protection, polyethylene-coated steel pipe according to claim 1. The polyethylene-coated steel pipe according to claim 1 or 2 , which is used for laying on the seabed or underground. It is a manufacturing method of the polyethylene covering steel pipe according to any one of claims 1 to 3 ,
After blasting the outer surface of the steel pipe, an adhesive medium is applied to the outer surface of the steel pipe by a method defined in JIS Z0313 (2004), then the adhesive medium is attached to white paper, and 256 of the adhesive medium is analyzed by image analysis. The primer with the primer layer, the adhesive polyethylene layer, and the polyethylene layer in order from the side in contact with the outer surface of the steel pipe is measured on the steel pipe whose average luminance value is 220 or more, and the primer is coated with the primer. The method for producing a polyethylene-coated steel pipe , wherein the layer contains 5 to 50% by mass of a vanadium compound and the primer layer further contains 2 to 20% by mass of zinc oxide . The method for producing a polyethylene-coated steel pipe according to claim 4 , wherein the polyethylene coating is performed after a silane coupling agent is applied to the outer surface of the steel pipe and dried. The method for producing a polyethylene-coated steel pipe according to claim 4 or 5 , wherein the primer is applied to the steel pipe and dried by infrared heating, induction heating, hot air heating, or a combination thereof.