JP7655341B2 - Manufacturing method of polyethylene coated steel pipe - Google Patents

Description

The present invention relates to a method for manufacturing polyethylene-coated steel pipes, and in particular to a method for manufacturing polyethylene-coated steel pipes that have a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer on the surface and have excellent cathodic peeling resistance and suppress changes in the mechanical properties of the steel pipes during manufacturing, and are used for water pipes, cable protection pipes, line pipes, civil engineering applications, etc.

Polyethylene-coated steel pipes, in which the pipe surface is coated with polyethylene as a corrosion-resistant layer, have excellent corrosion resistance and are used for various piping. Polyethylene-coated steel pipes are increasingly being laid on the seabed and buried underground, in which case they are often used in conjunction with cathodic protection. Cathodic protection further increases the corrosion resistance of polyethylene-coated steel pipes, but when cathodic protection is performed, there is a problem that the polyethylene coating tends to peel off from the outer surface of the steel pipe, a problem known as cathodic peeling.

Chromate treatment is known to be an effective method for preventing such cathodic peeling. For example, Patent Document 1 discloses a chromate-coated steel material having a chromate layer on the steel surface, which is a resin-coated heavy-duty corrosion-resistant steel material in which an epoxy primer layer, a maleic anhydride-modified polyolefin layer, and a polyolefin layer are laminated in that order.

JP 2005-35061 A

However, in recent years, in order to reduce the environmental impact, there is a demand for non-chromate treated organic coated steel materials with excellent corrosion resistance.

The method described in Patent Document 1 has the problem that, when coating steel pipes, a chromate treatment that places a high burden on the environment is required. In addition, when the mechanical properties of steel pipes change due to heating during coating, there is a general tendency for strength to increase while workability to decrease. In recent years, there has been an increase in applications in which workability is given greater importance from the standpoint of on-site workability and stability during use as piping, and technology that can respond to this has been in demand.

The present invention was made in consideration of the above circumstances, and aims to provide a method for manufacturing polyethylene-coated steel pipes that have excellent resistance to cathodic peeling and suppress changes in mechanical properties.

The inventors conducted extensive research to achieve the above object, and discovered that in a method for producing a polyethylene-coated steel pipe in which a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer are sequentially laminated on the surface of the steel pipe, by setting the surface temperature of the steel pipe during the powder epoxy resin primer coating to form the powder epoxy resin primer layer at 160 to 210°C, a polyethylene-coated steel pipe with excellent cathodic peeling resistance can be obtained while suppressing changes in the mechanical properties of the steel pipe.

The present invention has the following configuration.
[1] A method for producing a polyethylene-coated steel pipe in which a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer are sequentially laminated on a steel pipe surface,
A method for producing a polyethylene-coated steel pipe, characterized in that the surface temperature of the steel pipe during the powder epoxy resin primer coating when forming the powder epoxy resin primer layer is 160 to 210°C.
[2] A method for producing a polyethylene-coated steel pipe according to [1], characterized in that a blast treatment is performed on the steel pipe surface before forming a powder epoxy resin primer layer, and the surface roughness Rz of the steel pipe surface after the blast treatment is 60 to 100 μm and the peak count number is 20 or more.

The present invention provides a polyethylene-coated steel pipe that has excellent resistance to cathodic peeling and suppresses changes in mechanical properties.

According to the present invention, a polyethylene-coated steel pipe with excellent cathodic peeling resistance can be manufactured by non-chromate treatment, and the environmental load is reduced. Furthermore, according to the present invention, a polyethylene-coated steel pipe with excellent cathodic peeling resistance can be obtained while suppressing changes in the mechanical properties of the steel pipe by using a powder epoxy resin primer as the material for forming the primer layer and setting the steel pipe surface temperature at 160 to 210°C when forming the primer layer. The ability to suppress changes in the mechanical properties of the steel pipe has the advantage that the design of the steel pipe itself is easier. In other words, because changes in the mechanical properties of the steel pipe before and after coating are suppressed, it becomes easier to design and manage the mechanical properties of the polyethylene-coated steel pipe after coating based on the mechanical properties of the steel pipe before coating, and polyethylene-coated steel pipes with the desired mechanical properties can be efficiently manufactured.

The polyethylene-coated steel pipe of the present invention is manufactured by sequentially laminating a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer on the surface of the steel pipe.

The following describes one embodiment of the method for manufacturing polyethylene-coated steel pipes of the present invention.

The method for producing a polyethylene-coated steel pipe of this embodiment includes a step of forming a powder epoxy resin primer layer on a steel pipe surface, a step of forming a modified polyethylene resin adhesive layer on the steel pipe surface on which the powder epoxy resin primer layer has been formed, a step of forming a polyethylene layer on the steel pipe surface on which the modified polyethylene resin adhesive layer has been formed, and a step of cooling the steel pipe after the polyethylene layer has been formed. In this embodiment, these steps are continuously performed in a production line (coating line).
In this specification, the process of forming the powder epoxy resin primer layer, the process of forming the modified polyethylene resin adhesive layer, and the process of forming the polyethylene layer are collectively referred to as the coating process.

In the process of forming the powder epoxy resin primer layer, the steel pipe surface is blasted before the powder epoxy resin primer coating to clean the steel pipe surface and impart surface roughness. In order to remove salt adhering to the steel pipe surface, washing with water may be performed before the blasting. In addition, after the blasting, foreign matter such as iron powder may be removed by washing with water or pickling with phosphoric acid before heating the steel pipe.

The surface (outer surface) of the steel pipe after blasting preferably has a surface roughness Rz of 60 to 100 μm and a peak count of 20 or more. Here, the surface roughness Rz is the ten-point average roughness defined in JIS B0601:1994. The peak count is the number of the average length of the contour curve elements contained in the length L of the contour curve defined in JIS B0601:2013, where L = 10 mm. If the surface roughness Rz is 60 μm or more, the increase in the adhesive surface area due to the surface roughness and the effect of improving cathodic peeling resistance due to the anchor effect are further enhanced. If the surface roughness Rz is 100 μm or less, wetting is sufficient, making it easier to prevent the occurrence of voids between the coating film and the steel pipe surface, and also makes it easier to prevent the coating film thickness at the peak of the surface roughness from becoming smaller, thereby further enhancing the effect of improving cathodic peeling resistance. The surface roughness Rz is more preferably 60 to 90 μm. In addition, if the peak count on the steel pipe surface is 20 or more, the effect of improving cathodic peeling resistance due to the increase in adhesive surface area is further enhanced. The peak count is more preferably 25 or more. The upper limit of the peak count is not particularly limited, but it is preferably 50 or less from the viewpoint of ensuring the wettability of the paint.

This type of blasting can be carried out by using steel grit, steel shot, etc. as the abrasive material for blasting. By changing the type and particle size of the abrasive material, the surface roughness Rz and peak count number of the steel pipe surface can be changed. The abrasive material is not particularly limited, but for example, steel grit may be used alone, steel shot may be used alone, or a mixture of steel grit and steel shot may be used.

After the blasting process, the steel pipe is heated. The steel pipe can be heated by induction heating or the like. This heating is performed to melt and harden the powder epoxy resin primer, and the temperature of the steel pipe surface (outer surface of the steel pipe) is set to 160-210°C. If the temperature is less than 160°C, the powder epoxy resin primer will not harden sufficiently and good adhesion will not be obtained. If the temperature exceeds 210°C, strain aging hardening of the steel pipe will occur, resulting in a material change that increases the strength of the steel pipe. A more suitable range for the temperature is 170-200°C.

In the step of forming the powder epoxy resin primer layer, after the heating, the powder epoxy resin primer is applied to the surface of the steel pipe (the outer surface of the steel pipe) to form a powder epoxy resin primer layer. The coating may be electrostatic powder coating.

The powder epoxy resin primer may be selected according to the characteristics of the coating line. Specifically, the powder epoxy resin primer is a material that hardens at a steel plate surface temperature of 160 to 210°C. Furthermore, it is preferable that the material does not completely harden before the modified polyethylene resin is applied (second layer coating) when forming the second layer of modified polyethylene resin adhesive layer. By not completely hardening the powder epoxy resin primer layer before the second layer coating, the adhesion between the powder epoxy resin primer layer and the second layer of modified polyethylene resin adhesive layer is further improved, and the risk of poor adhesion can be reduced. Furthermore, it is preferable that the material is sufficiently hardened (hardening reaction is completed) by the time cooling is started after the coating process. However, since the hardening reaction may continue due to residual heat even after cooling has started, any material that is sufficiently hardened by the time cooling is completed can be used.

Such powder epoxy resin primers can be selected, for example, by determining the cure time (curing time) of candidate powder epoxy resin primers at a specified temperature (e.g., 180°C) according to CSA Standards (CSA: Canadian Standards Association, CSA Z245.20 Series 18, Section 12.1) and considering the relationship between this cure time and the characteristics of the coating line (time required for the coating process (line speed)). Such powder epoxy resin primers may be used with the cure time adjusted by a known method, or may be selected from commercially available products.

As described above, in the present invention, a powder epoxy resin primer is used as the primer. In addition to powder epoxy resin primers, liquid epoxy resin primers are generally known as primers. When a liquid epoxy resin primer is used, the thickness of the primer layer is about 50 μm, whereas in the present invention, a powder epoxy resin primer is used to form a primer layer with a thickness of about 150 μm to 500 μm. It is known that cathodic peeling occurs when oxygen that permeates the resin layer is reduced to hydroxide ions on the steel surface. In the present invention, a powder epoxy resin primer is used as the primer, and a thick primer layer can be formed, so that oxygen permeation through the resin layer can be suppressed and the cathodic peeling resistance can be further improved. Furthermore, in the present invention, a material that can be cured at low temperature is used as the powder epoxy resin primer, so that changes in the mechanical properties of the steel pipe due to coating (especially the increase in strength due to the thermal history during coating) can be avoided.

After the powder epoxy resin primer layer is formed, a modified polyethylene resin adhesive layer and a polyethylene layer are successively formed. In the step of forming the modified polyethylene resin adhesive layer, the modified polyethylene resin adhesive is applied to the steel pipe surface on which the powder epoxy resin primer layer has been formed by a method such as powder coating or extrusion coating, thereby forming the modified polyethylene resin adhesive layer.
In the step of forming the polyethylene layer, the polyethylene layer can be formed by extrusion coating the surface of the steel pipe on which the modified polyethylene resin adhesive layer has been formed with heated and melted polyethylene resin. Thereafter, the steel pipe on which the polyethylene layer has been formed is cooled (water-cooled).

The above process produces a polyethylene-coated steel pipe in which a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer are laminated in that order on the surface of the steel pipe. The polyethylene-coated steel pipe has suppressed changes in mechanical properties. Specifically, the rate of change in the yield strength of the steel pipe before and after the coating process (before and after coating) [100 x (yield strength of polyethylene-coated steel pipe (MPa) - yield strength of steel pipe before coating (MPa)) / yield strength of steel pipe before coating (MPa)] is less than 5%.

When mechanical property changes occur during the thermal history of the coating process and the strength of the steel pipe increases after coating, the workability of the steel pipe decreases. For this reason, mechanical testing may be performed after coating, but since it may be difficult to perform mechanical testing after coating due to process reasons, a polyethylene-coated steel pipe is required that does not change mechanical properties due to the thermal history of the coating process, i.e., in which the change in mechanical properties is suppressed. The polyethylene-coated steel pipe of the present invention has an excellent effect of suppressing the change in mechanical properties, and is particularly suitable for applications in which slight changes in mechanical properties such as those described above are problematic. An example of such an application is a gas pipe.

The steel pipe used was a carbon steel pipe of API SPECIFICATION 5L, X65 with an outer diameter of 610 mm, a pipe thickness of 17.6 mm, and a length of 12 m. Before coating (before applying powder epoxy resin primer), tensile test pieces were taken from the end of the steel pipe to check the change in mechanical properties. The outer surface of the steel pipe was blasted using steel grit with different particle sizes as an abrasive to clean the steel pipe surface and give it a surface roughness of Rz 52 to 95 μm and peak count number 18 to 35. The steel pipe surface (outer surface) was then heated to 132 to 265 °C by induction heating. The heated outer surface of the steel pipe was electrostatically powder coated with a powder epoxy resin primer (PIPECLAD 2000 LAT-Slow GEL manufactured by Valspar) to a thickness of about 300 μm. The cure time of the powder epoxy resin primer at 180°C (based on CSA Z245.20 Series 18 12.1) was 130 seconds. After that, a modified polyethylene resin adhesive layer and a polyethylene layer were successively formed. The modified polyethylene resin adhesive layer was formed by electrostatic powder coating with a modified polyethylene resin adhesive (ME0433 manufactured by Borealis) to a thickness of about 200 μm. The polyethylene layer was formed by wrapping polyethylene (HE3450 manufactured by Borouge) in a sheet shape using an extruder and a T-die, with a total thickness of 3.2 mm. In this example, the time required for the coating process (the time from coating of the powder epoxy resin primer to the start of cooling) was 130 seconds. After coating, water cooling was performed to produce a polyethylene-coated steel pipe.

(Tensile test)
Test pieces of the full wall thickness were taken from the steel pipe before coating and the steel pipe after coating (polyethylene-coated steel pipe) so that the longitudinal direction of the steel pipe and the test piece were aligned. Then, tensile tests were performed in accordance with API SPECIFICATION 5L for five tests, and the average value of the yield strength, which is likely to change due to strain aging hardening, was obtained. The yield strength of the steel pipe after coating was compared with that of the steel pipe before coating, and if the rate of change (rate of increase) was less than 5%, it was determined that the change in mechanical properties was suppressed. A rate of change of less than 5% has the advantage of being easier to design in manufacturing and increasing the yield. The test results are shown in Table 1.

(Cathodic Disbondment Test)
A test piece of 100 mm x 100 mm x total thickness was taken from the polyethylene-coated steel pipe manufactured by the above method. A circular artificial defect with a diameter of 6 mmφ was formed in the center of the test piece, and the outer surface of the steel pipe was exposed. An acrylic cylinder with an inner diameter of 70 mmφ was vertically placed on the polyethylene layer with the artificial defect as the center and fixed to the polyethylene layer with a sealant, and the inside of the cylinder was filled with a 3 mass% NaCl aqueous solution to create a cell. Platinum was used as the counter electrode, and the potential of the steel pipe at the artificial defect was maintained at -1.5 V against the silver-silver chloride electrode using a potentiostat. The test piece was left stationary in a thermostatic chamber at 80 ° C., and the potential was maintained for 28 days. Next, after recovering the test piece, the acrylic cylinder was removed, and the polyethylene coating was removed from around the artificial defect using a chisel and a cutter. The remaining primer layer was cut in eight directions with a cutter centered on the artificial defect. The cutter was inserted from the artificial defect under the primer layer, and the part where it was easily peeled off and the steel surface was exposed was the peeled part. The length from the end of the artificial defect to the tip of the peeled portion in each of the eight directions was measured, and the average value was taken as the cathodic peeling distance (mm). The smaller the value of this cathodic peeling distance, the better, and "15 mm or less" was considered to be a pass. Polyethylene-coated steel pipes that pass the test are suitable for use in laying on the seabed or burying underground. Table 1 shows the test results.

In the present invention examples No. 1 to 7, the rate of change in the yield strength of the steel pipe is less than the prescribed 5%, the change in the mechanical properties of the steel pipe is suppressed, and the cathodic peeling distance is less than the prescribed 15 mm, showing excellent cathodic peeling resistance. No. 8 to 14 are comparative examples. In No. 8 and No. 9, the steel pipe surface temperature is low when the powder epoxy resin primer is applied, the powder epoxy resin primer is insufficiently cured, good adhesion is not obtained, and the cathodic peeling distance is more than 15 mm. In No. 10 to No. 14, the steel pipe surface temperature is high when the powder epoxy resin primer is applied, and the rate of change in the yield strength of the steel pipe is 5% or more. In No. 11, 12, and 14, the steel pipe surface temperature is even higher when the powder epoxy resin primer is applied, the powder epoxy resin primer is overheated, and the resin deteriorates, and the cathodic peeling distance is more than 15 mm.

Thus, according to the present invention, a polyethylene-coated steel pipe with excellent cathodic peeling resistance and suppressed changes in mechanical properties can be obtained. According to the present invention, a polyethylene-coated steel pipe with excellent cathodic peeling resistance can be manufactured by non-chromate treatment, thereby reducing the environmental burden. Furthermore, the polyethylene-coated steel pipe manufactured according to the present invention suppresses changes in the mechanical properties of the steel pipe before and after coating, so it can be suitably applied to applications where slight changes in mechanical properties before and after coating are problematic.

Claims (1)

A method for manufacturing a polyethylene-coated steel pipe in which a powder epoxy resin primer layer, a modified polyethylene resin adhesive layer, and a polyethylene layer are sequentially laminated on the surface of the steel pipe,
Before forming the powder epoxy resin primer layer, a blast treatment is performed on the surface of the steel pipe, and the surface roughness Rz of the steel pipe surface after the blast treatment is 60 to 100 μm and the peak count number is 20 to 35,
A method for producing a polyethylene-coated steel pipe, characterized in that the surface temperature of the steel pipe after the blast treatment during the powder epoxy resin primer coating for forming the powder epoxy resin primer layer is set to 160 to 210°C.