JP2016050654A - Resin coated metal pipe and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resin coated metal pipe excellent in corrosion resistance and impact resistance, and having excellent adhesion between a zinc plated layer and a resin coated layer without using a primer layer, having a small number of processes in manufacturing, and having small environment load.SOLUTION: A resin coated metal pipe 10 includes a metal pipe 12, a zinc plated layer 14 formed on the outer peripheral surface of the metal pipe 12, and a resin coated layer 16 formed on the surface of the zinc plated layer 14 by fusion molding. The reins coated layer 16 comprises a reins material containing a fusion-moldable fluorine resin A having at least one kind of reactive functional group selected from a group comprising a carbonyl group-containing group, a hydroxy group, an epoxy group and an isocyanate group.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a resin-coated metal tube and a method for producing the same.

  Various pipes (fuel transport pipes, brake system pipes, etc.) in vehicles (automobiles, etc.) are required to have heat resistance in a high temperature environment, corrosion resistance against salt water, battery fluid, etc., and impact resistance against flint. Therefore, as the piping, a galvanized layer having corrosion resistance and impact resistance was formed on the outer peripheral surface of a metal tube having heat resistance, and a resin coating layer having corrosion resistance and impact resistance was formed on the surface of the galvanized layer. A resin-coated metal tube is used.

As the resin-coated metal tube, for example, the following are proposed.
(1) A resin coating in which a galvanized layer is formed on the outer peripheral surface of a metal tube by electroplating, a primer layer is formed on the surface of the galvanized layer, and polyamide is extruded on the surface of the primer layer to form a resin coating layer Metal tube (Patent Document 1).
(2) A resin in which a zinc coating layer is formed on the outer peripheral surface of a metal tube by electroplating, a primer layer is formed on the surface of the zinc plating layer, and a fluororesin paint is applied to the surface of the primer layer to form a resin coating layer Coated metal tube (Patent Document 2).

  In the resin-coated metal pipes (1) and (2), since the adhesion between the galvanized layer and the resin coated layer is poor, it is necessary to provide a primer layer between the galvanized layer and the resin coated layer. For this reason, the resin-coated metal pipes (1) and (2) have a problem that the number of steps for forming the primer layer is increased and the environmental load due to the solvent in forming the primer layer is increased. Further, the resin-coated metal pipe (1) has a problem that the corrosion resistance is inferior to that of a fluororesin because the resin coating layer is polyamide. In addition, in the resin-coated metal tube (2), the environmental burden due to the solvent when applying the fluororesin coating is increased, and the resin coating layer formed by the application is thin, so it is necessary to apply multiple coatings, and there are many processes. There is a problem of becoming.

Japanese Patent Laid-Open No. 10-315295 JP 2003-269660 A

  The present invention is a resin-coated metal tube that is excellent in corrosion resistance, has good adhesion between a galvanized layer and a resin coating layer without a primer layer, has a small number of steps in production, and has a small environmental load; Provided is a method capable of producing a resin-coated metal tube having the number of steps and a relatively small environmental load, excellent corrosion resistance, and good adhesion between a galvanized layer and a resin coating layer without a primer layer.

The present invention has the following aspects.
[1] A metal tube, a galvanized layer formed on the outer peripheral surface of the metal tube, and a resin coating layer formed on the surface of the galvanized layer by melt molding, wherein the resin coating layer is a carbonyl A resin-coated metal tube comprising a resin material containing a melt-moldable fluororesin (A) having at least one reactive functional group selected from the group consisting of a group-containing group, a hydroxy group, an epoxy group, and an isocyanate group.
[2] The resin-coated metal tube according to [1], wherein a peel strength at an interface between the galvanized layer and the resin coating layer is 20 N / cm or more.

[3] A method for producing a resin-coated metal tube, comprising the following step (a) and the following step (c).
(A) A step of preparing a plated metal tube having a galvanized layer on the outer peripheral surface of the metal tube.
(C) A melt moldable fluorine having at least one reactive functional group selected from the group consisting of a carbonyl group-containing group, a hydroxy group, an epoxy group and an isocyanate group on the surface of the galvanized layer of the plated metal tube A step of melt-molding a resin material containing the resin (A) to form a resin coating layer.
[4] The method for producing a resin-coated metal tube according to [3], further comprising the following step (b).
(B) A step of bringing an acidic aqueous solution into contact with the galvanized layer of the plated metal tube between the step (a) and the step (c).
[5] The method for producing a resin-coated metal tube according to [4], wherein the acidic aqueous solution is an aqueous nitric acid solution.
[6] The resin according to [5], wherein the concentration of the aqueous nitric acid solution is 0.5 to 5% by mass, and the contact time between the galvanized layer of the plated metal tube and the aqueous nitric acid solution is 1 to 30 seconds. A method for producing a coated metal tube.

The resin-coated metal tube of the present invention is excellent in corrosion resistance, has good adhesion between the galvanized layer and the resin coating layer without a primer layer, has a small number of steps in production, and has a small environmental load.
According to the method for producing a resin-coated metal tube of the present invention, the corrosion resistance is excellent with a relatively small number of steps and a relatively small environmental load, and the adhesion between the galvanized layer and the resin coating layer is good without a primer layer. A resin-coated metal tube can be manufactured.

It is sectional drawing which shows an example of the resin-coated metal tube of this invention.

The following definitions of terms apply throughout this specification and the claims.
“Hot plating” means a method of forming a metal coating on the surface of a material to be treated by immersing the material in a molten metal and then pulling it up.
“Electroplating” means a method of forming a metal coating on the surface of a material to be treated by immersing the material to be treated and a counter electrode in an aqueous solution containing a metal salt, using the material to be treated as a negative electrode, and using the counter electrode as a positive electrode. To do.
The “carbonyl group-containing group” means a group having a carbonyl group (—C (═O) —) in the structure.
“Melt moldable” means exhibiting melt fluidity.
“Showing melt fluidity” means that the melt flow rate (hereinafter referred to as MFR) is 0.1 to 1000 g / 10 min at a temperature higher by 20 ° C. than the melting point of the resin under a load of 49 N. Means that temperature exists.
“Structural unit” means a unit derived from a monomer formed by polymerization of the monomer. The structural unit may be a unit directly formed by a polymerization reaction, or may be a unit in which a part of the unit is converted into another structure by treating the polymer.
“Monomer” means a compound having a polymerizable carbon-carbon double bond.

<Resin-coated metal tube>
FIG. 1 is a cross-sectional view showing an example of a resin-coated metal tube according to the present invention.
The resin-coated metal tube 10 includes a metal tube 12, a galvanized layer 14 formed on the outer peripheral surface of the metal tube 12, and a resin-coated layer 16 formed on the surface of the galvanized layer 14.

(Metal pipe)
Examples of the metal pipe include a steel pipe, a copper pipe, a pipe made of another metal, and the like. In the case of a resin-coated metal pipe for vehicle piping, a steel pipe is preferable.
Examples of the steel pipe include a single winding pipe, a double winding pipe, and a seamless pipe. The steel pipe may have a copper layer on the surface, the polymerization surface, or the like. Examples of the steel of the steel pipe include low carbon steel, medium carbon steel, high carbon steel, and alloy steel.

Although the outer diameter of a metal pipe changes with uses and materials, in the case of the steel pipe of the resin coating metal pipe for vehicle piping, 20 mm or less is preferable.
The thickness of the metal pipe varies depending on the application and material, but in the case of a steel pipe of a resin-coated metal pipe for vehicle piping, 0.65 to 1.2 mm is preferable.

(Zinc plating layer)
Examples of the galvanized layer include those formed by hot dipping and those formed by electroplating, and those formed by hot dipping are preferred. In the case of a steel pipe of a resin-coated metal pipe for vehicle piping, a conventional galvanized layer is usually formed by electroplating. By forming the galvanized layer by hot dipping, it is possible to form a galvanized layer thicker than electroplating in a short time. A thick galvanized layer can sufficiently impart impact resistance to the resin-coated metal tube.

Examples of the material for the galvanized layer include zinc or a zinc alloy, and zinc having a purity of 100% is preferable from the viewpoint of economy and availability.
As the zinc alloy, an alloy containing zinc and one or more metals having a higher ionization tendency than zinc is preferable from the viewpoint of chemical stability of the plating layer itself, and a Zn—Al—Mg alloy is more preferable.

The zinc content in the zinc alloy (excluding the Zn—Al—Mg alloy) is preferably 70% by mass or more, and more preferably 75 to 95% by mass from the viewpoint of corrosion resistance to the steel pipe.
As the Zn—Al—Mg alloy, from the viewpoint of corrosion resistance and surface appearance, Mg: 0.05 to 10% by mass, Al: 4 to 22% by mass, and the balance consisting of Zn and inevitable impurities is preferable. .

A Zn—Al—Mg alloy plating layer formed by hot dipping has a ternary eutectic structure of Al / Zn / Zn ₂ Mg. The Al phase, Zn phase, and Zn ₂ Mg phase forming the ternary eutectic structure have irregular sizes and shapes, respectively, and are intricate with each other.
The Al phase is derived from the Al "phase at high temperature in the Al-Zn-Mg ternary equilibrium diagram. The Al" phase at high temperature is an Al solid solution that dissolves Zn and contains a small amount of Mg. . The Al "phase at high temperature usually appears separated into a fine Al phase and a fine Zn phase at room temperature.
The Zn phase is a Zn solid solution in which a small amount of Al is dissolved, and in some cases, Mg is further dissolved.
The Zn ₂ Mg phase is an intermetallic compound phase in which Zn is present in the vicinity of a point of about 84% by mass in the Zn—Mg binary equilibrium diagram.

The higher the area ratio of the ternary eutectic structure on the surface of the Zn—Al—Mg alloy plating layer, the better. By increasing the area ratio of the ternary eutectic structure, specifically by setting the area ratio of phases other than the Zn phase to 20% or more, the adhesiveness with the resin coating layer can be improved. Moreover, the corrosion resistance of the metal tube can be improved.
The area ratio of the Al phase on the surface of the Zn—Al—Mg alloy plating layer is preferably 15 to 45%, the area ratio of the Zn phase is preferably 50 to 80%, and the area ratio of the Zn ₂ Mg phase is 5 to 5%. 25% is preferred.
The area ratio of the ternary eutectic structure can be increased by, for example, decreasing the concentration of Al in the plating bath.
The ternary eutectic structure and other phases can be easily distinguished by observing the surface of the Zn—Al—Mg alloy plating layer with an electron microscope. The area ratio of each phase can be determined by analyzing the surface of the Zn—Al—Mg alloy plating layer using an electron beam microanalyzer (EPMA).

The thickness of the galvanized layer is preferably 10 to 50 μm from the viewpoint of corrosion resistance, impact resistance and economy.
The adhesion amount of the galvanized layer is preferably 10 to 600 g / m ² . When the adhesion amount of the galvanized layer is 10 g / m ² or more, the corrosion resistance is further improved. If the adhesion amount of the galvanized layer is 600 g / m ² or less, even if the ductility between the metal tube and the galvanized layer is different, at the interface between the metal tube and the galvanized layer in the processed part during bending, etc. Peeling hardly occurs.

(Resin coating layer)
The resin coating layer is formed by melt molding. By forming the resin coating layer by melt molding, it is possible to form a resin coating layer thicker than the coating in one step. A thick resin coating layer can sufficiently impart impact resistance to the resin-coated metal tube. Further, by forming the resin coating layer by melt molding, no solvent is required, and the environmental load during production is small compared to coating.

The film thickness of the resin coating layer is preferably 1 to 50 μm from the viewpoint of corrosion resistance, impact resistance and economy.
The peel strength at the interface between the galvanized layer and the resin coating layer is preferably 20 N / cm or more, more preferably 25 N / cm or more, and further preferably 30 N / cm or more.

(Resin material)
The resin coating layer is made of a resin material containing a fluororesin (A).
The resin material may contain components other than the fluororesin (A) as long as the effects of the present invention are not impaired.
Other components include other resins (B) that can be melt-molded other than the fluororesin (A), additives (C), and the like.

(Fluororesin (A))
The fluororesin (A) is a meltable fluororesin.
The fluororesin (A) has a temperature at which the MFR is 0.1 to 1000 g / 10 min at a temperature 20 ° C. or higher than the melting point of the fluororesin (A) under a load of 49 N. Those having a temperature of 5 to 100 g / 10 min are preferable, those having a temperature of 1 to 30 g / 10 min are more preferable, and those having a temperature of 5 to 20 g / 10 min are more preferable. . If MFR is more than the said lower limit, the fluidity | liquidity of a resin material will become favorable. If MFR is below the said upper limit, the mechanical strength of a resin coating layer will become favorable. In addition, as temperature at the time of measuring MFR, 297 degreeC or 372 degreeC is normally employ | adopted according to melting | fusing point of a fluororesin (A).

  The MFR of the fluororesin (A) is a measure of the molecular weight of the fluororesin (A). When the MFR is large, the molecular weight is low, and when the MFR is small, the molecular weight is large. The molecular weight of a fluororesin (A) can be adjusted with the manufacturing conditions of a fluororesin (A). For example, if the polymerization time is shortened during the polymerization of the monomer, the molecular weight tends to decrease and the MFR tends to increase. Moreover, when the fluororesin (A) obtained by the polymerization reaction is heat-treated, a crosslinked structure is formed, the molecular weight tends to increase, and the MFR tends to decrease.

  180-320 degreeC is preferable, as for melting | fusing point of a fluororesin (A), 200-300 degreeC is more preferable, and 230-300 degreeC is further more preferable. When the melting point of the fluororesin (A) is at least the lower limit of the above range, the heat resistance of the resin coating layer will be good. If the melting point of the fluororesin (A) is not more than the upper limit of the above range, the melt moldability of the resin material will be good.

The fluororesin (A) has at least one reactive functional group selected from the group consisting of a carbonyl group-containing group, a hydroxy group, an epoxy group, and an isocyanate group.
When the fluorine-containing copolymer (B) has a reactive functional group, the adhesion between the galvanized layer and the resin coating layer is improved.

The fluororesin (A) preferably has at least a carbonyl group-containing group as a reactive functional group.
A carbonyl group-containing group is a group containing a carbonyl group (—C (═O) —) in the structure. Examples of the carbonyl group-containing group include a group containing a carbonyl group between carbon atoms of a hydrocarbon group, a carbonate group, a carboxyl group, a haloformyl group, an alkoxycarbonyl group, an acid anhydride residue, and the like. An acid anhydride residue is preferable from the viewpoint of further improving the adhesion to the resin coating layer.

As a hydrocarbon group, a C2-C8 alkylene group etc. are mentioned, for example. In addition, carbon number of this alkylene group is carbon number in the state which does not contain a carbonyl group. The alkylene group may be linear or branched.
The haloformyl group is represented by —C (═O) —X (where X is a halogen atom). Examples of the halogen atom in the haloformyl group include a fluorine atom and a chlorine atom, and a fluorine atom is preferable. That is, the haloformyl group is preferably a fluoroformyl group (also referred to as a carbonyl fluoride group).
The alkoxy group in the alkoxycarbonyl group may be linear or branched. As this alkoxy group, a C1-C8 alkoxy group is preferable, and a methoxy group or an ethoxy group is more preferable.

The content of the reactive functional group in the fluororesin (A) is preferably 500 or more, more preferably 600 or more, and 800 or more with respect to 1 × 10 ⁶ main chain carbon atoms of the fluororesin (A). Is more preferable. The content of the reactive functional group in the fluororesin (A) is preferably 10,000 or less, more preferably 5000 or less, and more preferably 3000 or less with respect to 1 × 10 ⁶ main chain carbon atoms of the fluororesin (A). Is more preferable.
If content of a reactive functional group is more than the said lower limit, the adhesiveness of a zinc plating layer and a resin coating layer will become still better. If content of a reactive functional group is below the said upper limit, it is excellent in the balance of the heat resistance and adhesiveness of a resin coating layer.

The reactive functional group content of the fluororesin (A) can be easily adjusted within the above range by employing the method (1) described later to produce the fluororesin (A).
The content of the reactive functional group of the fluororesin (A) can be measured by a method such as nuclear magnetic resonance (NMR) analysis or infrared absorption spectrum analysis. For example, using a method such as infrared absorption spectrum analysis as described in JP-A-2007-314720, the proportion of structural units having reactive functional groups in all the structural units constituting the fluororesin (A) ( Mol%) and the content of the reactive functional group can be calculated from the ratio.

The fluororesin (A) has a reactive functional group derived from at least one selected from the group consisting of a monomer, a chain transfer agent, and a polymerization initiator used in the production of the fluororesin (A).
As the fluororesin (A), a fluororesin (A) having a reactive functional group derived from a monomer is preferable. The fluororesin (A) can be produced by the method (1) described later. According to the method (1), since the content of the reactive functional group can be easily controlled, it is easy to obtain a fluororesin (A) that can form a resin coating layer having good adhesion.

(Fluorine-containing copolymer (A1))
As the fluororesin (A) having a reactive functional group derived from a monomer, a structural unit based on the following monomer (m1) (from the viewpoint of further excellent corrosion resistance, impact resistance, and adhesion of the resin coating layer) The fluorine-containing copolymer (A1) having u1), a structural unit (u2) based on the following monomer (m2), and a structural unit (u3) based on the following monomer (m3) is preferable.
Monomer (m1): Tetrafluoroethylene.
Monomer (m2): A monomer having a reactive functional group.
Monomer (m3): A monomer other than the monomer (m1) and the monomer (m2).

Structural unit (u1):
The monomer (m1) constituting the structural unit (u1) is tetrafluoroethylene (hereinafter also referred to as TFE).
When the fluorinated copolymer (A1) has the structural unit (u1), the corrosion resistance and impact resistance of the resin coating layer are improved.

Structural unit (u2):
The monomer (m2) constituting the structural unit (u2) is a monomer having a reactive functional group.
When the fluorinated copolymer (A1) has the structural unit (u2), the adhesiveness of the resin coating layer is improved.

As the monomer (m2), a cyclic hydrocarbon monomer having an acid anhydride residue as a reactive functional group is preferable from the viewpoint that the adhesiveness of the resin coating layer is further improved.
Examples of the cyclic hydrocarbon monomer include itaconic anhydride (hereinafter also referred to as IAH), citraconic anhydride (hereinafter also referred to as CAH), 5-norbornene-2,3-dicarboxylic acid anhydride (hereinafter referred to as NAH). And maleic anhydride and the like. The cyclic hydrocarbon monomer may be used alone or in combination of two or more.

  As the cyclic hydrocarbon monomer, a fluorine-containing compound having an acid anhydride residue can be used without using a special polymerization method required when maleic anhydride is used (see JP-A No. 11-193132). One or more selected from the group consisting of IAH, CAH and NAH is preferable from the viewpoint of easy production of the copolymer (A1), and NAH is preferable from the viewpoint of further excellent adhesiveness of the resin coating layer.

Structural unit (u3):
The monomer (m3) constituting the structural unit (u3) is a monomer other than the monomer (m1) and the monomer (m2).

As a monomer (m3), the following monomer (m31) and the following monomer (m32) are mentioned. As the monomer (m3), the monomer (m31) is preferable from the viewpoint that the heat resistance of the resin coating layer is further improved. As the monomer (m3), the monomer (m32) is preferable from the viewpoint that the melt moldability of the resin material is excellent.
Monomer (m31): fluorinated monomer (excluding the monomer (m1)).
Monomer (m32): Non-fluorinated monomer (excluding the monomer (m2)).

The monomer (m31) is preferably a fluorine-containing monomer having one polymerizable double bond from the viewpoint of heat resistance and mechanical properties. Examples of the fluorine-containing monomer include the following.
Fluoroolefin (excluding monomer (m1)): vinyl fluoride, vinylidene fluoride (hereinafter also referred to as VdF), trifluoroethylene, chlorotrifluoroethylene (hereinafter also referred to as CTFE), hexa Fluoropropylene (hereinafter also referred to as HFP) and the like.
CF ₂ = CFOR ^f1 (where R ^f1 is a C 1-10 perfluoroalkyl group which may contain an oxygen atom between carbon atoms).
^{_{CF 2 = CFOR f2 SO 2 X}} 1 ( provided ^{that, R f2} is a perfluoroalkylene group having 1 to 10 carbon atoms which may contain an oxygen atom between carbon atoms, ^{X 1} is a halogen atom or a hydroxyl group.) .
CF ₂ = CFOR ^f3 CO ₂ X ² (where R ^f3 is a C 1-10 perfluoroalkylene group which may contain an oxygen atom between carbon atoms, and X ² is a hydrogen atom or a carbon atom of 3 or less. An alkyl group).
_{CF 2 = CF (CF 2)} p OCF = CF 2 ( Here, p is 1 or 2.).
^{_{CH 2 = CX 3 (CF 2}} ) q X 4 ( however, ^{X 3} is a hydrogen atom or a fluorine atom, q is an integer from 2 to 10, ^{X 4} is a hydrogen atom or a fluorine atom. ).
Perfluoro (2-methylene-4-methyl-1,3-dioxolane) and the like.

The monomer (m31) is at least 1 selected from the group consisting of VdF, CTFE, HFP, CF ₂ = CFOR ^f1 , and CH ₂ = CX ³ (CF ₂ ) _q X ⁴ from the viewpoint of further excellent heat resistance. preferably _seeds, CF 2 ^{= CFOR} _f1, HFP are more preferred.

Examples of CF ₂ = CFOR ^f1 include the following, and CF ₂ = CFOCF ₂ CF ₂ CF ₃ (hereinafter also referred to as PPVE) is preferable from the viewpoint of further excellent heat resistance.
CF ₂ = CFOCF ₂ CF ₃ ,
CF ₂ = CFOCF ₂ CF ₂ CF ₃ ,
CF ₂ = CFOCF ₂ CF ₂ CF ₂ CF ₃ ,
CF ₂ = CFO (CF ₂ ) ₈ F etc.

Examples of CH ₂ ═CX ³ (CF ₂ ) _q X ⁴ include the following, and CH ₂ ═CH (CF ₂ ) ₄ F or CH ₂ ═CH (CF ₂ ) from the viewpoint of further excellent heat resistance. ₂ F is preferred.
_{CH 2 = CH (CF 2)} 2 F,
_{CH 2 = CH (CF 2)} 3 F,
CH ₂ = CH (CF ₂ ) ₄ F,
_{CH 2 = CF (CF 2)} 3 H,
CH ₂ ═CF (CF ₂ ) ₄ H etc. The monomer (m31) may be used alone or in combination of two or more.

The monomer (m32) is preferably a non-fluorinated monomer having one polymerizable double bond from the viewpoint of melt moldability. Examples of the non-fluorinated monomer include the following, and ethylene, propylene, and vinyl acetate are preferable, and ethylene is more preferable from the viewpoint of melt moldability and availability.
Olefin having 3 or less carbon atoms: ethylene, propylene and the like.
Vinyl ester: vinyl acetate and the like.
A monomer (m32) may be used individually by 1 type, and may use 2 or more types together.

When the monomer (m3) constituting the structural unit (u3) is the monomer (m31), the total molar amount of the structural unit (u1), the structural unit (u2), and the structural unit (u3) The structural unit (u1) is 80 to 99.69 mol%, the structural unit (u2) is 0.01 to 0.5 mol%, and the structural unit (u3) is 0.3 to 19.99 mol%. The structural unit (u1) is 90 to 99.58 mol%, the structural unit (u2) is 0.02 to 0.3 mol%, and the structural unit (u3) is 0.4 to 0.4%. More preferably 9.98 mol%, the structural unit (u1) is 97.5-99.5 mol%, the structural unit (u2) is 0.05-0.3 mol%, and the structural unit (U3) is more preferably 0.45 to 2.45 mol%.
When the content of each structural unit is within the above range, the corrosion resistance, impact resistance, adhesion, and heat resistance of the resin coating layer will be good.

When the monomer (m3) constituting the structural unit (u3) is the monomer (m32), the total molar amount of the structural unit (u1), the structural unit (u2), and the structural unit (u3) The structural unit (u1) is 50 to 99.69 mol%, the structural unit (u2) is 0.01 to 1.0 mol%, and the structural unit (u3) is 0.3 to 49.99 mol%. The structural unit (u1) is 52 to 69.98 mol%, the structural unit (u2) is 0.02 to 1.0 mol%, and the structural unit (u3) is 30 to 47. It is more preferably 98 mol%, the structural unit (u1) is 53 to 64.95 mol%, the structural unit (u2) is 0.05 to 0.8 mol%, and the structural unit (u3) is More preferably, it is 35-46.95 mol%.
When the content of each structural unit is within the above range, the corrosion resistance, impact resistance, adhesiveness, and melt moldability of the resin material become good.

When the monomer (m3) constituting the structural unit (u3) is the monomer (m31) and the monomer (m32), the structural unit (u1), the structural unit (u2), the structural unit (u3), The structural unit (u1) is 50 to 99.69 mol%, the structural unit (u2) is 0.01 to 1.0 mol%, and the structural unit (u3) is 1. It is preferable that it is 0-49.99 mol%, a structural unit (u1) is 52-69.98 mol%, a structural unit (u2) is 0.02-1.0 mol%, and a structural unit ( More preferably, u3) is 30 to 47.98 mol%, the structural unit (u1) is 53 to 64.95 mol%, and the structural unit (u2) is 0.05 to 0.8 mol%. More preferably, the structural unit (u3) is 35 to 46.95 mol%.
If the content of each structural unit is within the above range, the corrosion resistance, impact resistance, adhesiveness, heat resistance, and melt moldability of the resin material will be good.

When the monomer (m3) constituting the structural unit (u3) is the monomer (m31) and the monomer (m32), the monomer with the structural unit (u31) based on the monomer (m31) The structural unit (u32) based on (m32) and the molar ratio (structural unit (u31) / structural unit (u32)) are preferably 0/100 to 100/0, and more preferably 1/99 to 10/90.
The content of each structural unit can be calculated by melt NMR analysis, fluorine content analysis, infrared absorption spectrum analysis, or the like of the fluorine-containing copolymer (A1).

The fluorine-containing copolymer (A1) is composed of the structural unit (u1), the structural unit (u2), and the structural unit (u3), and the structural unit (u2) has one acid anhydride residue. When based on the monomer, the content of the structural unit (u2) is 0.01 mol% with respect to the total molar amount of the structural unit (u1), the structural unit (u2), and the structural unit (u3). This corresponds to the content of the acid anhydride residue in the fluorine-containing copolymer (A1) being 100 with respect to 1 × 10 ⁶ main chain carbon atoms of the fluorine-containing copolymer (A1). The content of the structural unit (u2) is 5 mol% based on the total molar amount of the structural unit (u1), the structural unit (u2), and the structural unit (u3) in the fluorine-containing copolymer (A1). This corresponds to a content of acid anhydride residues of 50,000 per 1 × 10 ⁶ main chain carbon atoms of the fluorine-containing copolymer (A1).

  When the structural unit (u2) is based on a cyclic hydrocarbon monomer having an acid anhydride residue, the fluorinated copolymer (A1) is formed by hydrolysis of a part of the cyclic hydrocarbon monomer. May have a structural unit based on a dicarboxylic acid obtained (itaconic acid, citraconic acid, 5-norbornene-2,3-dicarboxylic acid, maleic acid, etc.). When it has a structural unit based on dicarboxylic acid, content of this structural unit shall be contained in content of a structural unit (u2).

When the monomer (m3) constituting the structural unit (u3) is the monomer (m31), preferred specific examples of the fluorine-containing copolymer (A1) include TFE / PPVE / NAH copolymer, TFE / PPVE / IAH copolymer, TFE / PPVE / CAH copolymer, TFE / HFP / IAH copolymer, TFE / HFP / CAH copolymer, TFE / VdF / IAH copolymer, TFE / VdF / CAH copolymer A polymer etc. are mentioned.
When the monomer (m3) constituting the structural unit (u3) is the monomer (m32), preferred specific examples of the fluorine-containing copolymer (A1) include TFE / IAH / ethylene copolymer and the like. Can be mentioned.
When the monomer (m3) constituting the structural unit (u3) is the monomer (m31) and the monomer (m32), preferred specific examples of the fluorinated copolymer (A1) include TFE / CH ₂ = CH (CF ₂ ) ₄ F / IAH / ethylene copolymer, TFE / CH ₂ ═CH (CF ₂ ) ₄ F / CAH / ethylene copolymer, TFE / CH ₂ ═CH (CF ₂ ) ₂ F / Examples thereof include IAH / ethylene copolymer, TFE / CH ₂ ═CH (CF ₂ ) ₂ F / CAH / ethylene copolymer, and the like.

(Method for producing fluororesin (A))
Examples of the method for producing the fluororesin (A) include the following methods (1) to (3). A resin coating layer in which the content of reactive functional groups can be easily controlled and the adhesiveness is good. The method (1) is preferable from the viewpoint that a fluororesin (A) that can be formed is easily obtained.
Method (1): A method of using a monomer having a reactive functional group when the fluororesin (A) is produced by a polymerization reaction.
Method (2): A method for producing a fluororesin (A) by a polymerization reaction using a chain transfer agent having a reactive reactive group. However, the chain transfer agent used here needs to have a reactive functional group that does not cleave as the chain transfer agent generates a radical.
Method (3): A method for producing the fluororesin (A) by a polymerization reaction using a polymerization initiator such as a radical polymerization initiator having a reactive reactive group. However, the radical polymerization initiator used here needs to have a reactive functional group that does not cleave as the polymerization initiator generates radicals.

According to the method (1), the fluororesin (A) having a reactive functional group derived from a monomer can be produced. In this case, a reactive functional group exists in the structural unit which comprises a fluororesin (A).
According to the method (2), the fluororesin (A) having a reactive functional group derived from a chain transfer agent can be produced. In this case, the reactive functional group exists as a terminal group at the main chain terminal of the fluororesin (A).
According to the method (3), the fluororesin (A) having a reactive functional group derived from the polymerization initiator can be produced. In this case, the reactive functional group exists as a terminal group at the main chain terminal of the fluororesin (A).

  When the reactive functional group possessed by the fluororesin (A) is derived from two or more of the monomer, chain transfer agent and polymerization initiator used in the production of the fluororesin (A), the fluororesin (A) can be produced by using two or more of the methods (1) to (3) in combination.

When the fluororesin (A) is produced by a polymerization reaction, the polymerization method is preferably a polymerization method using a radical polymerization initiator from the viewpoint of controlling the MFR of the fluororesin (A).
Polymerization method includes bulk polymerization method; solution polymerization method using organic solvent (fluorinated hydrocarbon, chlorinated hydrocarbon, fluorinated chlorinated hydrocarbon, alcohol, hydrocarbon, etc.); aqueous medium and appropriate organic solvent as required A suspension polymerization method using an aqueous medium and an emulsifier, and a solution polymerization method is preferable from the viewpoint of controlling the MFR of the fluororesin (A).

When producing a fluororesin (A) having a reactive functional group derived from a polymerization initiator, a radical polymerization initiator having a reactive functional group is used.
On the other hand, when manufacturing the fluororesin (A) which does not have the reactive functional group derived from a polymerization initiator, a polymerization initiator may be used and it is not necessary to use it. When using a polymerization initiator, a radical polymerization initiator having no reactive functional group is used.

As the radical polymerization initiator, an initiator having a 10-hour half-life temperature of 0 to 100 ° C is preferable, and an initiator having a 20 to 90 ° C is more preferable.
Examples of the radical polymerization initiator having a reactive functional group include di-n-propyl peroxydicarbonate, diisopropyl peroxycarbonate, tert-butyl peroxyisopropyl carbonate, bis (4-tert-butylcyclohexyl) peroxydicarbonate, di-2- Examples include ethylhexyl peroxydicarbonate.
Radical polymerization initiators that do not have reactive functional groups include azo compounds (azobisisobutyronitrile, etc.), non-fluorinated diacyl peroxides (isobutyryl peroxide, octanoyl peroxide, benzoyl peroxide, lauroyl peroxide, etc.), peroxy Dicarbonate (diisopropylperoxydicarbonate, etc.), peroxyester (tert-butylperoxypivalate, tert-butylperoxyisobutyrate, tert-butylperoxyacetate, etc.), fluorine-containing diacyl peroxide ((Z (CF ₂ ) _r COO ) ₂ (wherein Z is a hydrogen atom, fluorine atom or chlorine atom, r is an integer of 1 to 10), etc.), inorganic peroxide (potassium persulfate, persulfate) Sodium, persulfate Ammonium, etc.) and the like.

When producing a fluororesin (A) having a reactive functional group derived from a chain transfer agent, a chain transfer agent having a reactive functional group is used.
On the other hand, when manufacturing the fluororesin (A) which does not have the reactive functional group derived from a chain transfer agent, a chain transfer agent may be used and it is not necessary to use it. When a chain transfer agent is used, a chain transfer agent having no reactive functional group is used.

Examples of the chain transfer agent having a reactive functional group include acetic acid, acetic anhydride, methyl acetate, ethylene glycol, propylene glycol, etc., and acetic anhydride having an anhydride residue from the viewpoint of high reactivity with the counterpart material. Is preferred.
Examples of the chain transfer agent having no reactive functional group include alcohol (methanol, ethanol, etc.), chlorofluorohydrocarbon (1,3-dichloro-1,1,2,2,3-pentafluoropropane, 1,1- Dichloro-1-fluoroethane), hydrocarbons (pentane, hexane, cyclohexane, etc.) and the like.

Examples of the organic solvent used in the solution polymerization method include perfluorocarbon, hydrofluorocarbon, chlorohydrofluorocarbon, and hydrofluoroether. As for carbon number, 4-12 are preferable.
Examples of the perfluorocarbon include perfluorocyclobutane, perfluoropentane, perfluorohexane, perfluorocyclopentane, and perfluorocyclohexane.
Examples of the hydrofluorocarbon include 1-hydroperfluorohexane.
Examples of the chlorohydrofluorocarbon include 1,3-dichloro-1,1,2,2,3-pentafluoropropane.
Examples of the hydrofluoroether include methyl perfluorobutyl ether and 2,2,2-trifluoroethyl 2,2,1,1-tetrafluoroethyl ether.

The polymerization temperature is preferably 0 to 100 ° C, and more preferably 20 to 90 ° C.
The polymerization pressure is preferably from 0.1 to 10 MPa, more preferably from 0.5 to 3 MPa.
The polymerization time is preferably 1 to 30 hours.

  When a cyclic hydrocarbon monomer having an acid anhydride residue is used as the monomer (m2) constituting the structural unit (u2) of the fluorinated copolymer (A1), the cyclic hydrocarbon monomer The concentration during the polymerization is preferably from 0.01 to 5 mol%, more preferably from 0.1 to 3 mol%, still more preferably from 0.1 to 2 mol%, based on all monomers. When the concentration of the cyclic hydrocarbon monomer is within the above range, the polymerization rate during production is moderate. When the concentration of the cyclic hydrocarbon monomer is too high, the polymerization rate tends to decrease. As the cyclic hydrocarbon monomer is consumed during the polymerization, the consumed amount is continuously or intermittently supplied into the polymerization tank, and the concentration of the cyclic hydrocarbon monomer is maintained within the above range. It is preferable to do.

(Other resin (B))
Examples of the other resin (B) include polyetherimide, polyaryl ketone, aromatic polyester, polyamideimide, and thermoplastic polyimide.

(Additive (C))
Examples of the additive (C) include inorganic fillers and pigments.

(Use)
Applications of the resin-coated metal pipe of the present invention include various pipes (fuel transport pipes, brake system pipes) in transportation equipment (vehicles (automobiles, railway vehicles, etc.), aircrafts, etc.), heavy machinery (construction machinery, civil engineering machinery, etc.), etc. Etc.).
The resin-coated metal pipe of the present invention is useful as a fuel transport pipe / brake system pipe, and is particularly useful as a vehicle fuel transport pipe / vehicle brake system pipe.

(Mechanism of action)
The resin-coated metal tube of the present invention described above has heat resistance because it is based on a metal tube.
Moreover, since it has the resin coating layer formed by melt molding on the surface of a galvanization layer, it is not necessary to use a solvent in the case of manufacture, and an environmental load is small. Further, it is not necessary to perform overcoating during production, and the number of processes is small.
Moreover, since the resin coating layer is a fluororesin, it is excellent in corrosion resistance compared with polyamide.
A resin material comprising a melt-moldable fluororesin (A) in which the resin coating layer has at least one reactive functional group selected from the group consisting of a carbonyl group-containing group, a hydroxy group, an epoxy group, and an isocyanate group Therefore, the adhesion between the galvanized layer and the resin coating layer is good without the primer layer. In addition, since it is not necessary to form a primer layer during production, the number of steps is small and the environmental load is small.
By forming the resin coating layer by melt molding, the galvanized layer and the resin coating layer are firmly bonded. This is presumably due to the interaction (for example, hydrogen bonding) between the hydroxyl group on the surface of the galvanized layer and the reactive functional group of the fluororesin (A) contained in the resin coating layer.

<Method for producing resin-coated metal tube>
The manufacturing method of the resin-coated metal tube of the present invention is a method having the following steps (a) to (c).
(A) A step of preparing a plated metal tube having a galvanized layer formed by hot dipping on the outer peripheral surface of the metal tube.
(B) After the step (a), a step of bringing an acidic aqueous solution into contact with the galvanized layer of the plated metal tube as necessary.
(C) A step of forming a resin coating layer by melt-molding a resin material containing the fluororesin (A) on the surface of the galvanized layer of the plated metal tube after the step (a) or the step (b).

(Process (a))
As a method of preparing a plated metal tube, (a1) a method of obtaining a plated metal tube by forming a galvanized layer on the outer peripheral surface of the metal tube by hot dip plating or electroplating, and (a2) plating from a metal tube manufacturer. The method etc. which purchase a metal pipe are mentioned.
A known method may be used as a method of hot dipping or electroplating.

(Process (b))
Examples of the method of bringing the acidic aqueous solution into contact with the galvanized layer of the plated metal tube include (b1) a method of immersing the plated metal tube in the acidic aqueous solution, and (b2) a method of pouring the acidic aqueous solution over the galvanized layer of the plated metal tube. The method (b1) is preferable from the viewpoint that the acidic aqueous solution can be reliably brought into contact with the galvanized layer.
By bringing the acidic aqueous solution into contact with the galvanized layer of the plated metal tube, the adhesion between the galvanized layer and the resin coating layer is further improved.

  Examples of the acidic aqueous solution include an aqueous nitric acid solution, an aqueous hydrochloric acid solution, and an aqueous sulfuric acid solution, and an aqueous nitric acid solution is preferred from the viewpoint of surface cleanability in a short time.

  The concentration of the nitric acid aqueous solution is preferably 0.5 to 5% by mass, and more preferably 1 to 3% by mass. When the concentration of the nitric acid aqueous solution is equal to or higher than the lower limit, a sufficient surface cleaning effect can be obtained and impurities and the like can be sufficiently removed. If the concentration of the nitric acid aqueous solution is equal to or less than the upper limit, the appearance gloss of the plating layer is maintained.

  The contact time between the galvanized layer of the plated metal tube and the acidic aqueous solution is preferably 1 to 30 seconds, and more preferably 5 to 15 seconds. If the contact time is not less than the lower limit, a sufficient surface cleaning effect can be obtained. If the contact time is less than or equal to the upper limit, surface cleaning can be performed without reducing production efficiency.

(Process (c))
Examples of the melt molding method include an extrusion molding method.
In the extrusion method, an extruder or the like provided with a die cross head is used.
As a method for forming the resin coating layer by the extrusion molding method, the resin material containing the fluororesin (A) is melted with an extruder and the plated metal tube is passed through the die outlet while passing the plated metal tube through the die cross head. A method of forming a resin coating layer around the plated metal tube by extruding the molten resin material around the metal plate.
Extrusion molding conditions (cylinder temperature, die temperature, extrusion amount, line speed, etc.) may be appropriately set according to the type of fluororesin, the film thickness of the resin coating layer, and the like.

(Mechanism of action)
In the manufacturing method of the resin-coated metal tube of the present invention described above, since the metal tube is used as a base, a resin-coated metal tube having heat resistance can be manufactured.
Moreover, since the resin coating layer is formed on the surface of the galvanized layer by melt molding, it is not necessary to use a solvent and the environmental load is small. Further, it is not necessary to perform overcoating and the number of processes is small.
In addition, since the resin coating layer is a fluororesin, a resin-coated metal tube that is superior in corrosion resistance compared to polyamide can be produced.
Resin coating using a resin material containing a melt-moldable fluororesin (A) having at least one reactive functional group selected from the group consisting of a carbonyl group-containing group, a hydroxy group, an epoxy group and an isocyanate group Since the layers are formed, a resin-coated metal tube having good adhesion between the galvanized layer and the resin coating layer can be produced without a primer layer. Moreover, since it is not necessary to form a primer layer, the number of processes is small and the environmental load is small.

Hereinafter, the present invention will be specifically described with reference to examples, but the technical scope of the present invention is not limited thereto.
Examples 1 to 4 are experimental examples, and Example 5 is an example.

(Melting point)
Using a differential scanning calorimeter (DSC apparatus, manufactured by Seiko Instruments Inc.), the melting peak when the fluororesin was heated at a rate of 10 ° C./min was recorded, and the temperature corresponding to the maximum value was taken as the melting point.

(MFR)
Using a melt indexer (manufactured by Techno Seven Co., Ltd.), the mass (g) of the fluororesin flowing out from a nozzle having a diameter of 2 mm and a length of 8 mm under a load of 49 N at 297 ° C. for 10 minutes was measured. It was.

(Capacity flow rate)
The volume flow rate (Q value) is an index representing the melt fluidity of the fluororesin and is a measure of the molecular weight. A large Q value indicates a low molecular weight, and a small Q value indicates a high molecular weight. The Q value in this example is obtained when a fluororesin is extruded into an orifice having a diameter of 2.1 mm and a length of 8 mm under a load of 68.65 N at 297 ° C. using a flow tester manufactured by Shimadzu Corporation. Extrusion speed of

(Peel strength)
About 1 cm of the interface between the galvanized layer and the resin coating layer in the resin-coated metal plate was peeled off to prepare a test piece for peel strength measurement. The peel strength was calculated by dividing the load when the resin coating layer of the test piece was peeled off at 50 mm / min with a tensile tester by the width of the test piece.

(Production of fluorinated copolymer (A1))
A polymerization tank equipped with a stirrer with an internal volume of 94 L was degassed, 71.3 kg of 1-hydroperfluorohexane, 1,3-dichloro-1,1,2,2,3-pentafluoropropane (Asahi Glass Co., Ltd., AK225cb) 20.4 kg, CH ₂ ═CH (CF ₂ ) ₂ F, 562 g, and IAH 4.45 g, and the temperature inside the polymerization tank was raised to 66 ° C., and a single amount of 89/11 molar ratio of TFE / ethylene The pressure was increased to 1.5 MPa [gage] with the body mixed gas. As a polymerization initiator, 1 L of a 0.7 mass% 1-hydrotridecafluorohexane solution of tert-butylperoxypivalate was charged to initiate polymerization. During the polymerization, a monomer mixed gas of 60/40 molar ratio of TFE / ethylene was continuously charged so that the pressure was constant. Further, CH ₂ ═CH (CF ₂ ) ₂ F in an amount corresponding to 3.3 mol% and IAH in an amount corresponding to 0.5 mol% are continuously added to the total number of moles of TFE and ethylene charged during the polymerization. I was charged. 9.9 hours after the start of polymerization, when 7.28 kg of the monomer mixed gas was charged, the temperature in the polymerization tank was lowered to room temperature and purged to normal pressure.

The obtained slurry-like fluorine-containing copolymer (A1-1) was put into a 200 L granulation tank charged with 77 kg of water, heated to 105 ° C. with stirring, and the solvent was distilled off. The granulated product was granulated. The obtained granulated product was dried at 150 ° C. for 15 hours to obtain a granulated product of 6.9 kg of a fluorinated copolymer (A1-1).
The fluorine-containing copolymer (A1-1) was subjected to melt NMR analysis, fluorine content analysis, and infrared absorption spectrum analysis. From the results of the analysis, the structural unit based on TFE in the fluorine-containing copolymer (A1-1) / the structural unit based on CH ₂ ═CH (CF ₂ ) ₂ F / the structural unit based on IAH / the structural unit based on ethylene The ratio was 58.6 / 2.0 / 0.3 / 39.1 (molar ratio). The content of the reactive functional group in the fluorinated copolymer (A1-1) was 3000 with respect to 1 × 10 ⁶ main chain carbon atoms of the fluorinated copolymer (A1-1). The melting point of the fluorinated copolymer (A1-1) was 240 ° C., the MFR was 18.5 g / 10 min, and the Q value was 28 mm ³ / sec.

(Example 1)
Using a hot press (SA301, manufactured by Tester Sangyo Co., Ltd.), the fluorine-containing copolymer (A1-1) was molded to obtain a fluororesin sheet 1 (thickness: 200 μm).
Plated steel sheet having a galvanized layer formed by hot dipping on the surface of the steel sheet (JFE Steel, Galva Zinc, <steel plate> length: 10 cm, width: 10 cm, thickness: 1.0 mm, <plated layer> zinc purity: 100 %, Thickness: 30 μm).

The fluororesin sheet 1 was brought into close contact with the plated steel plate, and thermocompression bonding was performed for 120 seconds under the conditions of temperature: 350 ° C. and pressure: 5 MPa to obtain a resin-coated metal plate.
The peel strength at the interface between the resin coating layer and the galvanized layer in the resin-coated metal plate was 25 N / cm.

(Example 2)
The same plated steel sheet as in Example 1 was immersed in a 1% by mass nitric acid solution for 10 seconds, then washed with water and dried.
The fluororesin sheet 1 was brought into close contact with the plated steel sheet after being immersed in the nitric acid solution, and thermocompression bonding was performed for 120 seconds under conditions of temperature: 350 ° C. and pressure: 5 MPa to obtain a resin-coated metal plate.
The peel strength at the interface between the resin coating layer and the galvanized layer in the resin-coated metal plate was 41 N / cm.

(Example 3)
Resin coating in the same manner as in Example 1 except that the fluorinated copolymer (A1-1) was changed to a fluororesin having no reactive functional group (Fluon (registered trademark) ETFE LM-720AP, manufactured by Asahi Glass Co., Ltd.). A metal plate was obtained.
The peel strength at the interface between the resin coating layer and the galvanized layer in the resin-coated metal plate was 5 N / cm.

(Example 4)
Resin-coated in the same manner as in Example 1 except that the fluorinated copolymer (A1-1) was changed to a fluororesin having no reactive functional group (Nephron (registered trademark) ETFE EP-521, manufactured by Daikin). A metal plate was obtained.
The peel strength at the interface between the resin coating layer and the galvanized layer in the resin-coated metal plate was 7 N / cm.

(Example 5)
A plated steel pipe (<steel pipe> outer diameter: 480 mm, wall thickness: 1.0 mm, <plated layer> zinc purity: 100%, thickness: 30 μm) in which a zinc plating layer was formed on the surface of the steel pipe by hot dipping was prepared. .

An extruder (manufactured by Tanabe Plastics, VDC 40-100) provided with a die cross head was prepared. The extrusion molding conditions were: cylinder temperature: 300 ° C., die temperature: 300 ° C., extrusion amount: 0.5 kg / hour, line speed: 10 m / min.
The fluorine-containing copolymer (A1-1) was melted by an extruder and melted around the plated steel pipe from the discharge port of the die while passing the plated steel pipe through the die cross head (A1-1). ) Was extruded to form a resin coating layer (film thickness: 100 μm) around the plated steel pipe to obtain a resin-coated metal pipe.

  The resin-coated metal pipe of the present invention is used as various pipes (fuel transport pipes, brake system pipes, etc.) in transportation equipment (vehicles (automobiles, railway vehicles, etc.), aircrafts, etc.), heavy machinery (construction machinery, civil engineering machinery, etc.), etc. Useful.

DESCRIPTION OF SYMBOLS 10 Resin-coated metal pipe 12 Metal pipe 14 Zinc plating layer 16 Resin coating layer

Claims (6)

A metal tube,
A galvanized layer formed on the outer peripheral surface of the metal tube;
Having a resin coating layer formed by melt molding on the surface of the galvanized layer,
From the resin material in which the resin coating layer contains a melt-moldable fluororesin (A) having at least one reactive functional group selected from the group consisting of a carbonyl group-containing group, a hydroxy group, an epoxy group, and an isocyanate group A resin-coated metal tube.   The resin-coated metal tube according to claim 1, wherein a peel strength at an interface between the galvanized layer and the resin coating layer is 20 N / cm or more. The manufacturing method of the resin-coated metal pipe which has the following process (a) and the following process (c).
(A) A step of preparing a plated metal tube having a galvanized layer on the outer peripheral surface of the metal tube.
(C) A melt moldable fluorine having at least one reactive functional group selected from the group consisting of a carbonyl group-containing group, a hydroxy group, an epoxy group and an isocyanate group on the surface of the galvanized layer of the plated metal tube A step of melt-molding a resin material containing the resin (A) to form a resin coating layer. The method for producing a resin-coated metal tube according to claim 3, further comprising the following step (b).
(B) A step of bringing an acidic aqueous solution into contact with the galvanized layer of the plated metal tube between the step (a) and the step (c).   The method for producing a resin-coated metal tube according to claim 4, wherein the acidic aqueous solution is an aqueous nitric acid solution. The concentration of the aqueous nitric acid solution is 0.5 to 5% by mass,
The method for producing a resin-coated metal tube according to claim 5, wherein a contact time between the galvanized layer of the plated metal tube and the aqueous nitric acid solution is 1 to 30 seconds.

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