EP4678962A1

EP4678962A1 - Metal pipe for oil wells

Info

Publication number: EP4678962A1
Application number: EP24767010.2A
Authority: EP
Inventors: Masahiro Oshima; Masanari Kimoto
Original assignee: Vallourec Oil and Gas France SAS; Nippon Steel Corp
Current assignee: Vallourec Oil and Gas France SAS; Nippon Steel Corp
Priority date: 2023-03-03
Filing date: 2024-02-29
Publication date: 2026-01-14
Also published as: JP2024125004A; WO2024185640A1; AU2024231900A1; CN120752468A; AR132003A1; MX2025009841A

Abstract

An oil-well metal pipe that includes a Zn-Ni alloy plating layer having high hardness is provided. An oil-well metal pipe of the present disclosure includes a pipe main body including a pin and a box, and also includes a Zn-Ni alloy plating layer formed on a pin contact surface or a box contact surface. The Ni content in the Zn-Ni alloy plating layer is, in mass%, 14.8 to 25.0%, and the bulk density of the Zn-Ni alloy plating layer is 7.00 g/cm³ or more.

Description

TECHNICAL FIELD

The present disclosure relates to a metal pipe, and more particularly to an oil-well metal pipe.

BACKGROUND ART

Oil-well metal pipes are used for drilling in oil fields and gas wells (hereinafter, oil fields and gas wells are collectively referred to as "oil wells"). An oil-well metal pipe has a threaded connection. Specifically, at an oil well drilling site, in accordance with the depth of the oil well, a plurality of oil-well metal pipes are connected to form an oil country tubular goods connected body, which is typified by a casing pipe or a tubing pipe. An oil country tubular goods connected body is formed by fastening oil well pipes to each other. Inspections may in some cases be conducted on oil country tubular goods connected bodies. When conducting an inspection, the oil country tubular goods connected body is lifted up and loosened. Then, the oil-well metal pipes are detached from the oil country tubular goods connected body by loosening and are inspected. After the inspection, the oil-well metal pipes are refastened to each other again so that the oil-well metal pipes are reused as a part of the oil country tubular goods connected body.
The oil-well metal pipe includes a pin and a box. The pin has a pin contact surface including an external thread part on an outer peripheral surface of an end portion of the oil-well metal pipe. The box has a box contact surface including an internal thread part on an inner peripheral surface of an end portion of the oil-well metal pipe that is on the opposite side to the pin. When oil-well metal pipes are fastened together, the pin contact surface comes in contact with the box contact surface.
The pin contact surface and the box contact surface repeatedly experience strong friction during fastening and loosening of the oil-well metal pipe. For that reason, in the pin contact surface and the box contact surface, galling (unrepairable galling) is liable to occur when the fastening and loosening are repeated. Accordingly, the oil-well metal pipe is required to have sufficient durability with respect to friction, that is, to have excellent galling resistance.
Heretofore, compound greases containing heavy metal powder, which are referred to as "dopes", have been used to improve the galling resistance. Application of a compound grease to the pin contact surface and/or the box contact surface can improve the galling resistance of the oil-well metal pipe. However, heavy metal powder contained in the compound greases, such as Pb, Zn, and Cu, may affect the environments. For this reason, the development of the oil-well metal pipe that is excellent in galling resistance even without the use of a compound grease is desired.
In an oil-well metal pipe disclosed in Patent Literature 1 (International Application Publication No. WO2016/170031 ), a Zn-Ni alloy plating layer is formed on a pin contact surface or a box contact surface instead of using a compound grease. The Zn contained in the Zn-Ni alloy plating layer formed on a contact surface of the oil-well metal pipe enhances the corrosion resistance of the base metal of the oil-well metal pipe by sacrificial protection. In addition, it is described in Patent Literature 1 that the Zn-Ni alloy is also excellent in a wear resistance characteristic.

CITATION LIST

PATENT LITERATURE

Patent Literature 1: International Application Publication No. WO2016/170031

SUMMARY OF INVENTION

TECHNICAL PROBLEM

As described above, a Zn-Ni alloy plating layer enhances the corrosion resistance of an oil-well metal pipe by means of Zn. In addition, because the hardness of the Zn-Ni alloy plating layer itself is high, favorable galling resistance is obtained. In general, the higher that the hardness of a plating layer is, the higher the wear resistance characteristic will be and the higher the galling resistance will be. Therefore, with respect to a Zn-Ni alloy plating layer that is excellent in corrosion resistance, in order to further improve the galling resistance during fastening and loosening of an oil-well metal pipe, it is desirable to further increase the hardness. In particular, in a large-sized oil-well metal pipe, or an oil-well metal pipe composed of a high alloy, the susceptibility to galling is high. Therefore, in such kinds of oil-well metal pipe, in order to obtain excellent galling resistance while also obtaining corrosion resistance, it is desirable to further increase the hardness of a Zn-Ni alloy plating layer.
An objective of the present disclosure is to provide an oil-well metal pipe that includes a Zn-Ni alloy plating layer which has high hardness.

SOLUTION TO PROBLEM

An oil-well metal pipe of the present disclosure includes:

a pipe main body including a first end portion and a second end portion, wherein
the pipe main body includes:
- a pin formed at the first end portion, and
- a box formed at the second end portion;
- the pin includes:
  - a pin contact surface including an external thread part; and
  - the box includes:
    - a box contact surface including an internal thread part;
    - the oil-well metal pipe further including:
      - a Zn-Ni alloy plating layer formed on the pin contact surface or on the box contact surface, the Zn-Ni alloy plating layer being composed of a Zn-Ni alloy,
      - wherein
      - an Ni content in the Zn-Ni alloy plating layer is, in mass%, 14.8 to 25.0%, and
      - a bulk density of the Zn-Ni alloy plating layer is 7.00 g/cm³ or more.

ADVANTAGEOUS EFFECT OF INVENTION

An oil-well metal pipe of the present embodiment includes a Zn-Ni alloy plating layer which has high hardness.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] FIG. 1 is a graph illustrating the relation between the Ni content and the Vickers hardness of a conventional Zn-Ni alloy plating layer.
[FIG. 2] FIG. 2 is a graph illustrating the relation between the Ni content and the bulk density of a conventional Zn-Ni alloy plating layer.
[FIG. 3] FIG. 3 is a graph in which the relation between the Ni content and the bulk density of a Zn-Ni alloy plating layer of the present embodiment is added to the graph illustrated in FIG. 2.
[FIG. 4] FIG. 4 is a graph in which the relation between the Ni content and the Vickers hardness of a Zn-Ni alloy plating layer of the present embodiment is added to the graph illustrated in FIG. 1.
[FIG. 5] FIG. 5 is a side view of an oil-well metal pipe of the present embodiment.
[FIG. 6] FIG. 6 is a partial cross-sectional view illustrating a cross section (longitudinal cross section) along a pipe axis direction of a coupling of the oil-well metal pipe illustrated in FIG. 5.
[FIG. 7] FIG. 7 is a cross-sectional view parallel to the pipe axis direction of the oil-well metal pipe illustrated in FIG. 5, that illustrates a portion in the vicinity of a pin of the oil-well metal pipe.
[FIG. 8] FIG. 8 is a cross-sectional view parallel to the pipe axis direction of the oil-well metal pipe illustrated in FIG. 5, that illustrates a portion in the vicinity of a box of the oil-well metal pipe.
[FIG. 9] FIG. 9 is a partial cross-sectional view including a longitudinal cross section of the oil-well metal pipe of the present embodiment having a configuration different from that in FIG. 6.
[FIG. 10] FIG. 10 is a partial cross-sectional view including a longitudinal cross section of an integral type oil-well metal pipe according to the present embodiment.
[FIG. 11] FIG. 11 is an enlarged view of a pin contact surface illustrated in FIG. 7.
[FIG. 12] FIG. 12 is an enlarged view of a box contact surface illustrated in FIG. 8.
[FIG. 13] FIG. 13 is an enlarged view of a pin contact surface having a different structure to the pin contact surface illustrated in FIG. 11.
[FIG. 14] FIG. 14 is an enlarged view of a box contact surface having a different structure to the box contact surface illustrated in FIG. 12.

DESCRIPTION OF EMBODIMENTS

The present embodiment is described in detail below with reference to the accompanying drawings. The same reference symbols will be used throughout the drawings to refer to the same or like parts, and description thereof will not be repeated. In the following description, "%" regarding the elemental content in the Zn-Ni plating layer means mass percent.
The present inventors had the idea that if the Ni content in a Zn-Ni alloy plating layer is increased, the hardness of the Zn-Ni alloy plating layer will be further increased. Therefore, first, the present inventors investigated the relation between the Ni content of a Zn-Ni alloy plating layer formed on a contact surface (pin contact surface or box contact surface) of an oil-well metal pipe using the conventional method, and the hardness (Vickers hardness) of the Zn-Ni alloy plating layer, and obtained the results illustrated in FIG. 1.
Referring to FIG. 1, in the conventional Zn-Ni alloy plating layer, until the Ni content was 14.8%, the Vickers hardness increased together with an increase in the Ni content. However, when the Ni content was 14.8% or more, the Vickers hardness decreased as the Ni content increased.
Therefore, the present inventors investigated the reason why the hardness of the Zn-Ni alloy plating layer decreased when the Ni content was 14.8% or more. As a result, the present inventors clarified the following matters. When the Ni content in the Zn-Ni alloy plating layer was increased, a large number of porosities were formed in the Zn-Ni alloy plating layer. Therefore, the present inventors investigated the bulk density, which serves as an index of the proportion of porosities, with respect to the Zn-Ni alloy plating layers plotted in FIG. 1, and obtained the results illustrated in FIG. 2.
Referring to FIG. 2, as the result of the investigations, the present inventors found that in the conventional Zn-Ni alloy plating layer, the bulk density increased together with an increase in the Ni content until the Ni content was 14.8%. However, in a case where the Ni content was 14.8% or more, the bulk density rapidly decreased as the Ni content increased.
Based on the results of the investigations described above, the present inventors considered that the relation between the Zn-Ni alloy plating layer, the Ni content, and the bulk density is as follows. An increase in the Ni content in the Zn-Ni alloy plating layer contributes to an increase in the hardness of the Zn-Ni alloy plating layer. However, when the Ni content becomes high, the bulk density rapidly decreases. In FIG. 1 and FIG. 2, when the Ni content is 14.8% or more, the margin of decrease in hardness that accompanied a decrease in the bulk density is greater than the margin of increase in hardness that accompanied an increase in the Ni content. Hence, when the Ni content is 14.8% or more, the Vickers hardness of the Zn-Ni alloy plating layer decreases accompanying an increase in the Ni content (FIG. 1).
Based on the above findings, the present inventors had the idea that, in a Zn-Ni alloy plating layer, if a decrease in bulk density can be suppressed while increasing the Ni content, the Vickers hardness of the Zn-Ni alloy plating layer can be further increased accompanying an increase in the Ni content. Based on the aforementioned idea, the present inventors conducted tests that are described later, and obtained the results illustrated in FIG. 3 and FIG. 4.
FIG. 3 is a graph in which the relation between the Ni content and the bulk density of a Zn-Ni alloy plating layer of the present embodiment is added to the graph illustrated in FIG. 2. FIG. 4 is a graph in which the relation between the Ni content and the Vickers hardness of a Zn-Ni alloy plating layer of the present embodiment is added to the graph illustrated in FIG. 1. The "O" marks in FIG. 3 and FIG. 4 correspond to Inventive Examples among examples that are described later. Referring to FIG. 3, in the Zn-Ni alloy plating layer of the present embodiment, even when the Ni content is made 14.8% or more, the bulk density is 7.00 g/cm³ or more. In such case, as illustrated in FIG. 4, even when the Ni content is 14.8% or more, the Vickers hardness markedly increases accompanying an increase in the Ni content.
As described above, the present inventors discovered that, in a Zn-Ni alloy plating layer, if the Ni content is 14.8% or more and, furthermore, the bulk density is 7.00 g/cm³ or more, the hardness of the Zn-Ni alloy plating layer markedly increases.
The oil-well metal pipe of the present embodiment that was completed based on the above findings has the following structures.

[1] An oil-well metal pipe, including:
- a pipe main body including a first end portion and a second end portion, wherein
- the pipe main body includes:
  - a pin formed at the first end portion, and
  - a box formed at the second end portion;
  - the pin includes:
    - a pin contact surface including an external thread part; and
    - the box includes:
      - a box contact surface including an internal thread part;
      - the oil-well metal pipe further including:
        
        a Zn-Ni alloy plating layer formed on the pin contact surface or on the box contact surface, the Zn-Ni alloy plating layer being composed of a Zn-Ni alloy,
        
        wherein
        
        an Ni content in the Zn-Ni alloy plating layer is, in mass%, 14.8 to 25.0%, and
        
        a bulk density of the Zn-Ni alloy plating layer is 7.00 g/cm³ or more.
In the oil-well metal pipe of the present embodiment, the Ni content in the Zn-Ni alloy plating layer is, in mass%, 14.8 to 25.0%, and a bulk density of the Zn-Ni alloy plating layer is 7.00 g/cm³ or more. The hardness of a Zn-Ni alloy plating layer having this structure markedly increases. Therefore, the Zn-Ni alloy plating layer can suppress the occurrence of galling during fastening and loosening. In addition, the Zn-Ni alloy plating layer contains Zn. Therefore, a corrosion resistance property can be increased by sacrificial protection.
[2] The oil-well metal pipe of [1] above, wherein, in the Zn-Ni alloy plating layer:
the Ni content is, in mass%, 17.0% or more.

Hereunder, the oil-well metal pipe according to the present embodiment is described in detail.

[Structure of oil-well metal pipe]

First, the structure of the oil-well metal pipe of the present embodiment will be described. The oil-well metal pipe has a well-known structure. The available types of oil-well metal pipe are a T&C type oil-well metal pipe and an integral type oil-well metal pipe. Hereunder, each type of the oil-well metal pipe is described in detail.

[Case where oil-well metal pipe 1 is T&C type]

FIG. 5 is a configuration diagram illustrating one example of an oil-well metal pipe 1 of the present embodiment. FIG. 5 is a configuration diagram illustrating the oil-well metal pipe 1 of a so-called "T&C (threaded and coupled) type". Referring to FIG. 5, the oil-well metal pipe 1 includes a pipe main body 10.
The pipe main body 10 extends in the pipe axis direction. A cross section perpendicular to the pipe axis direction of the pipe main body 10 is a circular shape. The pipe main body 10 includes a first end portion 10A and a second end portion 10B. The first end portion 10A is an end portion on the opposite side to the second end portion 10B. In the T&C type oil-well metal pipe 1 illustrated in FIG. 5, the pipe main body 10 includes a pin tube body 11 and a coupling 12. The coupling 12 is attached to one end of the pin tube body 11. More specifically, the coupling 12 is fastened by threading to one end of the pin tube body 11.
FIG. 6 is a partial cross-sectional view illustrating a cross section (longitudinal cross section) that is parallel to the pipe axis direction of the coupling 12 of the oil-well metal pipe 1 illustrated in FIG. 5. Referring to FIG. 5 and FIG. 6, the pipe main body 10 includes a pin 40 and a box 50. The pin 40 is formed at the first end portion 10A of the pipe main body 10. When performing fastening, the pin 40 is inserted into the box 50 of another oil-well metal pipe 1 (not illustrated), and is fastened by threading to the box 50 of the other oil-well metal pipe 1.
The box 50 is formed at the second end portion 10B of the pipe main body 10. When performing fastening, the pin 40 of another oil-well metal pipe 1 is inserted into the box 50, and the box 50 is fastened by threading to the pin 40 of the other oil-well metal pipe 1.

[Regarding structure of pin 40]

FIG. 7 is a cross-sectional view of a portion in the vicinity of the pin 40 of the oil-well metal pipe 1 illustrated in FIG. 5, that is a cross-sectional view parallel to the pipe axis direction of the oil-well metal pipe 1. A dashed line portion in FIG. 7 represents the structure of the box 50 of another oil-well metal pipe in the case of fastening the oil-well metal pipe 1 to another oil-well metal pipe 1. Referring to FIG. 7, the pin 40 includes a pin contact surface 400 on the outer peripheral surface of the first end portion 10A of the pipe main body 10. At the time of fastening to another oil-well metal pipe 1, the pin contact surface 400 is screwed into the box 50 of the other oil-well metal pipe 1 to come into contact with the box contact surface 500 (to be described later) of the box 50.
The pin contact surface 400 includes at least an external thread part 41 formed on the outer peripheral surface of the first end portion 10A. The pin contact surface 400 may further include a pin sealing surface 42 and a pin shoulder surface 43. In FIG. 7, the pin shoulder surface 43 is disposed at the front end face of the first end portion 10A, on the outer peripheral surface of the first end portion 10A, the pin sealing surface 42 is disposed further on the front end side of the first end portion 10A than the external thread part 41. In other words, the pin sealing surface 42 is disposed between the external thread part 41 and the pin shoulder surface 43. The pin sealing surface 42 is provided in a tapered shape. Specifically, in the pin sealing surface 42, the external diameter of the pin 40 gradually decreases from the external thread part 41 toward the pin shoulder surface 43 in the longitudinal direction (pipe axis direction) of the first end portion 10A.
When performing fastening with another oil-well metal pipe 1, the pin sealing surface 42 contacts a box sealing surface 52 (described later) of the box 50 of the other oil-well metal pipe 1. More specifically, during fastening, when the pin 40 is inserted into the box 50 of the other oil-well metal pipe 1, the pin sealing surface 42 contacts the box sealing surface 52. Subsequently, when the pin 40 is screwed further into the box 50 of the other oil-well metal pipe 1, the pin sealing surface 42 closely contacts the box sealing surface 52. By this means, during fastening, the pin sealing surface 42 closely contacts the box sealing surface 52 to thereby form a seal that is based on metal-to-metal contact. Therefore, the gastightness can be increased in each of the oil-well metal pipe 1 that are fastened to each other.
In FIG. 7, the pin shoulder surface 43 is disposed at the front end face of the first end portion 10A. In other words, in the pin 40 illustrated in FIG. 7, the external thread part 41, the pin sealing surface 42 and the pin shoulder surface 43 are disposed sequentially in that order from the center of the pipe main body 10 toward the first end portion 10A. During fastening to another oil-well metal pipe 1, the pin shoulder surface 43 opposes and contacts a box shoulder surface 53 (described later) of the box 50 of the other oil-well metal pipe 1. More specifically, during fastening, the pin shoulder surface 43 contacts the box shoulder surface 53 as a result of the pin 40 being inserted into the box 50 of the other oil-well metal pipe 1. By this means, during fastening, a high torque can be obtained. Further, the positional relation between the pin 40 and the box 50 in the fastening state can be stabilized.
Note that, the pin contact surface 400 of the pin 40 includes at least the external thread part 41. In other words, the pin contact surface 400 may include the external thread part 41, and need not include the pin sealing surface 42 and the pin shoulder surface 43. The pin contact surface 400 may include the external thread part 41 and the pin shoulder surface 43, and need not include the pin sealing surface 42. The pin contact surface 400 may include the external thread part 41 and the pin sealing surface 42, and need not include the pin shoulder surface 43.

[Regarding structure of box 50]

FIG. 8 is a cross-sectional view of a portion in the vicinity of the box 50 of the oil-well metal pipe 1 illustrated in FIG. 5, that is a cross-sectional view parallel to the pipe axis direction of the oil-well metal pipe 1. A dashed line portion in FIG. 8 represents the structure of the pin 40 of another oil-well metal pipe 1 in the case of fastening the oil-well metal pipe 1 to another oil-well metal pipe 1. Referring to FIG. 8, the box 50 includes a box contact surface 500 on the inner peripheral surface of the second end portion 10B of the pipe main body 10. When performing fastening to another oil-well metal pipe 1, the box contact surface 500 contacts the pin contact surface 400 of the pin 40 of the other oil-well metal pipe 1 when the pin 40 is screwed into the box 50.
The box contact surface 500 includes at least an internal thread part 51 formed in the inner peripheral surface of the second end portion 10B. When performing fastening, the internal thread part 51 engages with the external thread part 41 of the pin 40 of the other oil-well metal pipe.
The box contact surface 500 may further include the box sealing surface 52 and the box shoulder surface 53. In FIG. 8, on the inner peripheral surface of the second end portion 10B, the box sealing surface 52 is disposed further on the pipe main body 10 side than the internal thread part 51. In other words, the box sealing surface 52 is disposed between the internal thread part 51 and the box shoulder surface 53. The box sealing surface 52 is provided in a tapered shape. Specifically, in the box sealing surface 52, the internal diameter of the box 50 gradually decreases from the internal thread part 51 toward the box shoulder surface 53 in the longitudinal direction (pipe axis direction) of the second end portion 10B.
When performing fastening to another oil-well metal pipe 1, the box sealing surface 52 contacts the pin sealing surface 42 of the pin 40 of the other oil-well metal pipe 1. More specifically, during fastening, when the pin 40 of the other oil-well metal pipe 1 is screwed into the box 50, the box sealing surface 52 contacts the pin sealing surface 42, and when the pin 40 is screwed in further, the box sealing surface 52 closely contacts the pin sealing surface 42. By this means, during fastening, the box sealing surface 52 closely contacts the pin sealing surface 42 to thereby form a seal that is based on metal-to-metal contact. Therefore, the gastightness can be increased in each of the oil-well metal pipe 1 that are fastened to each other.
The box shoulder surface 53 is disposed further on the pipe main body 10 side than the box sealing surface 52. In other words, in the box 50, the box shoulder surface 53, the box sealing surface 52 and the internal thread part 51 are disposed sequentially in that order from the center of the pipe main body 10 toward the front end of the second end portion 10B. When performing fastening to another oil-well metal pipe 1, the box shoulder surface 53 opposes and contacts the pin shoulder surface 43 of the pin 40 of the other oil-well metal pipe 1. More specifically, during fastening, the box shoulder surface 53 contacts the pin shoulder surface 43 as a result of the pin 40 of the other oil-well metal pipe 1 being inserted into the box 50. By this means, during fastening, a high torque can be obtained. Further, the positional relation between the pin 40 and the box 50 in the fastening state can be stabilized.
The box contact surface 500 includes at least the internal thread part 51. When performing fastening, the internal thread part 51 of the box contact surface 500 of the box 50 contacts the external thread part 41 of the pin contact surface 400 of the pin 40 in a manner such that the internal thread part 51 corresponds with the external thread part 41. The box sealing surface 52 contacts the pin sealing surface 42 in a manner such that the box sealing surface 52 corresponds with the pin sealing surface 42. The box shoulder surface 53 contacts the pin shoulder surface 43 in a manner such that the box shoulder surface 53 corresponds with the pin shoulder surface 43.
In a case where the pin contact surface 400 includes the external thread part 41 and does not include the pin sealing surface 42 and the pin shoulder surface 43, the box contact surface 500 includes the internal thread part 51 and does not include the box sealing surface 52 and the box shoulder surface 53. In a case where the pin contact surface 400 includes the external thread part 41 and the pin shoulder surface 43 and does not include the pin sealing surface 42, the box contact surface 500 includes the internal thread part 51 and the box shoulder surface 53 and does not include the box sealing surface 52. In a case where the pin contact surface 400 includes the external thread part 41 and the pin sealing surface 42 and does not include the pin shoulder surface 43, the box contact surface 500 includes the internal thread part 51 and the box sealing surface 52 and does not include the box shoulder surface 53.
The pin contact surface 400 may include a plurality of the external thread parts 41, may include a plurality of the pin sealing surfaces 42, and may include a plurality of the pin shoulder surfaces 43. For example, the pin shoulder surface 43, the pin sealing surface 42, the external thread part 41, the pin sealing surface 42, the pin shoulder surface 43, the pin sealing surface 42 and the external thread part 41 may be disposed in that order on the pin contact surface 400 of the pin 40 from the front end of the first end portion 10A toward the center of the pipe main body 10. In such case, the internal thread part 51, the box sealing surface 52, the box shoulder surface 53, the box sealing surface 52, the internal thread part 51, the box sealing surface 52 and the box shoulder surface 53 are disposed in that order on the box contact surface 500 of the box 50 from the front end of the second end portion 10B toward the center of the pipe main body 10.
In FIG. 7 and FIG. 8, a so-called "premium joint" is illustrated in which the pin 40 includes the external thread part 41, the pin sealing surface 42 and the pin shoulder surface 43, and the box 50 includes the internal thread part 51, the box sealing surface 52 and the box shoulder surface 53. However, as described above, the pin 40 may include the external thread part 41 and need not include the pin sealing surface 42 and the pin shoulder surface 43. In this case, the box 50 includes the internal thread part 51 and does not include the box sealing surface 52 and the box shoulder surface 53. FIG. 9 is a partial cross-sectional view illustrating a longitudinal cross section of the oil-well metal pipe of the present embodiment having a configuration different from that in FIG. 6. In the oil-well metal pipe 1 illustrated in FIG. 9, the pin 40 includes the external thread part 41 and does not include the pin sealing surface 42 and the pin shoulder surface 43. Further, the box 50 includes the internal thread part 51 and does not include the box sealing surface 52 and the box shoulder surface 53. The oil-well metal pipe 1 of the present embodiment may have the structure illustrated in FIG. 9.

[Case where oil-well metal pipe 1 is integral type]

The oil-well metal pipe 1 illustrated in FIG. 5, FIG. 6, and FIG. 9 is a so-called "T&C type" oil-well metal pipe 1, in which the pipe main body 10 includes the pin tube body 11 and the coupling 12. However, the oil-well metal pipe 1 according to the present embodiment may be an integral type instead of a T&C type.
FIG. 10 is a partial cross-sectional view including a longitudinal cross section of an integral type oil-well metal pipe 1 according to the present embodiment. Referring to FIG. 10, the integral type oil-well metal pipe 1 includes a pipe main body 10. The pipe main body 10 includes a first end portion 10A and a second end portion 10B. The first end portion 10A is disposed on the opposite side to the second end portion 10B. As described above, in the T&C type oil-well metal pipe 1, the pipe main body 10 includes the pin tube body 11 and the coupling 12. In other words, in the T&C type oil-well metal pipe 1, the pipe main body 10 is constituted by fastening two separate members (the pin tube body 11 and the coupling 12). In contrast, in the integral type oil-well metal pipe 1, the pipe main body 10 is formed in an integral manner.
The pin 40 is formed at the first end portion 10A of the pipe main body 10. When performing fastening, the pin 40 is inserted in and screwed into the box 50 of another integral type oil-well metal pipe 1, and thereby fastened to the box 50 of the other integral type oil-well metal pipe 1. The box 50 is formed at the second end portion 10B of the pipe main body 10. When performing fastening, the pin 40 of another integral type oil-well metal pipe 1 is inserted in and screwed into the box 50, to thereby fasten the box 50 to the pin 40 of the other integral type oil-well metal pipe 1.
The structure of the pin 40 of the integral type oil-well metal pipe 1 is the same as the structure of the pin 40 of the T&C type oil-well metal pipe 1 illustrated in FIG. 7. Similarly, the structure of the box 50 of the integral type oil-well metal pipe 1 is the same as the structure of the box 50 of the T&C type oil-well metal pipe 1 illustrated in FIG. 8. Note that, in FIG. 7 and FIG.8, the pin shoulder surface 43, the pin sealing surface 42 and the external thread part 41 in the pin 40 are disposed in that order from the front end of the first end portion 10A toward the center of the pipe main body 10. Therefore, the internal thread part 51, the box sealing surface 52 and the box shoulder surface 53 in the box 50 are disposed in that order from the front end of the second end portion 10B toward the center of the pipe main body 10. However, similarly to the pin contact surface 400 of the pin 40 of the T&C type oil-well metal pipe 1, it suffices that the pin contact surface 400 of the pin 40 of the integral type oil-well metal pipe 1 includes at least the external thread part 41. Further, similarly to the box contact surface 500 of the box 50 of the T&C type oil-well metal pipe 1, it suffices that the box contact surface 500 of the box 50 of the integral type oil-well metal pipe 1 includes at least the internal thread part 51.
In short, the oil-well metal pipe 1 of the present embodiment may be a T&C type or may be an integral type.

[Regarding Zn-Ni alloy plating layer]

In the oil-well metal pipe 1 of the present embodiment, a Zn-Ni alloy plating layer is formed on at least one contact surface among the pin contact surface 400 and the box contact surface 500. In other words, the Zn-Ni alloy plating layer may be formed on the pin contact surface 400, and need not be formed on the box contact surface 500. Alternatively, the Zn-Ni alloy plating layer may be formed on the box contact surface 500, and need not be formed on the pin contact surface 400. Further, the Zn-Ni alloy plating layer may be formed on the pin contact surface 400 and on the box contact surface 500.
In the following description, the structure on the pin contact surface 400 in a case where the Zn-Ni alloy plating layer is formed on the pin contact surface 400, and the structure on the box contact surface 500 in a case where the Zn-Ni alloy plating layer is formed on the box contact surface 500 are described.

[Structure on pin contact surface 400 in case where Zn-Ni alloy plating layer 100 is formed on pin contact surface 400]

FIG. 11 is a cross-sectional view of the vicinity of the pin contact surface 400 in a case where a Zn-Ni alloy plating layer 100 is formed on the pin contact surface 400. Referring to FIG. 11, the oil-well metal pipe 1 further includes the Zn-Ni alloy plating layer 100 formed on the pin contact surface 400 of the pin 40.
The Zn-Ni alloy plating layer 100 may be formed on one part of the pin contact surface 400 or may be formed on the entire pin contact surface 400. The interfacial pressure increases, in particular, in the final stage of fastening at the pin sealing surface. Therefore, in a case where the Zn-Ni alloy plating layer 100 is partially formed on the pin contact surface 400, the Zn-Ni alloy plating layer 100 is preferably formed on at least the pin sealing surface. As mentioned above, the Zn-Ni alloy plating layer 100 may be formed on the entire pin contact surface 400.

[Structure on box contact surface 500 in case where Zn-Ni alloy plating layer 100 is formed on box contact surface 500]

FIG. 12 is a cross-sectional view of the vicinity of the box contact surface 500 in a case where the Zn-Ni alloy plating layer 100 is formed on the box contact surface 500. Referring to FIG. 12, the Zn-Ni alloy plating layer 100 is formed on the box contact surface 500. The Zn-Ni alloy plating layer 100 may be formed on one part of the box contact surface 500 or may be formed on the entire box contact surface 500. The interfacial pressure increases, in particular, in the final stage of fastening at the box sealing surface. Therefore, in a case where the Zn-Ni alloy plating layer 100 is partially formed on the box contact surface 500, the Zn-Ni alloy plating layer 100 is preferably formed on at least the box sealing surface.

[Regarding Zn-Ni alloy plating layer 100]

As described above, the Zn-Ni alloy plating layer 100 is formed on at least one contact surface among the pin contact surface 400 and the box contact surface 500. The Zn-Ni alloy plating layer 100 may be formed in contact with the contact surface(s) (pin contact surface 400 and/or box contact surface 500). In other words, the Zn-Ni alloy plating layer 100 may be formed directly on the contact surface(s) (pin contact surface 400 and/or box contact surface 500). Alternatively, another plating layer may be formed between the Zn-Ni alloy plating layer 100 and the contact surface(s) (pin contact surface 400 and/or box contact surface 500). The other plating layer is, for example, an Ni plating layer.
The Zn-Ni alloy plating layer 100 is composed of a Zn-Ni alloy. The Zn-Ni alloy contains zinc (Zn) and nickel (Ni). Preferably, the Zn-Ni alloy consists of Zn and Ni, with the balance being impurities. Here, the term "impurities" of the Zn-Ni alloy refers to substances other than Zn and Ni that are contained in the Zn-Ni alloy plating layer 100 during production and the like of the oil-well metal pipe, and whose contents are within a range that does not influence the effects of the present embodiment.
The Zn-Ni alloy plating layer 100 contains Zn. Zn is a base metal in comparison to Fe. Therefore, the Zn-Ni alloy plating layer 100 is corroded with priority relative to the steel material (sacrificial protection). By this means, the corrosion resistance property of the oil-well metal pipe 1 is improved.

[Regarding Ni content and bulk density of Zn-Ni alloy plating layer 100]

The Ni content in the Zn-Ni alloy plating layer 100 of the present embodiment is within the range of, in mass%, 14.8 to 25.0%. Preferably, the chemical composition of the Zn-Ni alloy plating layer 100 contains Ni in an amount within the range of, in mass%, 14.8 to 25.0%, with the balance being Zn and impurities. In addition, the bulk density of the Zn-Ni alloy plating layer 100 in the present embodiment is 7.00 g/cm³ or more. In a conventional Zn-Ni alloy plating layer, in a case where the Ni content was raised to 14.8% or more, as illustrated in FIG. 1, the hardness decreased. However, in the Zn-Ni alloy plating layer 100 of the present embodiment, the bulk density is made 7.00 g/cm³ or more while also making the Ni content a high amount of 14.8% or more. As a result, as illustrated in FIG. 4, the hardness of the Zn-Ni alloy plating layer 100 is markedly higher than the hardness of the conventional Zn-Ni alloy plating layer.
If the Ni content in the Zn-Ni alloy plating layer 100 is too low, the hardness of the Zn-Ni alloy plating layer 100 will not be sufficiently high. On the other hand, if the Ni content in the Zn-Ni alloy plating layer 100 is too high, a large amount of hydrogen gas will be generated during formation of the Zn-Ni alloy plating layer 100. In this case, a large number of porosities will be present in the Zn-Ni alloy plating layer 100 formed, and the bulk density will excessively decrease. As a result, the hardness of the Zn-Ni alloy plating layer 100 will not be sufficiently obtained. Therefore, the Ni content in the Zn-Ni alloy plating layer 100 is within the range of 14.8 to 25.0%. A preferable lower limit of the Ni content in the Zn-Ni alloy plating layer 100 is 15.2%, more preferably is 15.5%, more preferably is 15.7%, more preferably is 16.0%, more, preferably is 16.2%, more preferably is 16.5%, more preferably is 17.0%, and further preferably is 17.5%. A preferable upper limit of the Ni content is 24.5%, more preferably is 24.0%, and more preferably is 23.5%.
If the bulk density of the Zn-Ni alloy plating layer 100 is too low, a large number of porosities will be present in the Zn-Ni alloy plating layer 100. In this case, the hardness of the Zn-Ni alloy plating layer 100 will not be sufficiently obtained. If the bulk density of the Zn-Ni alloy plating layer 100 is 7.00 g/cm³ or more, on the premise that the Ni content is within the range of 14.8 to 25.0%, sufficient hardness will be obtained in the Zn-Ni alloy plating layer 100. Therefore, the bulk density of the Zn-Ni alloy plating layer 100 is 7.00 g/cm³ or more. A preferable lower limit of the bulk density in the Zn-Ni alloy plating layer 100 is 7.30 g/cm³, more preferably is 7.50 g/cm³, further preferably is 7.60 g/cm³, more preferably is 7.70 g/cm³, further preferably is 7.80%, and more preferably is 7.90 g/cm³. The upper limit of the bulk density is not particularly limited. However, when the Ni content of the Zn-Ni alloy plating layer 100 is within the range of 14.8 to 25.0%, the upper limit of the bulk density is, for example, 10.00 g/cm³.

[Method for measuring chemical composition of Zn-Ni alloy plating layer 100]

The chemical composition of the Zn-Ni alloy plating layer 100 is measured by the following method. A sample including the Zn-Ni alloy plating layer 100 (sample including the contact surface on which the Zn-Ni alloy plating layer 100 is formed) is taken from the oil-well metal pipe 1. The Zn-Ni alloy plating layer 100 of the obtained sample is dissolved in hydrochloric acid at 10% concentration to obtain a liquid solution. The liquid solution is subjected to ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry), and the chemical composition of the Zn-Ni alloy plating layer 100 is obtained. By the above methods, the Ni content (mass%) and Zn content (mass%) in the Zn-Ni alloy plating layer 100 are determined.

[Method for measuring bulk density of Zn-Ni alloy plating layer 100]

The bulk density of the Zn-Ni alloy plating layer 100 is measured by the following method. The Zn-Ni alloy plating layer 100 of the aforementioned sample is dissolved in hydrochloric acid at 10% concentration of a predetermined volume to obtain a liquid solution. The liquid solution is subjected to elementary analysis of the chemical composition by ICP-AES, and the total mass (g) of Ni and Zn in the liquid solution are determined. The total mass of Ni and Zn in the liquid solution is divided by the surface area of the sample (the surface area of the surface on which the Zn-Ni alloy plating layer 100 was formed) to determine the coating mass per unit area (g/cm²) of the Zn-Ni alloy plating layer 100.
Further, the thickness of the Zn-Ni alloy plating layer 100 is determined by the following method. Before measuring the chemical composition of the Zn-Ni alloy plating layer 100 of the aforementioned sample, a specimen having a cross section in the depth direction of the Zn-Ni alloy plating layer 100 determined as the observation surface is taken from the sample. The observation surface of the specimen is observed in a backscattered electron image (BSE) at a magnification of 3,000 times using a scanning electron microscope. In the observation in a backscattered electron image (BSE) using a scanning electron microscope (SEM), the base metal (pipe) and the Zn-Ni alloy plating layer 100 are easily distinguishable by contrast. In the observation surface, the thickness of the Zn-Ni alloy plating layer 100 at an arbitrary five locations is measured. The arithmetic mean value of the measured thicknesses is defined as the thickness (µm) of the Zn-Ni alloy. The bulk density (g/cm³) of the Zn-Ni alloy plating layer 100 is determined based on the coating mass per unit area (g/cm²) of the Zn-Ni alloy plating layer 100 and the thickness (µm) of the Zn-Ni alloy plating layer 100.

[Thickness of Zn-Ni alloy plating layer 100]

The thickness of the Zn-Ni alloy plating layer 100 is not particularly limited. The thickness of the Zn-Ni alloy plating layer 100 is, for example, within the range of 1 to 20 µm. If the thickness of the Zn-Ni alloy plating layer 100 is 1 µm or more, the galling resistance can be further improved. Even if the thickness of the Zn-Ni alloy plating layer 100 is more than 20 µm, the aforementioned effect will be saturated. The lower limit of the thickness of the Zn-Ni alloy plating layer 100 is preferably 3 µm, and more preferably is 5 µm. The upper limit of the thickness of the Zn-Ni alloy plating layer 100 is preferably 18 µm, and more preferably is 15 µm.
As described above, in the oil-well metal pipe 1 of the present embodiment, the Ni content in the Zn-Ni alloy plating layer 100 is 14.8 to 25.0%, and in addition, the bulk density of the Zn-Ni alloy plating layer 100 is 7.00 g/cm³ or more. Therefore, as illustrated in FIG. 4, in the present embodiment, the Vickers hardness of the Zn-Ni alloy plating layer 100 can be increased. The hardness of the plating layer has a positive correlation with galling resistance during fastening and loosening. In other words, the higher that the hardness of the plating layer is, the higher that the galling resistance during fastening and loosening will be. Therefore, the oil-well metal pipe 1 of the present embodiment has excellent galling resistance. Furthermore, the Zn-Ni alloy plating layer 100 suppresses corrosion of the steel material by sacrificial protection. Therefore, the oil-well metal pipe 1 of the present embodiment is also excellent in corrosion resistance.

[Regarding other optional structures of oil-well metal pipe 1 of the present embodiment]

[Regarding chromate coating]

The oil-well metal pipe 1 of the present embodiment may further include a chromate coating 110 on the Zn-Ni alloy plating layer 100. Referring to FIG. 13, in a case where the Zn-Ni alloy plating layer 100 is formed on the pin contact surface 400, the chromate coating 110 may be formed on the Zn-Ni alloy plating layer 100. Further, referring to FIG. 14, in a case where the Zn-Ni alloy plating layer 100 is formed on the box contact surface 500, the chromate coating 110 may be formed on the Zn-Ni alloy plating layer 100.
In some cases the oil-well metal pipe 1 will be stored outdoors for a long period of time until actually being used at the oil well drilling site. In a case where the oil-well metal pipe 1 is exposed to the atmosphere for a long period of time outdoors, a chromate coating 110 enhances the corrosion resistance of the pin contact surface 400, and can suppress the occurrence of rust (white rust) at the pin contact surface 400. The chromate coating 110 is a coating containing trivalent chromium chromate. Preferably, the chromate coating 110 does not contain hexavalent chromium. The film thickness of the chromate coating 110 is not particularly limited. The film thickness of the chromate coating 110 is, for example, within the range of 10 to 200 nm. A preferable lower limit of the film thickness of the chromate coating 110 is 20 nm, more preferably is 30 nm. A preferable upper limit of the film thickness of the chromate coating 110 is 100 nm, more preferably is 90 nm.

[Lubricant coating]

The oil-well metal pipe 1 may further include a lubricant coating on the Zn-Ni alloy plating layer 100, on the chromate coating 110, or on a contact surface on which the Zn-Ni alloy plating layer 100 is not formed (on the pin contact surface 400 or on the box contact surface 500). The lubricant coating further enhances the lubricity of the oil-well metal pipe 1.
The lubricant coating may be solid, or may be in a semi-solid state or a liquid state. A commercially available lubricant can be used as the lubricant coating. The lubricant coating contains, for example, lubricating particles and a binder. As necessary, the lubricant coating may contain a solvent and other components.
The lubricating particles are not particularly limited as long as they are particles having lubricity. The lubricating particles are, for example, one or more types selected from the group consisting of particles of graphite, MoS₂ (molybdenum disulfide), WS₂ (tungsten disulfide), BN (boron nitride), PTFE (polytetrafluoroethylene), CFx (graphite fluoride), and CaCO₃ (calcium carbonate).
The binder, for example, is one or two types selected from the group consisting of an organic binder and an inorganic binder. The organic binder is, for example, one or two types selected from the group consisting of a thermosetting resin and a thermoplastic resin. The thermosetting resin, for example, is one or more types selected from the group consisting of polyethylene resin, polyimide resin and polyamide-imide resin. The inorganic binder, for example, is one or two types selected from the group consisting of compounds containing alkoxysilane and siloxane bonds.
An example of a commercially available lubricant is Seal-Guard ECF (trade name) manufactured by Jet-Lube LLC. Other examples of the lubricant coating include a lubricant coating containing rosin, metallic soap, wax or a lubricant powder.

[Regarding pipe main body 10 of oil-well metal pipe 1]

The chemical composition of the pipe main body 10 of the oil-well metal pipe 1 according to the present embodiment is not particularly limited. The pipe main body 10 may be formed, for example, by any of carbon steel, stainless steel, an alloy, or the like. That is, the oil-well metal pipe 1 may be a steel pipe made of a Febased alloy or an alloy pipe represented by a Ni-based alloy pipe. The steel pipe is, for example, a low-alloy pipe, a martensitic stainless steel pipe, and a duplex stainless steel pipe, etc.

[Method for producing oil-well metal pipe 1]

A method for producing the oil-well metal pipe 1 according to the present embodiment is described hereunder. Note that, as long as the oil-well metal pipe 1 of the present embodiment has the structure described above, a method for producing the oil-well metal pipe 1 is not limited to the following production method. However, the production method described hereunder is one favorable example for producing the oil-well metal pipe 1 of the present embodiment.
The method for producing the oil-well metal pipe 1 includes a preparation process (S1) of preparing a hollow shell in which the pin 40 or the box 50 is formed, and a Zn-Ni alloy plating layer formation process (S2). In the present embodiment, in the Zn-Ni alloy plating layer formation process (S2), electroplating is performed using a chloride bath containing nickel ions and a specific concentration of zinc ions as a plating bath. By this means, the Zn-Ni alloy plating layer 100 in which the Ni content is within the range of, in mass%, 14.8 to 25.0% and the bulk density is 7.00 g/cm³ or more can be formed on the pin contact surface 400 and/or the box contact surface 500 of the oil-well metal pipe 1. Hereunder, each process of the method for producing the oil-well metal pipe of the present embodiment is described in detail.

[Preparation process (S1)]

First, a hollow shell in which the pin 40 or the box 50 is formed is prepared. In the present description, the phrase "hollow shell in which the pin or the box is formed" means either of the pipe main body 10 and the pin tube body 11 in a T&C type oil-well metal pipe 1 and the pipe main body 10 in the integral type oil-well metal pipe 1.
The hollow shell in which the pin 40 or the box 50 is formed is produced, for example, by the following method. A starting material is produced using molten steel. Specifically, a cast piece (a bloom or billet) is produced by a continuous casting process using the molten steel. An ingot may also be produced by an ingotmaking process using the molten steel. As necessary, the bloom or ingot may be subjected to blooming to produce a billet. The starting material (a bloom or billet) is produced by the above-described process. The prepared starting material is subjected to hot working to produce a hollow shell. The hot working method may be piercing-rolling by means of the Mannesmann process, or may be a hot-extrusion process. The hollow shell after hot working is subjected to well-known quenching and well-known tempering to adjust the strength of the hollow shell. A hollow shell is produced by the above process. Note that, in a case where the oil-well metal pipe is the T&C type, a hollow shell for the coupling 12 is also prepared. The method for producing the hollow shell for the coupling 12 is the same as the method for producing the hollow shell that is described above.
In a case where the oil-well metal pipe 1 is the T&C type, threading is performed with respect to the outer surface of both end portions of the hollow shell for the pin tube body 11, to form the pin 40 that includes the pin contact surface 400. By means of the above process, a hollow shell (the pin tube body 11) in which the pin 40 is formed is prepared in a case where the oil-well metal pipe 1 is the T&C type. Note that, in a case where the oil-well metal pipe is the T&C type, the coupling 12 may also be prepared. Specifically, threading is performed with respect to the inner surface of both end portions of the hollow shell for the coupling 12, to form the box 50 that includes the box contact surface 500. The coupling 12 is produced by the above process.
In a case where the oil-well metal pipe 1 is the integral type, threading is performed on the outer surface of the first end portion 10A of the hollow shell to form the pin 40 that includes the pin contact surface 400. In addition, threading is performed with respect to the inner surface of the second end portion 10B of the hollow shell to form the box 50 that includes the box contact surface 500. By means of the above process, in a case where the oil-well metal pipe 1 is the integral type, a hollow shell (pipe main body 10) in which the pin 40 and the box 50 are formed is prepared.
The preparation process (S1) of the present embodiment may further include at least one of a grinding process and an Ni strike plating process.
In the case of performing a grinding process, a sandblasting treatment, and finishing by machine grinding are performed in the grinding process. The surface roughness of the contact surface can be increased by the sandblasting treatment. The sandblasting treatment can be carried out by a well-known method.
In the case of performing the Ni strike plating process, an Ni strike plating layer is formed on the surface of the hollow shell in the Ni strike plating process. The Ni strike plating layer is an extremely thin undercoat plating layer, and increases the adhesion of the Zn-Ni alloy plating layer 100 that is described later. Note that, the plating bath to be used in the Ni strike plating process is not particularly limited, and a well-known bath can be used. Further, the conditions for forming the Ni strike plating layer are not particularly limited, and can be appropriately adjusted and set.
Note that, in a case where an Ni strike plating process is performed, an Ni strike plating layer is formed between the pipe main body 10 and the Zn-Ni alloy plating layer 100. On the other hand, the thickness of the formed Ni strike plating layer is negligibly thin in comparison to the thickness of the Zn-Ni alloy plating layer 100. In other words, in the oil-well metal pipe 1 according to the present embodiment, an Ni strike plating layer may be included in the Zn-Ni alloy plating layer 100.

[Zn-Ni alloy plating layer formation process (S2)]

In the Zn-Ni alloy plating layer formation process (S2), the Zn-Ni alloy plating layer 100 is formed by electroplating on the pin contact surface 400 of the hollow shell in which the pin 40 is formed, or on the box contact surface 500 of the hollow shell in which the box 50 is formed.
In the Zn-Ni alloy plating layer formation process (S2), it is preferable to use a plating bath containing chloride ions (hereunder, referred to as "chloride bath") than to use a plating bath containing sulfate (hereunder, referred to as "sulfate bath"). As shown in examples that are described later, the Zn-Ni alloy plating layer 100 of the present embodiment can be formed by performing electroplating within a range satisfying conditions described later using a chloride bath.
Furthermore, in the plating bath, the Ni ion ratio is adjusted within the range of 50 to 70%. Here, the Ni ion ratio is defined by the following formula. Ni ion ratio = Ni concentration in plating bath/(Ni concentration in plating bath + Zn concentration in plating bath) × 100
The Zn-Ni alloy plating layer 100 in which the Ni content is within the range of 14.8 to 25.0% and the bulk density is 7.00 g/cm³ or more can be formed by adjusting within the ranges of electroplating conditions described later using a plating bath that contains chloride ions and in which the Ni ion ratio is adjusted within the range of 50 to 70%. The type and amount of support electrolyte and various additive agents (brightening agent, etc.) can also affect the plating obtained and is selected appropriately.
The Ni ion ratio in the aforementioned plating bath is lower than in a conventional Zn-Ni alloy plating bath. For example, in a commercially available plating bath with the trade name "DAIN Zinalloy N-PL" manufactured by Daiwa Fine Chemicals Co., Ltd that is a known Zn-Ni alloy plating bath, the Ni ion ratio is within the range of 75 to 90%, and thus the Ni ion ratio of the aforementioned plating bath is lower. In the present embodiment, as described above, the Zn-Ni alloy plating layer 100 having the aforementioned structure is formed by using a plating bath that has a low Ni ion ratio and that is a chloride bath. The chloride ion concentration of the plating bath is 215 to 230 g/L.
The Zn-Ni alloy plating layer 100 is formed by electroplating using the aforementioned plating bath. Taking the use of the aforementioned plating bath as a premise, the electroplating conditions can be appropriately adjusted according to well-known conditions. The electroplating conditions are, for example, a plating bath pH of more than 4.5 and less than 6.0, a plating solution temperature of 10 to 60°C, a current density of 1 to 15 A/dm², and a treatment time of 0.1 to 30 minutes. If the pH of the plating bath is outside the above range, sufficient bulk density and sufficient Ni content cannot be obtained. Further, if the current density is too high, sufficient bulk density cannot be obtained in the Zn-Ni alloy plating layer, nor can sufficient Ni content be obtained. It is adjusted that the plating solution and electroplating conditions so that the coating mass and composition of the Zn-Ni alloy plating layer 100 are appropriately obtained within the range that satisfies the aforementioned conditions. In the case of forming the Zn-Ni alloy plating layer 100 on the pin contact surface 400, the pin contact surface 400 is immersed in the aforementioned plating bath and electroplating is performed. On the other hand, in the case of forming the Zn-Ni alloy plating layer 100 on the box contact surface 500, the box contact surface 500 is immersed in the aforementioned plating bath and electroplating is performed.
The oil-well metal pipe 1 of the present embodiment having the structure described above is produced by the production processes that are described above.

[Other optional processes]

In the method for producing the oil-well metal pipe of the present embodiment, at least one process among the following chromating process, preconditioning treatment process and film formation process may also be performed. These processes are optional processes. Accordingly, these processes need not be performed.

[Chromating process]

A chromating process may be performed as necessary. In other words, the chromating process is an optional process. In the case of forming a chromate coating 110 on the Zn-Ni alloy plating layer 100, a well-known chromating process is performed after subjecting the Zn-Ni alloy plating layer 100 after the Zn-Ni alloy plating layer formation process (S2) to pickling. In the chromating process, first, a chromating solution is prepared. The chromating solution, for example, contains trivalent chromium ions. The chromating solution can be made to contain trivalent chromium ions by, for example, dissolving chromium chloride (III) and chromium sulfate (III) therein. Preferably the chromating solution does not contain hexavalent chromium. A commercially available chromating solution may be used for the chromating solution. The commercially available chromating solution is, for example, DAIN Chromate TR-02 (trade name) manufactured by Daiwa Fine Chemicals Co., Ltd. The contact surface (pin contact surface 400 and/or box contact surface 500) on which the Zn-Ni alloy plating layer 100 has been formed is immersed in the chromating solution, and chromating is performed to form a chromate coating 110 on the Zn-Ni alloy plating layer 100.

[Preconditioning treatment process]

As necessary, the production method of the present embodiment may include a preconditioning treatment process before the Zn-Ni alloy plating layer formation process (S2). In other words, the preconditioning treatment process is an optional process. The preconditioning treatment process includes, for example, pickling and alkali degreasing. In the preconditioning treatment process, oil or the like adhering to the contact surface is washed off. The preconditioning treatment process may further include performance of a grinding process such as sandblasting and/or finishing by machine grinding. Only one kind of these preconditioning treatments may be performed, or a plurality of the preconditioning treatments may be performed in combination.

[Film formation process]

As necessary, the production method of the present embodiment may include performance of a film formation process. In other words, the film formation process is an optional process. In the film formation process, a lubricant coating is formed on the Zn-Ni alloy plating layer 100 and/or on a contact surface (pin contact surface 400 or box contact surface 500) on which the Zn-Ni alloy plating layer 100 is not formed.
In the film formation process, a lubricant or a composition containing the components of the aforementioned lubricant coating is applied. By this means a lubricant coating is formed. The application method is not particularly limited. Examples of the application method include spray coating, brushing, and immersion. When adopting spray coating as the application method, the composition or lubricant may be heated and then sprayed in a state in which the flowability has been increased. The composition or lubricant is then dried to form a lubricant coating.

EXAMPLES

The advantageous effects of the oil-well metal pipe of the present embodiment are described more specifically hereunder by way of examples. The conditions adopted in the following examples are one example of conditions which are employed for confirming the workability and advantageous effects of the oil-well metal pipe of the present embodiment. Accordingly, the oil-well metal pipe of the present embodiment is not limited to this one example of the conditions.
In the present examples, a commercially available cold-rolled steel plate was used to simulate an oil-well metal pipe. The dimensions of the cold-rolled steel plate were a length of 150 mm × width of 100 mm (dimensions of region where Zn-Ni alloy plating layer was formed were a length of 100 mm × width of 100 mm). The steel grade was ultra-low carbon steel. The chemical composition of the cold-rolled steel plate was C: 0.19%, Si: 0.25%, Mn: 0.8%, P: 0.02%, S: 0.01%, Cu: 0.04%, Ni: 0.1%, Cr: 13% and Mo: 0.04%, with the balance being Fe and impurities.

[Zn-Ni alloy plating layer formation process]

A Zn-Ni alloy plating layer was formed by electroplating on the cold-rolled steel plates of the respective test numbers. The details of the conditions for forming the Zn-Ni alloy plating layer of the respective test numbers are as follows. For Test Numbers 1 to 7 in Table 1, a sulfate bath was utilized as a plating bath. Specifically, the plating bath for Test Numbers 1 to 7 contained 0.5 mol/L of sodium sulfate. Furthermore, an ammonium chloride is used as support electrolyte of the plating bath, and a brightening agent and a pH buffer material (for example, boric acid) are not added. In addition, the Ni ion ratio in the plating bath was changed for each of Test Numbers 1 to 7.

[Table 1]

TABLE1

Test Number	Plating Bath	Ni Content (mass%)	Bulk Density (g/cm³)	Vickers Hardness (Hv)
1	Sulfate Bath	0.0	6.77	109
2	Sulfate Bath	5.5	6.87	156
3	Sulfate Bath	7.9	7.36	277
4	Sulfate Bath	14.2	7.82	435
5	Sulfate Bath	16.6	6.63	392
6	Sulfate Bath	19.2	6.55	368
7	Sulfate Bath	63.3	6.28	114
8	Chloride Bath	10.7	7.59	271
9	Chloride Bath	11.3	8.06	308
10	Chloride Bath	11.4	7.75	305
11	Chloride Bath	11.6	7.59	319
12	Chloride Bath	12.0	7.81	330
13	Chloride Bath	12.3	7.94	396
14	Chloride Bath	12.6	8.01	366
15	Chloride Bath	12.6	8.08	407
16	Chloride Bath	13.2	8.01	414
17	Chloride Bath	14.1	7.99	418
18	Chloride Bath	14.8	8.30	448
19	Chloride Bath	15.5	7.78	438
20	Chloride Bath	16.1	7.86	471
21	Chloride Bath	16.8	7.99	454
22	Chloride Bath	17.4	8.49	476
23	Chloride Bath	18.6	8.71	497
24	Chloride Bath	22.6	7.55	547

For Test Numbers 8 to 24 in Table 1, a chloride bath was utilized as a plating bath. Specifically, the plating bath for each of Test Numbers 8 to 24 contained a chloride ion concentration within the range of 215 to 230 g/L. The Ni ion ratio in the plating bath was changed for each of Test Numbers 8 to 24. Note that, for Test Numbers 18 to 24, the Ni ion ratio in the plating bath was changed within the range of 50 to 70%. For Test Numbers 8 to 17, the Ni ion ratio in the plating bath was changed within a lower range than for Test Numbers 18 to 24 (that is, the Ni ion ratio was less than 50%).
Note that, it is adjusted so that the pH of the sulfate bath is 2.0 and the pH of the chloride bath is 5.5. The electroplating conditions other than the plating bath were appropriately adjusted within the following ranges: plating solution temperature: 10 to 60°C, current density: 1 to 15 A/dm², and treatment time 0.1 to 30 minutes.
Steel plates on which a Zn-Ni alloy plating layer was formed that simulated an oil-well metal pipe were produced by the above production method.

[Evaluation tests]

[Test to measure Ni content in Zn-Ni alloy plating layer]

The Ni content in the Zn-Ni alloy plating layer of each test number was measured by the following method. A sample including the Zn-Ni alloy plating layer (including a surface on which the Zn-Ni alloy plating layer 100 was formed) was taken from the steel plate of each test number. The Zn-Ni alloy plating layer of the obtained sample was dissolved in hydrochloric acid at 10% concentration, and a liquid solution was obtained. The liquid solution was subjected to ICP-AES, and elementary analysis of the chemical composition was performed to determine the Ni content (mass%) in the Zn-Ni alloy plating layer. The determined Ni contents are shown in the "Ni Content (mass%)" column in Table 1.

[Method for measuring bulk density of Zn-Ni alloy plating layer]

The bulk density of the Zn-Ni alloy plating layer of each test number was measured by the following method. The Zn-Ni alloy plating layer of the aforementioned sample was dissolved in hydrochloric acid at 10% concentration of a predetermined volume, and a liquid solution was obtained. The liquid solution was subjected to elementary analysis of the chemical composition by ICP-AES, and the total mass (g) of Ni and Zn in the liquid solution were determined. The total mass of Ni and Zn in the liquid solution was divided by the surface area of the sample to thereby determine the coating mass per unit area (g/cm²) of the Zn-Ni alloy plating layer. In addition, the thickness of the Zn-Ni alloy plating layer was determined by the following method. Before measuring the chemical composition of the Zn-Ni alloy plating layer of the aforementioned sample, a specimen having a cross section in the depth direction of the Zn-Ni alloy plating layer determined as the observation surface was taken from the sample. The observation surface of the specimen was observed in a backscattered electron image (BSE) at a magnification of 3,000 times using a scanning electron microscope. In the observation surface, the thickness of the Zn-Ni alloy plating layer was measured at an arbitrary five locations. The arithmetic mean value of the measured thicknesses was defined as the thickness (µm) of the Zn-Ni alloy. The bulk density (g/cm³) of the Zn-Ni alloy plating layer was determined based on the coating mass per unit area (g/cm²) of the Zn-Ni alloy plating layer and the thickness (µm) of the Zn-Ni alloy plating layer. The determined bulk densities are shown in the "Bulk Density (g/cm³)" column in Table 1.

[Vickers hardness test of Zn-Ni alloy plating layer]

The Vickers hardness (Hv) of the Zn-Ni alloy plating layer of each test number was determined by the following method. A sample having a cross section of the Zn-Ni alloy plating layer was taken. An arbitrary five points (measurement points) on the cross section of the Zn-Ni alloy plating layer were selected. The Vickers hardness at the selected measurement points was measured in conformity with JIS Z2244 (2009). A micro hardness tester with the trade name Fischerscope HM2000 manufactured by Fischer Instruments K. K. was used for the measurement. The test temperature was set to normal temperature (25°C), and the test force (F) was set to 0.01 N. Among the five measurement results that were obtained, the arithmetic mean value of measurement results obtained at three points that excluded the highest value and the lowest value was defined as the Vickers hardness (Hv) of the Zn-Ni alloy plating layer. The obtained Vickers hardnesses are shown in the "Vickers Hardness (Hv)" column in Table 1.

[Test results]

Referring to Table 1, in Test Numbers 18 to 24 the Ni content of the Zn-Ni alloy plating layer was, in mass %, 14.8% or more, and furthermore the bulk density of the Zn-Ni alloy plating layer was 7.00 g/cm³ or more. Therefore, the Vickers hardness of the respective Zn-Ni alloy plating layers of these test numbers was more than 435 Hv, and thus excellent hardness was obtained. Therefore, the steel plates of these test numbers could be predicted to be excellent in galling resistance during fastening and loosening.
On the other hand, for Test Numbers 1 to 7, a sulfate bath was used as the plating bath. Therefore, in the Zn-Ni alloy plating layers of Test Numbers 1 to 7, the Ni content was less than 14.8% or otherwise the bulk density was less than 7.00 g/cm³. As a result, the Vickers hardness of the Zn-Ni alloy plating layer of each of these test numbers was 435 Hv or less.
Further, for Test Numbers 8 to 17, although a chloride bath was used as the plating bath, the Ni ion ratio was lower than 50%. Therefore, in the Zn-Ni alloy plating layer of each of these test numbers, the Ni content was less than 14.8% and the Vickers hardness was 435 Hv or less.
An embodiment of the present invention has been described above. However, the foregoing embodiment is merely an example for implementing the present invention. Accordingly, the present invention is not limited to the above embodiment, and the above embodiment can be appropriately modified within a range which does not deviate from the gist of the present disclosure.

REFERENCE SIGNS LIST

1: Oil-Well Metal Pipe
10: Pipe Main Body
10A: First End Portion
10B: Second End Portion
40: Pin
41: External Thread Part
50: Box
51: Internal Thread part
100: Zn-Ni alloy Plating Layer
400: Pin Contact Surface
500: Box Contact Surface

Claims

An oil-well metal pipe, comprising:
a pipe main body including a first end portion and a second end portion, wherein

the pipe main body includes:
a pin formed at the first end portion, and

a box formed at the second end portion;

the pin includes:
a pin contact surface including an external thread part; and

the box includes:
a box contact surface including an internal thread part;

the oil-well metal pipe further comprising:
a Zn-Ni alloy plating layer formed on the pin contact surface or on the box contact surface, the Zn-Ni alloy plating layer being composed of a Zn-Ni alloy,

wherein

an Ni content in the Zn-Ni alloy plating layer is, in mass%, 14.8 to 25.0%, and

a bulk density of the Zn-Ni alloy plating layer is 7.00 g/cm³ or more.
The oil-well metal pipe according to claim 1, wherein, in the Zn-Ni alloy plating layer:
the Ni content is, in mass%, 17.0% or more.