A double-metal composite drill pipe with a special-shaped section and a manufacturing process thereof

Abstract

The application provides a double-metal composite drill rod with a special-shaped section and a manufacturing process thereof, and the double-metal composite drill rod with a special-shaped section comprises a steel-aluminum special-shaped section composite rod body, the steel-aluminum special-shaped section composite rod body comprises a steel outer tube, and an aluminum alloy inner tube is arranged in the steel outer tube; an inner-outer tube special-shaped torsion transmission section transmission structure is adopted between the aluminum alloy inner tube and the steel outer tube; or a special-shaped connecting pin torsion transmission structure is adopted between the aluminum alloy inner tube and the steel outer tube. The double-metal composite drill rod with a special-shaped section fully utilizes the high strength and wear resistance of steel and the high specific strength and corrosion resistance of light alloy, adopts a mechanical composite process, forms a light double-metal composite drill rod with a special-shaped section and a processing technology, and has the comprehensive performance of light weight, high strength, good processing performance, low cost, corrosion resistance and good wear resistance.

Description

Technical Field

[0001] This invention belongs to the field of geological drilling technology, and relates to drill pipes, specifically to a bimetallic composite drill pipe with an irregular cross-section and its manufacturing process. Background Technology

[0002] Drill pipe is an important channel in drilling projects for transporting mud, which has the functions of cooling and removing powder, from the surface to the drill bit at the bottom of the hole. It is also an essential medium for transmitting the pressure, speed and torque required for rock breaking to the drill bit. It is characterized by simple structure, complex stress, and frequent loading, unloading and handling during service.

[0003] Currently, steel is the primary material for drill pipes used in oil and gas geological exploration and development, mineral resource extraction, coal mine gas extraction and water hazard control, high-altitude tunnel exploration, and deep-earth and polar scientific drilling. Steel offers advantages such as high yield strength, strong torsional transmission capacity, good wear resistance, mature processing technology, and low cost. However, steel has a high density, resulting in a heavy self-weight for drill pipes of the same length, leading to high labor intensity during relocation and handling; high friction between horizontal and directional wells and the borehole wall places high demands on drilling rig capabilities; and it is not corrosion-resistant in acidic media containing H2S and CO2, making it prone to hydrogen embrittlement, which poses safety hazards and economic losses to drilling projects.

[0004] Lightweight, high-strength engineering materials such as aluminum alloys and titanium alloys have low density, low stiffness, high specific strength, good corrosion resistance, and non-magnetic properties. Aluminum alloy drill pipes and titanium alloy drill pipes can reduce the hook load of drilling rigs, which is beneficial for directional drilling operations. They also show significant improvements in terms of lightweighting and resistance to formation media and drilling fluid corrosion. However, aluminum alloy drill pipes have low surface hardness and poor wear resistance; titanium alloy drill pipes are too expensive and difficult to machine. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a bimetallic composite drill rod with an irregular cross section and its manufacturing process, so as to solve the technical problem that alloy drill rods in the prior art are difficult to balance wear resistance and processing difficulty.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A bimetallic composite drill pipe includes a steel-aluminum irregular cross-section composite rod body, which includes a steel outer tube and an aluminum alloy inner tube inside the steel outer tube.

[0008] The aluminum alloy inner tube and the steel outer tube adopt a torsion transmission structure with different torsion transmission cross sections between the inner and outer tubes.

[0009] Alternatively, a special-shaped connecting pin torsion transmission structure can be used between the aluminum alloy inner tube and the steel outer tube.

[0010] The present invention also has the following technical features:

[0011] Specifically, the torsion transmission structure of the inner and outer tubes with different torsion transmission sections is as follows: the inner surface of the steel outer tube and the outer surface of the aluminum alloy inner tube are in contact to form a mating surface, and the axial section of the mating surface is the torsion transmission section of the inner and outer tubes with different torsion transmission sections; the steel outer tube and the aluminum alloy inner tube form a torsion transmission structure through the mating of the inner and outer tubes with different torsion transmission sections.

[0012] In a preferred embodiment, the torsion-transmitting cross section of the inner and outer tubes is hexagonal.

[0013] In another preferred embodiment, the torsion transmission cross section of the inner and outer tubes is elliptical.

[0014] Specifically, the torsion transmission structure of the special-shaped connecting pin is as follows: the inner contour of the axial section of the steel outer tube is circular, and the outer contour of the axial section of the aluminum alloy inner tube is a regular hexagon, which is an inscribed regular hexagon of the circle; special-shaped connecting pins are installed in the six arc gaps between the steel outer tube and the aluminum alloy inner tube, the special-shaped connecting pins fill the arc gaps, and the special-shaped connecting pins are connected to the steel outer tube by axially arranged keys and keyways, thereby realizing the fixation and torsion transmission between the steel outer tube and the aluminum alloy inner tube.

[0015] Preferably, the aluminum alloy inner tube has thickened sections at both ends, and interference connection threads are machined on the thickened sections at both ends.

[0016] Furthermore, a drill pipe male connector and a drill pipe female connector are respectively installed on the interference fit thread of the aluminum alloy inner tube, and an insulating sealing gasket is respectively provided between the inner end of the drill pipe male connector and the inner end of the drill pipe female connector and the shoulder on the steel-aluminum special-shaped cross section composite rod.

[0017] This invention also protects a manufacturing process for the bimetallic composite drill pipe as described above, the process comprising the following steps:

[0018] Step 1: Assembly and forming of steel-aluminum composite bars with irregular cross-sections:

[0019] Check the fit dimensions of the steel outer tube and the aluminum alloy inner tube, and perform degreasing and rust removal treatment on the inner wall of the steel outer tube and the outer wall of the aluminum alloy inner tube. Align the fit phase of the non-circular torsional cross sections of the steel outer tube and the aluminum alloy inner tube. Insert the aluminum alloy inner tube into the inner hole of the steel outer tube, keeping the two end faces of the steel outer tube and the aluminum alloy inner tube flush, and then perform mechanical composite forming.

[0020] Step 2, end face connection thread machining:

[0021] One end of the drill pipe male connector is machined with a drill pipe male thread, and the other end of the drill pipe male connector is a male connector interference connection thread; both ends of the steel-aluminum bimetallic composite rod are machined with rod body interference connection threads, and the rod body interference connection threads are located on the aluminum alloy inner tube; one end of the drill pipe female connector is machined with a drill pipe female thread, and the other end of the drill pipe female connector is a female connector interference connection thread.

[0022] Step 3, Assembly of bimetallic composite drill pipe with irregular cross-section:

[0023] The drill pipe male connector is interference-fitted to the steel-aluminum composite rod body via an interference-fit thread. The male connector male connector is provided with a male connector limiting shoulder to limit the axial fit between the male connector interference-fit thread and the rod body interference-fit thread, thereby achieving the effects of sealing and torque transmission.

[0024] The drill pipe female joint is interference-connected to the steel-aluminum composite rod body via an interference-connection thread. The female joint limiting shoulder provided on the drill pipe female joint is used to limit the axial position of the interference-connection thread of the female joint and the interference-connection thread of the rod body, which can prevent high-pressure drilling fluid or water inside the drill pipe from entering the steel-aluminum bimetallic composite interface and improve the torque transmission effect between the drill pipe joint and the rod body.

[0025] Before pre-tightening the threads, apply thread-locking adhesive to the interference connection threads of the rod body. Insulating sealing gaskets are respectively fitted onto the root shoulders of the interference connection threads at both ends of the aluminum alloy inner tube. After the threads are pre-tightened, the insulating sealing gaskets play a sealing role to prevent drilling fluid from entering the steel-aluminum bimetallic composite interface and inducing electrochemical corrosion.

[0026] Compared with the prior art, the present invention has the following technical effects:

[0027] (I) The bimetallic composite drill pipe with irregular cross section of the present invention makes full use of the advantages of high strength and wear resistance of steel, as well as the characteristics of high specific strength and corrosion resistance of lightweight alloy. It adopts mechanical composite process to form a lightweight bimetallic composite drill pipe with irregular cross section and processing technology. It has comprehensive performance of light weight, high strength, good processing performance, low cost, good corrosion resistance and wear resistance.

[0028] (II) The bimetallic composite drill rod with irregular cross section of the present invention has alloy steel joints and lightweight bimetallic composite tube with irregular cross section. Compared with drill rods made of single material, it has excellent comprehensive performance, light weight, good surface wear resistance, and can drill deeper holes with the same drilling rig capacity. It is easy to transport and handle and is suitable for drilling construction in remote and complex areas such as high mountains, tunnels, polar regions, and mines.

[0029] (III) The bimetallic composite drill pipe with irregular cross section of the present invention uses a non-circular irregular cross section as the mating surface between the wear-resistant outer tube and the lightweight high-strength inner tube of the rod body. It has a better torsion transmission effect than conventional mechanical composite pipes such as rolling, extrusion and cold drawing. Moreover, compared with metallurgical composite pipes such as explosive welding and centrifugal casting, the processing technology is simple, the equipment investment is small and the cost is low.

[0030] (IV) The bimetallic composite drill pipe with irregular cross section of the present invention has an insulating sealing element provided on the joint and the connecting end face of the lightweight alloy tube body, which effectively prevents the flushing fluid from entering the steel-aluminum composite interface and causing electrochemical corrosion. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the steel-aluminum composite rod with a regular hexagonal cross section used for torsion transmission in Example 1.

[0032] Figure 2 This is a schematic diagram of the regular hexagonal cross-section in Example 1.

[0033] Figure 3 This is a schematic diagram of the steel-aluminum composite rod with an elliptical cross section used for torsion transmission in Example 2.

[0034] Figure 4 This is a schematic diagram of the elliptical cross-section in Example 2.

[0035] Figure 5 This is a schematic diagram of the irregular connecting pin in the steel-aluminum composite rod body using irregular connecting pin for torque transmission in Example 3.

[0036] Figure 6 This is a schematic diagram of the cross-sectional structure of the torsion transmission using a non-circular connecting pin in Example 3.

[0037] Figure 7 This is a schematic diagram of the overall structure of the bimetallic composite drill pipe with an irregular cross-section according to the present invention.

[0038] The meanings of the labels in the diagram are as follows: 1-Drill pipe male connector, 2-Steel-aluminum composite rod with special cross section, 3-Drill pipe female connector, 4-Insulating sealing gasket.

[0039] 11-Drill pipe male thread, 12-Interference fit thread for male connector, 13-Shoulder limit for male connector.

[0040] 21-Steel outer tube, 22-Aluminum alloy inner tube, 23-Interference fit thread on rod body, 24-Irregular torsion cross section of inner and outer tubes, 25-Irregular connecting pin.

[0041] 31-Drill pipe female thread, 32-Female connector interference fit thread, 33-Female connector limiting shoulder.

[0042] The specific content of the present invention will be further explained in detail below with reference to the embodiments. Detailed Implementation

[0043] It should be noted that, unless otherwise specified, all components in this invention are components known in the prior art.

[0044] As described in the background art, conventional single-material drill rods have disadvantages such as large weight, high material cost, difficult processing, insufficient corrosion resistance and wear life in drilling operations under complex conditions. This limits the use of traditional steel drill rods in deep hole, ultra-deep hole and ultra-long horizontal directional hole drilling. Existing lightweight drill rods such as aluminum alloy and titanium alloy are also not suitable for large-scale use in drilling projects.

[0045] Therefore, through dedicated research and drawing on experience in drilling operations and machining, the inventors of this invention have designed a lightweight bimetallic composite drill rod with an irregular cross-section. This design enhances torsion transmission capacity, reduces drill rod weight, improves corrosion and wear resistance, and reduces labor intensity during transportation and handling, thus contributing to the automation and intelligent development of drilling projects.

[0046] The following are specific embodiments of the present invention. It should be noted that the present invention is not limited to the following specific embodiments. All equivalent modifications made based on the technical solutions of this application fall within the protection scope of the present invention.

[0047] Example 1:

[0048] This embodiment provides a bimetallic composite drill pipe with an irregular cross-section, such as... Figure 1 and Figure 2 As shown, it includes a steel-aluminum irregular cross-section composite rod 2, which includes a steel outer tube 21 and an aluminum alloy inner tube 22 inside the steel outer tube 21.

[0049] The aluminum alloy inner tube 22 and the steel outer tube 21 adopt a torsion transmission structure with an inner and outer tube of different torsion transmission cross section 24.

[0050] The torsion transmission structure of the inner and outer tubes with different torsion transmission sections 24 is as follows: the inner surface of the steel outer tube 21 and the outer surface of the aluminum alloy inner tube 22 are in contact and fit to form a mating surface, and the axial section of the mating surface is the inner and outer tubes with different torsion transmission sections 24. The steel outer tube 21 and the aluminum alloy inner tube 22 form a torsion transmission structure through the fit of the inner and outer tubes with different torsion transmission sections 24.

[0051] In this embodiment, the outer steel tube 21 is made of high-precision drawn alloy steel tube; the inner aluminum alloy tube 22 is made of high-strength extruded aluminum alloy.

[0052] In this embodiment, the torsion transmission cross section 24 of the inner and outer tubes is hexagonal.

[0053] like Figure 1 As shown, the aluminum alloy inner tube 22 is thickened at both ends to form thickened sections, and interference connection threads 23 are machined on the thickened sections at both ends. The inner thickening structure is used to compensate for the adverse effect of the machining thread structure on the torsional section coefficient of the aluminum alloy tube body.

[0054] like Figure 7 As shown, a drill pipe male connector 1 and a drill pipe female connector 3 are respectively installed on the interference connection thread 23 of the aluminum alloy inner tube 22. Insulating sealing gaskets 4 are respectively provided between the inner end of the drill pipe male connector 1 and the inner end of the drill pipe female connector 3 and the shoulder on the steel-aluminum special-shaped cross section composite rod body 2.

[0055] In this embodiment, the drill pipe male connector 1 is made of alloy steel and consists of a drill pipe male thread 11, a male connector interference connection thread 12, and a male connector limiting shoulder 13.

[0056] In this embodiment, the drill pipe female connector 3 is made of alloy steel and consists of a drill pipe female thread 31, a female connector interference connection thread 32, and a female connector limiting shoulder 33.

[0057] In this embodiment, the insulating sealing gasket 4 is a ring-shaped insulating sheet structure made of high-temperature resistant polytetrafluoroethylene, used to prevent electrochemical corrosion caused by drilling fluid entering the bimetallic composite interface.

[0058] In this embodiment, the outer diameter of the drill rod is Φ114.3mm, and the length of a single rod varies from 1m to 3m.

[0059] This embodiment also provides a manufacturing process for the above-mentioned irregular cross-section bimetallic composite drill pipe, which includes the following steps:

[0060] Step 1: Assembly and forming of steel-aluminum composite bars with irregular cross-sections:

[0061] Check the fit dimensions of the steel outer tube 21 and the aluminum alloy inner tube 22. Degrease and derust the inner wall of the steel outer tube 21 and the outer wall of the aluminum alloy inner tube 22. Align the fit phase of the non-circular torsion transmission cross sections 24 of the steel outer tube 21 and the aluminum alloy inner tube 22. Insert the aluminum alloy inner tube 22 into the inner hole of the steel outer tube 21, keeping the two end faces of the steel outer tube 21 and the aluminum alloy inner tube 22 flush. Then perform mechanical composite forming.

[0062] Step 2, end face connection thread machining:

[0063] One end of the drill pipe male connector 1 is machined with a drill pipe male thread 11, and the other end of the drill pipe male connector 1 is a male connector interference connection thread 12; both ends of the steel-aluminum bimetallic composite rod body 2 are machined with rod body interference connection threads 23, and the rod body interference connection threads 23 are located on the aluminum alloy inner tube 22; one end of the drill pipe female connector 3 is machined with a drill pipe female thread 31, and the other end of the drill pipe female connector 3 is a female connector interference connection thread 32.

[0064] Step 3, Assembly of bimetallic composite drill pipe with irregular cross-section:

[0065] The drill pipe male connector 1 is interference-connected to the steel-aluminum composite rod body 2 via the male connector interference connection thread 12. The male connector limiting shoulder 13 provided on the drill pipe male connector 1 is used to limit the axial fit between the male connector interference connection thread 12 and the rod body interference connection thread 23, so as to achieve the effects of sealing and torque transmission.

[0066] The drill pipe female connector 3 is interference-connected to the steel-aluminum composite rod body 2 via the female connector interference connection thread 32. The female connector limiting shoulder 33 provided on the drill pipe female connector 3 is used to limit the axial position of the female connector interference connection thread 32 and the rod body interference connection thread 23, which can prevent high-pressure drilling fluid or water inside the drill pipe from entering the steel-aluminum bimetallic composite interface and improve the torsion transmission effect between the drill pipe connector and the rod body.

[0067] Before pre-tightening the threads, apply thread-locking adhesive to the interference connection threads 23 on the rod body. The insulating sealing gaskets 4 are respectively fitted onto the root shoulders of the interference connection threads 23 at both ends of the aluminum alloy inner tube 22. After the threads are pre-tightened, the insulating sealing gaskets 4 play a sealing role to prevent drilling fluid from entering the steel-aluminum bimetallic composite interface and inducing electrochemical corrosion.

[0068] In this embodiment, the bimetallic composite drill pipe with an irregular cross-section is used such that the male thread 11 is used to connect to the drill bit or other drill pipes, and the female thread 31 is used to connect to other drill pipes or water supply devices. After connection, the drilling pressure and torque loads are transmitted from the drill rig power head through the drill pipe female connector 3 to the aluminum alloy inner tube 22, then through the aluminum alloy inner tube 22 to the drill pipe male connector 1, and finally through the threaded connection pair between two adjacent drill pipes to the previous drill pipe.

[0069] Example 2:

[0070] This embodiment provides a bimetallic composite drill pipe with an irregular cross-section. Its main structure is the same as that of the bimetallic composite drill pipe with an irregular cross-section given in Embodiment 1, the only difference being that... Figure 3 and Figure 4 As shown in the figure, in this embodiment, the torsion transmission cross section 24 of the inner and outer tubes is elliptical.

[0071] The manufacturing process of the irregular cross-section bimetallic composite drill pipe in this embodiment is basically the same as that of the irregular cross-section bimetallic composite drill pipe in Embodiment 1, with the only difference being:

[0072] During assembly, the elliptical cross-sections of the stainless steel outer tube 21 and the aluminum alloy inner tube 22 are aligned, and then the aluminum alloy inner tube 22 is inserted into the stainless steel outer tube 21. The inner and outer tubes are mechanically combined by hydraulic action.

[0073] During use, the elliptical inner and outer tube irregular torsion transmission section 24 can transmit the torque of the aluminum alloy inner tube 22 to the stainless steel outer tube 21, driving the stainless steel outer tube 22 to rotate, which is conducive to the discharge of rock cuttings and coal powder in the borehole.

[0074] Example 3:

[0075] This embodiment provides a bimetallic composite drill pipe with an irregular cross section. Its main structure is the same as that of the bimetallic composite drill pipe with an irregular cross section given in Embodiment 1. The only difference is that in this embodiment, an irregular connecting pin 25 is used to transmit torque between the aluminum alloy inner tube 22 and the steel outer tube 21.

[0076] In this embodiment, as Figure 5 and Figure 6 As shown, the torsion transmission structure of the special-shaped connecting pin 25 is as follows: the inner contour of the axial section of the steel outer tube 21 is circular, and the outer contour of the axial section of the aluminum alloy inner tube 22 is a regular hexagon, which is a circular inscribed regular hexagon; special-shaped connecting pins 25 are installed in the six arc gaps between the steel outer tube 21 and the aluminum alloy inner tube 22, and the special-shaped connecting pins 25 fill the arc gaps. The special-shaped connecting pins 25 and the steel outer tube 21 are connected by axially arranged keys and keyways. The special-shaped connecting pins 25 realize the fixation and torsion transmission between the steel outer tube 21 and the aluminum alloy inner tube 22.

[0077] The manufacturing process of the irregular cross-section bimetallic composite drill pipe in this embodiment is basically the same as that in Embodiment 1, except that step one is different. Step one in this embodiment is as follows:

[0078] Step 1: Assembly and forming of steel-aluminum composite bars with irregular cross-sections:

[0079] Check the fit dimensions between the steel outer tube 21 and the aluminum alloy inner tube 22. Degrease and remove rust from the inner wall of the steel outer tube 21 and the outer wall of the aluminum alloy inner tube 22. Insert the aluminum alloy inner tube 22 into the inner hole of the steel outer tube 21, keeping the two end faces of the steel outer tube 21 and the aluminum alloy inner tube 22 flush. Use a special-shaped connecting pin 25 to fix and transmit torque between the steel outer tube 21 and the aluminum alloy inner tube 22, and then perform mechanical composite forming.

Claims

1. A bimetallic composite drill pipe, characterized in that, The system includes a steel-aluminum composite rod (2) with an outer steel tube (21) and an inner aluminum alloy tube (22) inside the outer steel tube (21). The inner ends of the inner aluminum alloy tube (22) are thickened to form thickened sections, and interference connection threads (23) are machined on the thickened sections at both ends. A drill pipe male connector (1) and a drill pipe female connector (3) are installed on the interference connection threads (23) of the inner aluminum alloy tube (22). The aluminum alloy inner tube (22) and the steel outer tube (21) are connected by a special-shaped connecting pin (25) to transmit torque. The torsion transmission structure of the special-shaped connecting pin (25) is as follows: the inner contour of the axial section of the steel outer tube (21) is circular, and the outer contour of the axial section of the aluminum alloy inner tube (22) is regular hexagonal, which is the inscribed regular hexagon of the circle; special-shaped connecting pins (25) are installed in the six arc gaps between the steel outer tube (21) and the aluminum alloy inner tube (22), the special-shaped connecting pins (25) fill the arc gaps, and the special-shaped connecting pins (25) and the steel outer tube (21) are connected by axially arranged keys and keyways, and the fixing and torsion transmission between the steel outer tube (21) and the aluminum alloy inner tube (22) are realized by the special-shaped connecting pins (25).

2. The bimetallic composite drill pipe as described in claim 1, characterized in that, Insulating sealing gaskets (4) are respectively provided between the inner end of the drill pipe male connector (1) and the inner end of the drill pipe female connector (3) and the shoulder on the steel-aluminum special-shaped cross section composite rod (2).

3. A manufacturing process for the bimetallic composite drill pipe as described in claim 2, characterized in that, The process includes the following steps: Step 1: Assembly and forming of steel-aluminum composite bars with irregular cross-sections: Check the fit dimensions of the steel outer tube (21) and the aluminum alloy inner tube (22), degrease and derust the inner wall of the steel outer tube (21) and the outer wall of the aluminum alloy inner tube (22), insert the aluminum alloy inner tube (22) into the inner hole of the steel outer tube (21), keep the two end faces of the steel outer tube (21) and the aluminum alloy inner tube (22) flush, use special-shaped connecting pins (25) to fix and transmit torsion between the steel outer tube (21) and the aluminum alloy inner tube (22), and then perform mechanical composite forming; Step 2, end face connection thread machining: One end of the drill pipe male connector (1) is machined with a drill pipe male thread (11), and the other end of the drill pipe male connector (1) is a male connector interference connection thread (12); both ends of the steel-aluminum bimetallic composite rod (2) are machined with rod body interference connection threads (23), and the rod body interference connection threads (23) are located on the aluminum alloy inner tube (22); one end of the drill pipe female connector (3) is machined with a drill pipe female thread (31), and the other end of the drill pipe female connector (3) is a female connector interference connection thread (32); Step 3, Assembly of bimetallic composite drill pipe with irregular cross-section: The drill pipe male connector (1) is connected to the steel-aluminum composite rod body (2) via the male connector interference connection thread (12). The male connector limiting shoulder (13) provided on the drill pipe male connector (1) is used to limit the axial fit between the male connector interference connection thread (12) and the rod body interference connection thread (23) to achieve the effects of sealing and torque transmission. The drill pipe female joint (3) is interference connected to the steel-aluminum composite rod body (2) via the female joint interference connection thread (32). The female joint limiting shoulder (33) provided on the drill pipe female joint (3) is used to limit the axial position of the female joint interference connection thread (32) and the rod body interference connection thread (23), which can prevent high-pressure drilling fluid or water inside the drill pipe from entering the steel-aluminum bimetallic composite interface and improve the torque transmission effect between the drill pipe joint and the rod body. Before pre-tightening the threads, apply thread-locking adhesive to the interference connection threads (23) of the rod body. The insulating sealing gaskets (4) are respectively fitted onto the root shoulders of the interference connection threads (23) of the rod body at both ends of the aluminum alloy inner tube (22). After the threads are pre-tightened, the insulating sealing gaskets (4) play a sealing role to prevent drilling fluid from entering the steel-aluminum bimetallic composite interface and inducing electrochemical corrosion.

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