JP7706783B2 - Copper alloys for corrosive environments - Google Patents

Description

Article 30, Paragraph 2 of the Patent Act applies ・Publication: Proceedings of JSCE Materials and Environments 2023 Materials and Environment 2023 Lecture Collection Publication date: May 1, 2023 ・Meeting name: Proceedings of JSCE Materials and Environments 2023 Materials and Environment 2023 Date: June 6-8, 2023

The present invention relates to a copper alloy for use in corrosive environments.

Copper (Cu) is the main electrical contact material used for electrical wiring connectors. However, in humid environments and with rising temperatures, corrosion products with high electrical resistance, mainly _Cu2O and CuO, are formed on the surface of Cu, increasing the electrical resistance of the surface and reducing its function as an electrical contact. For this reason, gold (Au) or silver (Ag), which has excellent corrosion resistance and oxidation resistance, is plated on the surface of Cu. However, because precious metal plating is expensive, tin (Sn), which is inexpensive and relatively corrosion resistant, is used for industrial equipment such as automobiles, which require economical efficiency.

However, since it is difficult for Cu-based electrical contact materials to suppress corrosion and oxidation and maintain low surface electrical resistance for a long period of time, improvements have been made to the Sn plating, such as a technique for forming an alloy layer of Cu and Sn on the outermost surface of an electrical contact material for connectors (see, for example, Patent Document 1), a technique for forming a Sn or Sn alloy layer on the outermost surface and forming an alloy layer containing an intermetallic compound of Cu and Sn underneath (see, for example, Patent Document 2), and a technique for forming an Ag ₃ Sn layer on a Sn plating layer (see, for example, Patent Document 3).

However, due to the need for weight reduction and resource conservation in automobiles and other transportation equipment, there is a demand to make the thickness of the Sn plating as thin as possible. Sn is a relatively soft metal, and when used in automotive connectors, it is necessary to assume that the Sn plating layer will be worn away and lost due to sliding caused by vibration, etc. As electric vehicles and autonomous driving become more common and widespread, poor contact in connectors can lead to serious accidents, and it is necessary to avoid this.

On the other hand, the specific conditions for the composition and microstructure of the Cu alloy to maintain low surface electrical resistance for a long period of time have not been clarified, and if the Sn plating wears away, the Cu-based electrical contact material will be exposed, and it will be difficult to avoid a state in which it corrodes.

JP 2010-267418 A JP 2011-12350 A JP 2011-26677 A

The present invention was made in consideration of the above circumstances, and its purpose is to provide a copper alloy for use in corrosive environments, such as those where dry and wet conditions are repeated, that can maintain a low surface electrical resistance even if the alloy surface corrodes, and has excellent electrical contact properties.

The inventors conducted various experimental research to overcome the limitations of the conventional technology and solve the unresolved problems described above, and completed the present invention. In particular, they discovered that by controlling the amounts of Sn, Zn, Al, and Ni added to Cu alloys, a Sn-rich phase is formed.

Furthermore, they found that the Sn-rich phase exerts a sacrificial protection effect on the entire Cu-based alloy in a corrosive environment. During this process, Sn corrosion products cover the alloy surface as a result of sacrificial protection, but they found that the Sn corrosion products (oxides, hydroxides, etc.) have excellent electrical conductivity and therefore do not significantly increase the electrical resistance of the alloy surface.

The present invention is based on the above new findings, and its gist is as follows. That is, the copper alloy for use in corrosive environments according to the present invention is characterized by having a Cu content of 35 atomic % or more, Sn and Zn contents each of which are 10 atomic % or more and 30 atomic % or less, and Al and Ni contents each of which are 1 atomic % or more and 20 atomic % or less, and having a Sn-rich phase in which the Sn content is 90 atomic % or more.

The copper alloy for use in corrosive environments according to the present invention preferably has an area ratio of the Sn-rich phase of 2.0% or more.

The copper alloy for use in corrosive environments according to the present invention can maintain a low surface electrical resistance even if the alloy surface corrodes in a corrosive environment where dry and wet conditions are repeated, and has excellent electrical contact properties. Therefore, according to the present invention, it is possible to dramatically reduce contact failures in connectors of electrical wiring in electrical and communication equipment installed outdoors, automobiles, etc., for a long period of time.

1 is a table showing the chemical compositions, the area ratios of Sn-rich phases, the surface electrical resistivities, and the evaluations thereof for copper alloys for use in corrosive environments (Examples 1 to 6) according to embodiments of the present invention and comparative examples. 2 is a scanning electron microscope image (S.E.) of the microstructure of Example 2 shown in FIG. 1 and elemental maps of Cu, Zn, Al, Ni, and Sn by EDS (energy dispersive X-ray spectroscopy).

Hereinafter, an embodiment of the present invention will be described.
Cu is a metal with relatively high corrosion resistance and low electrical resistance. It also has excellent workability, Sn plating property, and solderability. For this reason, it is an essential element for copper alloys for use in corrosive environments. However, if the Cu content is less than 35 atomic %, the corrosion resistance required for a copper alloy for use in corrosive environments cannot be obtained. For this reason, the copper alloy for use in corrosive environments according to the embodiment of the present invention contains 35 atomic % or more of Cu.

The copper alloy for use in a corrosive environment according to the embodiment of the present invention contains 35 atomic % or more of Cu, 10 atomic % to 30 atomic % or less of Sn and Zn, 1 atomic % to 20 atomic % or less of Al and Ni, and has a Sn-rich phase containing 90 atomic % or more of Sn. The area ratio of the Sn-rich phase is 0.5% or more. When Cu, Sn, Zn, Al, and Ni are within the above ranges, a microstructure having a Sn-rich phase containing 90 atomic % or more of Sn is formed. In the present invention, a microstructure having a Sn-rich phase containing 90 atomic % or more of Sn means a case where the area ratio of the Sn-rich phase is 0.5% or more. The area ratio here refers to the area ratio of the Sn-rich layer exposed on the alloy surface.

If the contents of Cu, Sn, Zn, Al, and Ni are outside the above ranges, the formation of the Sn-rich phase will be insufficient, making it impossible to maintain a low surface electrical resistance when corroded.

Sn, Zn, Al, and Ni are elements that dissolve in Cu, although in limited concentration ranges. Sn improves the malleability of Cu alloys in a solid solution state. Zn improves machinability and punchability in a solid solution state. Al and Ni are elements that improve the corrosion resistance of Cu alloys in a solid solution state. In addition, Al and Ni are elements that have excellent corrosion resistance as single metals, and are expected to improve corrosion resistance when alloyed, not only in Cu alloys. Therefore, in the copper alloy for corrosive environments according to the embodiment of the present invention, Sn, Zn, Al, and Ni contribute to improving various properties, including corrosion resistance, in phases other than the Sn-rich phase.

By the way, Sn is a less noble metal than Cu. Therefore, the Sn-rich phase exerts a sacrificial corrosion protection effect on the entire alloy mainly composed of Cu in a corrosive environment. With the sacrificial corrosion protection, the corrosion products of Sn cover the alloy surface, but since the corrosion products of Sn (oxides, etc.) have excellent electrical conductivity, they do not significantly increase the electrical resistance of the alloy surface. However, in the sacrificial corrosion protection effect, it is not preferable for metals other than Sn to dissolve and form corrosion products, because the electrical resistance of the corrosion products increases. Therefore, the copper alloy for corrosive environments of the embodiment of the present invention, which has excellent electrical contact characteristics, needs to be an Sn-rich phase containing 90 atomic % or more of Sn. If the Sn concentration in the Sn-rich phase is less than 90 atomic %, the electrical resistance of the corrosion products that are generated will be high.

From the viewpoint of improving corrosion resistance, it is preferable to have a large amount of Sn-rich phase containing 90 atomic % or more of Sn. When it is necessary to maintain low surface resistance for a long period of time in a severe corrosive environment, it is desirable for the area ratio of the Sn-rich phase containing 90 atomic % or more of Sn to be 2.0% or more. The area ratio here refers to the area ratio of the Sn-rich layer exposed on the alloy surface.

The copper alloy for use in corrosive environments according to the embodiment of the present invention contains Sn, Zn, Al, and Ni, and therefore in addition to an Sn-rich phase containing 90 atomic % or more of Sn, phases consisting of specific combinations of these metal elements may be generated. For example, a Ni-Al phase in which Ni is the main component and Al is in solid solution, a Cu-Sn-Zn phase or a Cu-Zn-Sn phase in which Cu is the main component and Sn and Zn are in solid solution, etc. may be generated. Here, this means that the Cu-Sn-Zn phase has a higher concentration of Sn than Zn, and the Cu-Zn-Sn phase has a higher concentration of Zn than Sn.

The copper alloy for use in corrosive environments according to the embodiment of the present invention is required to have a Sn-rich phase containing 90 atomic % or more of Sn, and does not specify the type or composition of other phases.

The following describes in detail the embodiments of the present invention based on examples, but the present invention is not limited to the description of the examples.

Copper alloys for corrosive environments, with the chemical compositions shown in Figure 1, were prepared by vacuum induction melting. Alloys of each composition were placed in a vacuum heat treatment furnace and heated to 500°C over the course of one hour. They were then subjected to homogenizing heat treatment at 500°C for five hours, after which they were immediately plunged into ice water (stirred at 5°C or less) for rapid cooling. The ice water was stirred so that the cooling rate was 900°C/min or more. Each alloy was then cut to a thickness of 5 mm, width of 15 mm, and length of 25 mm, and mirror-polished to prepare test pieces. For mirror polishing, the front, back, and end faces of the cut alloys were polished using diamond paste with a grain size of 1 μm.

The microstructure of each test piece was observed with an optical microscope, and the composition of each phase was analyzed with a scanning electron microscope and energy dispersive X-ray analysis. The average composition of the entire test piece was quantified by fluorescent X-ray analysis. The area ratio of the Sn-rich phase was calculated for a field of view of 1 mm x 1 mm. Figure 2 shows a scanning electron microscope image of the microstructure of the test piece of Example 2 and an element map obtained by EDS (energy dispersive X-ray analysis).

Each test piece was subjected to repeated wet-dry tests using artificial seawater adjusted to a pH of 8.2, and the electrical resistance of the surface was measured using the four-terminal method before and after the test.

This wet-dry cycle test simulates atmospheric corrosion caused by sea salt particles and is used to evaluate the durability of corrosion resistance in coastal areas.

For the wet-dry cycle test, a constant temperature and humidity chamber was used, where each test piece was repeatedly immersed in artificial seawater (25°C) for three and a half hours, followed by drying for 30 minutes in an environment of 25°C and 30% RH. The immersion time in artificial seawater was set so that it took 1 hour 20 minutes for the entire test piece to be immersed, 50 minutes for the entire test piece to be immersed, and 1 hour 20 minutes for the entire test piece to emerge from the artificial seawater. A combination of immersion in artificial seawater and drying constituted one cycle, and 20 cycles of wet-dry cycle tests were performed.

The four-terminal method uses a probe with rhodium-plated terminals arranged in a line at intervals of 2.0 mm, and measures electrical resistance by pressing the probe against the test piece with a constant force of 100 gf.

Figure 1 shows the surface electrical resistance of each test piece and its evaluation. The surface electrical resistance was evaluated based on the degree of increase in resistivity before and after the wet-dry cycle test. Compared to the resistivity in the mirror-polished state, an increase of one digit or less was rated as ◎ (very good), an increase of two digits or less was rated as ○ (good), and an increase of more than two digits was rated as × (bad).

As shown in Figure 1, it was found that the increase in resistivity during the wet-dry cycle test was smaller for the alloys of Examples 1 to 6 than for Comparative Examples 1 to 5. In particular, when the element concentration of any one of Cu, Sn, Zn, Al, and Ni falls outside the range of the copper alloy for use in corrosive environments according to the embodiment of the present invention, the area ratio of the Sn-rich phase containing 90 atomic % or more of Sn becomes less than 0.5%, and it is found that the surface electrical resistance increases significantly during the corrosion test. In addition, Comparative Example 5 is an example of pure copper, and it is found that the increase in resistivity is extremely large compared to the alloys obtained in Examples 1 to 6.

Possible applications of the copper alloy for use in corrosive environments according to the present invention include contact materials for electrical wiring connectors in automobiles and materials for electrical contacts in outdoor electrical equipment.

Claims (2)

Cu with a content of 36 atomic % or more and 78 atomic % or less ,
Sn and Zn each having a content of 10 atomic % or more and 30 atomic % or less,
The content of Al and Ni is 1 atomic % or more and 20 atomic % or less,
The copper alloy for use in a corrosive environment is characterized by having an Sn-rich phase in which the Sn content is 90 atomic % or more. The copper alloy for use in a corrosive environment according to claim 1, characterized in that the area ratio of the Sn-rich phase is 2.0% or more.