JP7706784B2 - Highly corrosion-resistant copper alloy - 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 highly corrosion-resistant copper alloy.

Copper (Cu) is the main material used as electrical contacts for connectors such as electrical wiring. However, when the material is repeatedly wet and dry under relatively high temperature conditions, the surface of the Cu corrodes, producing corrosion products with high electrical resistance, mainly consisting of _Cu2O and CuO. As a result, the electrical resistance of the surface increases, and its function as an electrical contact decreases. For this reason, the surface of the Cu is plated with gold (Au) or silver (Ag), which has excellent corrosion resistance and oxidation resistance. 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, 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 Cu alloys to maintain low surface electrical resistance over the long term have not been clarified, and if the Sn plating disappears, it will be difficult to avoid the Cu-based electrical contact material being exposed and corroding itself. In this context, there is a demand for highly corrosion-resistant copper alloys that maintain low surface electrical resistance even in corrosive environments and have excellent electrical contact properties.

In regards to improving the corrosion resistance of Cu, attempts have been made to add alloying elements. For example, aluminum bronze (Cu-6-12% Al-1.5-6% Fe-5% Ni), brass (Cu-40% Zn), phosphor bronze (Cu-4-10% Sn), cupro-nickel (Cu-10-30% Ni), and nickel silver (Cu-5-35% Ni-15-35% Zn) are used in corrosive environments. However, although these materials have excellent corrosion resistance, corrosion products are formed on the surface in corrosive environments, increasing the electrical resistance of the surface, making them unsuitable as electrical contact materials for connectors.

Therefore, a technique has been disclosed for connectors such as charging sockets for electric vehicles, in which a Cu alloy surface is coated with Sn or a Sn alloy, and then heat-treated to form a Cu-Sn intermetallic compound (Cu ₃ Sn, Cu ₄ Sn, etc.) on the surface (see, for example, Patent Document 1). However, Sn is an electrochemically less noble metal than Cu, and has poorer corrosion resistance than Cu, which is a noble metal. For this reason, the corrosion resistance of the Cu-Sn alloy is not very high.

In order to improve the corrosion resistance of Cu, it is presumed that it is suitable to add Sn, Zn, Ni, and Al in an appropriate composition, which have a composition range in which they dissolve in Cu. However, the specific conditions for the composition and microstructure of such Cu alloys have not been made clear. In particular, the technology to maintain not only high corrosion resistance but also the low surface electrical resistance required for electrical contact materials for connectors in electrical wiring is unclear.

Japanese Patent Application Publication No. 10-25562

The present invention was made in consideration of the above circumstances, and its purpose is to provide a highly corrosion-resistant copper alloy that exhibits high corrosion resistance in a corrosive environment where drying and wetting are repeated, can maintain a low surface electrical resistance, 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 controlling the amounts of Sn, Zn, Al, and Ni added to a Cu alloy produces a Sn-Cu-Ni phase containing 50 atomic % or more of Sn and 15 atomic % or more of Cu and Ni, and a Cu-Sn phase containing 65 atomic % or more of Cu and 5 atomic % or more of Sn.

Furthermore, it was found that the Cu alloy composed of these two phases shows very little corrosion weight loss in a dry-wet cycle test, and that the surface electrical resistance does not increase significantly as a result of the dry-wet cycle test.

The present invention is based on the above new findings, and its gist is as follows. That is, the highly corrosion-resistant copper alloy according to the present invention has a Cu content of 35 atomic % or more, a Sn content of 30 atomic % or more, a Ni content of 15 atomic % or more, and Zn and Al contents of 1 atomic % or more and 10 atomic % or less, and has a Sn-Cu-Ni phase and a Cu-Sn phase, the Sn-Cu-Ni phase contains 50 atomic % or more Sn, 15 atomic % or more Cu and 15 atomic % or more Ni, and the Cu-Sn phase contains 65 atomic % or more Cu and 5 atomic % or more Sn.

In the highly corrosion-resistant copper alloy according to the present invention, it is preferable that the Sn-Cu-Ni phase has an Sn content of 60 atomic % or more.

The highly corrosion-resistant copper alloy of the present invention exhibits high corrosion resistance in a corrosive environment where drying and wetting are repeated, and is capable of maintaining a low surface electrical resistance, providing 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, and the like, over a long period of time.

1 is a table showing the chemical compositions of highly corrosion-resistant copper alloys (Examples 1 to 6) according to embodiments of the present invention and comparative examples, as well as the compositions and area ratios of Sn--Cu--Ni phases and Cu--Sn phases. 2 is a scanning electron microscope image (S.E.) of the microstructure of Example 6 shown in FIG. 1, and elemental maps of Cu, Zn, Al, Ni, and Sn by EDS (energy dispersive X-ray spectroscopy). 2 is a table showing the corrosion resistance (average corrosion depth) and surface electrical resistivity of the examples and comparative examples shown in FIG. 1, as well as their evaluations.

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 Cu is less than 35 atomic %, the corrosion resistance required for a copper alloy for use in a corrosive environment cannot be obtained. For this reason, the highly corrosion-resistant copper alloy according to the embodiment of the present invention contains Cu at 35 atomic % or more.

The highly corrosion-resistant copper alloy according to the embodiment of the present invention contains 35 atomic % or more of Cu, 30 atomic % or more of Sn, 15 atomic % or more of Ni, and 1 atomic % or more and 10 atomic % or less of Zn and Al, and has a Sn-Cu-Ni phase containing 50 atomic % or more of Sn and 15 atomic % or more of Cu and Ni, and a Cu-Sn phase containing 65 atomic % or more of Cu and 5 atomic % or more of Sn.

When Cu, Sn, Zn, Al, and Ni are within the above ranges, a microstructure is formed that has a Sn-Cu-Ni phase containing 50 atomic % or more of Sn and 15 atomic % or more of Cu and Ni, and a Cu-Sn phase containing 65 atomic % or more of Cu and 5 atomic % or more of Sn.

The Sn-Cu-Ni phase is a phase that exhibits high corrosion resistance and low surface electrical resistance even after corrosion testing. To achieve this characteristic, it is necessary to contain 50 atomic % or more of Sn and 15 atomic % or more each of Cu and Ni. This phase exhibits relatively high corrosion resistance, which is an effect of Ni. If Ni is less than 15 atomic %, corrosion resistance decreases. If Cu is less than 50 atomic %, it becomes difficult to exhibit low surface electrical resistance even after corrosion testing. Similarly, if Sn is less than 15 atomic %, it becomes difficult to exhibit low surface electrical resistance even after corrosion testing.

The Cu-Sn phase also exhibits high corrosion resistance and low surface electrical resistance even after corrosion testing. To achieve this characteristic, the material must contain at least 65 atomic % Cu and at least 5 atomic % Sn. If the Cu concentration is less than 65 atomic %, corrosion resistance decreases. If the Sn concentration is less than 5 atomic %, the material will not be able to exhibit low surface electrical resistance even after corrosion testing.

The highly corrosion-resistant copper alloy according to the embodiment of the present invention is not limited in the ratio of the Sn-Cu-Ni phase and the Cu-Sn phase produced or in the overall alloy, but it is necessary that the alloy has both an Sn-Cu-Ni phase and a Cu-Sn phase. In terms of the area of the phases, it is preferable that the Sn-Cu-Ni phase is 50% or more and the Cu-Sn phase is 10% to 50%. This is because the Cu-Sn phase and the Sn-Cu-Ni phase exert different effects.

That is, the Cu-Sn phase has lower corrosion resistance than the Sn-Cu-Ni phase, but is excellent at maintaining the surface electrical resistance at a low value after the corrosion test. Conversely, the Sn-Cu-Ni phase has higher corrosion resistance than the Cu-Sn phase, but is inferior at maintaining the surface electrical resistance at a low value after the corrosion test. In order to ensure a balance between corrosion resistance and surface electrical resistance, it is preferable that the Sn-Cu-Ni phase is 50% or more and the Cu-Sn phase is 10% to 50%. In addition to these two phases, Ni-Al phases with an area ratio of 10% or less may be generated, and the highly corrosion-resistant copper alloy of the embodiment of the present invention does not exclude such generation, but it is preferable that the alloy is formed from two phases, the Sn-Cu-Ni phase and the Cu-Sn phase.

As mentioned above, the Sn-Cu-Ni phase has higher corrosion resistance than the Cu-Sn phase, but is inferior in maintaining the surface electrical resistance at a low value after a corrosion test. However, if you want to control the surface electrical resistance to a particularly low value, it is desirable for the Sn concentration in the Sn-Cu-Ni phase to be 60 atomic % or more. Sn has the effect of maintaining the surface electrical resistance of the Sn-Cu-Ni phase, and therefore the entire Cu alloy surface, at a low value after a corrosion test.

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.

Highly corrosion-resistant copper alloys 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 by an optical microscope, and the composition of each phase was analyzed by 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 ratios of the Sn-Cu-Ni phase and the Cu-Sn phase were calculated for a field of view of 1 mm x 1 mm. Note that Figure 2 shows a scanning electron microscope image of the microstructure of the test piece of Example 6 and an element map 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 3 shows the corrosion resistance (average erosion depth) and surface electrical resistance of each test piece, as well as their evaluation. Corrosion resistance was evaluated in terms of the average erosion depth before and after the wet-dry cycle test. 3 g of chromium trioxide (VI) and 12 mL of sulfuric acid were dissolved in pure water to make a total of 100 mL. Each test piece after the wet-dry cycle test was immersed in this solution at 25°C for 10 seconds to remove the corrosion products, and the average erosion depth was calculated from the change in mass before and after the test and the surface area of the test piece. Average erosion depths of 1 μm or less were rated as ◎ (very good), those between 1 μm and 3 μm or less were rated as ○ (good), and those exceeding 3 μm were rated as × (poor).

Surface electrical resistance was evaluated based on the degree of increase in resistivity before and after repeated wet-dry testing. Compared to the resistivity in a 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 3, the alloys of Examples 1 to 6 were found to have better corrosion resistance ratings than those of Comparative Examples 1 to 5, and also showed a slight increase in resistivity following repeated dry-wet testing. In particular, when the element concentration of any one of Cu, Sn, Zn, Al, and Ni falls outside the range of the highly corrosion-resistant copper alloy of the embodiment of the present invention, the Sn-Cu-Ni phase and Cu-Sn phase are not formed (see Figure 1), and not only is the corrosion resistance rating reduced, but the surface electrical resistance increases significantly following the corrosion test. Also, Comparative Example 5 is an example of pure copper, but it is found to have lower corrosion resistance and an extremely large increase in resistivity compared to the alloys of Examples 1 to 6.

Examples of applications for the highly corrosion-resistant copper alloy of 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 35 atomic % or more and 52 atomic % or less ,
Sn having a content of 30 atomic % or more and 36 atomic % or less ,
Ni content is 15 atomic % or more and 22 atomic % or less ,
The content of Zn and Al is 1 atomic % or more and 10 atomic % or less,
It has a Sn-Cu-Ni phase and a Cu-Sn phase,
The Sn-Cu-Ni phase contains 50 atomic % or more of Sn, 15 atomic % or more of Cu, and 15 atomic % or more of Ni;
The Cu-Sn phase is a highly corrosion-resistant copper alloy containing 65 atomic % or more of Cu and 5 atomic % or more of Sn. The highly corrosion-resistant copper alloy according to claim 1, characterized in that the Sn-Cu-Ni phase has an Sn content of 60 atomic % or more.