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  • 「No.6」Let us talk about the anti-rust technology of metal products used in our daily life.

Column 技術屋の解説

2021.05.17

「No.6」Let us talk about the anti-rust technology of metal products used in our daily life.

-“Gold” will not rust-

This time, it is not a technology to anti-rust, but I would like to talk about “gold” because it is a famous as metal that does not rust.

“Gold” has long been regarded as a decorative and expensive asset due to its clean appearance, rust-less, and non-discoloring characteristics.

“Gold” has high value as an industrial material and is in used for electronic circuit boards such as home appliances today.

Currently, the world’s main gold producing regions are countries with a lot of mineral resources such as China and Australia, but gold ore is also mined in Japan at “Hishikari Gold Mine” in Kagoshima Prefecture.

(In particular, gold ore mined from “Hishikari Gold Mine” contains 30-40g of gold per ton, which is famous for its concentration about 10 times that of the world’s average gold ore.)

 In addition, the reserves of the world are finite amounts, and their value is continuing to rise.

“Gold” is called “precious metal” together with silver and platinum.

Precious metals are difficult to ionize (compounds are difficult to make), so they are difficult to rust and discolor. Conversely, metals that are easily ionized are called “base metals”.

“Gold” is the most value metal that does not rust, but pure gold is not often used for accessory.

So why isn’t pure gold used for accessory?

Pure gold is soft and easily deformed. Gold leaf is manufactured by striking and stretching gold using its properties.

In other words, when used for accessory like a ring, pure gold is unsuitable used one, because it may be deformed by impact.

Instead, 18K with a gold percentage of 75% is generally used.

On a side note, the expression 18K represents the percentage of gold, 100% is 24K, and if the percentage of gold is reduced by a factor of 24 (about 4.17%) from there, it becomes 23K, and as it falls further, 22K, 21K… It becomes.

In other words, 18K means that the percentage of gold is 75%.

By setting it to 18K, it becomes possible to have hardness suitable for processing, and durability that can be used as a accessory can be obtained.

Then, if it is 18K, the percentage of gold is 75%, but what is included in the remaining ingredients?

General 18K is called yellow gold, which is an alloy of 75% gold, 15% silver and 10% copper.

In recent years, various gold alloys exist for accessory.

Below are some of them. * The color is an image.

The metal color can be changed by choose additive other material than gold.

For example, adding copper increases redness, and adding palladium increases whiteness.

From the standpoint of corrosion resistance, gold, platinum, palladium, and rhodium are extremely corrosion resistant and will not cause rust or discoloration in most environments.

Conversely, silver, among precious metals, can react with sulfur and turn black.

I would appreciate it if you could refer to it when choosing decorations.

Reference: Sumitomo Metal Mining Co., Ltd. HP (Hishikari): https://www.smm.co.jp/corp_info/location/domestic/hishikari/

Ministry of Foreign Affairs HP (Gold output): https://www.mofa.go.jp/mofaj/kids/ranking/gold.html

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