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  • 「No.8」Let us talk about a anti-rust technology of metal used in our daily life

Column 技術屋の解説

2021.05.22

「No.8」Let us talk about a anti-rust technology of metal used in our daily life

Let us talk about a anti-rust technology of metal used in our daily life
-The story of “coins” from the viewpoint of rust prevention-

The 8th article is a little different from anti-rust technologys too, but I would like to talk about metal products that are resistant to rust and corrosion.
One of your familiar metal products is a “coin”.

Japanese coins are well-made metal products and are said to have extremely high quality despite being widely available.

In particular, it is known that the 500-yen coin is subjected to more advanced metal processing to prevent counterfeiting.

By the way, do you know what kind of material is used in Japanese “coins”?

There are 6 types of “coins” (excluding commemorative coins) currently in Japan: 500 yen, 100 yen, 50 yen, 10 yen, 5 yen, 1 yen.

Below is a summary of the types of coins and their materials and compositions.

Table 1. Ordinary currency manufactured by Japan Mint (as of 2020)

CoinsMaterialComposition (percentage)
500 yen nickel brass coinNickel brassCopper: Nickel: Zinc (72:20:8)
100 yen white copper coin50 yen white copper coinWhite copperCopper: Nickel (75:25)
10 yen bronze coinBronzeCopper: Zinc: Tin (95:4-3:2-1)
5 yen brass coinBrassCopper: Zinc (60-70:40-30)
1 yen pure aluminum coinAluminumAluminum (100)

 
The new 10 yen has a metallic luster and has a reddish brown tint.

However, as it ages, it loses its metallic luster and turns red-black.

This means that copper is oxidized in the air, an oxide film is formed on the surface, and it grows to a visible thickness.

In particular, what you touch with your hands is covered with human sebum and sweat, which easily causes discoloration due to corrosion, and gradually changes to reddish black.

However, it is not a corrosion that greatly deforms the original metal like rust such as iron, so polishing with citric acid returns the original metallic luster.


Coins such as 500 yen and 100 yen have a white color tint but copper is the main component.

This is due to the nickel in the material.

Nickel is a metal with a white metallic luster, and nickel itself has high corrosion resistance and chemical resistance, and it is rarely used alone, but by adding it to stainless steel, etc., it will improve the corrosion resistance of those metals.

Therefore, compared to 10 yen (bronze), 100 yen(white copper) is less likely to lose its metallic luster.


It is said that there are various reasons why the above materials are used for coins.


One is said to have the antibacterial effect of copper. It is said that copper is used from a hygienic point of view because many people directly touch it or use it for a long time.


One is the stable value of copper. Gold coins and silver coins are expensive, and gold is also famous for its increasing value year by year. If the value of gold continues to rise, the value of the material as gold exceeds that of the gold coin, and it becomes meaningless as a “coin”. Therefore, by using stable and valuable copper and aluminum, it can be said that the value of the material does not exceed the value of “coins”. (Around 2004, the demand for copper as an electronic material increased mainly in China and the price increased.)

Finally, they have Characteristic that difficult to corrosion.

Considering the ionization tendency of a simple metal, it can be said that aluminum is likely to corrode, but in reality, aluminum quickly reacts with oxygen in the air to form an oxide film, and the oxide film forms a protective layer. By preventing further corrosion, it exhibits high corrosion resistance in daily life.

Similarly, copper and copper alloys exhibit high corrosion resistance by forming an oxide film on the surface quickly in the air.

In particular, copper has been used for a long time in human history, and it has been known for many years to use it to produce corrosion products with a blue or green color called patina on the surface. It can also be seen on the roofs of old temples. This patina is generated by the reaction with carbon dioxide and sulfur in the air, and since it is dense, it helps protect the underlying copper.

As an aside, our anti-rust paper “adpack-S” and “adpack-C” can be used for anti-corrosion storage of “coins” and “commemorative coins” that are mainly composed of copper and copper alloys. I would appreciate it if you could use it.

It’s easy, but the above is about anti-rust technology of metal used in our daily life part.4 -The story of “coins” from the viewpoint of rust prevention-

[Akiyama]

Reference: Japan Mint HP, https://www.mint.go.jp/

General corporate judicial person Nippon Copper Center HP http://www.jcda.or.jp/

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