Corrosion and anti-corrosion of metal
Even electroplated products intended for decoration lose their value if they become rusty.
It is necessary to prevent rust from appearing in the plating for any purpose.
I will explain in detail about the occurrence of rust.
Iron is naturally produced as an ore in the form of oxides (Feo3), hydroxides (Fe (OH) 3), and sulfides (FeS2).
By refining technology, a single piece of iron is taken out and used. When it comes to which of oxides, hydroxides and iron is more stable, oxides and hydroxides are more stable.
Therefore, when iron is left in the air, it naturally combines with oxygen and water in the air and changes into oxides and hydroxides. This phenomenon is called corrosion.
Corrosion is a phenomenon in which metal is demetallized, but corrosion does not necessarily progress unless the conditions for enclosing the metal are met.
For example, iron is less likely to rust in dry air or in the absence of oxygen supply.
In addition to water vapor, oxygen, and nitrogen, various gases such as sulfur, sulfur dioxide, nitrogen oxides (NOx), and carbon dioxide are present in the atmosphere, which are factors that corrode metals. Even in the same atmosphere, the air is dry and clean in the plateau area, so there are few corrosion factors and metals are hard to corrode. Become.
Metals are hard to corrode at room temperature without moisture, but at high temperatures, metals directly combine with oxygen, sulfur, chlorine, etc. in the air to form compounds such as oxides, sulfides, and chlorides, which corrode.
In the atmosphere, the metal surface is corroded by contact with rainwater and condensed water of water vapor, and by deposits such as dust.
[Local batteries and local corrosion]
Even high-purity metals contain impurities, and alloys contain two or more types of metals, so not all parts are uniform alloy components, and potentials are found in various non-uniform parts of the metal surface. You can make a difference in height.
The part with high potential becomes the local cathode (-pole), and the part with low potential becomes the local anode (+ pole), causing a reverse electric phenomenon.
The anode part melts due to the reverse electric phenomenon, and the cathode part is prevented from corroding. The formation of a short-circuit battery in this way is called a local battery, and the corrosion caused by it is called local corrosion.
This is why impurities-rich materials are prone to corrosion.
[Intergranular corrosion (intergranular corrosion)]
Metal crystals are often polycrystalline, and the boundary between crystals generally has a lower potential than the crystal part, so the grain boundary with a lower potential becomes the anode between the crystals and the grain boundaries, and becomes a local battery.
Intergranular corrosion progresses toward the inside of the metal, resulting in a shortened metal life.
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