Most of the rust on our swords belongs to atmospheric corrosion. Atmospheric corrosion is a kind of electrochemical corrosion. Due to the evaporation of environmental moisture, there is usually a layer of condensed water on the metal surface exposed to the atmosphere. When this thin layer of condensed water is formed, some gases (such as N2, O2, H2S, hydrogen chloride, SO2, CO2, etc. In the atmosphere) to form a conductive solution (electrolyte solution). The redox reaction between metal and medium leads to the destruction of metal. Therefore, the degree of atmospheric corrosion is different in different regions.
Different cities have different air pollution. For example, air pollution caused by industry, gases (sulfur and nitrogen oxides) formed by suspended particles in air and fuel oil, sea fog in coastal areas, etc., will all cause metal surface corrosion to varying degrees. All metals will react with oxygen in the atmosphere and form an oxide film on the surface. Unfortunately, the loose rust formed on ordinary carbon steel can't prevent the contact between metal and aqueous solution, so the metal surface will continue to be oxidized to spread the rust and eventually form holes.
It is hard to imagine that China could master such advanced modern science and technology for more than two thousand years in ancient times. At that time, how did people extract metal chromium, how to heat the fire to more than 4000 degrees, and how to master the oxidation treatment method of chromium salt (modern term), and evenly plate chromium salt compounds with a thickness of only 10 micron on the sword body. This technology was discovered and patented in the United States on 1950. However, it is really strange that China appeared more than 200 years ago.