Why does steel pipe get rust?

 Our daily production and life are inseparable from steel materials, but the amount of steel lost in the world due to corrosion is enormous. Therefore, how to protect steel from corrosion is of great significance.

What is the principle of steel corrosion? We can explore according to the small experiment shown in the figure below: add a small amount of calcium chloride in the first tube (calcium chloride can absorb the water vapor in the air, dry it), put a nail, plug it tightly Test tube mouth. A nail was placed in the second test tube, and the molten iron was immersed in distilled water which was boiled and rapidly cooled, and then the vegetable oil was poured to form an oil layer on the water surface. Place a nail in the third tube and add a small amount of distilled water to immerse a part of the nail in water. Three tubes were observed and recorded for a week. From the experimental results, we can find that the nails in the first and second tubes are not rusted, but the nails in the third tube are rusted, and reddish brown rust appears on the surface of the nail. It can be seen that iron rust requires the participation of water and oxygen.

The corrosion process of steel products is a complex chemical reaction process. The rust is usually reddish brown, and different forms of rust are formed under different conditions. The rust is mainly composed of iron oxide hydrate (Fe2O3·nH2O) and iron hydroxide [Fe(OH)3]. The rust structure on the steel surface is loose, and it cannot prevent the internal iron from coming into contact with oxygen, water vapor, etc., and eventually the iron is completely rusted.

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