Galvanized sheets and coils

Protection method is based on the following principle: most metals (e.g., zinc, tin, aluminum) are oxidized in the air, when a dense protective membrane of oxidized metal compounds appears on metal surface. This membrane prevents the penetration of oxygen into the metal and thus stops further oxidation. However, in the case of iron, the process works differently. The oxidized iron compounds (such as hydroxides) have a bigger volume than the original metal, as a result membrane immediately is ruined and becomes loose. This kind of loose and fragile membrane freely lets oxygen in to the deeper unoxidized metal and its oxidation process continues. Iron is unable to protect itself from further oxidation, so rust is formed. If iron has a metal cover layer (tin or zinc), which forms a protective membrane, such protective membrane will not allow oxygen to ruin both coating metal and iron under coating.