Zinc is easily soluble in acid and soluble in alkali, so it is called amphoteric metal. Zinc does not change much in dry air. In moist air or water containing carbon dioxide and oxygen, a layer of film mainly composed of alkali or zinc carbonate is formed on the surface of zinc, which can play a role in corrosion inhibition. The galvanized layer is in acid. The corrosion resistance of the aqueous solution of alkali, alkali and salt is poor, and it is also poor in the protection of atmosphere containing sulfur dioxide and hydrogen sulfide and humid air of marine nature, and it is easy to be in high temperature and high humidity air and in the atmosphere containing organic acid. Corroded. The galvanized layer contains metal impurities that will reduce its protective properties.
The standard electrode potential of zinc is -0.76 V, which is more negative than carbon steel, iron and low alloy steel. Therefore, when an iron-zinc primary battery is formed, the zinc coating is an anode and it will dissolve itself to protect the steel substrate even if the surface is galvanized. Incomplete layers also play this role, so the galvanized layer is called "anode plating." The galvanized layer has both mechanical protection and electrochemical protection to the iron matrix, so its corrosion resistance is quite good.
After the zinc coating passivated, because of the different passivation solution obtained passivation film of different colors or white passivation film. The resistance of the iridescent passivation film is more than five times higher than that of the colorless passivation film. This is because the iridescent passivation film is thicker than the white passivation film. On the other hand, when the surface of the iridescent passivation film is scratched, In moist air, the hexavalent chromium in the passivation film near the scratched area has "repassivation" on the scratched area, repairing the damage and restoring the passive film. Therefore, galvanization is mostly done with rainbow color passivation. The colorless passivation film has a white appearance and is used in daily hardware, architectural hardware, and other products that require a white uniform surface. In addition, there are black passivation, military green passivation, etc., also have applications in the industry.
Galvanizing has the advantages of low cost, good corrosion resistance, good appearance and resistance to storage, so it is widely used in a small number of industries such as light industry, instrumentation, and electromechanics. However, due to the low hardness of the galvanized layer, which is harmful to the human body and constitutes pollution to the environment, it should not be used in food, medical and other industries, and in recent years, the galvanizing process has been strictly controlled by the EPA, so it can replace the galvanized Environmental protection technology - A new generation of environmentally-friendly alloy catalytic fluid technology has become a hot "sweet potato."