The appearance of zinc coating on galvanized steel pipes is blue-white. The standard electrode potential is -0.76V. It is soluble in acid and alkali. It is a typical amphoteric metal. Zinc does not change in dry air. The critical humidity of zinc corrosion is more than 70%. Therefore, in humid atmosphere, zinc can react with carbon dioxide and oxygen to form a film consisting mainly of basic zinc carbonate, which can inhibit corrosion. Zinc reacts with sulfides such as hydrogen sulfide to form zinc sulfide. Zinc is vulnerable to chloride ions. Therefore, it is unstable in seawater.
Zinc of galvanized steel pipe is easy to be corroded when it contacts with volatiles of non-metallic materials (low molecular carboxylic acid, aldehyde, phenol, ammonia, etc.) under the conditions of airtight or bad ventilation and humid air, resulting in white loose corrosion products, commonly known as "white frost". Zinc has a tendency to grow filamentous single crystals, commonly known as "long hairs", in contact with volatiles released from bakelite, paint and wood under high temperature, high humidity and airtight conditions.
The standard potential of zinc is higher than that of iron, which is anodic coating for iron and steel. It can provide chemical protection for power supply. Zinc can be used as protective layer for steel products in industrial atmosphere, rural atmosphere and marine atmosphere. Its protective life is proportional to the thickness of zinc layer. In the air polluted by industrial exhaust gas, the corrosion resistance of zinc coatings on galvanized steel pipes is better than that of cadmium coatings, while in the sea or sea water, the opposite is true.
The corrosion resistance of zinc coating can be increased by 6--8 times after chromate passivation. Therefore, zinc passivation is an important step. Organic dyeing on passivation film can be used as protective coating for low paper products.