August 10, 2023
The appearance of the zinc coating is bluish white, the standard electrode potential is -0.76V, it is easily soluble in acid and alkali, and it is a curved amphoteric metal. Zinc hardly changes in dry air, and the critical humidity of zinc corrosion is greater than 70%, so it can react with carbon dioxide and oxygen to form a film mainly composed of basic zinc carbonate in a humid atmosphere. Zinc reacts with hydrogen sulfide and other sulfides to form zinc sulfide; zinc is easily corroded by chloride ions, so it is unstable in seawater.
When zinc comes into contact with volatiles of non-metallic materials (low molecular carboxylic acids, aldehydes, phenols, ammonia, etc.) in airtight or poorly ventilated, humid air conditions, zinc is easily corroded, forming white loose corrosion products, commonly known as " Hoarfrost". Zinc has a tendency to grow filamentous single crystals when it comes into contact with volatiles released from bakelite, paint, and wood under high temperature, high humidity, and airtight conditions, commonly known as "long hair".
The standard potential of zinc is negative than that of iron, and it is an anodic coating for steel, which can provide reliable electrochemical protection. Steel products used in industrial atmosphere, rural atmosphere and marine atmosphere can choose zinc as the protective layer, and its protective life is almost proportional to the thickness of the zinc layer. In the atmosphere polluted by industrial waste gas, the corrosion resistance of zinc coating is better than that of cadmium coating, and the opposite is true in sea or sea water.
After the zinc coating is passivated by chromate, the corrosion resistance can be increased by 6~~8 times. Therefore, galvanized passivation treatment is an indispensable step. Organic dyeing on the passivation film can be used as a protection for low-end products---decorative coating.
In order to reduce the thickness of the zinc coating and improve the corrosion resistance, the following two measures are taken at home and abroad: one is to improve the quality of the passivation film, such as using blue, green, army green, black passivation to replace the conventional iridescent passivation film , especially the military green passivation film has the best corrosion resistance; the second is the thinner iron containing 0.3%~~0.6% Zn-Fe alloy, or Zn-Ni alloy containing 6%~~10% nickel instead of pure zinc coating, the corrosion resistance is increased by more than three times, and it is widely used in automobile steel plates instead of galvanizing.
The service temperature of the zinc coating shall not exceed 250°C. At this temperature, the zinc crystal structure is deformed, becomes brittle, and the corrosion resistance decreases. Parts with corrosion resistance and heat resistance requirements should use Zn-Ni alloy layer.
Galvanizing electrolytes can be divided into two categories: alkaline and weakly acidic. Oxygen-free zincate galvanizing, oxidation galvanizing, salt galvanizing, oxide galvanizing, and sulfate galvanizing are commonly used at home and abroad. Domestically, oxygen-free zincate galvanizing is still the mainstream. In recent years, due to the improvement of the performance of galvanizing additives, oxide galvanizing and low-oxygen bright galvanizing have developed rapidly, while salt galvanizing has continued to decline.
The choice of plating solution should be based on the material, shape and appearance requirements of the plated parts, such as small punched and complex parts, especially deep holes or tubular parts that require auxiliary anodes, should use alkaline plating solution; parts and parts sensitive to hydrogen embrittlement Weak acid galvanizing solution should be used. It is very beneficial to production when two plating solutions with different properties are used.