galvanized
Characteristics: Zinc is relatively stable in dry air and does not easily change color. In water and humid atmosphere, it reacts with oxygen or carbon dioxide to form an alkaline zinc oxide or carbonate film, which can prevent zinc from continuing to oxidize and play . a protective role.
Zinc is very susceptible to corrosion by acids, alkalis and sulfides. The galvanized layer generally undergoes a passivation treatment. After passivation in chromic acid or chromate solution, since the formed passivation film does not easily interact with humid air, the anti-corrosion ability is significantly improved. For spring-loaded parts, thin-walled parts (wall thickness <0.5 m) and steel parts requiring high mechanical strength, hydrogen must not be removed. Copper and copper alloy parts do not need to be dehydrogenated.
Galvanizing has low cost, easy processing and good effect. The standard potential of zinc is relatively negative, so zinc coating is an anodic coating for many metals.
Application: Galvanizing is commonly used in atmospheric and other favorable environments. But it is not suitable for friction parts
Cadmiage
Characteristics: For parts in contact with the marine atmosphere or sea water and in hot water above 70°C, the cadmium coating is relatively stable, has strong corrosion resistance and good lubricating power. It dissolves very slowly in dilute hydrochloric acid, but is extremely soluble in water. nitric acid. Easily soluble, insoluble in alkalis and its oxides are insoluble in water. Cadmium plating is softer than zinc plating, has less hydrogen embrittlement and stronger adhesion under certain electrolysis conditions, the cadmium plating obtained is more beautiful than zinc plating. However, the gas produced when cadmium melts is toxic, as are soluble cadmium salts.
Under normal conditions, cadmium acts as a cathodic coating on steel and acts as an anodic coating in marine and high temperature atmospheres.
Application: It is mainly used to protect parts from atmospheric corrosion from seawater or similar salt solutions and saturated seawater vapor. Many parts, springs and threaded parts in the aviation, navigation and electronics industries are plated with cadmium. It can be polished, phosphated and used as a paint base, but it cannot be used as a tableware.
chromium
Characteristics: Chromium is very stable in humid atmosphere, alkalis, nitric acid, sulfides, carbonate solutions and organic acids, and is easily soluble in hydrochloric acid and hot concentrated sulfuric acid. Under the action of direct current, if the chromium layer serves as an anode, it is easily soluble in a caustic soda solution.
The chrome layer has strong adhesion, high hardness, 800~1000V, good wear resistance, strong light reflectivity and high heat resistance. It does not discolor below 480°C, begins to oxidize above 500°C and has a hardness of 700°C. Significant drop. Its disadvantages are that chrome is hard, brittle and easy to fall off, which is more obvious when subjected to alternating impact loads. And it’s porous.
Metallic chromium is easily passivated in air to form a passivation film, thereby changing the potential of the chromium. Therefore, chromium becomes a cathodic coating on iron.
Application: It is not ideal to directly plate chrome onto the surface of steel parts as an anti-corrosion layer. Generally, multi-layer electroplating (that is, copper → nickel → chrome plating) can be used to achieve the purpose of anti-rust and chrome. decoration. It is currently widely used to improve the wear resistance of parts, repair dimensions, light reflection and decorative lights.
Nickel plated
Characteristics: Nickel has good chemical stability in the atmosphere and alkaline liquids, is not easy to change color, and is only oxidized when the temperature is above 600°C. It dissolves very slowly in sulfuric acid and hydrochloric acid, but is easily soluble in dilute nitric acid. It is easy to passivate in concentrated nitric acid and has good corrosion resistance.
Nickel plating has high hardness, is easy to polish, has high light reflectivity, and can increase the appearance. Its disadvantage is that it is porous. To overcome this drawback, multi-layer metal plating can be used, with nickel as an interlayer.
Nickel is a cathodic plating on iron and an anodic plating on copper.
Application: Typically used to protect decorative coatings to prevent corrosion and increase aesthetics. Nickel plating on copper products is ideal for corrosion protection
However, because nickel is relatively expensive, copper-tin alloy plating is often used instead of nickel plating.
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