Zinc plating, cadmium plating, chromium plating, and nickel plating are all types of metal plating processes used to improve the corrosion resistance, wear resistance, and appearance of metal parts.
Zinc is relatively stable in dry air and does not easily change color. In water or humid environments, it reacts with oxygen or carbon dioxide to form zinc oxide or alkaline zinc carbonate film, which can prevent further oxidation of zinc and provide protection.
Zinc is highly susceptible to corrosion in acid, alkali, and sulfide. Therefore, galvanized coatings generally require passivation treatment. After passivation in chromic acid or chromate solution, the corrosion resistance is greatly improved because the formed passivation film is not easily affected by humid air. For spring parts, thin-walled parts (wall thickness <0.5m), and steel components that require high mechanical strength, dehydrogenation must be performed, but copper and copper alloy components may not need to be dehydrogenated.
Galvanizing has low cost, convenient processing, and good effect. Zinc has a more negative standard electrode potential, so zinc coating is an anodic coating for many metals.
Galvanizing is commonly used in atmospheric conditions and other favorable environments. However, it is not suitable for use as friction parts.
Cadmium-plated coatings are relatively stable when in contact with marine or salty solutions and in hot water above 70°C. They have strong corrosion resistance, good lubricity, slow dissolution in dilute hydrochloric acid, and are insoluble in alkali. Its oxides are also insoluble in water. Cadmium plating is softer than zinc plating, less prone to hydrogen embrittlement, and has strong adhesion. Under certain electrolytic conditions, the cadmium plating obtained is more beautiful than zinc plating. However, the gas produced during cadmium melting is toxic, and soluble cadmium salts are also toxic.
Under normal conditions, cadmium is a cathodic coating for steel. It is an anodic coating in marine and high-temperature environments.
It is mainly used to protect parts from atmospheric corrosion by seawater or similar salt solutions and saturated seawater vapor. Cadmium plating is widely used in the aerospace, navigation, and electronics industries, as well as in springs and threaded components. It can be polished, phosphated, and used as a paint base, but cannot be used for food utensils.
Chromium is very stable in humid air, alkali, nitric acid, sulfides, carbonate solutions, and organic acids. It is easily soluble in hydrochloric acid and hot concentrated sulfuric acid. Under DC current, if the chromium layer is used as the anode, it is easily dissolved in caustic soda solution.
The chromium layer has strong adhesion, high hardness (800~1000V), good wear resistance, high light reflectivity, and high heat resistance. It does not change color below 480°C, begins to oxidize at 500°C, and its hardness significantly decreases at 700°C. The disadvantage of chromium is that it is hard, brittle, and easy to fall off, especially when subjected to alternating impact loads. And it has porosity.
Metallic chromium is easily passivated to form a passivation film in the air, thereby changing the potential of chromium. Therefore, chromium is a cathodic coating for iron.
Directly electroplating chromium on steel parts as an anti-corrosion layer is not ideal. Generally, multiple layers of electroplating (i.e., copper plating → nickel plating → chromium plating) are needed to achieve rust prevention and decoration purposes. Currently, it is widely used to improve the wear resistance, repair dimensions, light reflection, and decorative lamps of parts.
Nickel has good chemical stability in the atmosphere and alkali solutions, is not easily discolored, and only begins to oxidize at temperatures above 600°C. It dissolves slowly in sulfuric acid and hydrochloric acid but is easily soluble in dilute nitric acid. It can be passivated easily in concentrated nitric acid, thus having good corrosion resistance.
Nickel-plated coatings have high hardness, easy polishing, high light reflectivity, and can increase beauty. However, they are porous. To overcome this disadvantage, multiple layers of metallic plating can be used, with nickel as the intermediate layer.
Nickel is a cathodic coating for iron and an anodic coating for copper.
Nickel plating is usually used to prevent corrosion and increase beauty, so it is generally used to protect decorative plating. Nickel plating on copper products is ideal for rust prevention. However, because nickel is relatively expensive, electroplating with copper-tin alloys is often used instead of nickel plating.