First, zinc plating:
Features: Zinc is relatively stable in dry air and is not easy to change color. In water and humid atmospheres, it reacts with oxygen or carbon dioxide to form oxides or alkaline zinc carbonate films, which can prevent zinc from continuing to be plated and oxidized, and play a protective role. Zinc is very susceptible to corrosion in acids, alkalis, and sulfides. The zinc coating generally needs to be passivated. After passivation in chromic acid or in chromate solution, the formed passivation film is not easy to react with humid air, and the anti-corrosion ability is greatly enhanced. For spring parts, thin-walled parts (wall thickness <0.5m), and steel parts requiring high mechanical strength, hydrogen removal must be carried out, and copper and copper alloy parts do not need hydrogen removal. Zinc plating has low cost, convenient processing, and good effect. The standard potential of zinc is relatively negative, so zinc plating is an anodic coating for many metals.
Application: Zinc plating is widely used in atmospheric conditions and other good environments. But it is not suitable for friction parts.
Second, cadmium plating
Features: For parts in contact with marine atmosphere or seawater and in hot water above 70℃, cadmium plating is relatively stable, has strong corrosion resistance, and good lubricity, dissolves very slowly in dilute hydrochloric acid, but is very easy to dissolve in nitric acid, is insoluble in alkali, and its oxide is also insoluble in water. Cadmium plating is softer than zinc plating, has less hydrogen embrittlement, has strong adhesion, and under certain electrolytic conditions, the resulting cadmium plating is more beautiful than zinc plating. However, the gas produced by cadmium when it melts is toxic, and soluble cadmium salts are also toxic. Under normal conditions, cadmium is a cathode coating on steel and 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 aviation, marine, and electronic industrial parts, springs, and threaded parts are plated with cadmium. It can be polished, phosphate, and used as a paint base, but cannot be used as tableware.
Third, chromium plating
Features: Chromium is very stable in humid atmospheres, alkali, nitric acid, sulfide, carbonate solution, and organic acid, and is easily soluble in hydrochloric acid and hot concentrated sulfuric acid. Under the action of direct current, if the chromium layer is used as an anode, it is easily soluble in caustic soda solution. The chromium layer has strong adhesion, high hardness, 800~1000V, good wear resistance, strong light reflectivity, and high heat resistance. It does not change color below 480℃, begins to oxidize above 500℃, and the hardness drops significantly at 700℃. Its disadvantages are that chromium is hard, brittle, and easy to fall off, which is more obvious when subjected to alternating impact loads. And it is porous. Metallic chromium is easily passivated in the air to form a passivation film, thereby changing the potential of chromium. Therefore, chromium becomes a cathodic coating on iron.
Application: It is not ideal to directly plate chromium on the surface of steel parts as an anti-corrosion layer. Generally, it can achieve the purpose of rust prevention and decoration through multi-layer electroplating (i.e. copper plating → nickel → chromium plating). At present, it is widely used to improve the wear resistance of parts, repair size, light reflection, and decorative lights.
Fourth, nickel plating
Features: Nickel has good chemical stability in the atmosphere and alkali solution, is not easy to change color, and is only oxidized at a temperature 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. The nickel coating has high hardness, is easy to polish, has high light reflectivity, and can increase beauty. Its disadvantage is that it is porous. To overcome this disadvantage, multiple metal coatings can be used, and nickel is the middle layer. Nickel is a cathode coating for iron and an anodic coating for copper.
Application: Usually used to protect decorative coatings to prevent corrosion and increase beauty. Nickel plating on copper products is ideal for corrosion protection, but because nickel is relatively expensive, copper-tin alloy plating is often used instead of nickel plating.
Post time: Oct-16-2024