Zinc is relatively stable in dry air and is not easy to change color. In water and humid atmosphere, it reacts with oxygen or carbon dioxide to form oxide or alkaline zinc carbonate film, which can prevent zinc continuous plating oxidation and play a protective role.
Zinc is easily corroded in acid, alkali and sulfide. Zinc coating is usually passivated. After passivation in chromic acid or chromate solution, the corrosion resistance is greatly enhanced because the passivation film formed is not easy to interact with humid air. For spring parts, thin-walled parts (wall thickness < 0.5m) and steel parts requiring high mechanical strength, dehydrogenation must be carried out. Copper and copper alloy parts can not be dehydrogenated.
Zinc plating has the advantages of low cost, convenient processing and good effect. The standard potential of zinc is relatively negative, so the zinc coating is anodic to many metals.
Application: zinc plating is widely used in atmospheric condition and other good environment, but it is not suitable for friction parts
For parts in contact with marine atmosphere or sea water and in hot water above 70 ℃, cadmium coating is relatively stable with strong corrosion resistance and good lubrication. It dissolves slowly in dilute hydrochloric acid, but it is very easy to dissolve in nitric acid, insoluble in alkali, and its oxide is also insoluble in water.
Cadmium coating is softer than zinc coating, hydrogen brittleness of coating is smaller, adhesion is stronger, and under certain electrolysis conditions, cadmium coating is more beautiful than zinc coating. But the gases produced by the melting of cadmium are toxic, and so are soluble cadmium salts. Under general conditions, cadmium is a cathodic coating on steel and an anode coating in marine and high temperature atmosphere.
Application: it is mainly used to protect parts from the atmospheric corrosion of sea water or similar salt solution and saturated seawater vapor. Many parts of aviation, navigation and electronic industry, spring and thread parts are plated with cadmium. It can be polished, phosphated and used as paint primer, but it can't be used as tableware.
Chromium is very stable in humid atmosphere, 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 chromium layer is used as anode, it is easy to dissolve in caustic soda solution.
The chromium coating has strong adhesion, high hardness, 800 ~ 1000V, good wear resistance, strong light reflection, and high heat resistance. It does not change color below 480 ℃ and begins to oxidize above 500 ℃. The hardness decreases significantly at 700 ℃. Its disadvantages are hard, brittle and easy to fall off, especially when subjected to alternating impact load.
It is porous. Chromium is easy to passivate in air and form passivation film, which changes the potential of chromium. Therefore, chromium on iron becomes a cathodic coating.
Application: it is not ideal to directly plating chromium on the surface of iron and steel parts as anti-corrosion coating. Generally, the purpose of rust prevention and decoration can be achieved by multi-layer electroplating (i.e., copper plating → nickel → chromium). At present, it is widely used to improve the wear resistance of parts, repair size, light reflection and decorative lights.
Nickel has good chemical stability in the atmosphere and alkali liquor, and is not easy to change color. It is oxidized only when the temperature is above 600 ℃. It dissolves slowly in sulfuric acid and hydrochloric acid, but soluble in dilute nitric acid. It is easy to passivate in concentrated nitric acid, so it has good corrosion resistance.
The nickel coating has high hardness, easy polishing, high light reflection and beautiful appearance. In order to overcome this shortcoming, multi-layer metal coating can be used, and nickel is the intermediate layer. Nickel is a cathodic coating on iron and an anode coating on copper.
Application: usually in order to prevent corrosion and increase aesthetic use, it is generally used to protect decorative coatings. Nickel plating on copper products is ideal, but because nickel is more expensive, copper tin alloy is often used instead of nickel plating.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)