Features: 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 from further plating and oxidation
and play a protective role.
Zinc is easy to be corroded in acid, alkali and sulfide. After passivation in chromic acid or chromate solution, the passivation film formed is not easy to react with humid air, so the anticorrosion ability is greatly
enhanced. For spring parts, thin-walled parts (wall thickness < 0.5m) and steel parts with high mechanical strength, hydrogen removal must be carried out, while copper and copper alloy parts can not.
Zinc coating has low cost, convenient processing
and good effect. The standard potential of zinc is negative, so zinc coating is anode coating for many metals.
Application: galvanizing is widely used in atmospheric conditions and other good environments. But it is not suitable for friction parts
Features: for parts in contact with marine atmosphere or sea water and hot water above 70 ℃, cadmium coating is relatively stable, with strong corrosion resistance and good lubricity. It dissolves very 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. The cadmium coating is softer than the zinc coating, the hydrogen brittleness of the coating is smaller, the adhesion is stronger, and
under certain electrolysis conditions, the obtained cadmium coating is more beautiful than the zinc coating. However, the gases produced by cadmium melting are toxic, and the soluble cadmium salt is also toxic.
Under general conditions, cadmium
is a cathode coating for steel, and an anode coating in marine and high temperature atmosphere.
Application: it is mainly used to protect parts from atmospheric corrosion caused by seawater or similar salt solution and saturated seawater vapor.
Many parts, springs and threaded parts in aviation, navigation and electronic industries are cadmium plated. It can be used for polishing, phosphating and painting, but not as tableware.
Features: chromium is very stable in
humid atmosphere, alkali, nitric acid, sulfide, carbonate solution and organic acid, and easy to dissolve 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 solution.
The chromium layer has strong adhesion, high hardness, 800 ~ 1000V, good wear resistance, strong light reflection, and high heat resistance. It does not change color below 480 ℃, oxidizes above 500 ℃, and decreases hardness
significantly at 700 ℃. The disadvantage of chromium is hard, brittle and easy to fall off, especially when subjected to alternating impact load. It is porous.
Chrmium metal is easy to passivate in the air to form a passivation film, thus changing
the potential of chromium. Therefore, chromium to iron is 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 anti rust and decoration
can be achieved by multi-layer plating (i.e. copper plating → nickel → chromium). At present, it is widely used to improve the wear resistance, repair size, light reflection and decorative lights of parts.