Galvanised

Characteristics

Zinc is stable in dry air and does not change colour easily. In water and humid atmosphere, it interacts with oxygen or carbon dioxide to produce oxide or alkaline zinc carbonate film, which prevents zinc from continuing to oxidise and protects it.

Zinc is very susceptible to corrosion in acids, alkalis and sulphides. Zinc plating layer should generally be passivated, in chromic acid or in chromate solution after passivation, due to the formation of the passivation film is not easy to act with moist air, corrosion resistance is greatly strengthened. Spring parts, thin-walled parts (wall thickness <0.5m) and require high mechanical strength of steel parts, must be dehydrogenated, copper and copper alloy parts can not be dehydrogenated.

Zinc plating is low cost, easy to process, good effect zinc standard potential is more negative, so zinc plating for many metals are anodic plating.

Applications

Zinc plating is commonly used in atmospheric conditions and other good environments. However, it is not suitable for friction parts.

Chromium plating,

Characteristics

Chromium is very stable in humid atmospheres, alkalis, nitric acid, sulphides, carbonate solutions and organic acids, but is soluble in hydrochloric acid and hot concentrated sulphuric acid. Under the action of direct current, if the chromium layer is used as anode, it is easily soluble in caustic soda solution.

Chromium layer has strong adhesion, high hardness, 800~1000V, good abrasion resistance, strong light reflection, and also has high heat resistance, no discolouration below 480℃, oxidation starts above 500℃, and the hardness decreases significantly at 700℃. The disadvantage of chromium is hard, brittle, easy to fall off, when subjected to alternating shock load is more obvious. And has a porous.

Chromium metal in the air is easy to passivate the generation of passivation film, thus changing the potential of chromium. Therefore, chromium becomes a cathodic coating for iron.

Application

Direct chromium plating on the surface of iron and steel parts as an anti-corrosion layer is not ideal, generally through multi-layer plating (i.e., copper → nickel → chromium plating) in order to achieve the purpose of rust prevention and decoration. At present, it is widely used to improve the wear resistance of parts, repair the size, light reflection and decorative lamps.

Cadmium Plating

Characteristics

With the oceanic atmosphere or seawater contact parts and in hot water above 70 ℃, cadmium plating is more stable, corrosion resistance, good lubrication, in dilute hydrochloric acid dissolves very slowly, but in nitric acid is very easy to dissolve, insoluble in alkali, and its oxides are also insoluble in water. Cadmium plating is softer than zinc plating, plating hydrogen embrittlement is small, strong adhesion, and under certain electrolysis conditions, the cadmium plating than zinc plating beautiful. But cadmium in the melting of the gas produced by toxic, soluble cadmium salts are also toxic. For environmental considerations, cadmium plating is now rare, basically replaced by nickel chromium plating. Because of the toxicity of cadmium, only in individual military products are used.

Under general conditions, cadmium is cathodic plating for steel, and anodic plating in the oceanic and high-temperature atmosphere.

Applications

It is mainly used to protect parts from seawater or similar salt solutions and saturated sea water vapour atmospheric corrosion, aviation, marine and electronic industry parts, springs, threaded parts are many cadmium plating. It can be polished, phosphated and used as a paint primer, but cannot be used for eating utensils.

Nickel plating

Characteristics

Nickel is chemically stable in the atmosphere and alkali, not easy to change colour, and is oxidised only when the temperature is above 600°C. Nickel is soluble in sulphuric and hydrochloric acids. It dissolves slowly in sulphuric acid and hydrochloric acid, but is easily soluble in dilute nitric acid. It is easy to passivate in concentrated nitric acid and thus has good corrosion resistance.

Nickel plating high hardness, easy to polish, high light reflection and can increase the beauty. Its disadvantage is that it is porous, in order to overcome this disadvantage, can be used in multi-layer metal plating, and nickel for the intermediate layer.

Nickel is cathodic for iron and anodic for copper.

Application

Nickel is usually used to protect decorative plating in order to prevent corrosion and to increase aesthetics. Nickel plating on copper products is ideal for corrosion prevention.

However, since nickel is more expensive, copper-tin alloy plating is used instead of nickel plating.