Galvanizing characteristics

Zinc is relatively stable in dry air and is not easily discolored. In water and humid atmospheres, it reacts with oxygen or carbon dioxide to form oxides or alkaline zinc carbonate films, which can prevent further oxidation of zinc plating and provide protection.

Zinc is susceptible to corrosion in acids, alkalis and sulfides. The galvanized layer is generally subjected to passivation treatment, after chromic acid or passivation in chromate solution, due to the formation of passivation film is not easy to act with humid air, the corrosion resistance is greatly strengthened. For spring parts, thin-walled parts (wall thickness <0.5m) and steel parts requiring high mechanical strength, hydrogen must be removed, and copper and copper alloy parts can not remove hydrogen.

Zinc plating has low cost, convenient processing and good effect. The standard potential of zinc is negative, so zinc coating is anodic coating for many metals.

Applications

Galvanizing is widely used in atmospheric conditions and other good environments, but not suitable for friction parts.

Nickel plating characteristics

Nickel has good chemical stability in the atmosphere and lye, is not easy to change color, and is oxidized at temperatures above 600°C. It dissolves very slowly in sulfuric acid and hydrochloric acid, but 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, easy polishing, high light reflection and can increase the appearance. Its disadvantage is that it has porosity, in order to overcome this shortcoming, multi-layer metal coating can be used, and nickel is the middle layer.

Nickel is a cathode coating on iron and an anode coating on copper.

Applications

It is usually used to protect decorative coatings in order to prevent corrosion and increase aesthetic use. Nickel plating on copper products is ideal for anti-corrosion.

However, because nickel is relatively valuable, copper-tin alloy is used instead of nickel plating.

Cadmium plating characteristics

Parts in contact with the Marine atmosphere or seawater and in hot water above 70 ° C, cadmium coating is relatively stable, strong corrosion resistance, good lubricity, it is very slow to dissolve in dilute hydrochloric acid, but it is easily dissolved in nitric acid, insoluble in alkali, and its oxide is 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 the obtained cadmium coating is more beautiful than the zinc coating under certain electrolytic conditions. However, the gas produced by cadmium when it is melted is toxic, and soluble cadmium salts are also toxic.

Under normal conditions, cadmium is a negative polar coating on steel, and an anode coating in Marine and high temperature atmosphere.

Applications

It is mainly used to protect parts from seawater or similar salt solutions and saturated seawater vapor atmospheric corrosion, aviation, Marine and electronic industry parts, springs, threaded parts are many cadmium plated. Can be polished, phosphating and used as paint base, but can not be used as food utensils.

Chromium plating characteristics

Chromium is very stable in humid atmosphere, alkali, nitric acid, sulfide, carbonate solution and organic acid, soluble in hydrochloric acid and hot concentrated sulfuric acid. Under the action of direct current, if the chromium layer as the anode, it is easily soluble in caustic soda solution.

The chromium layer has strong adhesion, high hardness, 800~1000V, good wear resistance, strong light reflection, and high heat resistance, does not change color below 480 ° C, begins oxidation above 500 ° C, and the hardness decreases significantly at 700 ° C. The disadvantage of chromium is hard, brittle and easy to fall off, which is more obvious when subjected to alternating impact loads. And it is porous.

Chromium metal is easily passivated in the air to form a passivated film, thus changing the potential of chromium. Therefore, chromium on iron becomes a negative polar coating.

Applications

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