What is the difference between zinc plating, cadmium plating, chromium plating and nickel plating?

Zinc plating

Characteristics.

Zinc is stable in dry air and does not change color easily, but 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 be oxidized and plays a protective role. Zinc in the acid and alkali, sulfide is very susceptible to corrosion. Zinc plating layer generally be passivated, in chromic acid or in chromate solution after passivation, due to the formation of 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.

Cadmium plating

Features.

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 not soluble in water. Cadmium plating is softer than zinc plating, plating hydrogen embrittlement is small, strong adhesion, and in certain electrolytic 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. Under general conditions, cadmium is cathodic plating for steel, and anodic plating in the oceanic and high-temperature atmosphere.

Application:

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

Chromium plating

Characteristics.

Chromium is very stable in moist 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 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, does not change color below 480 ℃, 500 ℃ or more began to oxidize, 700 ℃, the hardness of a significant decline. 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 plating for iron.

Applications.

Chromium plating directly on the surface of steel parts as an anti-corrosion layer is not ideal, generally by 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.

Nickel Plating

Characteristics.

Nickel is chemically stable in the atmosphere and alkali, not easy to change color, and is oxidized only when the temperature is above 600°C. Nickel can be dissolved in sulfuric acid and hydrochloric acid. It dissolves slowly in sulfuric and hydrochloric acids, 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 more ideal for corrosion prevention, but because nickel is more expensive, most of them are plated with copper-tin alloy instead of nickel plating.