The difference between Zinc plating, Cadmium plating, Chromium plating and Nickel plating!

Surface treatment processes such as zinc plating, cadmium plating, chromium plating, and nickel plating are widely used in industrial applications to improve the anti-corrosion performance and mechanical properties of materials. Each plating method is suitable for different application environments due to its unique chemical and physical properties. Here is an in-depth comparison and expansion of these processes:

Galvanized

Features: Galvanizing is coating zinc on metal surfaces and is often used to protect steel materials. Zinc has good stability in dry air and is not easy to change color. However, in a humid environment, zinc reacts with oxygen or carbon dioxide to form a zinc oxide or alkaline zinc carbonate film. The formation of this film acts as a barrier to prevent further oxidation and corrosion, giving the galvanized layer a better protective effect.

Zinc accelerates corrosion in acidic and alkaline media, especially in the presence of sulfides. Therefore, in order to improve the anti-corrosion performance of the galvanized layer, passivation treatment is usually performed. Passivation treatment is carried out in chromic acid or chromate solution to form a passivation film on the surface of the coating, which is difficult to react with moist air, thus significantly improving the anti-corrosion performance. It should be noted that for spring parts, thin-walled parts (wall thickness less than 0.5 mm) and steel parts requiring high mechanical strength, hydrogen removal treatment must be carried out to prevent the occurrence of hydrogen embrittlement. In contrast, copper and copper alloy parts generally do not require hydrogen removal treatment.

Application: Galvanizing is widely used in atmospheric conditions and other good environments due to its low cost, simple processing and remarkable effect. Common applications include construction, bridges, metal pipes and bolts, etc., but it is not suitable for surface treatment of friction parts because the zinc layer is soft and the anti-corrosion performance will significantly decrease after wear.

Cadmium plating

Features: The main feature of cadmium plating is its excellent stability and strong corrosion resistance in marine climate or seawater. Cadmium coating can well resist salt spray corrosion and high temperature and humidity environment. The coating surface is smooth and has good lubricity. Cadmium dissolves slowly in dilute hydrochloric acid but rapidly in nitric acid. The oxide of cadmium coating is insoluble in water, and the cadmium coating shows stability in neutral and weakly alkaline media. Compared with zinc plating, the cadmium plating layer is softer and less susceptible to hydrogen embrittlement, so it has stronger adhesion.

Cadmium plating has unique cathodic and anodic properties. Under normal atmospheric conditions, cadmium acts as a cathodic coating on steel, while in marine and high-temperature environments it acts as an anodic coating. This property allows cadmium plating to provide sacrificial protection to the substrate under certain conditions.

Application: Cadmium plating is widely used in the aviation, navigation, electronics industries and some key components such as springs and threaded parts for anti-corrosion. It can also be polished, phosphated, and even used as a base for paint. Although cadmium plating has outstanding effects, due to its toxicity, especially the toxic gases and soluble cadmium salts produced during melting, special attention must be paid to safety protection and its application in food-related equipment is limited.

Chrome plated

Features: Chrome plating is known for its high chemical stability in humid atmospheres, alkalis, and most organic acids. Its hardness is high, usually between 800 and 1000 HV, which gives it good wear resistance and heat resistance. Chromium does not change color when the temperature does not exceed 480°C. It begins to be oxidized when the temperature exceeds 500°C. The hardness drops significantly above 700°C.

The chrome plating layer has strong adhesion, extremely high light reflectivity, and a bright and beautiful surface. The disadvantage is that chromium is hard and brittle and easily falls off when subjected to alternating impact loads. In addition, the chromium layer is porous, so if chromium is plated directly on steel, the corrosion protection effect is not ideal. Therefore, multi-layer plating processes are often used, such as first copper plating, then nickel plating, and finally chromium plating, to improve the overall corrosion resistance and aesthetics.

Application: Chromium plating is mainly used to improve the wear resistance of parts, repair the size of worn parts, and for decorative purposes, such as automotive trim, home hardware, etc. Although chromium provides a shiny and beautiful appearance, its anti-corrosion properties often rely on a multi-layer coating process.

Nickel plated

Features: The nickel plating layer has good chemical stability, especially in the atmosphere and alkaline media, and is not easily oxidized and discolored. The nickel plating layer has high hardness, is easy to polish and has strong light reflection ability, making it excellent in both protective and decorative properties. The nickel layer begins to oxidize above 600°C but maintains its chemical stability at room temperature.

The disadvantage of nickel plating is its porosity, which makes it difficult to completely prevent corrosion of the substrate in humid and corrosive environments. To overcome this shortcoming, multi-layer electroplating is usually used, with a nickel layer often present as an intermediate layer. Nickel is a cathodic plating on iron and is an anodic plating on copper.

Application: Nickel plating is widely used in anti-corrosion and decoration fields, especially in household appliances, decorative accessories and industrial equipment. Nickel plating on copper products can effectively prevent corrosion. Although nickel is relatively expensive, in some cases, copper-tin alloy plating has become a substitute for nickel plating to reduce costs while maintaining protective effects.

Comprehensive comparison

Corrosion resistance: Cadmium plating performs best in marine and high-salt environments, while zinc plating is more economical and practical in ordinary atmosphere. Chrome plating itself does not provide excellent anti-corrosion properties and is often used to improve wear resistance. Nickel plating has a better balance in the fields of anti-corrosion and decoration.

Mechanical properties: Chromium plating has the highest hardness and is suitable for high wear resistance applications, cadmium and zinc plating are softer but have good adhesion, and nickel plating provides moderate hardness and machinability.

Cost: Zinc plating has the lowest cost, nickel plating and chromium plating are higher. Cadmium plating has greater cost and usage restrictions due to its toxicity and specific application requirements.