What are the differences between zinc plating, cadmium plating, chromium plating, and nickel plating?

Zinc plating

Features: Zinc is relatively stable in dry air and does not easily discolor. In water and humid atmospheres, it reacts with oxygen or carbon dioxide to form an oxide or alkaline zinc carbonate film, which prevents further oxidation of the zinc and provides protection.

Zinc is highly susceptible to corrosion in acids, alkalis, and sulfides. Zinc plating layers generally require passivation treatment. After passivation in chromic acid or chromate solutions, the resulting passivation film is less susceptible to reaction with humid air, greatly enhancing corrosion resistance. For spring parts, thin-walled parts (wall thickness <0.5 μm), and steel parts requiring high mechanical strength, hydrogen removal is necessary. Copper and copper alloy parts do not require hydrogen removal.

Zinc plating is low-cost, easy to process, and produces good results. Zinc has a relatively negative standard potential, so zinc plating is an anodic coating for many metals.

Applications: Zinc plating is widely used in atmospheric conditions and other favorable environments. However, it is not suitable for use on friction parts.

Cadmium plating

Characteristics: Cadmium plating is relatively stable in parts exposed to marine atmospheres or seawater, and in hot water above 70°C. It has strong corrosion resistance, good lubricity, dissolves very slowly in dilute hydrochloric acid, but dissolves very easily in nitric acid. It is insoluble in alkalis, and its oxides are also insoluble in water. Cadmium plating is softer than zinc plating, has less hydrogen embrittlement, and stronger adhesion. Furthermore, under certain electrolytic conditions, cadmium plating is more aesthetically pleasing than zinc plating. However, the gases produced when cadmium is melted are toxic, and soluble cadmium salts are also toxic.

Under normal conditions, cadmium is a cathodic plating for steel and an anodic plating in marine and high-temperature atmospheres.

Applications: It is mainly used to protect parts from atmospheric corrosion caused by seawater or similar salt solutions, as well as by saturated seawater vapor. Cadmium plating is widely used in aerospace, marine, and electronics industries for parts, springs, and threaded parts. It can be polished, phosphated, and used as a paint undercoat, but it is not suitable for food utensils.

Chromium Plating

Characteristics: Chromium is very stable in humid atmospheres, alkalis, nitric acid, sulfides, carbonate solutions, and organic acids. It is readily soluble in hydrochloric acid and hot concentrated sulfuric acid. Under direct current, if the chromium layer is used as the anode, it is readily soluble in caustic soda solution.

Chromium plating has strong adhesion, high hardness (800-1000V), good wear resistance, strong light reflectivity, and high heat resistance. It does not discolor below 480℃, begins to oxidize above 500℃, and its hardness decreases significantly at 700℃. Its disadvantages are that chromium is hard and brittle, and it peels easily, especially under alternating impact loads. It also has porosity.

Metallic chromium readily passivates in air, forming a passivation film that changes its potential. Therefore, chromium becomes a cathodic coating on iron.

Applications: Directly plating chromium onto the surface of steel parts as an anti-corrosion layer is not ideal. Generally, multiple electroplating layers (i.e., copper → nickel → chromium plating) are required to achieve rust prevention and decoration. Currently, nickel plating is widely used to improve the wear resistance of parts, repair dimensional accuracy, enhance light reflection, and for decorative lighting.

Nickel Plating

Characteristics: Nickel has good chemical stability in the atmosphere and in alkaline solutions, does not easily discolor, and oxidizes only at temperatures above 600°C. It dissolves slowly in sulfuric acid and hydrochloric acid, but readily dissolves in dilute nitric acid. It is easily passivated in concentrated nitric acid, thus exhibiting good corrosion resistance.

Nickel plating is hard, easy to polish, highly reflective, and aesthetically appealing. Its disadvantage is its porosity; to overcome this, multi-layer metal plating can be used, with nickel as an intermediate layer.

Nickel is a cathodic plating for iron and an anodic plating for copper.

Applications: Generally used for corrosion prevention and aesthetic enhancement, it is typically applied as a protective decorative coating. Nickel plating is ideal for corrosion protection on copper products.

However, due to the relatively high cost of nickel, copper-tin alloy plating is often used instead.