In the machining industry, it is common to see drawings requiring electroplating on the surface of parts, such as galvanizing, cadmium plating, chromium plating, and nickel plating. But do you know their specific differences?
Galvanizing:
Characteristics: Zinc is relatively stable in dry air and is not easily discolored. However, it reacts with oxygen or carbon dioxide in water and humid atmosphere to form zinc oxide or alkaline zinc carbonate film, which can prevent further oxidation of zinc and provide protection. Zinc is highly susceptible to corrosion in acids, alkalis, and sulfides. The galvanized layer generally needs to undergo passivation treatment. After passivation in chromic acid or chromate solution, the formed passivation film is not easily affected by humid air, greatly enhancing the corrosion resistance. For spring parts, thin-walled parts (wall thickness < 0.5mm), and steel parts with high mechanical strength requirements, dehydrogenation treatment is necessary, while copper and copper alloy parts may not require dehydrogenation. Galvanizing has low cost, convenient processing, and good results. Due to the negative standard electrode potential of zinc, the zinc coating is an anodic coating for many metals.
Application: Galvanizing is commonly used in atmospheric conditions and other favorable environments. However, it is not suitable for use on frictional parts.
Chromium Plating
Characteristics: Chromium is very stable in humid atmosphere, alkaline solutions, nitric acid, sulfides, carbonate solutions, and organic acids. It is soluble in hydrochloric acid and hot concentrated sulfuric acid. Under the action of direct current, if the chromium layer is used as the anode, it is easily dissolved 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 change color below 480°C, starts to oxidize above 500°C, and the hardness significantly decreases at 700°C. However, chromium is hard and brittle, making it prone to detachment, especially under alternating impact loads. It also has porosity.
Metal chromium easily passivates in the air, forming a passive film, which changes the potential of chromium. Therefore, chromium becomes a cathodic coating for iron.
Application: Directly plating chromium on the surface of steel parts as a corrosion-resistant layer is not ideal. Generally, multiple layers of electroplating (copper plating -> nickel plating -> chromium plating) are required to achieve rust prevention and decorative purposes. Currently, it is widely used in improving the wear resistance, repairing dimensions, light reflection, and decorative lighting aspects of parts.
Nickel Plating
Characteristics: Nickel has good chemical stability in the atmosphere and alkaline solutions, and it is not prone to discoloration. It is only oxidized at temperatures above 600°C. It dissolves slowly in sulfuric acid and hydrochloric acid, but is easily soluble in dilute nitric acid. It can passivate in concentrated nitric acid, thus demonstrating good corrosion resistance.
Nickel plating has high hardness, is easy to polish, has high light reflectivity, and can enhance aesthetics. However, it has porosity. To overcome this drawback, multiple layers of metal plating can be used, with nickel as the intermediate layer.
Nickel is a cathodic coating for iron and an anodic coating for copper.
Application: Nickel plating is commonly used for corrosion prevention and aesthetic enhancement. It is often used to protect decorative plating. Nickel plating on copper products is ideal for corrosion prevention. However, due to the relatively high cost of nickel, copper-tin alloy plating is often used as a substitute for nickel plating.