Metal plating can give components a refreshed appearance while improving strength and extending service life. Different plating options can be selected based on the desired look and performance. Five common plating processes for machined parts include nickel, chrome, zinc, copper, and tin plating.

Nickel offers good ductility, corrosion resistance, and magnetic properties, making it widely used in steel, nickel alloys, batteries, and the production of components for automobiles, aircraft, and industrial machinery. After nickel plating, parts have a clean, attractive appearance and improved resistance to rust.

Nickel plating is generally divided into two types: nickel electroplating and chemical nickel plating, also known as electroless nickel plating.

Nickel Electroplating

Nickel electroplating creates a stable nickel layer with fine crystallization and excellent polishing performance. The plated layer has relatively high hardness, which helps improve surface wear resistance. It is commonly used for optical instrument coatings, protective decorative coatings, casting crystallizers, and electronic components.

Chemical Nickel Plating / Electroless Nickel Plating

Chemical nickel plating provides excellent thickness uniformity and can be applied to a wide range of materials without the DC power supply or control equipment required for conventional electroplating. Many electroless nickel coatings offer strong corrosion resistance and high-temperature oxidation resistance.

This process is especially suitable for functional plating on complex-shaped parts that require wear resistance and corrosion protection. With low heat treatment temperatures and different holding times below 400°C, the coating can achieve varying levels of corrosion and wear resistance.

Because electroless nickel plating does not require electricity, it is a simpler and more environmentally friendly option. High-phosphorus nickel alloy coatings can further improve part strength and rust resistance. The process also avoids hazardous substances restricted by the EU RoHS directive, including lead, mercury, cadmium, hexavalent chromium, PBBs, and PBDEs.

Chrome plating is widely used as both an exterior finish and a functional coating because of its excellent protective and decorative properties.

Chrome plating forms a very thin passivated film on the surface of the part, creating a bright silver appearance. It helps reduce the risk of rust, improves surface friction resistance, and offers good heat resistance. When heated below 500°C, chrome plating shows no significant change in gloss or hardness. The plated layer also has strong chemical stability and resists alkalis, sulfides, nitric acid, and most organic acids.

In addition, chromium has a reflectivity of about 65% in the visible range and can maintain this reflectivity for a long time, performing better than silver and nickel in this regard.

Zinc is a highly corrosion-resistant metal. Applying a zinc layer to the surface of iron or steel helps isolate the base metal from oxygen and moisture, preventing the chemical reaction that causes rust. Compared with other protective coating methods, zinc plating or galvanizing is more cost-effective in both initial investment and long-term maintenance. It is one of the most widely used methods for rust prevention.

Copper plating is one of the most widely used processes in the electroplating industry. It is commonly applied before nickel, gold, or silver plating, and is also used on tin solder, lead-tin alloys, and zinc die-cast parts. Copper plating plays an important role in improving coating adhesion and corrosion resistance.

Copper has a positive standard electrode potential, good stability, soft texture, and strong toughness. It is also an excellent conductor of heat and electricity, making it widely used in industrial applications. Copper plating can give steel surfaces copper-like properties, helping reduce product costs. However, aside from certain applications that directly use its electrical conductivity, copper plating is most often used as an intermediate layer before other metal coatings are applied.

This is because copper layers can oxidize and discolor easily. In particular, when copper reacts with sulfides in the atmosphere, black copper sulfide corrosion products may form, causing the metal to lose its luster.

Because copper plating is relatively soft and has low hardness, it is generally not used as the outermost layer in metal plating.

Tin plating provides good solderability and a certain level of corrosion resistance, making it widely used for electronic components and printed circuit boards. In addition to physical methods such as hot dipping and spraying, processes like electroplating, immersion plating, and chemical plating are commonly used in industry because they are simple and practical.

Tin plating gives metal surfaces a silver-like appearance. Compared with most silver and nickel finishes, it has a slightly gray tone and can be produced with either a matte or shiny finish. Because tin plating is non-toxic, it is widely used for parts that may come into contact with food and beverages, such as components in kitchen appliances.

Tin is also easy to solder and offers good electrical conductivity, making tin plating especially suitable for electrical and electronic products that require soldering.