Metal plating is a manufacturing process that deposits a thin layer of one metal onto the surface of a different material, typically a metal substrate. This technique, often called electroplating, involves immersing the part into a chemical solution containing dissolved metal ions and applying an electrical current to create a precise metallic bond. The primary motivations for applying these metallic coatings are twofold: to significantly enhance the object’s surface properties, such as resistance to environmental damage, and to provide a desired appearance.
Sacrificial Coatings for Corrosion Control
A major application of metal plating is to prevent the base metal, often steel, from corroding using a coating that is preferentially consumed. This mechanism is known as sacrificial or galvanic protection, where the plated metal is electrochemically more active than the substrate.
Zinc is the most widely used metal for this purpose, often referred to as galvanizing. When the coating is damaged, exposing the underlying steel, the zinc layer sacrifices itself by oxidizing first, protecting the steel from rust until the zinc is consumed. Zinc coatings are common in the automotive, construction, and infrastructure sectors.
Alloys like zinc-nickel are increasingly employed because they offer superior corrosion resistance, especially in high-salt environments. The addition of nickel, usually between 12 and 16 weight percent, creates a highly protective layer that lasts longer than pure zinc.
Cadmium also provides excellent sacrificial protection for steel, particularly in marine environments where it outperforms zinc. This metal exhibits a low friction coefficient and good resistance to seizing, making it valuable for aerospace and military fasteners. Due to its toxicity and environmental impact, cadmium plating is heavily restricted and reserved only for specific, high-performance applications where no viable alternatives exist.
Durable Base Layers and Protective Finishes
Many plating metals serve primarily as a foundational layer, and Nickel is a workhorse in this category. It offers exceptional versatility and is frequently used as an undercoat in multi-layer systems to improve adhesion and provide a substantial barrier to corrosion. Nickel deposits can be engineered to be bright for aesthetic purposes or semi-bright for higher ductility and superior corrosion resistance.
A nickel layer also plays an important role in electronics, particularly when applied beneath tin plating to prevent undesirable material migration. Without this nickel barrier, the tin layer can diffuse into a copper substrate, forming intermetallic compounds that compromise solderability and increase the risk of tin whisker growth. Electroless nickel plating, which does not require an electric current, is valued for providing a coating of uniform thickness on parts with complex geometries, such as valves and printed circuit boards.
Another common metal for protective finishes is Tin, known for its non-toxic nature and excellent solderability, making it a preferred choice for the food and electronics industries. Tin plating protects food containers from rust and maintains electrical contact resistance on components like switchgear. Tin is a relatively soft metal and is typically applied over a nickel or copper underlayer to enhance its durability and functional performance.
Hardness and Aesthetic Appeal
The element Chromium is plated onto objects to achieve two distinct surface enhancements: a highly aesthetic appearance or extreme functional hardness. Decorative chrome is a thin, bright finish, typically less than one micrometer thick, applied over a nickel underlayer. This combination provides a brilliant, tarnish-resistant surface found on automotive trim, household fixtures, and consumer goods.
The nickel layer provides the bulk of the corrosion protection, while the final chromium layer adds the mirror-like finish. In contrast, hard chrome plating, also known as industrial or engineering chrome, is significantly thicker, often ranging from 10 to 500 micrometers. This thickness is applied directly for its functional properties, resulting in a non-reflective gray appearance.
Hard chrome boasts a high surface hardness, typically measuring between 65 and 69 on the Rockwell C scale. This exceptional hardness provides superior resistance to wear, abrasion, and low friction, making it suitable for industrial components such as:
- Hydraulic cylinders.
- Engine parts.
- Heavy-duty shafts.
- Worn machine parts (by restoring original dimensions).
Specialized Metals for Electrical and High-Wear Applications
Certain metals are selected for plating due to unique properties that justify their higher cost in specialized, high-reliability applications. Copper is frequently used as an initial layer in multi-step plating processes because of its excellent electrical conductivity and its ability to promote adhesion between the substrate and subsequent metal layers. It is applied directly in applications such as through-hole plating for printed circuits and for electromagnetic shielding.
For high-end electronics and medical devices, the precious metals Silver and Gold are indispensable. Silver possesses the highest electrical and thermal conductivity of all metals, making it valuable for high-frequency electronic components and power transmission. However, silver is prone to tarnishing from sulfur compounds in the air, which can compromise its surface conductivity over time.
Gold plating is used where stability and low contact resistance are necessary, such as in connectors, switches, and semiconductor components. Gold does not oxidize or tarnish, ensuring that electrical contacts remain stable and reliable over long periods. Though expensive, its chemical inertness and superior performance justify its use in aerospace, medical instruments, and high-reliability defense systems.