Metallic

Metallic coatings provide enhanced corrosion resistance of metals as either barrier coatings or sacrificial coatings. They are durable, usually easy to form, but sometimes porous, which can result in accelerated corrosion of the substrate metal. Some of the common metals used as coatings are nickel, lead, zinc, copper, cadmium, tin, chromium, and aluminum. Methods for applying metallic coatings include cladding, electrodeposition (electroplating), electroless plating, spraying, hot dipping, diffusion, chemical vapor deposition (CVD), and ion vapor deposition.

Nickel

Nickel is used as a coating for corrosion protection applications, and is also used as an undercoat for other coatings. Electrodeposition is the common method for applying nickel, but electroless plating can also be used. When nickel is used as a coating for steel, copper is sometimes used as an intermediate layer. Nickel is also used as an intermediate layer between steel and microcracked chromium to prevent the corrosion of steel. Nickel-phosphorous coatings have a superior corrosion resistance compared to nickel coatings, and can be electrodeposited or electrolessly deposited.

Aluminum

Hot-dipping, spraying, cementation, and ion vapor deposition processes are used to deposit aluminum coatings on steel. Hot-dipped aluminum coatings are used to protect the metal substrate from atmospheric corrosion and oxidation at elevated temperatures. Sprayed aluminum coatings are sometimes sealed with organic coatings to provide more uniform and impermeable protection. Ion vapor deposited aluminum coatings are soft and formable. Aluminum coatings have a minimum thickness of approximately 8-25 μm.

Lead

Electrodeposition and hot dipping are usually employed to apply lead coatings on steel, with tin sometimes added to improve bonding. Of course, lead compounds are toxic and therefore, the use of lead coatings is limited.

Copper

Copper is susceptible to atmospheric corrosion, and thus, it is not very useful as a protective coating when used alone. It is, however, useful when used in conjunction with subsequent coatings, as it has a low porosity and can work as a barrier coating with porous, corrosion resistant coating to protect the base metal from corrosion. In addition corrosion inhibitors, such as benzotriazole, can also improve the performance of copper coatings.

Cadmium

Cadmium is usually a preferred coating for the corrosion protection of steel in moist and marine environments; it is anodic to steel and therefore, will act as a sacrificial anode on steel. Cadmium coatings are smooth and conductive, and resist fretting and fatigue, but have been known to cause solid metal embrittlement of steel and titanium and exfoliation of susceptible aluminum alloys. Furthermore, the corrosion products of cadmium are toxic, so it should be avoided in applications that may contaminate the environment. There are alternatives to cadmium coatings however, such as zinc and tin coatings. Cadmium coatings are applied mostly by the electrodeposition process and are good for electrical applications. Minimum coating thickness is approximately 5 – 25 μm.

Zinc

Galvanization denotes the application of a zinc coating to the surface of a metal by any method. Hot dipping, electrodeposition, and spraying are a few methods used to galvanize a metal. Zinc is less expensive than cadmium, and is generally the preferred coating in industrial environments.

Chromium

Chromium coatings are hard and provide good wear resistance, but are typically used in conjunction with other coatings such as copper and nickel for corrosion protection applications.

Tin

Tin is another very common material used in coating applications and it provides good corrosion resistance to the metal substrates, either as a barrier or sacrificial coating. It is often used to coat steel and sometimes copper. Tin coatings are typically thin and porous; therefore, to achieve corrosion protection they should act as a sacrificial coating. Tin coatings are widely used in the food industry as coatings on steel containers.

Gold

Gold is often coated over other coatings to provide enhanced appearance or improved electrical properties. Gold coatings are used mostly for electrical applications (and jewelry) as they have a low contact resistance.