Principles of Electroplating

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Electroplating is the surface treatment and finishing of metals or nonmetals. An electrochemical reaction is used to form a metallic coating from an aqueous solution or a molten salt. Specifications such as deposition of pure metal or alloy coatings of any composition are met by selecting materials based on deposition rate, deposition efficiency and throwing power.

Electrochemical Cell

  • Elements of an electrochemical cell include the container, the melt and the electrodes. The anode and cathode are immersed in the melt in the container. Temperature, electrochemical limits and the atmosphere influence the operation of the cell. When a voltage is applied between the anode and the cathode, electrolysis occurs in the molten salt, and electrodeposition occurs.

Electrodeposition

  • The anode, or metal to be use for plating, and the cathode, or substrate to be plated, are immersed in a salt medium such as alkali metal halide. Silver (anode) is deposited on jewelry located on the cathode (substrate), for example, in silver electroplating. The metal to be plated dissolves in molten salt, and the solvent facilitates the plating process. The electrical charge (current passed through the solvent for a given time) determines the coating thickness. The uniformity of coating is influenced by the anode-cathode geometry. The coating does not become part of the substrate in electrodeposition; however, at elevated temperatures the coating and substrate interdiffuse with each other.

Electroforming

  • The coating formed by electroforming is so thick that a free-standing coating can be formed by removing the substrate. An artist can use a mandrel -- a rod made of wax, metal, or another material -- to make a replica of a statue, for example, coat it with gold and drain out the material or dissolve it away. Also, complicated shapes such as beautiful pieces of jewelry can be formed from ductile materials using electroforming techniques.

Protective Layers and Coatings

  • Types of protective layers and coatings based on electroplating include metallic, multilayered, alloy, composite, conversion, anodized and electroforming. Metals such as gold and silver, for example, can be used for metallic coatings. Several layers of materials such as copper and nickel are deposited in multilayered coatings. Alloys -- mixture of metals such as tin and lead -- can also be used for coatings. Composites, materials with different ingredients such as cobalt and chromium carbide, are used for specific coating applications. Conversion coatings have oxide, phosphate or chromate surfaces that provide improved corrosion resistance. A metal such as aluminum is used as the anode in anodized coatings, which are widely used in the food packaging and processing industry. Electroforming is a popular technique for jewelry making.

References

  • "Electroplating: Basic Principles, Processes and Practice"; Nasser Kanani; 2004
  • "Metallurgical and Ceramic Protective Coatings"; K.H.Stern; 1996
  • Photo Credit Medioimages/Photodisc/Photodisc/Getty Images
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