A-to-Z Guide to Thermodynamics,
Heat & Mass Transfer, and Fluids Engineering
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Electroplating is a method for the formation of a thin coating, typically of metal, alloy or composite 1–75 μm onto a suitable substrate (metal, alloy, polymer, ceramic or composite). The substrate to be plated forms the cathode of an Electrochemical Cell; the anode of the cell is preferably of pure metal which is electroplated. In this way the cathode reaction and anode reaction (metal dissolution) of a metal-metal ion couple M–Mn+

can be controlled to maintain a constant bath composition, which is important for the reproduction of plated workpieces.

The composition of the electroplating bath (cell) varies widely according to the required properties of the plate and the metal deposit. Metal can be electroplated from a simple aquo ion or from a metal complex; for example, cyanide. Complexing ions can serve several purposes: to prevent passivity of the anode; to make plating potentials more negative and prevent chemical reactions between substrate and the plating ion (e.g., Cu2+ with Fe); and to improve the throwing power. The throwing power is a measure of the uniformity of metal deposit, which is an important factor with objects of complex shape having variable contours and recesses. In addition to metal ion and electrolyte, other components (organic additives, brightness, levellers, structure modifiers and wetting agents) may be added to the bath to help impart specific properties or to avoid poor deposits. Preparation of the article prior to electroplating is extremely important to ensure a uniform, adherent deposit.

The process of electrodeposition occurs in two stages: the formation of nuclei of the new metal phase and their growth into crystals with the characteristic lattice, until a few atomic layers are formed, and then the gradual build-up of the electrodeposit layer itself.

There are various approaches to electroplating articles: jig or rack mounting (for large objects); barrel plating (for large batches of small objects); individual mounting; wire mounting and continuous cathode transfer. A range of metals (Sn, Cu, Ni, Cr, Zn, Cd, Pb, Ag, Au and Pt) are plated as well as several alloys (e.g., Ni-Fe, Pd-Ni) and newer composites of metals with polymers (polymets), ceramics (cermets) and other dispersed solids, e.g., SiC or diamond. Newer techniques in electroplating include the use of programmed current waveforms (e.g., pulsing) and periodic current reversal to produce dense, nonporous deposits.

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