Electroplating

Electroplating is an electrochemical process in which a metal coating is deposited onto a solid substrate using direct current. The technique relies on the redox reactions of dissolved metal cations within an electrolytic cell, allowing a thin and controlled layer of metal to form on the cathode surface. Widely applied in industrial manufacturing, engineering maintenance, electronics, and decorative arts, electroplating enhances properties such as corrosion resistance, hardness, reflectivity, electrical conductivity, and overall appearance. It is also integral to specialised processes including electroforming, printed circuit board fabrication, and metal purification.

Fundamental Principles and Mechanisms

Electroplating operates through an electrolytic cell consisting of a cathode, anode, and an electrolyte solution. The substrate to be coated functions as the cathode, where metal cations in the electrolyte undergo reduction and deposit as a solid metal layer. The electrolyte typically contains a salt of the metal being applied, ensuring a continuous supply of ions. For instance, a copper-plating bath containing copper(II) sulphate dissociates to yield Cu²⁺ ions, which gain electrons at the cathode to form metallic copper.
The anode material can be either soluble or inert. Soluble anodes, made of the same metal as the plating bath, dissolve to replenish metal ions in the solution, effectively transferring metal mass from the anode to the cathode. In contrast, inert anodes, such as carbon or lead, do not dissolve; instead, oxidation at the anode produces by-products such as oxygen or hydrogen peroxide. In such settings, metal ions must be added periodically to maintain bath composition.
The degree of uniformity in metal deposition is affected by the throwing power of the solution. This parameter reflects the ability of the plating bath to distribute current evenly across complex geometries. High throwing power results in more consistent thickness across regions located near and far from the anode, and it is strongly influenced by solution composition, temperature, and current density.

Materials, Bath Composition, and Chemical Factors

Many plating baths involve straightforward metal salt solutions, though additives are often incorporated to improve deposition quality, control grain structure, and modify conductivity. Cyanide-based baths, historically common for metals such as copper and silver, provide stable complexes that enhance anode dissolution and maintain consistent metal ion concentration. Additional chemicals, including carbonates and phosphates, may be introduced to adjust conductivity.
While electroplating typically produces single-metal coatings, selected alloy coatings—such as brass or solder—can also be electrodeposited. These alloy deposits usually form as fine intermixed crystals rather than true solid solutions. To achieve a fully homogeneous alloy, plated solder coatings may be heated post-deposition to allow proper fusion of metallic constituents.
Areas where plating is not required must be protected using stop-offs, such as waxes, tapes, lacquers, or foils. These prevent the electrolyte from contacting specific regions of the substrate and ensure controlled deposition.

Strike Coatings and Surface Preparation

A strike, or flash coating, is an ultra-thin initial layer deposited at high current density using a low metal ion concentration. This preliminary coating enhances adhesion, especially in systems where direct deposition is difficult. Strikes are particularly important when plating metals with poor natural adhesion to a given substrate. For example, electrolytic nickel adheres poorly to zinc alloy surfaces; therefore, a copper strike is applied first to create a compatible intermediate surface layer.
Strikes also serve as foundation coatings before further layers of different metals, enabling improved corrosion resistance, metallurgical compatibility, and coating durability.

Pulse Electroplating

Pulse electroplating, also termed pulse electrodeposition, modifies the deposition process by alternating the applied current or potential between high and zero values in a rapid, controlled sequence. Experimental parameters include peak current or potential, duty cycle, frequency, and the resulting average (effective) current or potential.
Key advantages include:

• Reduced internal stress in the deposited layer
• Improved structural uniformity
• Enhanced control over composition and thickness
• Reduced incidence of surface cracks when using short duty cycles and high frequencies

However, the method typically requires advanced power supplies capable of generating rapid switching and high peak currents. Additional practical considerations include the distance between anode and cathode, solution agitation, and operating temperature. Elevated temperatures often increase the deposition rate by accelerating reaction kinetics according to the Arrhenius relationship. Stirring influences ion transport, helping maintain consistent deposition quality.
One limitation is the tendency for inert anodes to become contaminated or even plated during reverse-current segments of the pulse cycle, which increases operational cost, particularly in systems using precious metal anodes.

Brush Electroplating

Brush electroplating, a portable and highly localised variant of the process, uses a brush electrode saturated with plating solution to apply metal selectively to specific areas. The brush, typically comprising a graphite core wrapped in an absorbent layer, serves as the anode, while the workpiece is the grounded cathode.
This method provides advantages such as:

• On-site repair of worn mechanical surfaces
• Minimal masking requirements
• Low volume of plating solution required
• Suitability for large or immovable structures

Brush electroplating is widely used for applying nickel or silver coatings to damaged or worn machine components. Modern developments allow deposits up to approximately 0.25 mm while maintaining uniformity. Disadvantages include the need for significant operator involvement, slower throughput compared to tank plating, and handling of potentially hazardous chemicals.

Barrel Plating

Barrel plating is one of the most efficient methods for coating large quantities of small items. Components are loaded into a rotating, non-conductive barrel that is submerged in the plating bath. Electrical contact is maintained through conductive elements inside the barrel, allowing each item to act as a cathode.
The tumbling motion ensures that all surfaces receive exposure to the plating solution and electrical contact, promoting uniform coating on components such as fasteners, pins, and small mechanical parts. Barrel plating is valued for its high productivity, cost-effectiveness, and ability to handle complex geometries, although it is not suitable for fragile or delicate items that may be damaged by the tumbling motion.

Applications and Industrial Significance

Electroplating is fundamental in areas requiring precision surface engineering. In electronics manufacturing, copper electroplating is essential in forming vias, tracks, and interconnects in printed circuit boards and integrated circuits. Electroforming enables the creation of precise metal components with intricate shapes by building up thickness to replicate mould patterns.
The process also has decorative and functional uses in jewellery production, automotive finishing, and household hardware. In industries handling corrosive environments, electroplated layers of chromium, nickel, zinc, and other metals provide essential protection against chemical attack, mechanical wear, and environmental degradation.
The ability to purify metals through controlled electrochemical deposition underlies several refining operations, especially in copper production, where impurities are removed as the metal is selectively plated from solution.