How Does Electroplating Work? Moving Metal Atom by Atom

An electroplating cell has the object to be coated connected as the cathode, the negative electrode, immersed in an electrolyte containing ions of the plating metal, with an anode completing the circuit. Applying a voltage drives metal ions in solution to the cathode, where each gains electrons and is reduced to metal, depositing as a solid layer. In most industrial baths the anode is made of the plating metal itself and dissolves as the process runs, replacing the ions consumed and keeping the bath at constant concentration, which is why the anode is a consumable. The thickness of the deposit is governed by Faraday's laws of electrolysis, meaning it is proportional to the current multiplied by the time and to the metal's atomic weight divided by the charge on its ion, so an operator can calculate the deposit from the electrical measurements alone. That quantitative control is the reason the process dominates: a coating can be specified in micrometres and delivered reliably.

Each metal is chosen for a property rather than for looks, though several supply both:

• •Zinc on steel, for corrosion protection, which works sacrificially: zinc is more reactive than iron, so it corrodes preferentially and protects the steel even where the coating is scratched through
• •Chromium, thin and decorative over nickel for brightness and tarnish resistance, or thick and hard for wear surfaces including hydraulic rods and engine components
• •Nickel, as an undercoat providing levelling and corrosion resistance, and as a finish
• •Gold, on electrical contacts, because it does not oxidise and therefore maintains low contact resistance indefinitely, which matters far more than its appearance in connectors
• •Silver, for the highest electrical conductivity of any metal, used in high-frequency components and switchgear
• •Copper, as a base layer that adheres well and levels the surface, and as the conductive layer built up in printed circuit board manufacture
• •Tin, for solderability and for food-contact corrosion resistance, which is what a tin can actually is

Two problems dominate the practice. The first is preparation: a deposit will not adhere to a surface carrying oil, oxide or dirt, and the great majority of plating failures are surface preparation failures, so the process is preceded by degreasing, acid pickling to remove oxide, rinsing between every stage and frequently an electrolytic cleaning step. The second is throwing power, the ability of a bath to deposit evenly on a shaped object, since current density is highest at edges and points and lowest in recesses, so a simple bath will plate corners thickly and the inside of a hole hardly at all. The remedies include bath chemistry formulated to raise throwing power, shaped auxiliary anodes placed inside recesses, shields to reduce deposition at high points, and agitation. Additives called brighteners and levellers, frequently proprietary organic compounds, adsorb preferentially onto high points and suppress deposition there, producing a mirror finish from a surface that was merely smooth.

Several techniques share the apparatus and differ in direction or mechanism. Electroless plating uses a chemical reducing agent instead of a current, so the deposit forms on any correctly prepared surface including plastics, coats uniformly regardless of shape, and is how non-conductive parts are metallised and how the copper is deposited in the holes of a circuit board. Anodising runs the current the other way for aluminium, making the workpiece the anode so that a thick controlled oxide layer grows into the surface rather than a metal layer sitting on it, which is why an anodised finish cannot chip off and why it accepts dye. Electropolishing is plating in reverse, removing material preferentially from high points to leave a smooth passive surface, used on stainless steel for hygienic and medical applications. Electroforming grows a thick deposit on a mandrel and then removes the mandrel, producing an object made entirely of plated metal, which is how some precision meshes and the master discs for record pressing were made.

Plating is among the more heavily regulated industrial processes because of what is in the baths. Hexavalent chromium, used in traditional hard and decorative chrome plating, is a confirmed human carcinogen, and its use has been progressively restricted, with European regulation requiring authorisation for continued use and a substantial effort to qualify trivalent chromium and other alternatives, which perform differently. Cyanide-based baths, historically standard for gold, silver, copper and zinc because they produce excellent deposits, are acutely lethal and generate hydrogen cyanide if acidified, so they are being replaced where an alternative chemistry performs adequately. Rinse water carries dissolved metals and must be treated, and spent baths are hazardous waste. The industry's response has been closed-loop rinsing, ion exchange recovery, substitution of chemistries and, in a great many countries, the relocation of plating to jurisdictions with lighter enforcement, which moves the exposure rather than removing it.

Electroplating deposits metal from solution onto a workpiece connected as the negative electrode, with a dissolving anode replacing the ions consumed, and the thickness follows directly from current multiplied by time, which is why coatings can be specified to the micrometre. Zinc protects steel sacrificially, gold maintains contact resistance, tin allows soldering and chrome supplies hardness. The practical difficulties are surface preparation and depositing evenly into recesses, and hexavalent chromium and cyanide baths make it a heavily regulated process.