Electroplating

Why electroplating matters

Almost every metal object you touch that resists rust, slides without galling, solders cleanly, or simply shines was electroplated. The process converts electrical charge into a precisely controllable film of metal, decoupling the surface a part needs from the cheap, strong, or lightweight substrate underneath. That decoupling is the whole point: a steel bolt gives you strength, a few micrometers of zinc gives you corrosion life, and you pay only for the thin skin that does the work.

• Corrosion protection. Zinc and zinc-nickel on steel act as sacrificial anodes — they corrode first and protect the base metal even at scratches.
• Wear and hardness. Hard chromium (800–1000 HV) armors hydraulic rods, piston rings, and injection-mold cavities against abrasion.
• Electrical contact. Gold and silver over nickel give low, stable contact resistance on connectors, edge fingers, and relays.
• Solderability. Tin and tin-lead keep leads and pads wettable for assembly long after storage.
• Appearance. Bright nickel plus a decorative chromium flash gives the durable "chrome" look on faucets, trim, and hardware.
• Build-up and repair. Electroplating restores dimension on worn or undersized shafts and bearing journals to microns.
• Electronics manufacturing. Copper electroplating fills vias and builds conductor traces in printed circuit boards and semiconductor interconnects.

How electroplating works, step by step

Strip the process to its physics and it is a controlled redox cell. Four ingredients matter: the part (cathode), a counter-electrode (anode), an ion-rich electrolyte, and a DC power supply that pushes electrons the way chemistry wouldn't do on its own.

1. Surface preparation. The single biggest cause of plating failure is dirt. Parts are alkaline-cleaned to remove oils, acid-activated (pickled) to strip oxide and smut, and rinsed. Any film blocks adhesion — the deposit will blister or peel.
2. Immersion and wiring. The part is racked and connected to the negative terminal (cathode); the anode connects to the positive terminal. Both hang in the electrolyte, which is a solution of metal salts (e.g., nickel sulfate + nickel chloride + boric acid in a Watts nickel bath), plus additives.
3. Cathode reaction — deposition. At the part, metal cations are reduced: Ni²⁺ + 2e⁻ → Ni(s), or Cu²⁺ + 2e⁻ → Cu(s). Each ion consumes n electrons and becomes an atom bonded to the growing film.
4. Anode reaction — replenishment. With a soluble anode, the metal dissolves: Ni(s) → Ni²⁺ + 2e⁻. The anode literally shrinks over time, feeding ions back into solution so the bath concentration holds steady.
5. Charge sets the mass. Every coulomb deposits a fixed amount of metal via Faraday's law. Multiply current by time to get charge; the deposit follows.
6. Agitation and temperature. Stirring, part motion, or air sparging refresh the diffusion layer at the surface, raising the usable current density and improving evenness. Baths run warm (nickel ~50–60 °C) to boost conductivity and deposit quality.
7. Rinse, passivate, and (if needed) bake. Parts are rinsed, sometimes chromate-passivated (over zinc), and high-strength steel is baked at ~190–220 °C for several hours to drive out absorbed hydrogen and prevent embrittlement.

Throwing power and current distribution

Charge sets the total metal deposited, but not where it goes. Current flows along the path of least resistance, so it crowds onto edges, points, and the faces closest to the anode, and starves deep recesses and the insides of holes. The result is over-plated corners and under-plated cavities. A bath's ability to even this out is its throwing power, and it depends on high solution conductivity, cathode polarization (the tendency for over-plated areas to resist more current), and often lower metal-ion concentration. High-current-density burning at edges and low-current-density dullness in recesses are two sides of the same non-uniform-current coin. Chromium baths throw poorly and need conforming or auxiliary anodes; alkaline and additive-rich baths throw well.

Worked example: Faraday's law and plating time

The design workhorse is Faraday's law of electrolysis, which links deposited mass to charge passed:

m = (M · I · t) / (n · F)

where the symbols are:

Symbol	Quantity	Units
m	Mass of metal deposited	g
M	Molar mass of the metal	g/mol
I	Plating current	A (= C/s)
t	Plating time	s
n	Electrons transferred per ion (valence)	—
F	Faraday's constant	96,485 C/mol
η	Cathode current efficiency	fraction (0–1)
ρ	Deposit density	g/cm³
A	Plated surface area	cm² (or dm²)

Converting mass to a thickness δ over an area A and including current efficiency η gives the practical form:

δ = (η · M · I · t) / (n · F · ρ · A)

Problem. Plate a nickel layer onto a part with 1.0 dm² (100 cm²) of surface at a current density of 3 A/dm² (so I = 3 A). Nickel: M = 58.69 g/mol, n = 2, ρ = 8.90 g/cm³. Assume η = 0.95. How long to reach 12 µm?

Solve. Target mass per unit area: δ·ρ = (12 × 10⁻⁴ cm)(8.90 g/cm³) = 1.068 × 10⁻² g/cm². Over 100 cm² that is m = 1.068 g. Rearranging Faraday's law for time: t = (n·F·m)/(η·M·I) = (2 × 96,485 × 1.068)/(0.95 × 58.69 × 3) ≈ 1,233 s ≈ 20.5 minutes. Equivalently, nickel's electrochemical equivalent is about 1.095 g per amp-hour at 100 % efficiency, so at 3 A and 95 % efficiency you deposit ~3.12 g/h — reaching ~12 µm/dm² in about 20 minutes. Halve the current density and you double the time; that linear trade between current and rate is the process's defining lever.

Common plated metals and their jobs

Metal	Typical bath	Primary purpose	Typical thickness
Zinc (+ chromate)	Acid chloride or alkaline	Sacrificial corrosion protection of steel	5–25 µm
Nickel (bright/semi-bright)	Watts sulfate-chloride	Corrosion, leveling, decorative undercoat	5–40 µm
Chromium (decorative)	Hexavalent or trivalent	Bright, tarnish-resistant flash over nickel	0.1–0.5 µm
Chromium (hard/engineering)	Hexavalent chromic acid	Wear/abrasion resistance, dimensional build	2–500 µm
Copper	Acid sulfate or cyanide	Adhesion/leveling underlayer, PCB conductors	2–35 µm
Gold	Cyanide or sulfite	Contact resistance, corrosion, bonding	0.05–5 µm
Tin / tin-lead	Methanesulfonate or fluoborate	Solderability, whisker-managed finish	3–15 µm

Common misconceptions and failure modes

• "More current always plates faster and better." Above the bath's limiting current density, hydrogen evolves and the deposit burns — rough, dark, powdery, and non-adherent.
• "The deposit is uniform everywhere." Without good throwing power, edges are thick and recesses are thin; a "12 µm average" can mean 25 µm on corners and 3 µm in a bore.
• "Any anode works." Inert anodes deplete the bath and demand chemical replenishment; soluble anodes of the coating metal self-refill — but only if they dissolve cleanly and don't passivate.
• "Adhesion is about the plating bath." Adhesion is almost entirely won or lost in cleaning and activation before the part ever enters the plating tank.
• "Plating strengthens the part." On high-strength steel, absorbed hydrogen can cause delayed hydrogen embrittlement and sudden brittle fracture unless the part is baked promptly after plating.
• "Chrome plating is a thick chromium layer." Decorative "chrome" is a sub-micron chromium flash over a much thicker bright-nickel layer that does most of the corrosion work.
• "You can plate plastic directly." Non-conductors need a conductive seed (electroless copper or nickel, or sputtered metal) before electroplating can carry current to the surface.