Electroplating

Why electroplating matters

• Corrosion protection on a budget. Galvanized steel (zinc plate) carries 5-25 μm of zinc that sacrificially corrodes before the steel beneath; the entire global automotive sheet-steel supply runs through electrogalvanizing or hot-dip lines. Replace 25 μm of zinc with 25 μm of paint and you halve the outdoor lifetime.
• Wear resistance you cannot get from bulk material. Hard chrome at 50 μm gives a Vickers hardness of HV 800-1100 — comparable to tool steel — on top of cheap mild-steel hydraulic rams, engine cylinder liners, and printing rolls. The steel substrate stays tough; the chrome takes the wear.
• Decorative finish at micrometric cost. A typical 'gold-plated' watch case carries 0.5-2 μm of gold. At ~$70/g and ~19 g/cm³, a 30 cm² case totals ~$2 of gold versus hundreds for a solid-gold equivalent.
• Electrical contact engineering. Connector pins use 0.1-2 μm of hard gold (cobalt-hardened) over 1-3 μm of nickel barrier over the brass body. The nickel blocks copper diffusion into the gold; the gold delivers <10 mΩ contact resistance and 1000-cycle durability.
• Selective additive build-up. Through-hole and via plating in PCBs adds 25-35 μm of copper specifically inside drilled holes, completing electrical paths between layers; the same acid-copper bath deposits the via copper and the trace copper in one step.
• Refining at scale. Copper electrorefining processes 20+ Mt/yr globally and is the only economical route to 99.99% Cu wire-grade copper. The anode-slime gold and silver byproducts are sold separately; they often dwarf the refining electricity cost.
• Substrate-independent decorative options. Plate on plastic (POP) lets car-makers chrome ABS components after a chemical pre-etch and electroless nickel/copper strike — saving 60% versus die-cast zinc parts on grilles, badges, and bezels.

Common misconceptions

• The anode metal automatically becomes the coating. Only with soluble anodes (copper into copper sulfate, nickel into Watts bath). With insoluble lead anodes in chrome plating, the metal comes only from the dissolved CrO3, and the bath depletes over time and must be replenished.
• Higher current density gives a thicker coating per unit time. True only up to the limiting current density; above it you get burnt deposits, dendritic powder, and falling current efficiency as hydrogen evolution dominates. Each bath has a sweet spot — 3-5 A/dm² for bright nickel, 30-60 A/dm² for hard chrome, 0.1-1 A/dm² for hard gold.
• Plating the inside of a deep hole works the same as the outside. Throwing power can be terrible. Acid copper at 4 A/dm² will plate the rim of a 10 mm hole at 5x the rate of its center. PCB shops solve this with leveling additives, pulse-reverse waveforms, and air agitation; mechanical engineers fall back to electroless nickel for blind bores.
• Cathode efficiency means coating quality. Chromium plating runs at 12-25% cathode efficiency yet produces a hard, uniform coating; the inefficiency is by design — the hydrogen evolution polishes the surface and breaks oxide skins. High-efficiency nickel baths can still produce porous, dull deposits if bath chemistry is off.
• Hexavalent and trivalent chrome are interchangeable. Hexavalent (CrO3) is the historical hard-chrome bath but is a CMR (carcinogenic, mutagenic, reprotoxic) substance restricted under REACH Annex XIV. Trivalent chrome plates a similar-looking deposit at 0.3 μm decorative thickness but cannot match hard-chrome thickness or hardness for wear applications.
• Faraday's law gives the actual mass deposited. It gives the theoretical mass at 100% current efficiency. Real cells lose current to hydrogen evolution, oxygen reduction at the cathode, anode side reactions, and shunt paths. Multiply the Faraday number by efficiency (12-100% depending on bath) for the real coating mass.

Mechanism: cation migration, charge transfer, crystallization

Three steps run in series at every cathode surface element. First, mass transport: the metal cation M^n+ moves from bulk electrolyte to the diffusion layer (~10-100 μm thick) by migration in the field, by convection in stirred baths, and by Fick's-law diffusion across the unstirred layer. Second, charge transfer: at the metal-electrolyte interface, the cation sheds its solvation shell (commonly six water molecules for transition metals), tunnels electrons from the cathode, and becomes an adsorbed neutral atom (an 'adatom'). The Butler-Volmer equation links the rate of this step to overpotential, and at low current the deposition rate doubles for roughly every 30-60 mV of additional cathodic overpotential. Third, crystallization: the adatom diffuses across the surface to a kink site at a step edge, where it locks into the lattice. Bath additives (saccharin in nickel, brightener+leveler in copper) adsorb on these step edges to control grain size and brightness.

Faraday's two laws set the bookkeeping. Law 1: mass deposited is proportional to charge, m = M·I·t/(n·F). Law 2: at the same charge, the deposited masses of different metals are in proportion to their equivalent weights (M/n). Combined: 1 ampere-hour deposits 1.186 g Cu (n=2), 1.095 g Ni (n=2), 4.025 g Ag (n=1), 2.452 g Au (n=1), or only 0.323 g Cr (n=6). At 100% efficiency this means a 25 μm copper layer on a 10 cm² area requires 0.224 g, hence 38 A·s = 38 coulombs — about 1 minute at 0.6 A. Hard chrome at 25% efficiency on the same area for 25 μm Cr (0.181 g) requires roughly 1080 coulombs, or 60 minutes at 0.3 A: chrome simply costs more electricity per micron than nearly any other plate.

Bath chemistry must (a) keep the metal in soluble cation form across operating temperature and pH, (b) provide ionic conductivity, (c) dissolve the soluble anode at the same rate metal plates out, and (d) include grain-refining and leveling additives that suppress dendrites. The Watts nickel bath (NiSO4 280 g/L + NiCl2 60 g/L + H3BO3 40 g/L) is the historical workhorse: sulfate carries the bulk cation, chloride drives anode dissolution, boric acid buffers pH near 4. Saccharin and 1,4-butynediol added at ~0.5 g/L turn matte deposits into mirror-bright finish. Modern plating engineers spec the entire stack — substrate prep, strike, primary deposit, post-bake — together.

Hard chrome vs decorative chrome vs Ni vs Cu vs Au plating bath compositions and thicknesses

Bath	Key chemistry	Current density	Temperature	Typical thickness	Application
Hard chrome	CrO3 ~250 g/L + H2SO4 catalyst (100:1)	30-60 A/dm²	50-60 °C	20-500 μm	Engine cylinder liners, hydraulic rams, print rolls
Decorative chrome	CrO3 ~150 g/L + sulfate or trivalent	10-15 A/dm²	35-45 °C	0.25-0.5 μm	Automotive trim, plumbing fixtures (over Cu+Ni)
Bright nickel (Watts)	NiSO4 280 + NiCl2 60 + H3BO3 40 g/L + brighteners	3-5 A/dm²	50-60 °C	5-30 μm	Underplate for chrome, decorative finish
Acid copper	CuSO4 200 g/L + H2SO4 50 g/L + Cl- 50 ppm + leveler	2-7 A/dm²	25-30 °C	10-50 μm	PCB through-hole, electroforming, copper underplate
Hard gold (Co-hardened)	K[Au(CN)2] 8-12 g/L + citrate buffer + Co 0.2 g/L	0.1-1 A/dm²	30-50 °C	0.1-2.5 μm	Connector contacts, watch cases
Silver (cyanide)	K[Ag(CN)2] 30 g/L + KCN 60 g/L	0.5-3 A/dm²	20-30 °C	2-25 μm	Tableware, RF reflectors, electrical contacts
Zinc (acid chloride)	ZnCl2 70 g/L + KCl 200 g/L + boric acid	1-5 A/dm²	20-30 °C	5-25 μm	Galvanized fasteners, automotive sheet

Applications and case studies

• Hard chrome on engine cylinders. Caterpillar, Cummins, and motorcycle engine builders deposit 25-50 μm of hard chrome on grey-iron cylinder bores running at 30-60 A/dm² in CrO3/H2SO4 baths. The Vickers hardness of HV 900+ extends piston-ring life by 3-5x versus untreated bore. Hours per micron: ~1 minute at 50 A/dm² for one micron, with the bath running 30 minutes per cylinder for a 30 μm coat.
• Copper electrorefining at Codelco Chuquicamata. The world's largest single copper-refining operation strips ~600 kt/yr of cathode copper at 99.99% purity from 200-300 A/m² cells. Each cell holds ~50 anodes and ~51 cathodes in a 4 m × 1 m × 1.3 m tank; total cell-house current loads run into hundreds of kA. Anode slime delivers ~150 t/yr of gold and silver byproduct.
• PCB through-hole plating. Standard 1.6 mm FR-4 boards with 0.3 mm vias receive 25-35 μm of acid copper inside every via after a palladium-activated electroless copper strike. Pulse-reverse waveforms (forward 20 ms / reverse 1 ms at 3:1 current ratio) selectively dissolve the rim deposit and force throwing power into 5:1 aspect ratio vias.
• Watch case gold plating. Swiss watch shops apply 5-10 μm of cobalt-hardened gold to brass cases at 0.5 A/dm² from K[Au(CN)2] bath. Faraday's law gives ~17 A·hr/m² for 1 μm of gold; a 30 cm² case at 5 μm consumes ~25 mg of gold.
• Galvanized automotive sheet at ArcelorMittal. Continuous electrogalvanizing lines coat both sides of cold-rolled steel strip with 7-10 μm of zinc at line speeds of 100-180 m/min. Cell currents reach 30-80 kA per cell; the line carries 25+ cells. Zinc sacrificially protects the steel for ~15-25 years on body panels.