electroplating

name	electroplating
description	Electroplating — Faraday's law, plating rate, current efficiency, hull cell, coating thickness, hardness, adhesion, chromium/nickel/gold/zinc plating, ASTM B633/B456, hydrogen embrittlement risk, post-bake.
metadata	{"priority":7,"promptSignals":{"phrases":["electroplating","plating","chromium plating","nickel plating","zinc plating","Faraday plating"],"minScore":3}}

Electroplating — Complete Skill

Faraday's Law (Plating Rate)

Mass deposited: m = (M × I × t) / (n × F) [g; M = atomic weight [g/mol]; I = current [A]; t = time [s]; n = valence; F = 96,485 C/mol]

Plating thickness: δ = m / (ρ × A) = (M × I × t) / (n × F × ρ × A) [m; ρ = density [g/cm³]; A = plating area [cm²]]

Plating rate: R_plate = δ / t = (M × i) / (n × F × ρ) [μm/hr; i = current density [A/cm²]] Or: R_plate = (Equivalent weight / F) × i × 3600 × 10,000 [μm/hr; EW = M/n]

Current efficiency η_c: η_c = (mass deposited) / (theoretical mass from Faraday) Actual: η_c = 0.60–0.99 depending on plating bath and conditions Remaining current: H₂ evolution (HER) or O₂ evolution (competing side reactions) m_actual = η_c × (M × I × t) / (n × F)

Plating Rate Examples

Hard chromium (from CrO₃ bath): EW = Cr/6 = 52/6 = 8.67 g/eq; ρ = 7.19 g/cm³; η_c = 0.10–0.20 (low efficiency) R_Cr = (8.67 × i × 3600) / (96,485 × 7.19) × 10,000 ≈ 0.45 μm/(A/dm²)/hr [at η=100%] Actual (η=0.15): R_Cr ≈ 0.07 μm/(A/dm²)/hr Typical: i = 25–60 A/dm²; R ≈ 10–30 μm/hr → 50 μm takes 1.5–5 hr

Nickel (Watts bath): EW = 58.7/2 = 29.35; ρ = 8.9; η_c = 0.92–0.97 R_Ni ≈ 1.08 μm/(A/dm²)/hr (at η=100%); typical i = 2–5 A/dm² → R ≈ 2.5–5 μm/hr

Zinc (acid zinc): EW = 65.4/2 = 32.7; ρ = 7.13; η_c = 0.93–0.98 R_Zn ≈ 1.22 μm/(A/dm²)/hr; typical i = 1–3 A/dm² → R ≈ 1.5–3.5 μm/hr

Gold (from gold cyanide): EW = 197/1 = 197; ρ = 19.3; η_c = 0.90–0.98 R_Au ≈ 0.28 μm/(A/dm²)/hr; typically i = 0.5–2 A/dm² (expensive); PCB: 1–3 μm typical thickness

Common Platings

Hard Chromium (ASTM B177, MIL-STD-1611)

Bath: CrO₃ 225–270 g/L + H₂SO₄ 2.25–2.70 g/L; T = 55–65°C Current density: 25–60 A/dm²; voltage 6–12 V Hardness: 850–1000 HV (harder than most base metals; near diamond) Applications: hydraulic rods, landing gear, gun barrels, dies, rolls (wear + corrosion) Thickness: 25–500 μm (typically 50–250 μm for wear applications) Issues: hexavalent Cr (Cr⁶⁺) — highly toxic, carcinogenic → EU banned (some exemptions) Replacement: Trivalent Cr (TCP process), HVOF WC-CoCr, EN (electroless nickel), physical spray coatings

Electroless Nickel (EN, ASTM B733)

Not electrolytic: autocatalytic; Ni-P or Ni-B alloy; no external current Bath: nickel salt + reducing agent (hypophosphite); deposits uniformly on any geometry Hardness: as-deposited 450–550 HV; after 400°C heat treat: 950–1100 HV (Ni₃P precipitation) Phosphorus content: Low P (< 4%): high hardness; High P (> 9%): better corrosion, less hard Thickness uniformity: ±5% (much better than electrolytic) Applications: uniform complex geometries, corrosion resistance, electrical contacts

Zinc (ASTM B633)

Primary use: corrosion protection (sacrificial anode for steel) Neutral salt spray (NSS) test: uncoated zinc 96+ hrs; ASTM B117 Thickness by service class (ASTM B633):

Service	Minimum thickness	NSS hrs
SC1 (indoor)	5 μm	48
SC2 (mild outdoor)	8 μm	96
SC3 (moderate)	12 μm	120
SC4 (severe)	25 μm	240

Chromate conversion (passivation): applied over zinc → improves corrosion by 2–5× Yellow chromate: Cr⁶⁺ (banned in EU for automotive since RoHS); Clear/trivalent chromate: compliant replacement

Decorative Nickel-Chrome (ASTM B456)

Stack: Cu (5–10 μm) + Bright Ni (10–20 μm) + Decorative Cr (0.25–0.5 μm thin) Corrosion resistance from multilayer stack; bright Ni provides cathodic protection for Cr CASS test (ASTM B368): copper accelerated acetic acid-salt spray; harder than NSS

Gold (ASTM B488)

Applications: PCBs, connectors (no corrosion, low contact resistance) Hard gold (Ni-alloyed): Type III (99.7% Au) or Type I/II with cobalt/nickel alloy; connector contacts Pure gold: Type I (99.9%); soft; for wire bonding Thickness classes: Grade A = 1.25 μm; Grade B = 0.5 μm; Grade C = 1.5–2.5 μm

Hull Cell

Hull cell: small (267 mL) trapezoidal tank; deposits at varying current densities in one test Visualizes plating quality across i range; identify low-current burning, high-current burning, pitting Current density range on cathode: from 0.1 to 1.0 × max CD in single run Use: bath troubleshooting, additive optimization without full production bath test

Hydrogen Embrittlement

H₂ embrittlement (HE): atomic hydrogen absorbed during acid cleaning or electroplating → diffuses into steel lattice → reduces ductility and fracture toughness → delayed fracture Risk: high-strength steel (σ_u > 1000 MPa; > 34 HRC); hydrogen can reduce elongation from 20% to <5%

ASTM F519 test: tests plated high-strength fasteners for HE; notched tensile specimen loaded to 75% of UTS; hold 200 hr; check for fracture Post-bake relief (ASTM F519, SAE AS1820): Bake at 190°C for 8–24 hr within 4 hr of plating → drives H out of lattice → restores ductility For very high strength (> 50 HRC): bake 23 hr at 190°C

Cadmium plating: historically used for aerospace (AMS 2400); similar HE risk; bake required Replacement for Cd: Zn-Ni alloy; ionic liquid plating; AMS 2417

Standards

Standard	Scope
ASTM B633	Zinc electroplating
ASTM B456	Decorative Ni-Cr plating
ASTM B177	Hard chromium plating
ASTM B733	Electroless nickel
ASTM B488	Gold electroplating
ASTM F519	Hydrogen embrittlement testing
AMS 2406	Hard chromium (aerospace)
AMS 2404	Electroless nickel (aerospace)

Output

Provide: plating type (hard Cr/EN/Zn/Au/Ni), bath chemistry, current density i [A/dm²], current efficiency η_c [%], plating rate R [μm/hr], required thickness δ [μm], plating time t [hr], current I required [A] for plating area A [dm²], adhesion test result (tape test/bend test per ASTM B571), hardness [HV], ASTM B633 service class (Zn), hydrogen embrittlement risk (material HRC > 34?) and post-bake spec (190°C, hours), and applicable standard (ASTM B633/B456/B177/B733, ASTM F519 for HE).