Chrome Plating
Part of Electrochemistry
How to deposit hard, corrosion-resistant chromium coatings through electrolysis, and why it is the most demanding common plating process.
Why This Matters
Chromium is the hardest metal that can be practically electrodeposited. Hard chrome plating (20–250 μm thick) extends the service life of tools, cutting edges, bearing surfaces, hydraulic cylinders, and injection molds by factors of 10–100 compared to bare steel. Decorative chrome plating provides corrosion resistance and the characteristic bright surface seen on automotive trim, plumbing fixtures, and hand tools.
In a rebuilding context, chrome plating extends the service life of irreplaceable precision parts — machined components that took significant effort to produce and cannot be replaced quickly. A well-plated chromium bearing race or cutting tool lasts for decades of hard use.
The process is more demanding than most plating operations: it requires extremely high current densities, a bath that is highly corrosive and generates toxic hexavalent chromium mist, and precise temperature control. These challenges are manageable with proper equipment and safety measures, but they make chrome plating a late-stage capability — something to develop after mastering simpler processes.
Chromium Chemistry
Chrome plating does not use a dissolving anode (unlike copper or nickel plating). Instead, chromium deposits from chromic acid (CrO₃ dissolved in water) with a lead or lead-tin insoluble anode. The bath must be replenished periodically with chromic acid to replace the deposited chromium.
Cathode reaction (deposition): Cr₂O₇²⁻ + 14 H⁺ + 12 e⁻ → 2 Cr + 7 H₂O
Anode reaction (oxygen evolution, not chromium dissolution): 2 H₂O → O₂ + 4 H⁺ + 4 e⁻
Efficiency: Only 12–25% of the current deposites chromium. The rest generates hydrogen at the cathode. This low efficiency is what requires the extremely high current densities.
Hexavalent vs. Trivalent Chromium
| Type | Chemistry | Hazard | Deposit Quality |
|---|---|---|---|
| Hexavalent (Cr⁶⁺) | CrO₃ + H₂SO₄ | Highly toxic, carcinogenic | Excellent hardness and corrosion resistance |
| Trivalent (Cr³⁺) | CrCl₃ or Cr₂(SO₄)₃ + complexing agents | Lower toxicity | Good decorative, lower performance than Cr⁶⁺ |
Hexavalent chromium is classified as a human carcinogen and is being phased out in many jurisdictions. Trivalent chrome plating produces acceptable decorative deposits but is inferior for hard chrome applications. Where only decorative chrome is needed, trivalent baths are strongly preferred due to significantly reduced health risk.
Bath Formulation (Hexavalent, Decorative/Hard Chrome)
Standard bath composition:
- Chromic acid (CrO₃): 250–400 g/L
- Sulfuric acid (H₂SO₄): 2.5–4 g/L (ratio CrO₃:H₂SO₄ = 100:1 by weight)
- Temperature: 45–55°C (decorative) or 50–65°C (hard chrome)
- Current density: 1,500–5,000 A/m² (very high — 10–25× copper plating)
Preparation: Dissolve CrO₃ in water. Add H₂SO₄ carefully. The bath looks dark red-orange. The sulfate catalyst is essential — without it, no deposit forms.
Electrodes
Anodes: Lead-tin alloy (6–8% tin) or pure lead. Lead-tin provides better oxygen evolution efficiency and longer anode life. The anode must be larger than in most plating processes — area ratio 1:1 to 1.5:1 (anode:cathode) due to high current density requirements.
Cathode: The part being plated. Must be carefully prepared (polished, cleaned, activated) or adhesion fails.
Surface Preparation for Chrome Plating
Chrome adhesion is highly sensitive to surface preparation:
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Polish the substrate to the desired final finish roughness. Chrome is very thin and replicates surface texture exactly.
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Degrease thoroughly — alkaline soak, then ultrasonic if available.
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Etch/activate — immerse in chromic acid bath without current for 30–60 seconds. This dissolves the passive oxide layer on the base metal.
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Immediate plating — begin current flow immediately after activation. Any delay allows the passive layer to reform.
For steel substrates, a copper or nickel undercoat is often applied first to improve adhesion and provide a corrosion barrier beneath the chrome. Chrome is porous at thin deposits; the undercoat prevents corrosion from reaching the steel through the pores.
Health and Safety
Hexavalent Chromium is a Human Carcinogen
Chronic inhalation of Cr(VI) mist causes lung cancer and nasal septum damage. The chromic acid bath generates a fine mist during electrolysis that must not be inhaled.
Mandatory controls:
- Fume suppression: Add a surface active agent (PFAS-based historically; alternatives include hydrocarbon-based suppressants) to the bath to reduce misting. The suppressant creates a foam layer on the bath surface.
- Ventilation: Local exhaust ventilation capturing mist at the tank surface. Minimum face velocity 0.5 m/s across the tank surface.
- Respiratory protection: Minimum P100 particulate respirator. Supplied-air respirator for frequent or prolonged use.
- Skin protection: Double-glove (nitrile under neoprene), face shield, apron.
- Avoid skin contact — Cr(VI) causes skin sensitization and non-healing ulcers (“chrome holes”).
Waste disposal: Chrome plating waste (drag-out, rinse water, spent bath) contains Cr(VI). Must be chemically reduced to Cr(III) with sodium bisulfite, then precipitated as Cr(OH)₃ and removed as solid waste. Never discharge to water bodies.
Trivalent Chrome as Safer Alternative
For decorative applications where maximum hardness is not required, trivalent chrome baths are substantially safer:
- Bath: CrCl₃ with complexing agents (oxalic acid, glycine), sodium or potassium sulfate, pH 2.8–4.0, 25–35°C
- Hazard: Cr(III) is not a carcinogen in normal doses (though skin contact should still be avoided)
- Deposit: Appears identical to Cr(VI) deposits aesthetically; hardness and wear resistance are lower
For any application where decorative appearance is the primary goal, use trivalent chromium and eliminate the carcinogenic risk entirely.