---
type: process
name: "Electroplating"
category: "Finishing"
subcategory: "Additive"
materials: ["Metal", "Plastic"]
tolerances: "Zinc 0.0002–0.001 in (5–25 µm), electroless nickel 0.0005–0.002 in (13–50 µm), hard chrome 0.0002–0.010 in (5–250 µm) per surface. Electrolytic deposits vary 2:1 or more between high- and low-current areas; electroless nickel holds about ±10%"
volumes: "Barrel plating is economical from a few pounds of hardware to millions of pieces; rack plating from 1 part upward"
lead_time: "3–10 business days at a job shop; add 1 day for an embrittlement-relief bake"
url: https://manufacturingprocesses.org/processes/finishing/electroplating
---

# Electroplating

Electroplating deposits a metal coating from solution onto a conductive part by passing current through an electrolyte bath.

- **Category**: [Finishing](https://manufacturingprocesses.org/processes/finishing.md)
- **Family**: Additive
- **Materials**: Metal, Plastic
- **Typical tolerances**: Zinc 0.0002–0.001 in (5–25 µm), electroless nickel 0.0005–0.002 in (13–50 µm), hard chrome 0.0002–0.010 in (5–250 µm) per surface. Electrolytic deposits vary 2:1 or more between high- and low-current areas; electroless nickel holds about ±10%
- **Surface finish**: Reproduces or slightly amplifies the substrate finish; bright baths add gloss but do not level deep tool marks
- **Typical volumes**: Barrel plating is economical from a few pounds of hardware to millions of pieces; rack plating from 1 part upward
- **Lead time**: 3–10 business days at a job shop; add 1 day for an embrittlement-relief bake

## Overview

Electroplating deposits a metal coating onto a conductive part by making it the cathode in an electrolyte containing dissolved ions of the plating metal. Current drives ions out of solution onto the surface, and Faraday's law makes thickness a direct function of current and time — which is why plating specifications are written in thickness, not in dip time.

Common deposits and their working thicknesses: zinc for sacrificial corrosion protection at 0.0002–0.001 in (5–25 µm), electroless nickel for uniform hardness and corrosion resistance at 0.0005–0.002 in (13–50 µm), hard chrome for wear surfaces at 0.0002–0.010 in (5–250 µm), and decorative chrome at 0.00001–0.00002 in (0.25–0.5 µm) over a nickel underlayer that does the actual protecting. ASTM B633 codifies zinc thickness by service condition: 5 µm for mild, 8 µm moderate, 13 µm severe and 25 µm very severe exposure. Any base metal that conducts can be plated; plastics require an electroless metallization step first.

## How it works

1. **Clean.** Alkaline soak and electroclean remove oil and shop soil. Plating adhesion failures are almost always cleaning failures, not bath failures.

2. **Activate.** An acid dip strips oxide and leaves an electrochemically active surface. Stainless steel, aluminum and titanium re-oxidize within seconds, so they need a special strike: a Wood's nickel strike for stainless, a double zincate for aluminum.

3. **Strike.** A thin, high-adhesion first layer from a low-efficiency bath anchors the main deposit. Copper strike under nickel on steel, nickel strike under chrome.

4. **Plate.** The part is racked or loaded into a barrel and made the cathode. Thickness follows current density × time, so the rectifier setting and the dwell define the deposit. Rack plating gives control and a cosmetic surface; barrel plating tumbles thousands of small parts at once for a fraction of the cost, with more thickness scatter and part-to-part contact marks.

5. **Post-treat.** Zinc deposits are chromate-conversion passivated immediately — trivalent clear, blue, black or iridescent — because bare zinc white-rusts within days. Electroless nickel is often heat treated at 750 °F (400 °C) for 1 hour to raise as-plated hardness from roughly 500–550 HV to about 900–1000 HV.

6. **Bake for hydrogen embrittlement.** Acid cleaning and plating charge atomic hydrogen into high-strength steel. Parts at or above roughly 40 HRC must be baked, typically at 375 ± 25 °F (190 ± 14 °C), starting within 4 hours of plating; durations run from 3 hours for moderate-strength parts to 23 hours or more for the highest-strength grades. This step is mandatory, not optional, and skipping it produces delayed brittle fracture days or weeks into service.

### Throwing power

Current density is not uniform. Edges, corners and outside features see high current and plate thick; recesses, bore interiors and the inside of a tube see low current and plate thin — 2:1 or worse across one part is normal for bright acid baths. Electroless nickel is the exception: it is an autocatalytic chemical reaction with no current, so it deposits within about ±10% of nominal on every wetted surface including deep bores and blind holes.

## Design guidelines

### Specify thickness at the worst-case location

A drawing that says "zinc plate per ASTM B633 SC3" is asking for 13 µm minimum, and the inspector measures at the low-current-density point. Call out where thickness is measured, or the shop will plate to hit the number on an edge and leave the recess bare.

### Allow 4 × the radial deposit on threads

Coating on a 60° thread changes pitch diameter by roughly four times the radial thickness. A 0.0002 in (5 µm) zinc deposit consumes about 0.0008 in of pitch-diameter clearance, which is why fastener specifications limit plating thickness rather than maximizing it. On tapped holes, either plate before tapping or specify an oversize tap.

### Break sharp edges and avoid deep blind recesses

Sharp corners build a thick, rough, sometimes nodular deposit ("treeing"); deep recesses starve. Break edges to 0.010–0.030 in (0.25–0.75 mm) and open up narrow slots. Where geometry cannot change, the shop must add auxiliary anodes or current thieves, which is setup labor.

### Design in a rack or contact point

Racked parts carry a contact mark; barrel parts carry random contact marks and can nest. Add a hanging hole or specify an acceptable contact area. Parts that nest — thin discs, cupped stampings, springs — should be rack plated even though it costs more.

### Choose the deposit for the job, not the look

Zinc protects steel sacrificially and is the default for fasteners and brackets. Zinc-nickel gives substantially longer salt-spray life than plain zinc and is the automotive under-hood standard. Electroless nickel gives uniform thickness on complex geometry plus hardness. Hard chrome gives roughly 850–1100 HV for wear surfaces but is deposited with a micro-crack network and does not by itself protect steel from corrosion. Decorative chrome is a flash over nickel — the nickel provides the corrosion barrier.

### Plan for aluminum and plastic separately

Aluminum needs a zincate pretreatment because its oxide reforms instantly. Plastics need an etched, catalyzed surface and an electroless copper or nickel layer before electroplating; ABS and ABS/PC are the standard platable resins, and glass-filled or highly crystalline resins generally are not.

| Feature | Recommended | Limit | Why |
| --- | --- | --- | --- |
| Zinc thickness (ASTM B633) | 8–13 µm (SC2–SC3) | 5 µm (SC1) | Below SC1 the coating cannot protect the substrate |
| Thread pitch-dia. allowance | 4 × radial thickness | Plate before tapping | Thread geometry multiplies radial build |
| Edge break before plating | 0.010–0.030 in (0.25–0.75 mm) | — | Sharp edges grow thick, rough, nodular deposits |
| Recess depth vs. width | ≤ 1 : 1 for electrolytic baths | Use electroless nickel | Low current density starves deep recesses |
| Embrittlement relief bake | 375 ± 25 °F (190 ± 14 °C) | Start within 4 h of plating | Absorbed hydrogen causes delayed fracture above ~40 HRC |
| Electroless nickel uniformity | ±10% of nominal | — | Chemical, not electrical, deposition

## Cost drivers

Plating is quoted by weight or area for barrel work and per rack for rack work, over a lot minimum.

- **Rack vs. barrel.** Barrel plating is the cheapest metal finish available for small hardware because thousands of parts share one cycle. Rack plating costs many times more per part and is required for anything cosmetic, fragile, or prone to nesting.
- **Deposit metal.** Zinc is inexpensive. Electroless nickel is expensive because the bath is consumed as it plates and has a finite number of metal turnovers. Gold and silver are priced on the metal.
- **Thickness.** Time on the rectifier scales with thickness, so an SC4 (25 µm) zinc call-out costs materially more than SC1 (5 µm) — specify the service condition the part actually sees.
- **Masking and secondary operations.** Masking, embrittlement-relief baking, and post-plate thread chasing are separate labor lines.
- **Specification burden.** Certifications, thickness reports, salt-spray testing and adhesion testing per lot add cost that has nothing to do with the plating itself.

Cost-reduction tactics:

1. Design small parts so they can be barrel plated — no nesting shapes, no fragile projections, no deep cups.
2. Specify the ASTM B633 service condition your application needs rather than defaulting to the thickest option.
3. Use zinc-nickel or a better chromate rather than more zinc when salt-spray life is the requirement.
4. Keep base-metal hardness below about 40 HRC where the design allows, and the embrittlement bake disappears.
5. Where geometry is deeply recessed, price electroless nickel against electrolytic plating plus auxiliary anodes — it is often cheaper overall.

## FAQ

### How thick is electroplating?

It depends on the deposit and the specification. ASTM B633 sets zinc at 5 µm (0.0002 in) for mild service up to 25 µm (0.001 in) for very severe service. Electroless nickel typically runs 0.0005–0.002 in (13–50 µm), hard chrome 0.0002–0.010 in (5–250 µm), and decorative chrome only 0.00001–0.00002 in (0.25–0.5 µm) over nickel.

### What is hydrogen embrittlement relief baking and when is it required?

Acid cleaning and plating drive atomic hydrogen into high-strength steel, which later causes delayed brittle fracture. Parts at or above roughly 40 HRC must be baked, typically at 375 ± 25 °F (190 ± 14 °C), beginning within 4 hours of plating. Bake times run from about 3 hours for moderate-strength parts to 23 hours or more for the highest-strength grades.

### Why does plating come out thicker on edges than in recesses?

Current density concentrates at edges and corners and starves in recesses, so a typical bright acid bath deposits 2:1 or more thickness between the two. Auxiliary anodes and current thieves help. Electroless nickel avoids the problem entirely because it plates chemically, holding about ±10% on every wetted surface including blind holes.

### Can you electroplate aluminum or plastic?

Yes, but neither goes straight into the bath. Aluminum needs a double zincate pretreatment because its oxide reforms in seconds. Plastic needs a chemical etch, a catalyst, and an electroless copper or nickel layer to make it conductive — ABS and ABS/PC are the standard platable resins.

### How much does plating add to a thread?

Thread geometry multiplies radial coating thickness by roughly four on pitch diameter, so a 0.0002 in (5 µm) zinc deposit consumes about 0.0008 in of pitch-diameter clearance. That is why fastener standards cap plating thickness. For tapped holes, plate before tapping or use an oversize tap.

### What is the difference between hard chrome and decorative chrome?

Hard chrome is a functional wear coating deposited 0.0002–0.010 in (5–250 µm) thick at roughly 850–1100 HV, plated directly for hydraulic rods and molds. Decorative chrome is a 0.00001–0.00002 in (0.25–0.5 µm) flash over bright nickel — the nickel underneath provides the corrosion protection, and the chrome only supplies color and tarnish resistance.

## Alternative processes

- [Galvanizing](https://manufacturingprocesses.org/processes/finishing/galvanizing.md): Galvanizing coats steel with zinc, usually by hot dipping, so the zinc corrodes sacrificially and protects the steel beneath it.
- [PVD Coating](https://manufacturingprocesses.org/processes/finishing/pvd-coating.md): PVD coating vaporises a solid target inside a vacuum chamber so it condenses on the part as a very thin, extremely hard film.
- [Powder Coating](https://manufacturingprocesses.org/processes/finishing/powder-coating.md): Powder coating sprays electrostatically charged dry polymer powder onto an earthed part, then bakes it into a tough continuous film.
- [Chemical Conversion Coating](https://manufacturingprocesses.org/processes/finishing/chemical-conversion-coating.md): Chemical conversion coating reacts the metal surface with a chemical bath to form a thin protective, conductive or paint-adhering film.
- [E-Coating (Electrophoretic Deposition)](https://manufacturingprocesses.org/processes/finishing/e-coating-electrophoretic-deposition.md): E-coating deposits paint electrically from a bath so it plates evenly into every recess, then cures to a uniform film.

## Related processes

- [Galvanizing](https://manufacturingprocesses.org/processes/finishing/galvanizing.md): Galvanizing coats steel with zinc, usually by hot dipping, so the zinc corrodes sacrificially and protects the steel beneath it.
- [Electroforming](https://manufacturingprocesses.org/processes/forming/electroforming.md): Electroforming grows a metal shell by electrodeposition onto a mandrel that is afterwards removed, producing thin parts with sub-micron detail.
- [Chemical Conversion Coating](https://manufacturingprocesses.org/processes/finishing/chemical-conversion-coating.md): Chemical conversion coating reacts the metal surface with a chemical bath to form a thin protective, conductive or paint-adhering film.
- [Electropolishing](https://manufacturingprocesses.org/processes/finishing/electropolishing.md): Electropolishing reverses plating: current dissolves the surface fastest at the peaks, leaving a bright, deburred, passive finish.
- [Passivation](https://manufacturingprocesses.org/processes/finishing/passivation.md): Passivation removes free iron from a stainless steel surface with an acid bath so the chromium oxide layer can reform unbroken.

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*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/finishing/electroplating)*

*Last updated: August 11, 2026*