Lesson 11.5CoatChapter 11 · Coatings and surface finishing

Electroplating (chrome, nickel, zinc)

Electric current deposits metal from a bath

Electroplating uses electric current to coat a part with a thin layer of another metal, such as chrome, nickel or zinc. The shiny chrome on a bathroom tap and the yellowish zinc on a bolt are both electroplated. It adds looks, rust protection or wear resistance while the part keeps its exact shape.

Photo: Encik Tekateki, CC BY-SA 4.0, via Wikimedia Commons

Electroplating · plating tank, power supply and a decorative chrome stackCircled numbers match the steps below. The inset shows the layers of a typical decorative chrome finish, not to scale.

How it works

Clean and rack the part. The part is degreased, rinsed and dipped briefly in acid to remove oxide (activation). It is then hung on a rack or tumbled in a barrel that conducts electricity.
Immerse it in the electrolyte. The tank holds an electrolyte (water with a dissolved salt of the coating metal, such as nickel sulfate). The bath is often warmed and stirred.
Switch on the current. A DC power supply (rectifier) connects the part to the negative side, making it the cathode. Bars of the coating metal on the positive side are the anodes.
Ions travel and plate. Metal ions carry a positive charge, so they drift to the part. There they pick up electrons and turn back into solid metal, atom by atom, on every wetted surface.
The anode dissolves. As metal leaves the solution, the nickel or zinc anode dissolves to replace it. In chrome baths the anodes do not dissolve, so chemicals are added instead.
Rinse and repeat. The part is rinsed between tanks; decorative chrome goes through copper, nickel and chrome baths in turn. Zinc-plated parts usually get a passivation dip (a thin conversion film that adds corrosion resistance and the blue or yellow tint).

Thickness depends on current and time: more current or more minutes give a thicker coat. Plating builds up faster on edges and corners, where the electric field is strongest, and slower in holes and recesses.

Key facts

Materials: Coatings of chrome, nickel, zinc, copper, tin, silver, gold; on steel, brass, copper, aluminum, and plastics made conductive first
Typical parts: Taps, car trim, bolts and screws, hydraulic piston rods, electrical contacts, jewelry
Part size: From tiny connector pins to rolls and rods several meters long; the tank sets the limit
Precision: Very thin and controllable: zinc typically 5–25 µm, decorative chrome well under 1 µm, hard chrome tens to hundreds of µm
Surface finish: Copies the surface below: polished parts come out mirror-bright, matte parts stay matte
Batch size it suits: From single repairs to millions of fasteners in barrel lines
Tooling and machine cost: Racks and fixtures are cheap; an automated plating line with tanks, hoists and waste treatment costs hundreds of thousands to millions
Speed: Minutes for thin zinc or decorative chrome, up to many hours for thick hard chrome

Strengths and limits

Strengths

Thin, even coats that keep the part's dimensions
Many metals for looks, corrosion, wear or conductivity
Mirror-bright decorative finishes
Cheap per part in large batches, especially in barrels

Limits

Uneven in deep holes and recesses; edges build up more
Uses toxic chemicals (for example hexavalent chromium and cyanide baths) that need strict waste treatment
Hydrogen made during plating can embrittle high-strength steel, so parts are baked afterward
Every step depends on perfect cleaning; poor prep makes the coat peel or blister

Where it's used

AutomotiveZinc-plated fasteners and brackets, chrome trim, nickel-plated parts
Plumbing and homeChrome taps, shower heads, door handles
Hydraulics and machineryHard-chrome piston rods, rolls, molds
ElectronicsGold and tin on connectors, copper and nickel on circuit boards
AerospaceCadmium or zinc-nickel on landing-gear parts and fasteners
Jewelry and coinsGold and silver plating over cheaper metals

How to recognize it

A very thin coat that follows every detail, scratch and polish line of the part underneath.
Mirror-bright chrome with a slightly blue tint, or the rainbow, yellow or blue sheen of passivated zinc on screws.
Contact marks: a small bare or dull spot where the part touched its rack.
When it fails, tiny pits or blisters under the shine, or flaking at sharp edges.

Compare with

Hot-dip galvanizing11.4zinc from a molten bath instead of current; much thicker and longer-lasting outdoors, but not precise.Anodizing11.6also an electrochemical bath, but the part is the anode and its own surface turns into oxide instead of receiving new metal.Electroforming11.7the same chemistry run much longer to build a free-standing metal part on a mold.

Watch it

VIEW CHEM · Animated anode, cathode and ion flow

Mayer Motorsports · A shop walkthrough: polishing, then copper, nickel and chrome

Proinvest Group | Unlimited Steel Solutions · Industrial hard chrome on hydraulic bars

Riley Surface World · An automatic barrel zinc line with its gantry and tanks

See real photos

Wikipedia: Electroplating — principle diagrams and plating-line photos
Wikipedia: Chrome plating — chromed car and motorcycle parts, decorative versus hard chrome
Wikimedia Commons photo search

Remember

The part is the negative electrode; positive metal ions in the bath land on it and become metal.
Current and time set the thickness, from under 1 µm to hundreds of µm.
Clean parts and safe handling of toxic baths are what make or break plating.