Plating
Also: electroplating, metal plating, electrodeposition, metal finishing, plating services
Plating deposits a thin metal layer (zinc, nickel, chrome, tin, gold, silver) on a part for corrosion resistance, wear, conductivity or appearance. Which metal, and how thick, is the decision.
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Plating covers a family of processes that deposit metal on a part from a solution, either electrolytically (the part is the cathode, a current drives the deposit) or by an autocatalytic chemical reaction (electroless). The deposit is a different metal from the substrate, bonded to it atom by atom, a few micrometres to a few hundred micrometres thick. It changes what the surface does without changing what the part is made of.
The choice of metal follows the job. Zinc (ASTM B633) is the cheap sacrificial coating for steel hardware. Electroless nickel (ASTM B733) gives uniform thickness, hardness and corrosion resistance on any shape. Electrolytic nickel (ASTM B689 engineering, ASTM B456 decorative nickel-chrome) gives ductile build-ups and bright decorative finishes. Hard chrome (AMS 2460) is the wear coating for shafts and hydraulics. Tin (ASTM B545) is for solderability; gold (ASTM B488, MIL-DTL-45204) and silver (ASTM B700) for contacts and conductivity; copper as an underplate and for conductivity; cadmium (restricted, aerospace only) for galvanic compatibility with aluminum.
Two things trip up every buyer. Electrolytic deposits follow the current, so they pile up on edges and corners and thin out in bores and recesses; electroless nickel does not, which is why it costs more. And plating on high-strength steel introduces hydrogen: parts above about 1,000 MPa (145 ksi) or 31 HRC must be baked at 190-220 °C (375-430 °F) within a few hours of plating (ASTM B850) or they can crack in service. Say the hardness on the RFQ.
At a glance
| Typical tolerances | Thickness by metal and purpose: zinc 5-25 µm (0.0002-0.001"), decorative nickel-chrome 10-40 µm, electroless nickel 5-75 µm, hard chrome 25-250 µm (0.001-0.010") or more for salvage. Electrolytic deposits vary 2:1 or worse between edges and recesses; electroless is uniform to about ±10%. Every micrometre adds to the surface and four times that to a thread pitch diameter. Plate on a dimension only when you state the plated size. |
|---|---|
| Size limits | Barrel plating takes small parts (fasteners, clips, under about 100 mm) in bulk. Rack plating handles parts up to the tank: 2-3 m (7-10 ft) is common, 6 m and more at large shops for hydraulic cylinders and rolls. Weight is limited by racking and hoists, typically a few hundred kilograms. Internal surfaces are plated only where an anode can reach or where electroless nickel is used. |
| Surface finish | Follows the incoming surface: a polished part plates bright, a machined part plates matte in the same texture. Bright nickel and chrome are mirror-like and decorative; zinc is silver to iridescent depending on the chromate; electroless nickel is a semi-bright grey; hard chrome is matte grey until ground. Hardness ranges from 40 HV (soft gold) to 500-1,000 HV (electroless nickel, hard chrome). |
| Lead time | Zinc and tin on standard parts in 2-5 business days; nickel, electroless nickel and decorative chrome in 5-10 days; hard chrome with grinding in 1-3 weeks. Certification and salt-spray testing add a week. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Carbon, alloy and tool steels, cast iron, stainless (with a nickel strike to activate the surface), copper alloys, and zinc die castings (copper underplate first). Aluminum is plated after a zincate immersion, then usually electroless nickel. Titanium and magnesium need specialist pre-treatments. Plastics can be plated (ABS, some nylons) by an etch-and-seed process at dedicated shops. Sintered and powder-metal parts trap solution and must be sealed first.
What drives the cost
- The metal: zinc and tin are cheap; nickel and chrome cost more; gold and silver are priced by the gram of deposit
- Thickness and surface area, which set the metal and the tank time
- Rack versus barrel: racked parts cost several times barrel parts
- Masking, plugging and selective plating of only some surfaces
- Substrate preparation: stainless strikes, zincate on aluminum, sealing of castings and powder metal
- Hydrogen embrittlement relief baking and the paperwork that proves it
- Testing and certs: thickness by X-ray fluorescence, adhesion, salt spray, RoHS or ELV compliance
When to use it
- Corrosion protection on steel parts that must stay near their machined size (zinc, electroless nickel)
- A hard, wear-resistant surface on a part that cannot be heat treated or is not steel (hard chrome, electroless nickel)
- Electrical contacts, RF surfaces and solder joints (gold, silver, tin, copper)
- Appearance: bright nickel-chrome on consumer and automotive trim, satin nickel on hardware
- Restoring worn shafts and bores to size, or building up a mis-machined dimension
When not to
- Aluminum parts that only need corrosion protection or colour: anodize or chem film is cheaper and needs no zincate
- Parts with hidden internal passages or deep blind holes on an electrolytic process; use electroless nickel or another finish
- High-strength steel above 1,400 MPa (200 ksi) without an embrittlement plan; consider a non-electrolytic coating
- Large weldments and structural steel: hot-dip galvanizing or powder coating is cheaper per square metre
- Parts that will be welded afterwards: the deposit contaminates the weld and zinc fumes are toxic
Design tips
- Name the spec, the class and the thickness on the drawing: "Zinc plate per ASTM B633 Fe/Zn 8, Type III" says everything a plater needs.
- State the base metal hardness or tensile strength so the plater knows whether to bake for hydrogen embrittlement.
- Radius outside edges and corners; electrolytic deposits build up there and can be two to three times the nominal.
- Design threads to accept the plating: internal threads are usually plated after tapping, and a 10 µm deposit closes the pitch diameter by about 40 µm.
- Give the plater a rack point that can carry a small bare mark, and specify a plug or mask for every bore, thread or face that must stay bare.
- Do not plate over a sharp, unbroken machined edge that will be handled; plating chips there.
- Ask for salt-spray hours, not "corrosion resistant": zinc with a clear trivalent passivate is 24-96 h to white rust, yellow chromate is more, electroless nickel is 100-1,000 h depending on thickness and phosphorus.
Frequently asked questions
- What is the difference between electroplating and electroless plating?
- Electroplating uses an external current, so the deposit follows the current and thickens on edges and thins in recesses. Electroless plating uses a chemical reducing agent in the bath, so the deposit is the same thickness everywhere the solution touches, including bores and blind holes, at a higher price.
- Which plating is best for corrosion resistance on steel?
- For low-cost hardware, zinc or zinc-nickel with a trivalent passivate. For machined parts that must hold size, high-phosphorus electroless nickel at 25-50 µm. For structural steel outdoors, hot-dip galvanizing beats any plating.
- Does plating cause hydrogen embrittlement?
- It can, on hardened steel. Acid cleaning and electroplating charge the steel with hydrogen, and parts harder than about 31 HRC (1,000 MPa) can crack under load days later. The fix is a bake at 190-220 °C within hours of plating, per ASTM B850, which the drawing must call out.
- How much does plating add to part dimensions?
- The full deposit thickness on every plated surface: 5-25 µm for zinc, up to 250 µm for hard chrome. On a 60-degree thread, the pitch diameter grows by about four times the plating thickness, so most external threads are plated undersize or gauged after plating.
