Zinc Plating
Also: electro-galvanizing, electrogalvanizing, zinc electroplating, ASTM B633, clear zinc, yellow zinc, zinc chromate plating, zinc-nickel plating, trivalent zinc
Zinc plating puts a thin sacrificial zinc layer (5-25 µm) on steel parts, usually with a chromate passivate: the cheapest corrosion finish for hardware and small parts.
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Zinc plating electrodeposits zinc on steel from an alkaline or acid chloride bath, then dips the part in a chromate (or now mostly trivalent chromium) passivate that slows the zinc from corroding. Zinc is anodic to steel, so it protects by corroding first: a scratch to bare steel stays rust-free as long as zinc remains nearby. It is the default finish on fasteners, brackets, clips and stampings because it is cheap, thin and runs in barrels by the thousand.
ASTM B633 is the spec. The service condition sets the thickness: SC1 (mild) 5 µm, SC2 (moderate) 8 µm, SC3 (severe) 13 µm, SC4 (very severe) 25 µm. The type sets the passivate: Type I is as-plated, Type II is coloured hexavalent chromate (yellow, olive drab, black), Type III is clear hexavalent, and Types V and VI are the clear and coloured trivalent versions that RoHS and ELV now require. A clear trivalent passivate gives 24-96 hours of salt spray to white rust; a thick-film trivalent or a yellow chromate gives 96-200 hours; a topcoat sealer more. Zinc-nickel (12-15% Ni, ASTM B841) triples that and is replacing plain zinc under the hood and in aerospace.
The catch is hydrogen. The acid cleaning and the plating charge the steel with hydrogen, and hardened parts (above about 31 HRC or 1,000 MPa) can crack days later under load. Those parts must be baked at 190-220 °C (375-430 °F) for 2-24 hours within a few hours of plating, per ASTM B850, and spring steel and case-hardened parts are the classic victims when the bake is missed.
At a glance
| Typical tolerances | Thickness 5-25 µm (0.0002-0.001") per ASTM B633 service condition, measured as a minimum on significant surfaces; a rack-plated part varies about 2:1 between edges and recesses, a barrel-plated part more. Add the thickness to every surface and four times it to a thread pitch diameter: a 8 µm coat closes an internal thread by about 30 µm on pitch diameter, within the allowance of most class 2B threads, but not class 3. |
|---|---|
| Size limits | Barrel plating handles fasteners and small stampings under about 100-150 mm (4-6") in lots of thousands. Rack plating takes parts to 2-3 m (7-10 ft) and a few hundred kilograms. Deep blind holes, tubes and box sections plate only near the opening; zinc-plated tubing is usually plated as strip before forming. |
| Surface finish | Bright or satin silver as plated, then clear-blue (Type III/V), yellow-iridescent (Type II/VI), olive drab or black by the passivate. Follows the base surface with slight brightening; barrel parts pick up a light tumbled sheen. Zinc is soft (about 70-90 HV) and scratches easily; the corrosion protection is in the sacrificial action, not the hardness. Salt spray to white rust: 24-96 h clear trivalent, 96-200 h yellow or thick-film, 500-1,000 h zinc-nickel. |
| Lead time | 2-5 business days for barrel and rack lots; add a day for the embrittlement bake. Zinc-nickel and black passivates may route to a different shop and add a few days. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Carbon, alloy and case-hardened steels, cast and malleable iron (with care on porous castings), and powder metal after sealing. Stainless is not zinc plated in practice. Brass and copper can be zinc plated but rarely are. Aluminum and zinc die castings are not candidates. Hardened steel above 31 HRC requires embrittlement relief baking; above about 1,400 MPa (200 ksi), many specs forbid electrolytic zinc altogether.
What drives the cost
- Barrel versus rack: barrel parts are priced by the pound; racked parts by the piece
- Lot size: a minimum lot charge dominates small orders
- Thickness (SC1 through SC4) and the passivate (clear trivalent is cheapest, black and thick-film cost more)
- Hydrogen embrittlement bake for hardened parts
- Zinc-nickel or zinc-iron alloy baths instead of plain zinc
- Masking, plugging and any selective plating
- Certifications: thickness reports, salt spray, RoHS or ELV declarations
When to use it
- Fasteners, clips, brackets, stampings and small machined parts in mild to moderate corrosion service
- Steel parts that must stay near size, indoors or under a paint or powder topcoat
- Volume hardware where the price per piece must be cents, not dollars
- A base for a coloured passivate (yellow, black, olive drab) for identification or appearance
- Zinc-nickel for automotive, brake and aerospace hardware needing 500+ hours of salt spray
When not to
- Structural steel outdoors: hot-dip galvanizing is ten times thicker and lasts decades
- Very high-strength steel (above 1,400 MPa, 200 ksi) springs and fasteners, where embrittlement risk is unacceptable
- Wear surfaces: zinc is soft; use electroless nickel or hard chrome
- Marine and chloride exposure with no topcoat; plain zinc goes white in weeks
- Parts that will be welded afterwards, both for weld quality and for the fumes
Design tips
- Call out "ASTM B633, Fe/Zn 8, Type III" (or the SC and Type you need) rather than "zinc plate"; the plater cannot guess the thickness or the passivate.
- State the steel hardness or strength; anything above 31 HRC needs the ASTM B850 bake called out on the drawing.
- Design external threads to a class 2A with the plating allowance, or gauge them after plating; plating internal threads after tapping is normal.
- Add a rack point or a hole the plater can hang from on parts too big for the barrel.
- Specify a trivalent passivate (Type V or VI) for RoHS or ELV compliance; hexavalent yellow is still available but restricted.
- Radius sharp edges; zinc builds up there and rubs off in handling.
- For any real outdoor life, add a sealer or a paint topcoat over the passivate, or move to zinc-nickel.
Frequently asked questions
- How long does zinc plating last?
- Indoors, indefinitely. In salt spray a clear trivalent zinc lasts 24-96 hours to white rust and a yellow or thick-film passivate 96-200 hours; outdoors that translates to a year or two before white corrosion shows. Zinc-nickel goes 500-1,000 hours; hot-dip galvanizing lasts decades.
- What is the difference between clear zinc and yellow zinc?
- The passivate over the zinc. Clear (blue-bright) is a thin chromate with modest corrosion resistance; yellow (iridescent) is a thicker chromate with roughly twice the salt-spray life. Yellow was hexavalent; trivalent yellow-tinted passivates now cover RoHS jobs.
- Does zinc plating cause hydrogen embrittlement?
- It can on hardened steel. Parts above about 31 HRC or 1,000 MPa must be baked at 190-220 °C within hours of plating to drive out hydrogen (ASTM B850). Springs, lock washers, case-hardened pins and grade 10.9 and 12.9 fasteners are the usual failures when the bake is skipped.
- Zinc plating vs galvanizing?
- Zinc plating is electroplated, 5-25 µm, smooth, bright and cheap on small parts. Hot-dip galvanizing is dipped in molten zinc, 50-100 µm and more, rough and grey, and lasts far longer outdoors. Plate hardware; galvanize structure.
