Zinc Plating
Zinc plating is an electroplated zinc coating, usually 5–25 µm thick, that protects steel from rust by corroding in its place. With a clear/blue, yellow or black passivation it is the most common and economical anti-corrosion finish for fasteners, brackets and pressed steel parts.
What it is
Zinc is more reactive than steel, so when the coating is scratched the zinc corrodes first and protects the exposed steel. This "sacrificial" protection is why zinc plating is so widely used on mild steel parts.
After plating, parts are dipped in a passivation (chromate conversion) layer that slows white rust and sets the colour: clear or blue, yellow/iridescent, black or olive. Modern trivalent passivations are RoHS-compliant. A sealer or top coat can extend salt-spray life further.
Typical applications
- Bolts, nuts, screws, washers and other fasteners
- Brackets, clamps and sheet metal pressings
- Automotive and two-wheeler under-body and interior parts
- Electrical panel hardware and enclosures
- General machined mild steel components
Suitable materials
- Mild steel and low-carbon steel
- Alloy and medium-carbon steels (baking may be needed)
- Cast iron and malleable iron
- Sintered steel parts (with care)
Points to consider
- State the thickness and passivation colour (for example "Zn 8 µm, trivalent blue") rather than just "zinc plated".
- High-strength parts (above about 31 HRC, such as 10.9 and 12.9 fasteners or hardened springs) must be baked after plating to avoid hydrogen embrittlement.
- Standard 6g/6H thread tolerances only accommodate a thin coating; heavier thicknesses need undersized threads.
- Zinc plating is not recommended above about 100 °C in service, as passivation protection drops.
- For long outdoor life or road-salt exposure, zinc-nickel usually gives far better value.
Frequently asked questions
How long does zinc plating last?
It depends on thickness, passivation and environment. Indoors it can last many years; in salt-spray testing a typical 8–12 µm coating with trivalent passivation reaches red rust in roughly 96–240 hours.
What is the difference between blue, yellow and black zinc plating?
The colour comes from the passivation layer after plating. Yellow traditionally gives more corrosion resistance than clear/blue; black is chosen for appearance. Performance depends on the specific passivation used.
Is zinc plating RoHS compliant?
Yes, when trivalent passivation is used. Older hexavalent (Cr6+) yellow and olive passivations are restricted under RoHS and ELV regulations.
Does zinc plating cause hydrogen embrittlement?
It can in high-strength steels above about 31 HRC. Those parts are baked, typically at around 200 °C for several hours soon after plating, to drive out hydrogen.