Zinc plating vs hot-dip galvanizing: which one does your fastener need?
We run Cr3+ zinc electroplating in-house and we do not own a galvanizing kettle. This page compares them straight, including the cases where the answer is not us.
Electroplated zinc vs Hot-dip galvanizing
Short answer: fine threads and bulk parts go electroplated; long outdoor life on heavy steel goes hot-dip
Electroplated zinc deposits a thin, even layer of about 5–12 µm, so threads still assemble with standard nuts and the finish can be white, blue, yellow or black. Hot-dip galvanizing immerses the part in molten zinc at roughly 445–465 °C and builds 45–85 µm or more, which lasts far longer outdoors but forces nuts to be tapped oversize after coating, leaves an uneven surface, and heats the part close to the tempering range of high-strength bolts. We only offer the electroplated route; if the drawing calls for hot dip we will say so and point you to a galvanizer.
Electroplated zinc versus hot-dip galvanizing
| Decision factor | Cr3+ zinc electroplating (our line) | Hot-dip galvanizing (not offered here) |
|---|---|---|
| Principle | Electrolytic deposit in barrel or on rack, then Cr3+ passivate | Immersion in molten zinc, then centrifuged to throw off the excess |
| Standards usually quoted | ISO 4042, ISO 2081, ASTM B633 | ISO 10684 (fasteners), ISO 1461, ASTM A153 Class C |
| Typical thickness | About 5–12 µm, even across the part | About 45–85 µm and up, uneven, heaviest at corners and thread ends |
| Effect on threads and assembly | Thickness is capped by the thread tolerance; standard nuts still fit | Nuts must be tapped oversize after coating and supplied matched to that bolt lot |
| Temperature the part sees | Near room temperature — existing hardness is untouched | Roughly 445–465 °C, close to the tempering range of high-strength bolts |
| Property-class limits | High classes are possible with a de-embrittlement bake | The fastener hot-dip standard restricts which classes may be dipped and some structural specs forbid it on the highest-strength bolts — check the governing spec |
| Hydrogen embrittlement risk | Present: acid pickling plus electrolytic hydrogen — controlled by baking (we bake in-house) | No electrolytic step, but acid pre-treatment and high heat still apply |
| Outdoor corrosion life | Shorter — indoor, semi-exposed or painted-over service | Much longer, because the layer is thick and alloyed into the steel |
| Colour and finish | White, blue, yellow, black; smooth and uniform | Matte grey, spangled, with possible zinc drips at thread ends |
| Parts it suits | Screws, nuts, bolts, washers, pins, springs — small parts in bulk | Structural steelwork, large parts, and fasteners that stay outdoors for decades |
| Who can run it | We run it in-house, with de-embrittlement ovens on site | We have no kettle — this goes to a hot-dip galvanizing plant |
How to choose
Electroplated zinc
Choose electroplated zinc for small fasteners in volume where the thread must still assemble with a standard nut, the colour has to be consistent, coating thickness must stay inside the thread tolerance, and service is indoor, semi-exposed or painted over. High-hardness parts get their de-embrittlement bake in the same building.
Hot-dip galvanizing
Choose hot-dip galvanizing when the drawing or project standard calls for tens of microns of zinc for long outdoor life — structural fasteners, posts, railings, or anything bolting into steelwork that was itself dipped. That work belongs at a galvanizing plant, not here.
Limits before this goes on the drawing
- We do not offer hot-dip galvanizing and will not substitute electroplated zinc for it without the customer approving the change in writing — different thickness, different standard, different service life.
- Thickness and salt-spray figures are typical guidance for small parts, not guarantees; confirm them against the actual part and the governing specification.
- Hot-dip nuts are deliberately tapped oversize, so bolt and nut must stay together as a matched pair; do not mix them with electroplated nuts.
- Electroplated parts at roughly property class 10.9 and above must carry a de-embrittlement bake, started as soon after plating as the process allows.
- The two systems do not share a friction coefficient — if you switch systems, re-check the tightening torque with the assembly engineer first.
What to send us for review
- 1Part type, thread size, material and property class or as-hardened hardness
- 2The coating standard the drawing quotes, with thickness and the thread tolerance to protect
- 3Real service conditions: indoor, outdoor, coastal, or painted over
- 4De-embrittlement bake requirement and any test evidence you need reported
- 5Lot quantity or weight, plus sample pieces for a trial run
Frequently asked questions
Do you offer hot-dip galvanizing?
No. We have no molten-zinc kettle. Work that specifies hot dip has to go to a galvanizing plant. What we run is Cr3+ zinc electroplating with hydrogen de-embrittlement baking, for screws, nuts and bolts in bulk.
Can electroplated zinc replace hot-dip galvanizing on a drawing?
Not as a shop decision. The layers differ by several times in thickness and the two are called out under different standards. If the change is genuinely wanted, the drawing owner has to approve in writing that the thinner layer and shorter outdoor life are acceptable.
Why do hot-dip galvanized nuts feel loose?
Because the zinc layer is thick, the nut is tapped oversize after coating so it will still run onto the thicker bolt thread. The looseness is deliberate and the pair must stay matched.
Which is cheaper for small parts in bulk?
Electroplating, because small fasteners are processed in barrels by the lot and the thickness can be held inside the thread tolerance. Hot dip is more economical per kilogram of heavy steel, not per small piece.