In this guide
The coating on a bolt often decides how long it lasts. A grade 8.8 bolt that rusts through in two years has failed just as surely as one that snapped. Here is how the most common fastener finishes compare, so you can specify the right one in your enquiry.
Quick comparison
| Finish | Typical thickness | Corrosion resistance | Best for |
|---|---|---|---|
| Plain / oiled | None | Very low | Parts that will be painted or used indoors in dry conditions |
| Black oxide | Very thin conversion layer | Low (relies on oil) | Machinery, tooling, indoor use where appearance matters |
| Zinc electroplated | About 5–12 µm | Moderate | Indoor and sheltered use, general assembly |
| Zinc flake | About 8–12 µm | High | Automotive, high-strength bolts, sheltered outdoor use |
| Hot-dip galvanised | About 45–85 µm | High (long outdoor life) | Structural steel, outdoor, construction, solar, utilities |
Thickness values are typical ranges; the actual requirement should be set by the applicable standard and your design.
Zinc electroplating
The most common general-purpose finish. A thin layer of zinc is deposited electrically, then passivated, usually with a clear/blue or yellow trivalent passivation, which also affects corrosion performance and appearance. It is neat, inexpensive and keeps thread tolerances, but the thin layer is not designed for long outdoor exposure. Specify RoHS-compliant trivalent passivation if you sell into Europe.
Hot-dip galvanising (HDG)
Bolts are dipped in molten zinc, forming a thick, metallurgically bonded coating. This is the standard for outdoor steelwork, transmission towers, solar mounting and construction anchors. Two things to remember:
- Nuts must be tapped oversize after galvanising so they fit the thicker bolt thread. Always buy HDG bolts and nuts as matched sets.
- The finish is matte grey and slightly uneven; it is functional rather than decorative.
Black oxide
A chemical conversion finish that gives a uniform black appearance with almost no change in dimensions. On its own it offers little corrosion protection, so it is normally supplied with an oil or wax. Common for socket head cap screws, machine parts and tooling used indoors.
Zinc flake coatings
Zinc and aluminium flakes in a binder, applied by dip-spin and cured. They give good corrosion resistance at low thickness and are applied without electrolytic hydrogen, which makes them popular for high-strength automotive fasteners. They cost more than electroplating.
A warning on high-strength bolts
Electroplating can introduce hydrogen into hardened steel. For property class 10.9 and 12.9 bolts this creates a risk of hydrogen embrittlement, a delayed brittle fracture after installation. If you need plated high-strength bolts, ask for post-plating baking (de-embrittlement) or choose a non-electrolytic coating such as zinc flake. A good supplier will raise this with you; if they don't, raise it yourself.
How to choose
- Indoor, dry, low cost: zinc plated.
- Machinery and appearance: black oxide (oiled).
- Outdoor structures: hot-dip galvanised, or stainless steel A2/A4 in coastal or chemical environments.
- High-strength automotive or critical joints: zinc flake.
Tell us the environment and the service life you expect, and we'll suggest the finish and get comparable quotes from Indian manufacturers. See also our fastener import guide.
Frequently asked questions
Which bolt coating lasts longest outdoors?
Hot-dip galvanising usually gives the longest outdoor life for carbon-steel bolts because of its thick zinc layer. In coastal or chemical environments, stainless steel A2 or A4 is often specified instead.
Why do HDG nuts need to be oversize?
The galvanised coating adds thickness to the bolt thread, so nuts are tapped oversize after galvanising to fit. HDG bolts and nuts should be ordered as matched sets.
Can 10.9 or 12.9 bolts be zinc plated?
They can, but electroplating creates a risk of hydrogen embrittlement. Ask for post-plating baking, or choose a non-electrolytic coating such as zinc flake.
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