Black Oxide vs Dacromet vs Zinc Plating: How to Choose a Fastener Finish
A bolt's finish is not decoration. It decides how long the fastener survives its environment, how consistently it tightens, and — at high property classes — whether the treatment process itself can damage the bolt. Yet finish is often the last line filled in on a drawing, sometimes copied from the previous order without a second thought.
We offer four surface treatments for custom fasteners: black oxide, zinc plating, colour zinc plating, and Dacromet. This guide explains what each one actually does, where each belongs, and the order in which to decide.
Start with the environment, not the finish name
Every finish decision should begin with three questions:
- Where does the bolt live? Indoor and dry, indoor and humid, outdoor sheltered, outdoor exposed, or in contact with chemicals, fertiliser, or road salt?
- How long must it last, and what does failure look like? Surface rust on a machine frame is cosmetic; a seized or weakened wheel fastener is not.
- What property class is the bolt? At class 10.9 and especially 12.9, the treatment process itself becomes a selection factor (see the note on hydrogen below).
A finish chosen without the environment is a guess. The same zinc-plated bolt that serves for years indoors can be the wrong answer outdoors.
The four finishes
Black oxide
Black oxide is a conversion coating: the surface of the steel is chemically converted to a black oxide layer. It adds almost no thickness, so dimensions and thread fit are essentially unchanged, and it gives a uniform matte black appearance.
- Choose it for: indoor, dry, or regularly oiled environments — machine internals, tools, fixtures, and assemblies where appearance matters and corrosion exposure is minimal
- Do not choose it for: outdoor or humid service on its own. Black oxide provides only mild corrosion resistance and relies on the oil or wax applied over it; unprotected, it is the weakest of the four against rust
Zinc plating
Electroplated zinc is the general-purpose finish for fasteners. The zinc layer protects the steel sacrificially — it corrodes preferentially — and it is the finish most buyers mean when they say a bolt is "galvanised-looking" on a standard order (true hot-dip galvanizing is a different, much thicker process, and is not among our standard treatments).
- Choose it for: general machinery, indoor and sheltered outdoor use, and the broad middle of applications where moderate corrosion protection at moderate cost is the right balance
- Watch for: white rust (zinc corrosion product) appearing before red rust in long humid storage — cosmetic first, but a sign the layer is being consumed
Colour zinc plating
Colour zinc is zinc plating with a coloured chromate passivation layer on top, giving the familiar yellow-iridescent appearance. The passivation adds protection over plain zinc and makes the finish easy to identify at a glance — useful when two otherwise similar bolts must not be mixed on an assembly line.
- Choose it for: the same environments as zinc plating where additional passivation protection or visual identification is valuable, including automotive and equipment applications
- Do not choose it for: applications with restrictions on hexavalent-chromate-type passivation in your market's regulations — if your product must meet a specific environmental regulation, state it in the inquiry so the finish can be reviewed against it
Dacromet
Dacromet is a zinc-aluminium flake coating, applied as a liquid and baked rather than electroplated. Two properties make it the high-demand option in our range: it provides the strongest corrosion protection of the four, and because it is not an electroplating process, it avoids the hydrogen-embrittlement risk that electroplating creates for high-strength fasteners.
- Choose it for: outdoor exposure, automotive and wheel fasteners, agricultural and construction equipment, and high property classes (10.9, 12.9) that need strong corrosion protection
- Do not choose it for: indoor dry applications where black oxide or zinc already does the job — you would be paying for protection the bolt will never need
Side-by-side
| Finish | Corrosion protection | Best environment | Dimensional effect | Notes |
|---|---|---|---|---|
| Black oxide | Low (relies on oil/wax) | Indoor, dry, oiled | Negligible | Best appearance; weakest rust defence |
| Zinc plating | Moderate | Indoor, sheltered outdoor | Thin electroplated layer | General-purpose default |
| Colour zinc | Moderate-plus | Indoor, sheltered outdoor | Thin layer + passivation | Easy visual identification |
| Dacromet | Highest of the four | Outdoor, exposed, automotive | Thin baked flake coating | No electroplating hydrogen risk; preferred for 10.9/12.9 with corrosion exposure |
A note on numbers: we deliberately do not publish a single "hours of salt spray" or "years of protection" figure for these finishes. Corrosion performance depends on coating condition, environment, and maintenance, and a single number quoted without those conditions would be a slogan, not engineering data. If your application has a specific corrosion test requirement, state it in your inquiry and it will be reviewed against your actual conditions.
The hydrogen question, once more
As covered in our property class guide, high-strength fasteners (class 10.9 and above) are sensitive to hydrogen introduced during electroplating; without correct process control this can lead to delayed, sudden fastener failure. This is why the class decision and the finish decision belong together: a 12.9 bolt that needs serious corrosion protection is a strong argument for a non-electrolytic coating such as Dacromet, or for a plating route that has been explicitly agreed with the manufacturer.
A practical selection order
- Indoor, dry, appearance matters → black oxide.
- General machinery, indoor or sheltered → zinc plating; choose colour zinc if passivation or line-side identification adds value.
- Outdoor exposure, automotive, agriculture, or high class with corrosion risk → Dacromet.
- Regulated market or a specific corrosion test to meet → say so in the inquiry; the finish gets reviewed against the requirement instead of assumed.
Common mistakes
- Copying the finish from the last order after the application or environment changed.
- Asking for "galvanised" without saying which process you mean — electroplated zinc and hot-dip galvanizing differ greatly in thickness, thread fit, and price; if you mean hot-dip, say so explicitly so it can be reviewed.
- Choosing black oxide for outdoor use because it looks good in the sample photo.
- Specifying a high class and a plated finish independently, without raising the hydrogen question.
FAQ
Which finish lasts longest outdoors?
Of the four treatments we offer, Dacromet provides the highest corrosion protection and is our standard recommendation for exposed outdoor service.
Can you match the finish of a bolt we already use?
Usually, if the existing finish can be identified. Send a sample or clear photos with your inquiry, and describe the environment — the review will confirm the match or recommend the closest of our four treatments.
Does the finish change the price much?
Finish is a real cost factor, and Dacromet sits above the zinc options, which sit above black oxide. Exact differences depend on part size and quantity — another reason the finish belongs in the first inquiry, not added later. Our checklist of what to send for a custom bolt quote shows where to state it.
What finishes can HyHaven apply?
Black oxide, Dacromet, colour zinc plating, and zinc plating, across our custom fastener range in property classes 4.8 to 12.9, from 2,000 pieces. Send your drawing via the custom fasteners page with the environment described, and the review will confirm the finish with the quotation.
HyHaven manufactures custom fasteners to your drawing or sample, with black oxide, Dacromet, colour zinc and zinc finishes, from 2,000 pieces. Request a quote.