BZP, hot-dip galvanised, sherardised and zinc flake compared
All four finishes are zinc, and what separates them is how the zinc is applied and how much of it ends up on the part. BZP is a few microns of electroplated zinc for indoor use. Hot-dip galvanising is a thick molten-zinc layer for exposed structural work. Sherardising is a diffused zinc layer that keeps thread dimensions. Zinc flake is a thin cured coating that suits high-strength fasteners. Process and thickness, not the metal, decide where each belongs.
This is the most expensive small decision in fixings, because the fastener costs pennies and replacing it after it fails costs access, downtime and labour. The comparison below covers protection, thread fit and the hot-dip clearance problem that catches out mixed stock.
- BZP is an indoor finish. Outdoors it shows white corrosion within months and red rust soon after.
- HDG nuts are tapped oversize after galvanising, so HDG nuts and plain bolts are a loose, unreliable fit.
- Sherardising is dimensionally predictable and thin classes take standard nuts, which is why rail and civils specify it.
- Zinc flake is not electroplated, so it avoids the hydrogen embrittlement risk on 10.9 and above.
- No zinc coating matches A4 stainless where chlorides are present.
Bright zinc plated (BZP)
Zinc deposited electrolytically and then passivated, giving the familiar bright, faintly blue finish. The coating is thin, typically only a few microns, applied under BS EN ISO 4042. That thinness is the commercial attraction and the technical problem.
- Where it belongs: indoors, in dry conditions, on light-duty and general assembly work.
- Where it fails: anything external. White corrosion products appear within months and red rust follows.
- Thread fit: no practical issue, since standard tolerances allow for the plating thickness.
- Watch for hydrogen embrittlement. Electroplating drives hydrogen into the steel. BS EN ISO 4042 sets the thresholds by hardness: at or below 360 HV no baking is necessary; above 390 HV baking is required; between the two it may be omitted where supplementary process verification is carried out. By property class that makes class 10.9 optional with testing and class 12.9 and above required.
Using BZP outdoors to save money is the most common false economy in fixings.
Hot-dip galvanised (HDG)
The part is immersed in molten zinc, producing a thick, metallurgically bonded and deliberately uneven coating with far more zinc on it than plating can deposit. Fasteners are covered by BS EN ISO 10684. The thickness is what gives the outdoor life, and it is also what creates a thread problem.
- Bolts are galvanised after threading, so zinc adds to the male thread.
- Nuts are therefore tapped after galvanising, oversize, to a special tolerance class, and carry an extra letter in the marking to say so.
- The nut thread itself is left effectively bare, protected by the zinc around it and usually by a lubricant.
- So an HDG nut on a plain or BZP bolt is a sloppy fit with reduced thread engagement, and a standard nut will generally not run onto an HDG bolt. The exception is a bolt threaded undersize before galvanising and marked U, which is made to take a standard nut. Otherwise buy HDG nuts and bolts as a matched pair, never from separate bins.
HDG suits exposed structural steelwork, external plant and civils. It is a coarse finish, it is not a precision one, and its use on higher property classes needs confirming rather than assuming.
Sherardised
Parts are tumbled in zinc powder in a sealed drum below the melting point of zinc, so the zinc diffuses into the steel surface instead of being deposited on top of it. The result is an even matt grey zinc-iron layer that follows the thread form closely rather than pooling in the roots.
- The advantage is dimensional. Sherardising is far more predictable than hot dip because the deposit is even and follows the thread form. It is not zero thickness: BS EN ISO 17668 offers classes from roughly 10 to 75 microns, and the thin classes usually take standard nuts while thicker ones need the thread sizing agreed with the processor. Confirm the class rather than assuming.
- Frequently specified in rail and civils for exactly that reason, and worth asking about whenever HDG thread clearance is causing trouble.
- Thickness is selectable by class, so the level of protection can be specified rather than accepted.
- Trade-offs: a matt grey appearance rather than a bright one, a batch process with a lead time, and fewer stockists than BZP or HDG.
Zinc flake
Zinc and aluminium flakes suspended in a binder, applied by dip-spin or spray and then cured in an oven rather than plated. Covered by BS EN ISO 10683 and sold under several proprietary names. Coatings are thin, in the same order as plating, yet reach salt spray performance many times that of BZP.
- No electrolysis, so no hydrogen is charged into the steel. That is why it is a common choice at property class 10.9 and above, where the alternative would be plating a hard fastener.
- Thin and dimensionally predictable, so thread fit stays manageable and no oversize tapping is needed.
- Topcoats can be added for friction control, colour coding or extra life.
- Less robust to mechanical damage than hot-dip zinc, and a factory finish rather than something applied to cut or modified parts on site.
Choosing between them
- Indoor, dry, general assembly: BZP.
- Exposed steelwork and external civils: HDG, bought as matched nut and bolt sets.
- External work where threads must stay to size, or where the specification calls for it: sherardised.
- High-strength fasteners, or where a thin high-performance coating is required: zinc flake.
- Chlorides, marine, coastal or de-icing salt: none of these on their own. Use A4 stainless.
Two cautions. A zinc-coated fastener in contact with a dissimilar metal can still set up galvanic corrosion, so the coating decision does not remove the material compatibility one. And coating and grade are independent: the finish decides how long the fastener survives the environment, while the property class decides what load it carries. Property classes are covered in our guide to choosing fasteners.
Common questions
Is BZP the same as galvanised?
Both are zinc, but they are not interchangeable. BZP is electroplated and only a few microns thick, so it is an indoor finish. Hot-dip galvanising immerses the part in molten zinc and leaves a much thicker layer that survives outdoors for years. Loose trade use of the word galvanised for anything zinc-coloured is a frequent cause of the wrong finish arriving on site.
Why will my nut not fit a hot-dip galvanised bolt?
Because the zinc has added thickness to the male thread. Under BS EN ISO 10684, the mating nut is tapped oversize after galvanising to clear that build-up and is marked accordingly. A standard nut will therefore bind or refuse to start on an HDG bolt, and an HDG nut on a plain bolt is loose with reduced engagement. Buy them as a matched pair.
Which zinc finish lasts longest outdoors?
Hot-dip galvanising, on thickness alone, because it puts far more zinc on the part than any of the alternatives and zinc protects sacrificially in proportion to how much there is. Zinc flake achieves very high salt spray figures at a fraction of the thickness and is better on precision threads, while sherardising sits between the two and holds thread dimensions.
What coating should be used on 10.9 and 12.9 fasteners?
Usually zinc flake. Electroplating charges hydrogen into hardened steel and creates a delayed brittle fracture risk. Under BS EN ISO 4042 baking is required above 390 HV and may be omitted with supplementary verification between 360 and 390 HV, so class 12.9 requires it while class 10.9 does not automatically. Zinc flake involves no electrolysis, so the risk is avoided rather than managed. Where a high-strength fastener does need plating, confirm the baking regime with the supplier before ordering.
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