Zinc Plating for Fasteners: Spec, Thickness, Salt Spray

Look at the coating column on almost any fastener purchase order and you will see two words: “zinc plated”. That is not a specification. Two suppliers can both quote “zinc plated” and deliver parts that survive 120 hours and 900 hours of neutral salt spray respectively — because coating thickness, passivation chemistry and the standard they are gauged against are all left open.

This guide is the factory-side view of zinc-based coatings for fasteners: which system delivers which corrosion performance, what each one does to thread fit, where hydrogen embrittlement becomes a safety issue rather than a footnote, and the exact wording to put on a purchase order so the coating you receive matches the coating you tested.

The six zinc-based coating systems you will be quoted

Almost every anti-corrosion coating in the fastener trade is a variation on one of these six. They are not interchangeable — they differ by an order of magnitude in cost and in protection.

System	Typical thickness	Protection mechanism	Governing standard
Electroplated zinc (barrel or rack)	5–12 µm	Sacrificial zinc + chromate conversion layer	ISO 4042 / ASTM B633
Zinc flake (Dacromet, Geomet, Delta-Tone)	8–12 µm	Zinc + aluminium flakes in an inorganic binder, plus topcoat	ISO 10683 / ASTM F1136
Hot-dip galvanizing (HDG)	45–85 µm	Thick zinc-iron alloy layers, metallurgically bonded	ISO 10684 / ASTM F2329
Mechanical zinc plating (peen plating)	5–75 µm	Cold-welded zinc powder, no electrolysis	ASTM B695
Zinc-nickel alloy	8–12 µm	Zinc with 10–15% nickel, much slower corrosion rate	ISO 4042 / automotive specs
Stainless A2 / A4 (passivated, no coating)	—	Passive chromium-oxide film, not sacrificial	ISO 3506

Note the last row. Stainless is not “a coating” — it is a different material answer to the same problem, and it behaves completely differently in service. We cover the material side in Standards & Materials; this article stays on coatings.

Thickness and salt spray hours: the number your customer actually bought

Corrosion performance is expressed as hours of neutral salt spray (ASTM B117) to first red rust — that is, the first sign of corrosion of the base steel, not the white rust of the zinc itself. White rust appears earlier and is normal; red rust is failure.

Coating	Thickness	Typical salt spray to red rust	Relative cost
Electroplated zinc, clear/blue trivalent	5–8 µm	96–200 h	Lowest
Electroplated zinc, yellow / iridescent trivalent	8–12 µm	200–400 h	Low
Mechanical zinc plating	5–75 µm	200–500 h	Medium
Zinc-nickel alloy	8–12 µm	720–1,000 h	Medium-high
Zinc flake (Dacromet / Geomet)	8–12 µm	500–1,000+ h	Medium-high
Hot-dip galvanized	45–85 µm	500–1,500+ h	High

Read this table as indicative, not as a warranty. Actual results depend on the coating supplier, the passivation being genuinely trivalent and correctly cured, and the salt spray cabinet itself. That is exactly why the specification must name the standard and the thickness class, not just the finish colour.

One practical trap: a buyer asks for “yellow zinc” believing yellow equals better protection. It usually does correlate with a thicker deposit and a different passivation, but the colour is a by-product of the passivation chemistry — not a measurable spec. Order Fe/Zn 8, trivalent yellow passivation to ISO 4042, and let the colour follow.

Trivalent vs hexavalent chromate — and why you should care

Zinc alone corrodes quickly in humid air. The chromate conversion layer on top is what gives electroplated zinc its real service life, and there are two families:

Hexavalent chromium (Cr6) — the traditional yellow/olive passivation, excellent performance, but hexavalent chromium is a classified carcinogen. It has been restricted under RoHS and the EU End-of-Life Vehicles Directive, and exemptions have largely expired. Sourcing it now creates a compliance liability for your customer.
Trivalent chromium (Cr3) — the current default. Clear/blue, yellow or black passivation, all produced without hexavalent chromium. Salt spray performance is close to Cr6 in most grades and fully compliant.

If your drawing or an old part number still says “Cr6”, the safe move is to request a trivalent equivalent and have the change documented. Suppliers who still offer Cr6 voluntarily are worth a second look at their compliance process as well as their pricing.

What coating does to thread fit

Every coating adds material to the thread flanks. ISO 965-1 sets the tolerance classes for uncoated threads, and the coating standards define the allowance that keeps the coated product within gauge. In practice:

Electroplated zinc, thin deposits (5–8 µm): threads are pre-made slightly under nominal so the coated part still passes the 6g/6h gauge. Normal practice, no special action from the buyer.
Zinc flake and mechanical zinc: thin and uniform enough that standard fits are generally preserved — one reason these systems are popular for precision parts.
Hot-dip galvanizing: 45–85 µm is far too thick for a standard fit. HDG fasteners follow a different scheme — bolts are tapped/rolled before coating and nuts are tapped oversize (6AZ / 6AX per ISO 965-5), so a standard nut and a galvanized nut are not interchangeable. This is why HDG orders must state both the bolt and the nut coating.

Our hot-dip galvanized hex bolts DIN 933 and the matching hot-dip galvanized square washers DIN 434 are supplied as a matched set for exactly this reason. Contrast that with our zinc plated threaded rod DIN 975/976, where the electroplated deposit is thin enough to keep standard metric fits intact.

Hydrogen embrittlement: the coating risk that is a safety issue

Electroplating generates atomic hydrogen that can be absorbed into the steel. On high-strength parts this hydrogen migrates to stress concentrations and causes sudden, brittle failure — often weeks after installation, with no visible corrosion.

The rule of thumb used across the industry: any fastener of property class 10.9 or above, or any hardened part above roughly 32 HRC, must be baked after electroplating — typically 190–230 °C for 4–24 hours, started within a few hours of plating. The relevant requirements sit in ASTM B850 and the embrittlement test methods in ASTM F1940 and ISO 15330.

There is a cleaner route for high-strength fasteners: choose a coating that involves no electrolysis at all. Zinc flake coatings and mechanical zinc plating avoid the hydrogen-exposure mechanism entirely, which is why zinc flake has become the default on automotive class 10.9 and 12.9 hardware. Our DIN 912 socket head cap screws, class 12.9, are coated on this principle.

Choosing a coating by service environment

Service environment	Practical recommendation
Indoor, dry, controlled	Electroplated zinc 5–8 µm, trivalent clear
Indoor, humid or urban	Electroplated zinc 8–12 µm, trivalent yellow; or zinc flake
Outdoor, industrial / urban	Zinc flake, or hot-dip galvanizing for long service life
Coastal / marine chloride exposure	Zinc flake, hot-dip galvanizing, or A4 (316) stainless
Chemical, food, pharmaceutical	A2/A4 stainless, passivated — zinc-based coatings are unsuitable
Class 10.9 / 12.9 high-strength	Zinc flake or mechanical zinc to avoid hydrogen embrittlement

Watch for galvanic coupling as well. Zinc coatings sacrifice themselves to protect steel, but contact with stainless or aluminium in a wet environment can accelerate attack on the less noble metal. If your assembly mixes materials, say so in the enquiry — the coating choice may need to change.

Non-zinc alternatives worth knowing

Three finishes come up often enough to recognise, even if they are not zinc:

Black oxide — a conversion coating with almost no sacrificial protection; it is an appearance and light-oil-retention finish, not a corrosion finish. See our black oxide eye bolts DIN 444.
Phosphate — usually a base for paint or oil, common on drywall screws and internal parts.
Passivated stainless — for A2/A4 parts, the “coating” question becomes a passivation and pickling question. Compare our stainless steel eye bolts DIN 444 A2/A4 with the zinc plated forged eye bolts DIN 444 — same geometry, very different service envelope.

If you want a direct comparison of the three most common heavy-duty systems, our earlier article on hot-dip galvanizing vs electro-galvanizing vs Dacromet goes deeper on the trade-offs between them.

How to write the coating callout on a purchase order

Four elements turn “zinc plated” into a specification: the standard, the thickness class, the passivation type, and the salt spray requirement. A clean line item looks like this:

Hex Bolts — M12 × 60, DIN 933, Class 8.8, Carbon Steel, Zinc Plated to ISO 4042, Fe/Zn 8, Trivalent Blue Passivation (Cr6-free), Salt Spray ≥ 200 h to red rust per ASTM B117, EN 10204 3.1 Certificate, Bulk MOQ 500 pcs

Swap the coating block for the system you actually need — “Zinc Flake to ISO 10683, 8–10 µm, Salt Spray ≥ 720 h” or “Hot-Dip Galvanized to ISO 10684, 45 µm minimum, oversize tapped nuts included”. For anything at class 10.9 and above, add one further line: “Hydrogen embrittlement relief per ASTM B850”.

Verifying the coating on arrival

Coating thickness — magnetic gauge per ISO 2178, measured on flats away from threads and edges; take readings at several points.
Salt spray report — a third-party ASTM B117 report stating hours to red rust, not a marketing claim on the packing list.
Thread gauging — GO/NO-GO gauges for the class on the drawing, checked after coating, not before.
Appearance — uniform coverage, no bare patches, no white corrosion product on delivery.
Embrittlement record — for class 10.9 and above, the bake temperature and duration on the process record.
Material certificate — EN 10204 3.1 remains the B2B default, and it should quote the coating standard alongside the steel grade.

Every one of these is a standard document we issue with the shipment rather than a favour you have to negotiate for.

Coating recommendations by product family

In practice the same coating logic repeats across our range, so here is how it maps onto the products buyers most often ask about:

Anchors for concrete — zinc plated carbon steel wedge anchors for indoor work, hot-dip galvanized concrete expansion bolts for outdoor and coastal, and A2/A4 stainless wedge anchors for wet or corrosive environments.
Hollow wall and elevator fixings — zinc plated hollow wall anchors and zinc plated elevator anchor bolts, both supplied with a trivalent passivation as standard.
Special coating requests — for non-standard combinations we keep a dedicated range under special coatings fasteners.

Across the wider catalogue, our bolts, expansion bolts and rigging hardware families are all available in the coating systems above. You can browse the full range on the products page or look at everything we have written on the topic under surface treatment & coating.

Get the coating right before the tooling is cut

Coating is the last variable to be decided and the first to be blamed. Naming the standard, the thickness class, the passivation and the salt spray target on the PO costs one extra line and removes almost every dispute at the receiving dock — and it is the only way to compare two suppliers’ quotes on equal terms.

Rongrun Fasteners manufactures and exports fasteners with electroplated zinc, zinc flake, hot-dip galvanizing, mechanical zinc, zinc-nickel and stainless A2/A4 finishes from an ISO 9001:2015 certified factory, with full OEM/ODM support, bulk MOQ, custom packaging and EN 10204 3.1 traceability. Send us your drawing, the service environment and the quantity — we will come back with the coating recommendation, price, lead time and certificate in one working day. Contact our engineering team here.

Zinc Plating for Fasteners: Thickness, Salt Spray & Specification