Zinc-flake coating
Zinc-flake is a paint-like coating of zinc flakes, often with aluminium, that is spun or sprayed on and cured in an oven instead of plated. No electric current and no acid are involved, so it is the coating the bolting standards turn to for high-strength steel. It is applied by outside Houston coaters.
Degrease, blast, coat, cure.
- STEP 01
Degrease
Parts are alkaline-cleaned or vapour-degreased.
- STEP 02
Blast
Shot blasting removes rust and scale. Acid cleaning and phosphating are left out, so the process adds no hydrogen to the steel.
- STEP 03
Coat
Bulk parts are dipped in a basket and spun to throw off the surplus (dip-spin). Others are dipped and drained, or sprayed.
- STEP 04
Cure
The coating is baked to set its binder. The parts must be able to take baking temperatures up to 626 °F (330 °C) without harm.
- STEP 05
Build up and top-coat
Further coats are applied and cured the same way, plus a sealer or a top coat with built-in lubricant where the system has one.
- Acid cleaning or phosphating before coating brings the hydrogen risk back. Specify blast cleaning for hardened parts.
- Heat: zinc-coated bolting should stay below about 390 °F (210 °C) in service (ASTM A193, A194).
- Twist-off (tension-control) bolt assemblies: coatings must be agreed with the bolt maker and applied under its direction (ASTM F3125).
- Thickness and salt spray life vary from system to system. Quote the system, not a generic figure.
Finishes are applied by outside coaters and platers in Houston, to the specification on the order.
No hydrogen from the coating, a little room in the nut.
No current and no acid are used, so the coating process itself puts no hydrogen into the steel, provided the parts are not acid-cleaned or phosphated first. That is why ISO 10683 points to zinc-flake for fasteners of 1000 MPa and above. It is also why ASTM F3125 lists a zinc-flake coating (ASTM F3393) as the qualified coating for its 150 ksi grades, after hydrogen embrittlement testing.
The coating still takes up thread space. Nuts are tapped oversize before coating, by about half the galvanizing allowance: 0.009 in. on the diameter for 1/2-13, 0.012 in. for 1-8 and 0.014 in. for 1-1/2-6 (ASTM A563 Table 5). Overtapped nuts for 150 ksi bolts are then proof-loaded to at least 175 ksi (ASTM F3125).
Top coats can carry a lubricant, and ASTM F3393 classes a coating by corrosion, appearance and coefficient of friction, so the friction class can be named on the order. Thickness and salt spray life depend on the system, so they are taken from the coating system's data.
Designations and classes.
How to call it out.
- Name the specification and classes, for example zinc-flake to ASTM F3393, corrosion D, appearance 1, friction R (the codes ASTM F3125 lists for structural bolting), or the ISO 10683 system on your drawing.
- Say whether the nuts are coated too. They are tapped oversize before coating (ASTM A563 Table 5).
- For F3125 Grade A490, the coating is applied by the bolt manufacturer, with its performance agreed in writing (ASTM F3125 Annex A1).
The tests behind it.
Coating thickness and salt spray
Coating thickness is measured with a magnetic or eddy-current gauge, or by X-ray fluorescence for thin platings, and checked against the class ordered. Salt-spray exposure checks corrosion protection on a sample from the lot when the specification calls for it.
Will a plated high-strength bolt crack in service?Hydrogen embrittlement relief and testing
High-strength steel can pick up hydrogen during pickling and plating and fail days later under load. Parts above 39 HRC are baked after plating to drive the hydrogen out, and a sustained-load test on fasteners from the lot proves the relief worked.
Will the nut carry the bolt's full load?Proof load testing of nuts
The nut is threaded on a hardened mandrel and loaded to the proof stress for its grade and size. It passes if the threads neither strip nor break and it can be removed by hand afterwards; a wrench may start it, for up to half a turn.
Zinc-flake coating: common questions.
Does zinc-flake cause hydrogen embrittlement?
The coating process adds no hydrogen, as long as the parts are blast-cleaned rather than acid-cleaned or phosphated. That is why it is used on 10.9, 12.9 and 150 ksi bolting.
What happened to ASTM F1136 and F2833?
ASTM withdrew both in 2025. ASTM F3393 consolidates and replaces them, together with F3019.
Are the nuts coated too?
They can be. Nuts for zinc-flake bolts are tapped oversize before coating so they run on the coated bolt: 0.012 in. on the diameter for a 1-8 nut (ASTM A563).
Can it go on A490 bolts?
Yes. ASTM F3125 lists zinc-flake to F3393 as the qualified coating for its 150 ksi grades, applied by the bolt manufacturer.
Compare with.
Hot-dip galvanizing
Parts are dipped in molten zinc, which alloys with the steel to form a thick, tough coating with a long life outdoors. The coating is far heavier than electroplating, so nuts are tapped oversize after galvanizing to run on the coated bolt.
Economical sacrificial protection for indoor and mild serviceZinc electroplating
A thin electroplated zinc layer with a clear, yellow or black chromate, or a chromium-free passivate. Zinc corrodes in preference to the steel underneath, so a scratch does not rust. It is the general-purpose finish for hardware that lives indoors or under cover.
Legacy aerospace and military finish, on requestCadmium plating
A soft, lubricious sacrificial coating with excellent salt-fog resistance and low galvanic action against aluminium. Its use is restricted by environmental rules, so we supply it only where a drawing or military specification still calls for it.