Electro zinc plating, hot-dip galvanising, zinc flake, black oxide and A2 / A4 stainless fasteners compared — plus hydrogen embrittlement, galvanic corrosion and which finish to use where.
A bolt that rusts is not just ugly — it can seize, lose its preload or snap. Choosing the right finish for a fastener is as important as choosing its size and grade. This guide compares the common fastener coatings, explains hydrogen embrittlement and galvanic corrosion, and shows which finish to use where.
Carbon and alloy steel fasteners in grades 4.6 to 12.9 rust without protection. The coating has three jobs: protect the steel from corrosion, keep the thread working so the joint can be tightened and undone, and give a predictable friction so the tightening torque produces the right clamping force. A change of coating changes the friction, so the torque value must change too — something our bolt torque calculator takes into account.
For the basics of bolt, nut and screw types and grades, see our guide to types of industrial fasteners.
The most common finish for general fasteners. A thin layer of zinc — usually around 5–12 microns — is electroplated onto the part and then passivated. Modern passivates are trivalent chromium (clear/blue, iridescent or black); older yellow and olive finishes used hexavalent chromium, which is now restricted in many markets. Electroplating of fasteners is covered by ISO 4042.
The fastener is dipped in molten zinc, giving a much thicker coating — typically several tens of microns. ISO 10684 covers hot-dip galvanised fasteners. Because the coating is so thick, nuts are tapped oversize after galvanising so they still fit the coated bolt, which means HDG bolts must be used with matching HDG nuts.
A non-electrolytic coating of zinc and aluminium flakes in a binder, applied by dip-spin or spray and baked. ISO 10683 covers zinc flake coatings on fasteners. Well-known proprietary systems include Geomet and Dacromet-type coatings.
Common on socket head cap screws and machine parts. The black finish itself gives very little corrosion protection — most of it comes from the oil film. Suitable for indoor, oiled machinery, not for outdoor exposure.
Instead of coating carbon steel, the fastener can be made of stainless steel to ISO 3506:
Stainless threads can gall — the threads cold-weld and seize during tightening. Using anti-seize, tightening slowly and choosing a nut of a different grade or with a coating all help.
| Finish | Typical thickness | Corrosion resistance | Hydrogen embrittlement risk | Typical use |
|---|---|---|---|---|
| Plain / black oxide + oil | — | Very low | None | Indoor, oiled machinery |
| Electro zinc + passivation | 5–12 µm | Low–medium | Yes, for high-strength parts | Indoor panels and assemblies |
| Zinc flake | 8–15 µm | High | None from coating | High-strength, automotive, outdoor |
| Hot-dip galvanised | Several tens of µm | High | Low (larger sizes) | Structural, towers, outdoor supports |
| Stainless A2 | Solid material | High | None | Outdoor, food, general |
| Stainless A4 | Solid material | Very high | None | Coastal, marine, chemical |
Actual performance depends on the passivate, any sealer or top coat, the coating thickness achieved and the site conditions, so specify the coating system and test requirement rather than just "zinc plated".
Acid cleaning and electroplating can let atomic hydrogen into the steel. In hardened, high-strength fasteners — generally property class 10.9 and above, and hardened washers and spring parts — this hydrogen can cause sudden, brittle fracture hours or days after tightening, with no warning.
When two different metals touch in the presence of moisture, the less noble one corrodes faster. With fasteners, the area ratio matters most:
Keep joints dry, use insulating washers or sealants where metals differ, and match fastener material to the enclosure — see our guide to choosing enclosure material.
| Environment (ISO 9223 corrosivity) | Suggested fastener finish |
|---|---|
| C1–C2: indoor, dry or heated buildings | Electro zinc with passivation, black oxide + oil for machinery |
| C3: urban outdoor, sheltered industrial | Zinc flake, hot-dip galvanised or A2 stainless |
| C4: industrial areas and coastal areas with moderate salt | Hot-dip galvanised, high-grade zinc flake or A4 stainless |
| C5 / CX: high-salt coastal, offshore, aggressive industrial | A4 stainless; or duplex systems specified by the project |
Friction varies a lot between finishes: a dry zinc-plated bolt, a lubricated zinc flake bolt and a stainless bolt with anti-seize can need very different torques for the same preload. Using one torque chart for every finish either leaves joints loose or overstretches bolts. Use the finish-specific friction in our bolt torque & joint design calculator to get the right wrench setting.
VERCHO Engineering Solutions supplies nut bolts, studs, screws and washers in mild steel, high-tensile grades 8.8, 10.9 and 12.9, and A2 / A4 stainless steel, with the finish your site needs. Tell us the application, environment and grade, and we will recommend the right coating. Write to info@verchogroup.com.
Related reading: Indian Standards for fasteners and types of industrial fasteners.
For any question, please write to us at info@verchogroup.com — or send it straight from this page. Our engineering team will reply by email.
