⚡ Key takeaways
- The finish decides the life of the fastener, not the fastener itself. The same class 8.8 bolt rusts within three years indoors in blue-white zinc, and survives ten years outdoors in zinc-nickel. The wrong finish wastes a good bolt.
- Salt spray hours are the hard specification. Indoor cabinets need 96 h, outdoor cabinets 480 h, chassis 720 h, battery packs 1,000 h. Set the requirement from the environment.
- Electroplating is cheap but brings hydrogen risk. Above class 10.9, plated bolts must be baked or they fail in delayed fracture. Hydrogen-sensitive parts should go straight to zinc flake.
- Zinc flake (Geomet) is the no-hydrogen option. Water-based coating, no electrolysis, no hydrogen, over 1,000 h of salt spray and 300 °C resistance: the chassis and EV default.
- Zinc-nickel (12–15 % Ni) is the best all-rounder. 720–1,000 h, low hydrogen risk, good appearance, dyeable black or grey: the mainstream choice for EV battery packs.
- Stainless is its own finish. 304 indoors, 316 on the coast, no plating needed — but stainless galls, so lubricate.
- PVD and DLC are the premium tier. A black DLC coating on moving parts, eyewear and medical devices gives very low friction at five to ten times the cost of zinc.
Contents
2. Electroplated zinc: blue-white, yellow, black
3. Zinc-nickel: the EV mainstream
4. Zinc flake / Geomet: no hydrogen
5. Anodising: for aluminium only
6. Black oxide: the cheapest option
7. PVD and DLC: premium moving parts
8. Stainless, plain and passivated
9. Hydrogen embrittlement: the invisible killer
10. Decision tree
1. Why the finish matters so much
Eighty percent of fastener failures are not fracture but corrosion. A bolt that snaps is usually already in the late stages: rust expands the thread, preload falls and vibration finishes the job. A finish does four things:
- Physical barrier: keeps steel away from air and water.
- Sacrificial protection: zinc is more active than steel and corrodes first.
- Appearance: visible parts need a consistent colour.
- Function: lower friction (DLC), conductivity (nickel), insulation (anodising).
2. Electroplated zinc
The most common and cheapest finish, with modest performance.
| Type | Appearance | Salt spray | Cost | Use |
|---|---|---|---|---|
| Blue-white zinc | Silver, slightly iridescent | 48–96 h | Low | Indoor cabinets, furniture, general sheet |
| Yellow (rainbow) zinc | Rainbow | 120–200 h | Low | Older automotive standards, indoor and outdoor |
| Black zinc | Black | 96 h | Low | Visible parts, black equipment |
| Black zinc over black nickel | Deep black | 200 h | Medium | Outdoor visible parts |
3. Zinc-nickel (12–15 % Ni): the EV mainstream
Adding 12–15 % nickel to the zinc deposit transforms the performance:
- 720–1,000 h of salt spray, five to ten times plain zinc.
- Resists 200 °C.
- Low hydrogen risk because the bath sits near neutral pH.
- Dyeable black, grey or blue with good appearance.
- Stable torque coefficient.
It is the standard for EV battery packs, chassis and outdoor cabinets.
4. Zinc flake / Geomet: no hydrogen
Zinc flake is a water-based coating of zinc and aluminium flakes with no electrolytic step, so there is no hydrogen at all.
- Over 1,000 h of salt spray.
- Resists 300 °C.
- No hydrogen risk, so class 12.9 is safe.
- Thin (5–8 μm), so thread tolerances barely change.
Drawbacks: poor conductivity, so unsuitable for grounding; two to three times the cost of zinc; cannot be welded.
5. Anodising: for aluminium only
Anodising applies to aluminium parts (see the aluminium fastener guide):
- Natural: 5–10 μm, indoor.
- Sulphuric: 10–20 μm, dyeable.
- Hard: 25–50 μm, wear and galling resistant.
- Chemical conversion: 1–3 μm, gold, for grounding.
6. Black oxide: the cheapest option
Black oxide grows a 1–3 μm layer of Fe₃O₄ on steel:
- Salt spray under 24 h; oiling is mandatory.
- Very low cost.
- Even black appearance.
- Use: internal engine parts, firearms, traditional black finishes.
7. PVD and DLC: premium moving parts
| Coating | Colour | Friction coefficient | Hardness (HV) | Relative cost | Use |
|---|---|---|---|---|---|
| PVD TiN | Gold | 0.4 | 2,000 | 5× | Cutting tools, decorative |
| PVD TiCN | Blue-grey | 0.3 | 3,000 | 6× | Wear parts |
| DLC | Black | 0.1 | 3,000 | 8× | Moving parts, eyewear, medical, premium screws |
DLC's very low friction suits threads that must run smoothly and resist seizure: spectacle hinge screws, high-end medical instruments, tooling screws.
8. Stainless, plain and passivated
Stainless is effectively its own finish:
- 304 for indoor and damp environments, no plating.
- 316 for coastal, chemical and medical.
- Passivation is still required to remove free iron and rebuild a dense oxide film.
9. Hydrogen embrittlement
Hydrogen embrittlement is the most insidious high-strength failure: the bolt assembles perfectly and fractures months later with no warning.
Three conditions must coincide
- Strength above 1,000 MPa, so class 10.9 and above.
- A hydrogen source: plating, pickling or phosphating.
- Tensile stress in service.
Prevention
- Bake at 190–220 °C for 4–8 hours after plating.
- Use zinc flake, which involves no electrolysis, for sensitive parts.
- Never use acid pickling for de-rusting.
10. Decision tree
- Dry indoor? Blue-white zinc, or black oxide with oil.
- Outdoor or damp? Zinc-nickel at 720 h, or 304 stainless.
- Coastal or chemical? 316 stainless or zinc flake.
- Class 10.9 and above? Zinc flake, or plate plus bake.
- Grounding required? Nickel plating or plain stainless.
- Black appearance? Black zinc indoors, DLC for premium.
- Above 200 °C? Zinc flake or stainless.
FAQ
Q. Zinc flake or electroplated zinc? Zinc flake has no hydrogen risk and over 1,000 h of salt spray but does not conduct and costs two to three times more. Electroplating is cheap but carries hydrogen risk.
Q. Can class 10.9 and above be electroplated? Avoid it. If it is unavoidable, bake at 200 °C within four hours and for four hours. Zinc flake is the better answer.
Q. Where does DLC belong? High wear and low friction: tooling screws, injection moulding machine parts, premium consumer products. Cost is high.
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