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About This Tool
The SBS Finish Guide is an internal engineering reference for selecting, comparing, and specifying fastener surface finishes. Use the linked consensus standards on each finish card and in the table below for complete technical requirements and specification details.
Referenced Standards
| Standard | Description |
|---|
Engineering Best Practice
Finish Selection Decision Framework
A structured approach to selecting the right surface finish for fastener applications. Work through the decision criteria in order of priority to identify the best solution for your specific environment and requirements.
Step 1
Define the Environment
Start with the harshest condition the fastener will face throughout its service life.
- Indoor / dry: Minimal protection needed. Zinc clear or black oxide often sufficient.
- High humidity / mild outdoor: Zinc clear or yellow trivalent with sealant (96+ hr salt spray).
- Automotive / under-hood: Zinc Nickel or Geomet 321. Must survive thermal cycling.
- Marine / severe corrosion: Geomet 720+, Zinc Nickel 12ES, or stainless with passivation.
- Chemical / acid exposure: Consult SBS engineering. Organic coatings or speciality platings required.
Step 2
Confirm Base Material
The fastener material constrains which finishes are safe and effective.
- Low-carbon steel: All zinc electroplate options applicable. HE risk managed by baking.
- High-hardness / PC 10.9, 12.9: Avoid standard zinc electroplate. Use Geomet 321, Zinc-Tin, or mechanical zinc (no HE risk).
- Case-hardened tapping screws: Geomet 321 + ML preferred to prevent cold welding.
- Stainless steel: Passivation standard. Electropolish for galling risk. RIE 4037A for black aesthetic.
- Aluminum: Anodize. Never apply zinc electroplate directly to aluminum.
Step 3
Check Regulatory Requirements
Regulatory compliance can immediately eliminate options.
- RoHS / ELV (automotive, EU): Eliminates all Cr6 (hexavalent) finishes. Use trivalent (Cr3) or non-chrome alternatives only.
- REACH / SVHC: Cadmium and hexavalent chrome are SVHC. Avoid entirely unless military exemption applies.
- Torque-tension spec (K-factor): Dry film lubricants and topcoats (Geomet PLUS ML/VL) give tighter CoF range. Specify CoF target in callout.
- Food / medical / NSF: Consult SBS for FDA-compliant coating options.
Step 4
Assess Hydrogen Embrittlement Risk
HE can cause delayed fracture with no visible warning. It is critical for high-strength fasteners.
- HE occurs during: Acid pre-treatment (pickling) and electroplating. Atomic hydrogen diffuses into steel lattice.
- Most susceptible: Steels above 39 HRC (approx. Grade 10.9+), case-hardened, and spring steel.
- Prevention: Bake at 190–220°C within 4 hrs of plating (ASTM F519 / ISO 4042).
- Preferred alternatives: Geomet 321, Mechanical Zinc, Zinc-Tin. Zero HE risk from process.
- Note: Zinc-Nickel has lower HE risk than plain zinc due to plating permeability, but is not zero.
Step 5
Consider Torque and Assembly
Coating thickness and lubricity directly affect the torque-tension relationship.
- Thread tolerance: Thick coatings (HDG, heavy zinc) may require over-tapped nuts or special clearances.
- Coefficient of Friction (CoF): Dry zinc can have CoF 0.12–0.20. Wax or PLUS ML topcoats target 0.09–0.16.
- Galling risk (stainless): Use wax lube, nickel strike, electropolish, or Molykote for A2/A4 fasteners in stainless assemblies.
- Re-use: Geomet 321 with ML topcoat maintains CoF through multiple installation cycles. Plain zinc degrades.
Step 6
Balance Cost vs. Performance
Over-engineering is as costly as under-engineering. Match the finish to the actual service life requirement.
- $ (Zinc Clear 3–5µm): Interior, dry, non-critical. Short service life acceptable.
- $$ (Zinc Yellow with sealant, Mechanical Zinc): Outdoor, moderate exposure, 5–10 yr life.
- $$$ (Zinc Nickel, Geomet 321): Automotive, 10+ yr, or high-hardness requirements.
- $$$$ (Zinc Nickel 12ES, GEOBLACK): Premium aesthetics + extreme corrosion. Marine, luxury automotive, architecture.
Finish Performance Matrix
| Finish | Corrosion Resist. | HE Risk | RoHS | Cost | Best For |
|---|---|---|---|---|---|
| Zinc Clear (Cr3 Trivalent) | Yes | Yes | $$ | Interior, general purpose | |
| Zinc Yellow (Cr3 Trivalent) | Yes | Yes | $$ | Outdoor, moderate exposure | |
| Black Zinc (Cr3 + Sealant) | Yes | Yes | $$ | Black aesthetic, moderate protection | |
| Zinc Nickel Clear (12B+S) | Low | Yes | $$$ | Automotive, high-cycle assemblies | |
| Black Zinc Nickel (12ES) | Low | Yes | $$$ | Premium black, 1000+ hr protection | |
| Geomet 321 + ML Clear | None | Yes | $$$$ | High-hardness, automotive critical | |
| Mechanical Zinc (Clear) | None | Yes | $$ | Washers, HE-sensitive parts | |
| Hot Dip Galvanized | None | Yes | $$ | Structural, outdoor, large fasteners | |
| Passivation (Stainless) | None | Yes | $ | Stainless steel standard treatment | |
| Cadmium | Yes | No | $$$$ | Military only. Avoid in commercial. |
Key Trade-offs by Finish Type
Zinc Electroplate: Pros
Low cost, widely available from many suppliers
Wide range of colors and thicknesses available
Cr3 (trivalent) versions are RoHS compliant
Sealant topcoats push performance above 96 hr salt spray
Good base for paint and adhesive bonding
Zinc Electroplate: Cons
Hydrogen Embrittlement risk on hardened steel (>39 HRC)
Thinner sacrificial layer than Geomet or HDG
Performance degrades rapidly with scratches or damage
CoF variability makes torque-tension control difficult
Cr6 versions no longer acceptable in most applications
Zinc Flake (Geomet): Pros
Zero HE risk: safe for all hardness levels including 12.9
Excellent corrosion resistance: 720+ hr standard salt spray
Consistent CoF with topcoat selection (PLUS ML, VL, XL)
RoHS and REACH compliant, chrome-free
Good re-use performance vs. electroplate
Zinc Flake (Geomet): Cons
Premium cost relative to electroplate
Limited color options (silver/black; no true yellow)
Minimum batch sizes may be required by applicators
Fewer approved applicators than zinc electroplate
Washers may stick together during dip-spin process
Zinc Nickel: Pros
Superior corrosion resistance vs. plain zinc (500–1080 hr)
Black version combines aesthetics and performance
Reduced (not zero) HE risk due to plating permeability
Preferred in European automotive OEM specifications
RoHS compliant with trivalent passivation
Zinc Nickel: Cons
Higher cost than plain zinc electroplate
NOT recommended on PC 12.9 or case-hardened parts due to residual HE risk
Plating bath management is more complex than zinc
Fewer applicators than standard zinc plate
Nickel is SVHC in EU; process handling requirements apply
Quick Reference: Finish Selection by Application
| Application | Recommended Finish | Why |
|---|---|---|
| General hardware, Grade 5/8, indoor | Zinc Clear Cr3 (Fe/Zn 5AN) | Low cost, RoHS, adequate protection for environment |
| Outdoor structural, Grade 5/8 | Zinc Yellow Cr3 + Sealant (Fe/Zn 8CNS) | 96+ hr white rust, RoHS, visible yellow for inspection |
| Automotive body, Grade 10.9 | Geomet 321 + PLUS ML | No HE risk on high-hardness, 720 hr salt spray, controlled CoF |
| High-hardness tapping screw | Geomet 321 + PLUS ML | Prevents cold welding, no HE, consistent drive torque |
| Black premium aesthetic + protection | Black Zinc Nickel (Fe/Zn-Ni 8ES) | 840 hr salt spray, RoHS, premium black appearance |
| Stainless fastener, standard | Passivation (ASTM A967) | Restores oxide layer, removes free iron, standard requirement |
| Stainless in stainless assembly | Passivation + Wax Lube | Mitigates galling risk on stainless-to-stainless thread engagement |
| Washer (HE sensitive) | Mechanical Zinc (ISO 12683) | No HE from process, adequate protection, cost effective |
| Marine / salt water exposure | Stainless A4 + Passivation | Stainless provides inherent corrosion resistance without coating |
| Military / extreme environment | Consult SBS Engineering | Application-specific; cadmium, special organics, or specialty alloys |