Scope
Cadmium plating for fasteners has historically been specified by:
- QQ-P-416F — US Federal Specification "Plating, Cadmium (Electrodeposited)"
- AMS-QQ-P-416 — SAE AMS adoption of the federal spec
- MIL-STD-171 — Finishing of Metal and Wood Surfaces (historical reference)
QQ-P-416 was cancelled as an active federal specification but remains in use on legacy drawings and in MRO/depot repair of in-service aircraft and defense equipment. AMS-QQ-P-416 was the industry adoption of the federal spec and is the reference on many current defense drawings.
Why cadmium was the gold standard
Cadmium plating combined several hard-to-match properties:
- Exceptional corrosion resistance — 200+ hours salt spray for standard coatings; 500+ hours with chromate supplementary treatment
- Galvanic compatibility with aluminum — cadmium and aluminum have similar electrode potentials, minimizing galvanic corrosion at dissimilar-metal joints (a critical advantage on aluminum aircraft structure)
- Natural lubricity — the cadmium surface is naturally low-friction; K-factors are favorable without added lubricants
- Anti-galling — stainless-on-cadmium and titanium-on-cadmium thread pairs resist galling
- Ductility — doesn't crack or spall during forming, assembly, or thermal cycling
- Dimensional precision — thin coating (5–25 μm) doesn't interfere with precision threads
Across aerospace structural hardware, defense equipment, and precision mechanisms, cadmium plating became the default specification.
Classes and types per QQ-P-416
QQ-P-416 defines:
Classes (thickness):
- Class 1: 0.0005" (13 μm) — standard
- Class 2: 0.0003" (8 μm) — lighter
- Class 3: 0.0002" (5 μm) — very light (precision threads)
Types (supplementary treatment):
- Type I: plain / no chromate
- Type II: chromate conversion (clear, yellow, or olive drab)
- Type III: phosphate conversion (paint base)
Type II yellow (iridescent chromate) is what most legacy aerospace drawings specify — the distinctive yellow-gold cadmium-plated fastener appearance.
Hydrogen embrittlement
Cadmium plating, being electrolytically deposited, carries hydrogen embrittlement risk similar to zinc electroplating. High-strength steels require mandatory hydrogen relief baking:
- ≥ HRC 40: mandatory 375°F bake for 23 hours per QQ-P-416
- HRC 33–40: bake strongly recommended
On aerospace structural bolts (MS21250-series, NAS6700-series, etc.), every cadmium-plated fastener is baked and the bake is documented on the traceability package.
Why cadmium is being phased out
Cadmium is a known human carcinogen and environmental toxin:
- Occupational exposure — cadmium plating workers face elevated risk of lung disease and kidney damage. OSHA permissible exposure limits are stringent; containment and ventilation are expensive.
- End-user exposure — worn or damaged cadmium surfaces can release cadmium dust. Handling and machining cadmium-plated parts generates cadmium-contaminated debris.
- Environmental release — plating wastewater, air emissions, and disposal of cadmium-plated scrap all release cadmium into water and soil.
- Regulatory pressure — EU RoHS, REACH, ELV (automotive), and California Proposition 65 all restrict cadmium. Global automotive has essentially eliminated cadmium; aerospace is in late-stage phase-out for new programs.
The US DoD Qualified Products List (QPL) for cadmium plating has shrunk as shops either consolidate or close. Finding a qualified cadmium plating source for new orders is increasingly difficult and expensive.
Approved replacements
Modern aerospace and defense programs replace cadmium with:
| Replacement | Best for | Limitations |
|---|---|---|
| Zinc-nickel (AMS 2417) | High-strength steel aerospace hardware | Different galvanic behavior; requires engineering assessment for Al-contact |
| Aluminum IVD (AMS 2429 / 2430) | Aerospace fasteners | Cost; lower throughput |
| Zinc-flake coatings (F3019, F1136, F2833) | Structural and general aerospace | Newer in aerospace; qualification effort |
| Sermatel / SermaGard (diffusion alloys) | High-temperature aerospace | Specific-service applications |
| Modern stainless fasteners | Many structural applications | Weight; different galvanic behavior |
For any legacy drawing calling out QQ-P-416 cadmium on new production, the engineering question is always: is the cadmium callout essential, or is a current-spec replacement acceptable? The answer is engineering-assessment-specific but almost always points to a modern alternative.
Repair and MRO
For in-service aircraft and equipment that already has cadmium plating from original manufacture, field repair practices vary:
- Touch-up of minor coating damage per QQ-P-416 remains allowed on specific aircraft per MRO procedures
- Replacement of coated hardware with cadmium may require new cadmium-plated parts (the replacement logic preserves galvanic and mechanical consistency with the original assembly)
- Some MRO depots still maintain cadmium plating capability specifically for legacy fleet support
Applications (historical)
- Aircraft structural fasteners (MS21250, MS21297, NAS6700 series)
- Engine attachment hardware
- Defense vehicle and weapons-system bolts
- Aerospace landing gear hardware
- Legacy marine naval hardware
- Some industrial and agricultural hardware (mostly phased out by 1990s)
Related specifications
- AMS-QQ-P-416 — The current AMS adoption of the federal spec
- AMS 2417 — Zinc-nickel plating (replacement)
- AMS 2429 / AMS 2430 — Aluminum ion-vapor-deposition (IVD) plating (replacement)
- F3019, F1136, F2833 — Zinc-flake coatings (replacement)
- MIL-STD-171 — Finishes historical reference
- F519 — HE testing method
Documentation
California Fastener's position on cadmium plating: cadmium-plated hardware is available for legacy defense and aerospace MRO applications through qualified plating sources, but is not a recommended coating for new designs. For any new specification work, we recommend discussion of zinc-nickel, aluminum IVD, or zinc-flake alternatives based on the specific service environment, galvanic compatibility, and HE risk profile of the application.