A fastener's base material sets its strength. The surface finish determines how long it survives the environment it lives in. Getting the finish wrong introduces corrosion risk, potential hydrogen embrittlement in high-strength steel, and galvanic incompatibility with adjacent structure. This guide covers the five finishes found on the vast majority of aerospace fasteners, what the governing specifications actually require, and the failure modes associated with incorrect or missing finish treatments.
Why Finish Selection Matters
Two specific failure modes are finish-driven and not intuitive until you've seen them in service:
- Galvanic corrosion: When two dissimilar metals contact in the presence of an electrolyte (water, fuel, hydraulic fluid), current flows from the anode to the cathode. The anode corrodes preferentially. A steel bolt through an aluminum fitting in a wet compartment, with no sealant and no compatible finish, will corrode the aluminum fitting much faster than either material would corrode alone.
- Hydrogen embrittlement: High-strength steel fasteners (above approximately 150,000 psi) are susceptible to hydrogen uptake during electroplating processes. Hydrogen diffuses into the steel lattice and can cause delayed brittle fracture under sustained tensile load — sometimes hours or days after installation. The part looks fine, torques normally, and then fractures under service stress at a load well below its rated tensile strength.
Both of these failure modes are process-controlled, not visible in the finished hardware without testing. The certifications and process documentation that accompany properly sourced aerospace hardware are how you verify the finish was applied correctly and that bake-out requirements were met.
Aerospace Fastener Finishes Comparison
| Finish | Spec | Base Material | Salt Spray (hrs) | Notes |
|---|---|---|---|---|
| Cadmium Plate Type I | AMS QQ-P-416 | Alloy Steel | 96–200 | Standard AN bolt finish (no chromate) |
| Cadmium Plate Type II | AMS QQ-P-416 | Alloy Steel | 200–500 | Chromate conversion coat over cadmium |
| Passivation (stainless) | AMS 2700 / ASTM A967 | CRES 300-series | 200+ | No dimensional change; restores passive layer |
| Sulfuric Anodize | MIL-A-8625 Type II | Aluminum | 168 | Dye-able; moderate wear resistance |
| Hard Anodize | MIL-A-8625 Type III | Aluminum | 300+ | High wear resistance; adds ~0.001" |
| Zinc Phosphate | MIL-DTL-16232 | Alloy Steel | 48 | Primer base coat only; not standalone protection |
| Dry Film Lubricant | MIL-PRF-46010 | Various | N/A | Anti-galling on titanium/SS threaded joints |
Cadmium Plate — AMS QQ-P-416
Cadmium plating is the standard corrosion protection for AN alloy steel hardware. The specification has two types and three classes that determine deposit thickness and supplementary treatment:
- Type I: No supplementary treatment after plating. The cadmium deposit itself provides corrosion protection. Minimum salt spray resistance approximately 96 hours before white corrosion products appear.
- Type II: Chromate conversion coating applied over the cadmium deposit. The chromate film adds corrosion inhibition and gives the surface a faint yellow-gold tint. Salt spray resistance extends to 200–500 hours depending on deposit thickness and chromate treatment quality. Standard AN hardware is typically Type II.
- Class 1: 0.0002" minimum deposit — decorative/thin. Class 2: 0.0005" minimum — general purpose, standard for structural hardware. Class 3: 0.0008" minimum — maximum protection, used in severe environments.
Cadmium plate hazards to know: cadmium softens and oxidizes above approximately 450°F, making it unsuitable for exhaust-adjacent hardware. Cadmium oxide fumes produced during welding or cutting are toxic — ventilate thoroughly and use respiratory protection. Cadmium is not compatible with certain fuels and some hydraulic fluids — verify engineering data when specifying cadmium-plated hardware in direct fluid contact.
Passivation of CRES — AMS 2700
Passivation is a chemical treatment, not a coating. It removes free iron contamination from the stainless steel surface using a nitric acid or citric acid bath, then allows the natural chromium oxide passive layer to form uniformly across the surface. No material is added — there is no dimensional change and no change in visual appearance after processing.
- The passive chromium oxide layer is what gives stainless steel its corrosion resistance. If that layer is disrupted by machining, grinding, or contamination with carbon steel particles (from tooling), the underlying iron is exposed and will rust.
- Passivation is required after any machining, grinding, or forming operation on CRES hardware to restore the passive layer
- ASTM A967 specifies the test methods for verifying passivation quality (copper sulfate test, ferroxyl test, water immersion). AMS 2700 is the aerospace-qualified version of the same process.
- Request AMS 2700 passivation certs on CRES fasteners being used in certified aircraft — this documents that the passive layer is intact and the corrosion protection is what you expect
Anodize — MIL-A-8625
Anodizing is an electrochemical process that converts the aluminum surface into aluminum oxide. Unlike plating (which deposits material), anodizing grows the oxide layer outward and inward from the original surface — roughly half the layer thickness is additive, half is formed by conversion of base metal.
- Type II (Sulfuric Anodize): 18–25 micrometers typical layer thickness. 168-hour salt spray minimum. Dye-able: can be colored gold, black, clear, or any standard color for part identification. Hardness is moderate — adequate for most airframe hardware applications.
- Type III (Hard Anodize / Hardcoat): 25–75 micrometer layer, grown using lower temperature and higher current density than Type II. Significantly harder surface (Rockwell C 60–70 range equivalent). Typical dimensional addition is approximately 0.001" per surface — must be accounted for in close-tolerance applications. 300+ hours salt spray. Cannot be dyed due to dense pore structure.
- Aluminum fasteners (AN series with "A" suffix, two raised dashes on head) are typically anodized. Hard anodize is used on aluminum structural components in shear or wear applications.
- Important for electrical bonding: Anodize is electrically insulating. If you anodize hardware that is part of a bonding path, the bond resistance will be unacceptably high. Bond connections require bare metal contact — apply sealant around the bond point, not through it.
Galvanic Compatibility
When you install a fastener through a structure, you're creating a metal-to-metal contact that will be exposed to varying degrees of moisture throughout the aircraft's life. The galvanic couple between fastener and structure determines whether accelerated corrosion will occur at that interface.
- Cadmium and aluminum: Compatible — both fall in Group A per MIL-STD-889. Standard cadmium-plated AN bolts are designed to work with aluminum structure.
- CRES (300-series) and aluminum: Borderline compatibility — wet contact can preferentially corrode the aluminum at the interface. Wet-sealant assembly (PR-1422 or equivalent) or electrical isolation tape is recommended for CRES fasteners in aluminum structure in wet compartments.
- Titanium and carbon fiber composite: Excellent — titanium is the preferred fastener material for carbon fiber composite structure precisely because of this galvanic compatibility.
- Titanium and aluminum: Compatible (similar galvanic potential in the table).
- Carbon steel and aluminum: Serious galvanic couple — corrosion of the aluminum accelerates significantly in the presence of moisture. This is why bare carbon steel fasteners are not used in aluminum airframe work without cadmium plating or other protection.
For certified repairs involving mixed materials, the applicable structural repair manual or engineer's data governs the approved material combination. MIL-STD-889 provides the reference table; use it to evaluate combinations not explicitly covered in the repair documentation.
Hydrogen Embrittlement Caution
High-strength steel fasteners (above approximately 150,000 psi tensile, Rockwell C 40 and above) are susceptible to hydrogen embrittlement from electroplating processes. During cadmium electroplating, hydrogen ions in the plating bath can be driven into the steel lattice by the electrochemical process. Once absorbed, the hydrogen migrates to high-stress regions of the part — thread roots, head-to-shank fillet radii — where it reduces ductility and enables crack initiation at loads well below normal tensile limits.
- The required remedy is thermal baking: 375°F (190°C) for a minimum of 8 hours after plating, per AMS 2759/9. This drives the diffusible hydrogen out of the steel before it can cause damage.
- NAS close-tolerance bolts at 160,000 psi and above require documented bake-out after cadmium plating. Request process certifications specifically calling out AMS 2759/9 compliance when ordering high-strength hardware.
- A bolt that was not properly baked after plating will look, measure, and torque identically to one that was. The difference only appears under sustained tensile load — and the failure mode is delayed brittle fracture, not a clean ductile yield that would give you warning.
- For surplus hardware: the 8130-3 from the original PAH implicitly covers compliance with all applicable process specifications for that part number. A missing bake would have been a production non-conformance documented at manufacture. This is one of the reasons documented traceability to a PAH matters for high-strength hardware specifically.