Metallic coatings in one place
Where to buyA fastener or a part carries a coating for one of five reasons: to stop corrosion, to be a paint base, to control torque and stop seizing, to survive heat, or to wear. Most aircraft coatings are metallic or metal-derived: a plated metal, an oxide grown on aluminium, a conversion film. This guide puts them side by side with their specification, what each is for, and the figures that have a public source.
It sits next to the Alodine and MIL-DTL-5541 page, which covers the conversion coat bath in detail, and it feeds the electrical bonding guide, where the question is which coatings conduct. Figures that no public document confirms are left out and listed under the limits.
Metallic coatings at a glance
| Coating | Specification | What it is for | Figures with a public source |
|---|---|---|---|
| Cadmium plating | AMS-QQ-P-416 (legacy QQ-P-416) | Sacrificial corrosion protection on steel; paint base; no stick-slip on threads | Not recommended above 450 °F (232 °C, our conversion). Classes by minimum thickness: 0.0005, 0.0003 and 0.0002 in (about 13, 8 and 5 µm) |
| Zinc plating | ASTM B633 (legacy QQ-Z-325) | Sacrificial, like cadmium; little used on aircraft structure | Less compatible with aluminium than cadmium |
| Zinc-nickel | AMS 2417 | The common cadmium replacement | Provisional: nickel range, temperature types and bake rule are not confirmed here, so no figures are given |
| IVD aluminium | MIL-DTL-83488 | Ion vapour deposited aluminium: a non-toxic alternative to cadmium on steel | Classes by minimum thickness: 0.0010, 0.0005 and 0.0003 in (0.026, 0.013 and 0.008 mm) |
| Conversion coat | MIL-DTL-5541; material MIL-DTL-81706 | Chemical conversion on aluminium (Alodine, Iridite): corrosion resistance and a paint base; Class 3 keeps the electrical resistance low | 168 h of 5 % neutral salt spray (ASTM B117) required |
| Anodizing | MIL-PRF-8625 | An oxide film on aluminium, a good paint base, can be dyed | Types I and IB: 0.5–18 µm (0.00002–0.0007 in). Type II: 1.8–25 µm (0.00007–0.001 in) |
| Passivation | AMS 2700; ASTM A967, A380 | Stainless steel: removes free iron and restores the oxide film. Not a coating | Nitric or citric acid, ambient to 60 °C (140 °F, our conversion), usually 20 to 30 minutes or more |
| Chromium and nickel | QQ-C-320; QQ-N-290 | Barrier and wear coatings on high-strength steel, such as landing-gear journals | Light buffing only, never below the minimum thickness |
| Silver plating | AMS-QQ-S-365 | Locknuts at higher temperature; resists galling | Resists heat to 1,400 °F (760 °C, our conversion) |
| Black oxide | MIL-DTL-13924 | Minimal corrosion protection; needs oil or wax | Class 1 for carbon and low-alloy steel, Class 4 for 300-series stainless |
| Dry film lubricant | MIL-PRF-46010 | Controlled torque; over cadmium or silver it stops thread seizing | Cure: Type I 150 ± 3 °C (302 °F), Type II 204 ± 3 °C (399 °F), °F our conversion |
| Thermal spray | Process, no single specification | Flame, wire-arc, plasma, HVOF and cold-spray coatings | About 20 µm to several mm thick. Aerospace uses are not confirmed here |
US specifications state inches and °F; microns and °C are conversions or are printed by the source next to the inch value. Salt-spray hours are hours, not lengths. The specification text governs.
Cadmium plating
Cadmium is an electrodeposited, silver-gray coating with very good corrosion protection, particularly in a salty atmosphere. It is sacrificial: the cadmium corrodes instead of the steel. White, brown or black mottling is normal and is not removed for appearance, and a wire brush has no place on it. A plated part cannot be restored in the field: touch up with primer or a temporary preservative. Over the plate, the finishes are a chromic clear dip (bright), an olive-drab dichromate or an iridescent dichromate (yellow to brown).
Heat. AC 43.13-1B does not recommend cadmium above 450 °F (232 °C, our conversion): above that the cadmium diffuses into the base metal and embrittles it, a known problem with nuts. Plating is specified by type (I as plated, II with a chromate treatment, III with a phosphate treatment for paint) and by class, the minimum thickness: Class 1 is 0.0005 in (about 13 µm), Class 2 is 0.0003 in (about 8 µm) and Class 3 is 0.0002 in (about 5 µm).
Hydrogen embrittlement. High-strength steel parts are baked after plating, replating included; the drawing and the specification set the temperature and time. Brush (selective) plating can cadmium-plate ultrahigh-strength steel without hydrogen embrittlement. Cadmium is toxic and is being phased out; on fasteners it gives corrosion resistance at low thickness, no stick-slip (so a reliable torque), compatibility with aluminium and a paint base.
Zinc and zinc-nickel
Zinc coatings protect like cadmium, by sacrifice, and get the same corrective treatment; there is little zinc on aircraft structure. ASTM B633 types are I as plated, II with a coloured chromate and V with a phosphate. Zinc is less compatible with aluminium than cadmium.
Zinc-nickel (AMS 2417) is the usual cadmium replacement. It is dull to bright silver-gray. Putting it on an existing drawing in place of cadmium needs the customer's engineering approval. The nickel content, the service-temperature types, the salt-spray hours and the hydrogen-embrittlement bake rule are quoted differently by different sources and are not confirmed, so this guide gives none of them: read AMS 2417 and your drawing.
IVD aluminium
Ion vapour deposited aluminium (MIL-DTL-83488) is applied in a vacuum chamber and is used as a non-toxic alternative to cadmium on steel. Type I is as coated and Type II has a supplementary chromate treatment, which is the default when none is specified. The classes are minimum thicknesses: Class 1, 0.0010 in (0.026 mm); Class 2, 0.0005 in (0.013 mm); Class 3, 0.0003 in (0.008 mm). Class 1 is chosen where the tolerance allows it, Class 2 if Class 1 does not fit dimensionally, and Class 3 for close-tolerance parts such as fine threads.
The salt-spray hours quoted for these classes in some tables are not on the public text of the specification, so they are not published here.
Chemical conversion coat on aluminium
MIL-DTL-5541 (formerly MIL-C-5541) covers the chromate conversion coat on aluminium; the material is qualified under MIL-DTL-81706. Type I is hexavalent chromate and Type II is non-hexavalent (trivalent or non-chromate), the RoHS and REACH route, at the same performance requirement of 168 hours of 5 % neutral salt spray (ASTM B117). Class 1A gives the best corrosion resistance, painted or not; Class 3 gives corrosion protection with a low electrical resistance, the contact resistance being set by the order. Colour alone does not prove the process, because dyes exist: Alodine 1200S is light gold at 1 minute and golden brown at 3 minutes.
In aircraft practice a conversion coat goes on immediately after corrosion is removed, wet for 2 to 5 minutes, then rinsed, with hidden areas rinsed thoroughly. Magnesium gets a chrome-pickle treatment, and steel a phosphate (Parkerizing, Bonderizing, Granodizing) before painting. The bath, times and the 48 hour rule are on the Alodine page.
Anodizing
Anodizing grows an electrolytic oxide film on aluminium: corrosion resistant, a good paint base and open to dye. MIL-PRF-8625 (formerly MIL-A-8625) defines Types I, IB, IC, II, IIB and III, and Class 1 (not dyed) and Class 2 (dyed). Type I and IB (chromic acid) are 0.5 to 18 µm (0.00002 to 0.0007 in), thin, soft and slightly self-healing, a pretreatment for paint; Type II (sulfuric) is 1.8 to 25 µm (0.00007 to 0.001 in). Both need sealing. Type III is the hard coat, thicker than Type II, with sealing optional; the sources disagree on its range, so no range is given here.
Sealing is done in boiling water at 96 to 100 °C, mid-temperature at 70 to 80 °C, or cold; hot-water sealing cuts abrasion resistance by about 20 %. AC 43.13-1B covers the special treatment of anodized surfaces and the galvanic anodizing treatment (paragraphs 6-36 and 6-133). An anodic film does not conduct: strip it where a joint is bonded (see the bonding guide).
Passivation, chromium, nickel, silver, black oxide and the rest
Passivation (AMS 2700, which replaced AMS-QQ-P-35 in 2005; ASTM A967 and A380) cleans free iron from stainless steel and restores the passive oxide film in nitric or citric acid, then neutralise, rinse and dry. It is not a coating, it does not help carbon steel and it shows no visible change. Austenitic steels get their corrosion resistance from that film: do not remove it because it is discoloured.
Chromium (QQ-C-320) and nickel (QQ-N-290) protect high-strength steel as a barrier, so a break destroys the protection and rework is limited to light buffing, never below the minimum thickness. A metal that corrodes slower than steel, such as chromium, relies on a tight coating; one that corrodes first, such as zinc or cadmium, tolerates cracks. Electroless nickel can replace much hard chrome.
Silver (AMS-QQ-S-365) goes on locknuts for higher temperature: it resists heat to 1,400 °F (760 °C, our conversion), has excellent lubricity and resists galling. A molybdenum disulfide dry film over cadmium or silver prevents thread seizing (MIL-PRF-46010). Black oxide (MIL-DTL-13924) gives little protection and needs oil or wax. Thermal spray is a family of processes (flame, wire arc, plasma, HVOF and cold spray) for coatings from about 20 µm to several mm; its aerospace uses are not confirmed in the sources used here.
Finish letters in fastener numbers
| Letter | Usually means | Where |
|---|---|---|
| W | Cadmium plated | NAS1919 and NAS1921 blind rivets: cadmium on the rivet sleeve. NAS1398 and NAS1399 write CW for stainless, cadmium plated |
| P | Lubricated | NAS1919 and NAS1921. Some supplier suffixes use P for passivated: that is a supplier convention, not a standard |
| J | Chemical conversion coat (Alodine type) | ASNA solid rivets in 2017A-T4 (DX) and 2117 (DC) |
| No letter | Depends on the standard | Many NAS bolts imply cadmium (QQ-P-416 Type II) when no finish letter is added; NAS1580 listings cite cadmium plus chromate, or passivation, depending on the material |
No single public table of dash letters exists for NAS and MS fasteners: each standard defines its own, and distributor code schemes are theirs. Check the standard of the part. Nut finish colour and plating identify the application (a cadmium nut is gold per the FAA handbook); consult the IPB for the lock nut type.
Questions people actually ask
Is cadmium plating still used on aircraft fasteners?
Yes, on a great many existing designs: it resists corrosion at low thickness, gives a reliable torque and is compatible with aluminium. It is toxic and being phased out, and changing the coating on an existing drawing needs engineering approval.
What temperature can a cadmium-plated nut take?
AC 43.13-1B does not recommend cadmium above 450 °F (232 °C). Above that the cadmium diffuses into the base metal and embrittles it. Silver-plated locknuts are used for higher temperatures, up to 1,400 °F (760 °C).
What is the difference between Class 1A and Class 3 conversion coat?
Class 1A gives the best corrosion resistance, painted or unpainted. Class 3 protects against corrosion while keeping the electrical resistance low, which is why it is used where a joint is bonded.
Is zinc-nickel a drop-in replacement for cadmium?
It is the common replacement, but not a drop-in one: a change on an existing drawing needs the customer's engineering approval, and the bake rule for high-strength steel depends on the process route. Follow AMS 2417 and the drawing.
Does anodizing conduct electricity?
No. The anodic film is an insulator. A bonded joint has the film removed where the current passes, and the bare aluminium is treated with a conductive chemical film within 24 hours.
Why does aluminium corrode next to a carbon fibre panel?
Carbon is cathodic against aluminium, so in a wet joint the aluminium is the anode and corrodes. Cadmium plating behaves the same way. Isolate the joint with sealant and use a titanium fastener, usually Ti-6Al-4V.
Limits — when NOT to use this
What the sources support, and no further.
- Figures come from public summaries, not from the specification texts. AMS-QQ-P-416, MIL-DTL-5541, MIL-PRF-8625, AMS 2417 and AMS 2700 were not read in full. The specification and the drawing govern.
- Not published, because no public document confirms it: the salt-spray hours of the IVD classes; the nickel range, temperature types, salt-spray hours and bake rule of zinc-nickel; the thickness range of Type III anodizing (the sources disagree); the aerospace uses of thermal spray.
- Legacy names. QQ-P-416, MIL-C-5541 and MIL-A-8625 became AMS-QQ-P-416, MIL-DTL-5541 and MIL-PRF-8625, so the revision letters in vendor tables are out of date.
- Not covered. Process control, bath chemistry (except through the Alodine page) and the finish tables of specific connector families.
Where to buy
Distributors coming soon.
Sources
- FAA AC 43.13-1B, Change 1, Chapters 6 and 7 — Cadmium and its limit at 450 °F, finishes over cadmium, silver plating to 1,400 °F, chromium and nickel plating, chromate conversion coat practice, anodizing treatments (paragraphs 6-36, 6-133), nut finishes.
- FAA-H-8083-31B, Aviation Maintenance Technician Handbook: Airframe — Nut colour and finish, carbon fibre against aluminium (chapter 7), corrosion treatment.
- Alumiplate, MIL-DTL-5541 approvals page (vendor summary of the specification) — Types, classes and the 168 hour salt-spray requirement of the conversion coat.
- MIL-DTL-83488D, third-party mirror — IVD aluminium types and the thickness of Classes 1 to 3.
- Wikipedia: Cadmium plating; Anodizing; Chromate conversion coating; Passivation; Thermal spraying — Secondary summaries of the specifications for cadmium classes, anodizing types and thicknesses, sealing, passivation and spray processes. The specification texts were not read.
- Distributor finish guides (MW Components, Eugene Fastener) — Finish-code conventions and legacy specification names. Vendor conventions, not standards.