Aerospace Sourcing
Special processes

Aerospace Surface Finishing: Specs, Callouts and Pitfalls

Most aerospace finishes are called out by a handful of specifications: MIL-PRF-8625 for anodize, MIL-DTL-5541 for chemical conversion coating, AMS 2700 for stainless passivation, AMS 2404 for electroless nickel, and MIL-PRF-23377 primer under MIL-PRF-85285 topcoat. A complete callout names the spec, type, class and any thickness, sealing or masking requirement.

Request a quote

Write the callout so nothing is left to default

Every one of these specs has defaults, and the defaults are rarely what the designer pictured. A note that says "anodize per MIL-PRF-8625" leaves the type, class, thickness and seal open. A note that says "chem film per MIL-DTL-5541" leaves the type and class open. The finisher will pick something defensible, and receiving inspection will then argue about whether it matches intent.

A complete finish callout has five parts: specification, type, class, thickness or coating weight where relevant, and masking. Add color for dyed anodize and paint, and add the sequence when more than one finish applies, for example chem film the bore, hard anodize the outside diameter, mask the threads.

FinishSpecKey selectionsTypical callout
Sulfuric anodizeMIL-PRF-8625Type II, Class 1 undyed or Class 2 dyedAnodize per MIL-PRF-8625, Type II, Class 2, black, sealed
Hard anodizeMIL-PRF-8625Type III, thickness, sealed or unsealedHard anodize per MIL-PRF-8625, Type III, Class 1, 0.0020 in nominal, unsealed
Chem filmMIL-DTL-5541Type I (hexavalent) or Type II (non hexavalent), Class 1A or 3Chem film per MIL-DTL-5541, Type II, Class 3
PassivationAMS 2700Method 1 (nitric) or Method 2 (citric), type, verification testPassivate per AMS 2700, Method 1 or 2
Electroless nickelAMS 2404Class by post plate heat treatment, thicknessElectroless nickel per AMS 2404, Class 1, thickness per drawing
Primer and topcoatMIL-PRF-23377, MIL-PRF-85285Primer class (C1, C2, N), topcoat colorPrime per MIL-PRF-23377, Class N, topcoat per MIL-PRF-85285, color per drawing

Anodize: MIL-PRF-8625

Anodize converts the aluminum surface into aluminum oxide. Type I is chromic acid anodize, Type II is sulfuric acid anodize and Type III is hard anodize. Class 1 is undyed and Class 2 is dyed. Do not confuse these with the architectural Class I and Class II designations, which are a different system.

Thickness is where most problems start. For Type III, the specification assumes 0.002 in (2.0 mils) when the drawing gives no thickness, and practitioners point out that it is on the customer to state the thickness wanted. Published typical ranges for Type II from finishers run a few tenths of a mil. Because roughly half the anodic layer grows into the base metal and half builds outward, a 0.002 in hard coat changes a diameter by about 0.002 in, and threaded or close tolerance features need either masking or pre-plate dimensions.

Hard anodize affects fatigue. Finishers report fatigue debits on the order of 30 to 50 percent on some high strength alloys, a figure that varies by alloy and test. For fatigue critical parts, the stress engineer should accept the finish explicitly. Alloy matters too: high copper alloys such as 2024 anodize less uniformly than 6061, and cast alloys with high silicon give darker, less consistent coatings. The aluminum alloys page covers anodize response by alloy.

Sealed or unsealed

Sealing closes the pores and improves corrosion resistance. Hard anodize used for wear is often specified unsealed because sealing lowers hardness. If the part will be painted or bonded, ask the finisher and your adhesive or paint data sheet which surface condition is required.

Chem film: MIL-DTL-5541

Chemical conversion coating is a thin film that improves corrosion resistance and paint adhesion while keeping the surface electrically conductive. It adds negligible dimension, which is why it is the default on aluminum electronics housings and bonding surfaces.

  • Type I uses hexavalent chromium and typically looks gold or brown. Type II contains no hexavalent chromium and typically looks clear. If the type is not specified, paragraph 3.1 of the spec defaults to Type I.
  • Class 1A is for maximum corrosion protection, painted or unpainted. Class 3 is a lighter coating for protection where low electrical resistance is required.
  • A testing laboratory summary of the Class 3 requirement gives a contact resistance limit of 5,000 microohms per square inch at 200 psi electrode pressure, and less than 10,000 microohms per square inch after 168 hours of salt spray. Confirm these against the current revision before using them for acceptance.
  • Products must be on the qualified products list, so the finisher should be able to name the QPL product used.

MIL-DTL-5541 Class 1A is also the pretreatment most commonly used under military aircraft paint systems unless the part manufacturer specifies otherwise.

Passivation: AMS 2700

Passivation removes free iron and other surface contamination from stainless steel so the chromium oxide layer can form uniformly. It does not change dimensions in any useful sense and it is not a coating. AMS 2700 replaced the cancelled federal spec QQ-P-35, and revision F (2018) is the edition cited in industry summaries.

Method 1 uses nitric acid and Method 2 uses citric acid. Either is allowed unless the purchaser specifies one. Method 1 has eight types that vary acid concentration, temperature and the use of sodium dichromate, including a type intended for free machining grades, and the standard defines five classes. Free machining stainless such as 303 and 416 is prone to pitting in aggressive baths, so let the processor choose the type for the alloy unless you have a reason not to.

Verification tests named in AMS 2700 and the parallel commercial standard ASTM A967 include water immersion, high humidity, salt spray, copper sulfate and a free iron test. Vendors often blur the two documents, so make sure the cert names the one your drawing calls out. Alloy selection guidance is on the stainless and specialty steels page.

Plating and hydrogen embrittlement

AMS 2404 electroless nickel is the preferred replacement for AMS-C-26074 (formerly MIL-C-26074). Its classes are defined by post plate heat treatment: Class 1 has no heat treatment other than embrittlement relief, and Classes 2, 3 and 4 are heat treated at progressively lower temperatures, roughly 450, 375 and 250 °F or above. Heat treatment raises hardness and improves adhesion on some substrates. Sources disagree on the current revision letter, so state the revision on the PO.

Any plating on high strength steel raises hydrogen embrittlement risk. AMS 2759/9 covers the relief bake, and ASTM B850 is the commercial guide, which selects the bake from the actual tensile strength of the steel. The plater needs the final heat treat condition to choose the cycle. See the heat treatment page for how the bake fits in the process sequence.

Primer and topcoat

The common military aircraft system is a MIL-PRF-23377 epoxy primer under a MIL-PRF-85285 polyurethane topcoat. MIL-PRF-23377 classes differ by inhibitor: C1 is barium chromate, C2 is strontium chromate and N is non chromate. Where low VOC limits apply, MIL-PRF-85582 water borne epoxy primer is substituted.

Paint failures are usually surface prep failures. Require the paint shop to record pretreatment, cure times, dry film thickness and an adhesion test on a witness panel or the part if allowed. If the drawing has masking requirements for grounding points or bearing bores, show them on a sketch rather than in words.

Nadcap and the cert package

Chemical processing is a Nadcap commodity under the AC7108 criteria, with sub-criteria for anodizing, electroplating, electroless plating, conversion coating, painting and dry film coatings, and solution analysis and testing. As with heat treat, read the certificate scope, and check whether your prime has its own approval list for finish processors.

A finish cert should show the specification and revision, type, class, thickness measurement method and result where applicable, seal status, bath or lot identification, and the embrittlement bake record for plated steel. Coating thickness on anodize is usually measured by eddy current, which is covered on the nondestructive testing page.

Get finishing sourced with the right paperwork

If you need anodize, chem film, passivation, plating or paint to a military or AMS spec, send the drawing and finish notes. We source the work from qualified processors, flow down the spec, revision and accreditation requirements, check the certs before parts ship, and a person replies within one business day. We sign a mutual NDA before files. Controlled drawings follow the process on controlled programs. Back to the processes hub.

Questions

What does MIL-PRF-8625 default to if the drawing gives no thickness?

For Type III hard anodize, the specification assumes 0.002 in (2.0 mils) when no thickness is called out, and finishers note that the customer is expected to specify the thickness wanted. For Type II there is no single number most engineers can rely on, so specify thickness when it matters for fit or wear. Remember that about half the coating grows into the base metal, which changes the dimensional allowance.

Should I specify MIL-DTL-5541 Type I or Type II?

Type I uses hexavalent chromium and Type II does not. Many programs are moving to Type II for environmental and worker safety reasons, but some legacy drawings and qualification data still assume Type I. If the drawing gives no type, paragraph 3.1 of the specification makes Type I the default. State the type explicitly and confirm your program accepts Type II before changing an existing callout.

What is the difference between Class 1A and Class 3 chem film?

Class 1A gives maximum corrosion protection and is the usual pretreatment under paint. Class 3 is a lighter film used where low electrical resistance is needed, such as grounding surfaces, EMI gaskets and electronics enclosures. Class 3 trades some corrosion resistance for conductivity. A lab summary gives the Class 3 contact resistance limit as 5,000 microohms per square inch, which should be checked against the current revision.

Nitric or citric passivation?

AMS 2700 allows either Method 1 (nitric) or Method 2 (citric) unless the purchaser specifies one. Citric is often preferred for safety and waste handling, while nitric types allow tuning for specific alloys, including free machining grades. Unless you have qualification data tied to one method, let the processor choose and require that the cert state the method, type and verification test used.

Does hard anodize hurt fatigue life?

It can. Finishers report fatigue debits of roughly 30 to 50 percent on some high strength aluminum alloys, and the effect depends on alloy, thickness and stress level. For fatigue critical parts, the stress analyst should approve the finish, and the drawing should limit hard anodize to the surfaces that need wear resistance. Masking and thinner coatings reduce the penalty.

When does a plated part need a bake?

When the base metal is high strength steel above the threshold in the governing specification. AMS 2759/9 covers relief baking, and earlier revisions applied it to steels heat treated to 180 ksi minimum or higher. ASTM B850 selects the bake from actual tensile strength. The plater needs the final heat treat condition to choose the cycle, and the bake must happen within the specified time after plating.

Related

Sources

Ready to
source it?

A person replies within 1 business day. NDA first.

Request a quote