Aerospace Finishes: Types, Standards, and How to Specify for Your Parts

An aerospace part can hit every dimension on the drawing, and the finish still decides whether it survives the service environment. Get that right at the drawing stage, and the rest of the program runs smoother.

The finish callout is where it starts. A complete callout, a finish the substrate accepts, and a NADCAP requirement confirmed early all keep tooling, lead time, and the first article on track.

The hard part is knowing which finish the part actually needs and how to say it clearly on the drawing. That’s a judgment call we make often, and it pays off most when it happens before the part is cut.

The sections below cover the common aerospace finish types, the standards that govern them, and a framework for selecting and calling out the right finish. The principles apply whether the part is machined, sheet metal, or cast.

What Makes Aerospace Finishing Different?

Every aerospace finish comes with a traceable process, a documented thickness range, and an acceptance standard that the finisher must demonstrate compliance with. That traceability separates aerospace finishing from general industrial coating work.

The finish performance on a given part depends on more than coating chemistry alone. The substrate material, part geometry, surface preparation before coating, process control during application, and post-coat inspection all influence the result. Two parts coated in the same finishing line can perform differently if one carries residual machining oil or if the anodize bath chemistry drifts between runs. Finish selection is a system decision, not just a coating choice.

Types of Aerospace Finishes

The table below summarizes the most commonly specified aerospace finishes by substrate, thickness, hardness, governing specification, salt spray performance, and typical application. Each finish family is covered in detail below.

Anodizing

Anodizing is an electrochemical process that converts the aluminum surface itself into a hard, porous aluminum oxide layer. It is not a coating applied on top. The oxide grows from the base metal, and that distinction matters for tolerancing: roughly half the layer grows into the substrate and half grows outward, with variation depending on the alloy and process parameters. If you are machining a shaft to a tight OD tolerance before hard anodizing, the pre-anodize machining dimension has to account for that outward growth. Missing that step produces oversized parts after finishing.

Type II sulfuric acid anodizing produces a coating of 5 to 25 microns. It provides corrosion resistance, dye acceptance for color coding, and moderate wear resistance, per MIL-A-8625 Type II. Housings, brackets, and non-wear surfaces across commercial and defense programs are the typical candidates.

Type III hard anodizing builds a thicker layer of 25 to 75 microns with surface hardness in the 400 to 600 HV range, depending on the alloy, which puts it in the territory of some hardened steels, per MIL-A-8625 Type III. Pistons, valve bodies, sliding contact surfaces, and aluminum parts that see repeated mechanical engagement are typical applications.

Applicable substrates are aluminum alloys, primarily 6061, 7075, and 2024. High-copper and high-silicon alloys anodize with less uniformity and can produce soft spots or color variation. Confirm alloy compatibility with the finishing house before committing to a hard anodize callout on an unusual grade.

Chemical conversion coating

Chemical conversion coating forms a thin protective film on aluminum or magnesium through a chemical reaction at the surface. The film is typically under 1 micron, so it does not meaningfully change part dimensions. Its primary roles are corrosion protection and paint adhesion.

Chromate conversion coatings, sold under trade names like Alodine and Iridite, fall under MIL-DTL-5541. Class 1A provides corrosion protection. Class 3 gives low electrical resistance for grounding surfaces. Non-chromate alternatives based on trivalent chromium or zirconium chemistry are specified under MIL-DTL-81706, and their use is increasing as REACH and similar regulations restrict hexavalent chromium in new designs.

Electroless nickel plating

Electroless nickel is an autocatalytic chemical plating process that deposits a nickel-phosphorus alloy layer without electrical current. Because there is no current involved, coating thickness is uniform across the entire surface, including bores, recesses, and complex internal features. That uniformity is why electroless nickel appears so often on hydraulic fittings and sensor housings, where irregular plating would create weak points.

For aerospace applications, the governing spec is AMS 2404. MIL-C-26074 covers general electroless nickel use. Thickness ranges from 5 to 75 microns, depending on the application. As-deposited hardness sits in the 500 to 700 HV range; heat treatment at around 400°C can push this to 900 to 1000 HV for applications that demand serious wear resistance.

Phosphorus content selection matters: medium phosphorus works for general corrosion and wear; high phosphorus provides maximum corrosion resistance in acidic or chloride environments; low phosphorus delivers the highest hardness for non-corrosive service. Specifying the wrong phosphorus grade for the operating environment is one of the more common errors on electroless nickel callouts.

Cadmium plating and alternatives

Cadmium is an electroplated sacrificial coating that protects steel substrates through galvanic action. The cadmium corrodes preferentially, shielding the base metal beneath. Per QQ-P-416 or AMS-QQ-P-416, thickness runs 5 to 25 microns.

Cadmium is restricted under REACH and regional regulations, and new aerospace designs commonly specify alternatives when the program allows. Legacy defense and aerospace programs still mandate cadmium for specific part numbers, and replacing it requires formal re-qualification. Common alternatives include zinc-nickel plating per AMS 2417, IVD aluminum per MIL-DTL-83488, and tin-zinc.

High-strength steel parts require hydrogen embrittlement relief baking after plating, typically per ASTM F519 or AMS 2759/9. Baking adds at least 23 hours to the finishing cycle at 190 to 205°C, depending on the steel’s tensile strength. For prototype schedules, this baking requirement alone can add a full day to the overall part lead time.

Passivation

Passivation is a chemical treatment with nitric or citric acid that removes free iron and surface contaminants from stainless steel, restoring the natural chromium oxide film that gives stainless steel its corrosion resistance. Per ASTM A967 or AMS 2700. The process does not change dimensions or surface appearance in any significant way.

If a stainless aerospace part leaves the shop without passivation, the surface carries free iron from the machining process that can initiate pitting under service conditions. Passivation is often the final step in the finishing sequence after machining and any preceding processes, and it is common on aerospace stainless parts, fluid handling components, fittings, and fasteners.

Primer and topcoat systems

Primer and topcoat systems are multi-layer organic coating systems. A typical stack-up is a chromate or non-chromate primer plus a polyurethane or epoxy topcoat, covering airframes, fairings, and any surface exposed to UV, rain, chemical de-icing fluids, and aerodynamic loads.

Primer specs include MIL-PRF-23377 (epoxy chromate) and MIL-PRF-85582 (non-chromate). Topcoat: MIL-PRF-85285 (polyurethane). Total system thickness runs 75 to 200 microns, depending on the spec and number of coats.

Interior cabin components require flame-retardant coating systems per FAR 25.853. These cabin-rated systems are a separate specification family from exterior coatings. Confusing the two on a drawing callout creates a compliance problem that surfaces during cabin certification testing.

Thermal spray coatings

Thermal spray is a family of processes, including plasma spray, HVOF, and flame spray, that deposit metallic, ceramic, or cermet coatings by propelling heated particles onto the part surface. Two processes dominate aerospace applications.

HVOF tungsten carbide produces coatings with hardness in the 1000 to 1400 HV range at thicknesses of 50 to 500 microns, per AMS 2448. HVOF tungsten carbide has replaced hard chrome on landing gear cylinders and hydraulic actuator rods across many programs, driven by both performance gains and regulatory pressure on hexavalent chromium.

Plasma-sprayed yttria-stabilized zirconia is the standard thermal barrier coating for turbine blades and combustor liners, per AMS 2447, at operating temperatures above 1000°C. NADCAP accreditation is standard for aerospace thermal spray facilities.

Dry film lubricants

Dry film lubricants are solid lubricant coatings based on PTFE, MoS2, or graphite, applied as thin films for low-friction or anti-galling service, per MIL-PRF-46010 or SAE AS1701. Thickness typically sits at 5 to 25 microns. Coefficient of friction ranges from 0.03 to 0.15 depending on type.

Common applications include threaded fasteners, sliding interfaces, and actuator components, particularly on titanium and stainless steel parts, where galling is a real and expensive risk during assembly torque-down.

Aerospace Finish Standards and Specifications

Understanding which standard governs which finish type and how that standard appears on an aerospace drawing is foundational to specifying finishes correctly.

MIL and AMS specification families

MIL specs govern most established aerospace finishes. MIL-A-8625 covers anodizing. MIL-DTL-5541 covers chromate conversion. MIL-C-26074 covers electroless nickel. QQ-P-416 covers cadmium plating. MIL-PRF-23377 and MIL-PRF-85285 cover primer and topcoat systems. Each spec defines process parameters, acceptable thickness ranges, adhesion tests, and acceptance criteria, including salt spray performance.

AMS specs, issued by SAE International, cover newer finishes and updated procedures. AMS 2404 governs electroless nickel for aerospace. AMS 2417 covers zinc-nickel plating. AMS 2447 and AMS 2448 cover thermal spray processes. On a typical aerospace drawing, the engineer references the spec number, the class, and any type designators that apply to the specific part.

NADCAP accreditation

The National Aerospace and Defense Contractors Accreditation Program, administered by the Performance Review Institute, audits and accredits suppliers performing special processes, including plating, anodizing, chemical processing, and thermal spray.

When a finish appears on a Boeing, Airbus, or defense prime drawing, the finishing supplier must hold current NADCAP accreditation for that specific process. Verifying this belongs at supplier qualification, not at first article inspection. Discovering an accreditation gap at FAI means the parts need to be refinished at an accredited shop, adding cost and weeks to the schedule.

How finishes appear on aerospace drawings

The standard callout format follows a consistent pattern: spec number, class, type, and thickness range. For example, MIL-A-8625 Type III, Class 1, 0.0010 to 0.0020 inch. The drawing should also specify masking zones if multiple finishes appear on the same part and any post-finish baking requirements, such as hydrogen embrittlement relief.

Incomplete or ambiguous callouts are among the most common causes of finishing rework. The mistakes below account for the majority:

• Calling “anodize” without specifying Type, Class, or thickness range—the finisher defaults to whatever is most convenient
• Specifying a finish without noting whether dimensions apply pre- or post-coating, which creates conflicting inspection results
• Omitting masking zones when multiple finishes appear on the same part
• Missing hydrogen embrittlement relief baking requirements on high-strength steel callouts
• Referencing an outdated spec revision when the program requires the current edition

How to Specify Aerospace Finishes for Your Parts

Finish specification works best as a structured decision rather than a preference. The five steps below move from operating requirements through to a drawing callout, in the order that produces the most reliable finish selections.

Step 1: Identify the operating environment

Start with the service conditions the part will face. Temperature range, chemical exposure, salt spray or marine atmosphere, UV exposure, sliding contact, and electrical requirements each narrow the finish options differently. A part inside a pressurized cabin has different finishing needs from one mounted on an external airframe surface or sitting inside an engine hot section.

Starting from environment requirements rather than a preferred finish avoids a common trap: selecting a coating first and reverse-engineering the justification afterward. Environment-first selections hold up better at qualification testing.

Step 2: Match the substrate to compatible finishes

1. Aluminum:Anodize Type II or III, chemical conversion, electroless nickel, primer plus topcoat over conversion coating
2. Steel: Cadmium or zinc-nickel plating, electroless nickel, primer plus topcoat. Hydrogen embrittlement relief baking is required for high-strength grades after any electroplating process
3. Stainless steel: Passivation is the baseline for most aerospace stainless steel applications. Electroless nickel or thermal spray may be added for specific wear or shielding requirements
4. Titanium: Specialized anodizing using different chemistry from aluminum anodizing, electroless nickel, and dry film lubricants for anti-galling at fastened joints. Titanium galls aggressively against itself and against steel during assembly, so fastener interfaces almost always need a lubricant coating

Step 3: Check OEM and program specification requirements

Many aerospace programs dictate specific finishes by part number or drawing callout. Boeing, Airbus, and defense primes each maintain approved process lists and qualified supplier lists. For a part that replaces an existing one, match the legacy finish spec unless a formal substitution has been approved through the program’s engineering change process. For a new design, verify the program-level material and process standards before specifying a finish that falls outside the approved list.

Step 4: Evaluate cost and lead time trade-offs

The finishing process and complexity drive the cost more than the material alone. The tiers below give a reference for planning prototype and production schedules.

• Anodizing and passivation: fast and relatively low cost; typically adds 1 to 2 days to the finishing cycle
• Electroless nickel and chemical conversion: moderate cost and lead time; typically adds 2 to 4 days, including post-plate baking
• Thermal spray and specialized plating: high added cost and schedule; factor these in separately when planning overall part lead time
• Cadmium plating with hydrogen embrittlement relief baking: at least 24 hours total.

Build finish lead time into the total part schedule from the start, particularly for prototyping programs where parts often move through finishing one at a time rather than in batches. A prototype quoted at 5 days for machining can become 8 or 9 days delivered when finishing, inspection, and shipping are included.

Step 5: Confirm with the supplier at the DFM stage

A DFM review should include finish selection alongside machining feasibility. Confirm that the supplier’s finishing partners hold the required NADCAP accreditations and can meet the spec’s acceptance criteria. Confirm masking requirements for any zone-specific finishes. For CNC-machined parts that also require finishing, the DFM review is the point at which the two processes get aligned, and dimensional sequencing is confirmed. Catching a finishing-incompatible geometry at DFM costs a drawing revision. Catching it at first article inspection costs rework, refinishing, and schedule delays.

Where Aerospace Finishes are Applied on Real Parts

Mapping finish types to real part categories helps engineers move from general knowledge to specific drawing decisions. The four categories below cover the major aerospace component families and their typical finish requirements.

Structural and airframe components

Aluminum skins, frames, fittings, and brackets form the bulk of structural airframe hardware. External surfaces typically receive chromate conversion coating followed by primer and topcoat for corrosion and UV protection. Hard anodizing appears on wear interfaces such as hinge fittings and lock pins, where repeated mechanical contact demands surface hardness. Typical substrates include Aluminum 2024, 7075, and 6061.

Landing gear and hydraulic systems

Actuator rods, valve bodies, hydraulic fittings, and gear components operate under high loads in corrosive environments. Cadmium or zinc-nickel plating protects high-strength steel parts. Electroless nickel covers hydraulic valve bodies for uniform internal coverage on complex bore geometries. Typical substrates: 4340, 300M, 15-5PH stainless, and aluminum bronze. The finish specification and the machining tolerance plan need to be coordinated from the start for precision components in these systems.

Engine and hot-section parts

Turbine blades, combustor liners, and exhaust components operate at the extreme end of aerospace service conditions. Plasma-sprayed yttria-stabilized zirconia serves as a thermal barrier coating on blades and combustors, insulating the base alloy from gas temperatures above 1000°C. Diffusion aluminide coatings on nickel superalloy airfoils provide oxidation resistance. Typical substrates include Inconel 718, Inconel 625, René alloys, and single-crystal nickel superalloys.

Avionics housings, hardware, and fasteners

Aluminum avionics enclosures commonly receive chromate conversion Class 3 for electrical grounding, maintaining low contact resistance across mating surfaces. Electroless nickel provides EMI shielding on copper or steel housings. Cadmium or zinc-nickel plating protects high-strength steel fasteners. Dry film lubricant on titanium fasteners prevents galling during torque-down—a real concern on Ti-6Al-4V bolts that can seize and snap if assembled dry. Typical substrates: Aluminum 6061 and 2024 for housings; A286, Inconel 718, and Ti-6Al-4V for fasteners.

Working with an Aerospace-Finishing Capable Supplier

Aerospace finish decisions made before production begins reduce qualification risk and keep the schedule intact. Getting the finish specification, substrate compatibility, and NADCAP requirements aligned at the drawing stage is where most rework is prevented.

Yijin Solution machines aerospace components under AS9100D across aluminum, stainless steel, titanium, and high-strength steel, with finish recommendations and NADCAP-aware supplier coordination included in the DFM review at the quoting stage. Send your CAD file with your material and operating environment, and our engineers will return a DFM review and quote within 24 hours.

FAQs on Aerospace Finishes

What is the most common aerospace finish for aluminum parts?

Sulfuric acid anodizing per MIL-A-8625 Type II is the most widely specified aerospace finish for aluminum. It provides corrosion resistance, accepts dye for part identification and color coding, and serves as a base for additional coatings when needed. For applications involving wear or repeated mechanical contact, Type III hard anodization adds surface hardness and abrasion resistance at higher thickness buildup.

Do aerospace finishes affect part tolerances?

Most coatings add measurable thickness to the part’s surface. Hard anodizing grows roughly half into the substrate and half outward, so a 50 micron coating changes an outer diameter by approximately 25 microns per side, with some alloy-to-alloy variation. Electroless nickel adds the full coating thickness on top of the original surface. Chemical conversion coatings are typically under 1 micron and have minimal dimensional impact. Coordinate with the finisher on coating growth and adjust machining dimensions before production.

What is NADCAP, and why does it matter for aerospace finishing?

NADCAP is the National Aerospace and Defense Contractors Accreditation Program, administered by the Performance Review Institute. It audits and accredits suppliers performing special processes, including plating, anodizing, chemical processing, and coatings. Most aerospace primes, including Boeing, Airbus, and Lockheed Martin, require finishing suppliers to hold current NADCAP accreditation for the relevant process. Working with a NADCAP-accredited finisher reduces qualification risk and audit burden on the buyer’s side.

Can the same part have multiple aerospace finishes?

Many aerospace parts carry multiple finishes on different surfaces. A common configuration is an aluminum housing with hard anodizing on wear surfaces, chemical conversion on mating and grounding surfaces, and primer plus topcoat on external exposed areas. Each finish zone is masked during adjacent processes. Specifying zone callouts and masking instructions clearly on the drawing prevent finishing errors and avoid the rework that adds weeks to a delivery schedule.

How does finishing affect aerospace part lead time?

Anodizing and passivation typically add 1 to 2 days. Electroless nickel with post-plate baking adds 2 to 4 days. Cadmium plating with hydrogen embrittlement relief baking adds at least one full day beyond the plating itself. Thermal spraying with post-spray grinding and inspection can add a week or more. Build finish lead time into the total part schedule when planning prototype-to-production transitions, particularly when multiple finishing steps are required on the same part.

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