Aluminum Anodizing Specification Guide for Machined Parts: CNC Compass

Aluminum Anodizing Specification Guide for Machined Parts

Specify anodizing by coating requirement, class, sealing, masking, appearance, dimensional impact, and inspection instead of relying on a color name.

Short answer Specify the governing anodizing standard, coating type and class, sealing, masking, appearance acceptance, and post-coating dimensional requirements. Do not use “black anodize” as the whole specification: color alone does not define coating performance or thickness.

Decision factor	Recommendation	Notes
General corrosion and cosmetic finish	Define the applicable conventional anodic coating type and class	Confirm alloy, seal, color, and acceptance sample.
Wear-focused surface	Evaluate hard anodic coating	Review thickness, dimensions, fatigue implications, and sealing with engineering.
Threads, fits, electrical contacts	Define masking or post-coating size	Coating growth can affect functional interfaces.
Cosmetic color	Approve a representative limit sample	Alloy, temper, surface preparation, and lot variation influence appearance.

Begin with performance

Anodizing converts the aluminum surface into an anodic coating. The correct callout depends on what the part must resist or achieve: corrosion, wear, electrical isolation, paint adhesion, or a controlled cosmetic appearance.

MIL-PRF-8625 is an active U.S. performance specification covering multiple anodic coating types and two classes for non-architectural aluminum applications. If a contract invokes it, use the current controlled document and specify the required type and class rather than copying an informal shop description.

Control the interfaces

Coating thickness and growth matter at bearing seats, close fits, threads, sealing surfaces, and electrical contact points. The drawing should identify masked surfaces and state whether a dimension applies before or after coating. Do not leave the finishing supplier to infer which interfaces are functional.

Hard anodic coatings may be considered where wear or abrasion resistance is important. Engineering should also review alloy suitability, required thickness, fatigue-sensitive service, sealing, and the final dimensional condition.

Treat color as an acceptance requirement

Color can vary with alloy, temper, stock source, surface preparation, coating process, dye, sealing, geometry, and viewing conditions. If appearance matters, define a controlled finish direction and approve representative limit samples. A digital color value by itself is rarely enough for metallic finishes.

Put these items on the RFQ

Governing specification and revision
Coating type and class
Alloy and temper
Sealing requirement
Masked and electrically conductive areas
Pre- or post-coating dimensional basis
Cosmetic acceptance method
Required test reports or certificates
Packaging controls for cosmetic surfaces

Ask the machine shop whether finishing is in-house or subcontracted and how lot traceability is maintained across the handoff.

Frequently asked questions

Is black anodize a complete specification?

No. It states a color intention but does not define the coating type, class, thickness or performance, sealing, masking, dimensional treatment, or inspection criteria.

Does anodizing change dimensions?

An anodic coating has thickness and part of the coating grows outward from the original surface. Critical fits, threads, and sealing surfaces need an agreed dimensional strategy.

Should hard anodize always be sealed?

Not automatically. Sealing can improve corrosion behavior but may affect other properties. The governing specification and functional requirement should determine the treatment.

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Editorial guidance should be checked against its cited first-party sources before publication. Updated 2026-09-17.