Specifying anodizing correctly isn’t just about calling out “anodize per spec” — it’s about understanding which alloys anodize well, how much dimensional buildup to expect, and whether your part geometry can handle the process. With extensive experience anodizing precision components for audio enclosures, medical housings, and aerospace brackets, small specification oversights can turn into expensive rework or assembly problems.
Anodizing aluminum involves cleaning the part, creating a controlled oxide layer through electrolytic process, and sealing the surface. Type II sulfuric anodizing (0.0007″ buildup) works for most applications, while Type III hardcoat (0.001-0.002″) provides superior wear resistance. Proper drawing specifications and compatible alloys like 6061-T6 are essential for success.
Learn how to specify anodizing on drawings, choose the right type, and avoid pitfalls that cause part rejection or color mismatch in production.
Table of Contents
What do I need to include in my drawing to specify anodizing?
Include anodizing type (Type II or III), coating thickness, color requirements, and masking callouts directly on your drawing. Use standard finish symbols like “Anodize Type II, Clear, 0.0007″ thick” with specific notes for threads, datums, or areas requiring masking. Without these details, shops make assumptions that often don’t match your intent.
Most anodizing failures trace back to incomplete drawing specifications. From our experience with audio faceplates and medical housings, engineers often assume “natural clear” means the same thing across different shops, or forget that critical threads need masking callouts. Type II sulfuric anodizing typically builds 0.0007″ per surface, while Type III hardcoat adds 0.001-0.002″ — validated through our CMM measurements on production parts.
Quick Decision Matrix:
Application Type Thickness Color Spec Masking Priority
Audio/Consumer Type II 0.0007″ RAL color code Threads, logos
Medical/Aerospace Type II/III 0.0007–0.002″ Per MIL-A-8625 Datums, contacts
Industrial Type II 0.0007″ Clear/black Threads only
Complete Drawing Specification Template: Essential callouts we recommend for every anodized part:
- Surface finish note: “Anodize Type II per MIL-A-8625, Class 1, 0.0007″ nominal, clear finish”
- Color specification: “Color per RAL 5015 ±ΔE 2.0” (not vague terms like “blue”)
- Masking callouts: “Mask threads M6x1.0 and electrical contact areas per drawing”
- Post-process dimensions: Adjust hole tolerances from Ø6.00 +0.05/-0.00 to Ø6.00 +0.05/-0.014
For regulated industries, add compliance references like “biocompatible per ISO 10993” or “per AS9100 documentation requirements” to meet traceability standards.
Drawing Checklist (30-Second Verification):
- Anodizing type specified (Type II most common)
- Thickness called out: 0.0007″ for Type II, 0.001-0.002″ for Type III
- Color per standard reference (RAL, Pantone, or MIL spec)
- Masking areas marked with crosshatch and “DO NOT ANODIZE”
- Critical dimensions adjusted for coating buildup
Design Takeaway: Always specify complete anodizing requirements using MIL-A-8625 or ISO standards, include explicit masking callouts for threads and contact areas, and adjust critical dimensions for coating thickness. This prevents rework and ensures parts meet assembly requirements on first delivery.
Can I anodize all aluminum alloys — or are some not compatible?
6061-T6 and 7075 anodize excellently with consistent color and finish quality. Avoid high-silicon casting alloys (A380, A383) and 2024 aluminum — silicon content above 5% creates blotchy, uneven coatings that look unprofessional.
Silicon content is the primary factor determining anodizing quality. We’ve processed thousands of parts across different alloys, and the results are predictable: wrought alloys like 6061 and 7075 produce uniform, attractive finishes, while casting alloys often result in rejected parts due to streaking or poor color consistency. High-silicon content interferes with the oxide layer formation during the anodizing process.
The copper content in 2024 aluminum (3.8-4.9%) creates galvanic reactions during anodizing, leading to streaky, discolored surfaces that are impossible to polish out. High-silicon casting alloys like A380 contain 7.5-9.5% silicon, creating porous, uneven oxide layers that produce mottled appearances no matter how carefully the process is controlled.
Alloy Performance Summary:
- 6061-T6: Best overall choice — consistent color, excellent dye uptake, cost-effective for most applications
- 7075-T6: High strength applications, anodizes well but shows natural gray undertone compared to 6061
- 5052-H32: Marine environments, excellent corrosion resistance due to magnesium content
- 2024-T4: Avoid — copper content causes streaky, blotchy finish regardless of process controls
- A380 casting: Avoid — 7.5-9.5% silicon creates porous, inconsistent coating that traps dye poorly
From our experience with precision instrumentation housings, 6061-T6 provides the most predictable results for critical cosmetic applications and remains the industry standard. Robotics components requiring higher strength use 7075, which anodizes well but the gray undertone becomes noticeable on decorative surfaces where color consistency matters.
These materials consistently fail cosmetic inspection due to their alloy chemistry, not process issues. Many anodizing shops cannot produce acceptable hardcoat on 2024 aluminum, requiring careful supplier selection if this alloy is unavoidable.
Design Takeaway: Specify 6061-T6 for optimal anodizing results and cost-effectiveness. Use 7075 when strength requirements exceed 6061 capabilities but expect slight color differences. Completely avoid 2024 and casting alloys if anodizing is required — switch to powder coating or plating for these materials.
Which type of anodizing should I use for wear, corrosion, or color?
Type II sulfuric anodizing handles most applications including colored finishes and moderate corrosion protection. Choose Type III hardcoat for high-wear environments, sliding surfaces, or when abrasion resistance exceeds decorative requirements.
The decision between anodizing types depends on your primary functional requirement. Type II creates a 0.0005″ porous oxide layer that accepts dyes well and provides good corrosion resistance for indoor applications. Type III builds a denser, harder coating of 0.001-0.002″ with superior wear resistance but limited color options — typically natural gray or black only.
We’ve tested anodized samples across different applications to validate these performance claims. Type II shows excellent salt spray resistance per ASTM B117 testing and delivers 20+ year outdoor service life when properly sealed. Type III hardcoat achieves Rockwell hardness of 60-70 HRC on the surface, compared to 15 HRC for untreated aluminum, translating to significantly better wear resistance in sliding applications.
Performance Comparison by Type:
- Type II: 0.0005″ thickness, full color range, 336+ hour salt spray resistance, moderate wear resistance
- Type III: 0.001-0.002″ thickness, gray/black colors only, 60-70 HRC surface hardness, high wear resistance
- Type I: 0.0002-0.0007″ thickness, natural finish only, aerospace paint adhesion applications
For color and appearance, Type II offers the full spectrum of dye options while preserving sharp details and fine features. Consumer electronics and decorative housings benefit from Type II’s ability to maintain tight tolerances while providing adequate protection and full color options. For heavy-duty applications, Type III delivers surface hardness comparable to tool steel — we’ve seen 10x improvement in wear testing on industrial automation components compared to Type II.
Type III’s thicker coating can affect tight tolerances and fine features like engraved text or small threaded holes. Military and semiconductor equipment requiring chemical resistance often specify Type III despite the dimensional impact from coating buildup of 0.0005-0.001″ per surface.
Design Takeaway: Select Type II for decorative parts, color requirements, and general corrosion protection. Specify Type III hardcoat only when wear resistance justifies the added thickness and tighter tolerance requirements. Match anodizing type to your primary functional need rather than over-specifying.
Will anodizing change my aluminum part's dimensions or fit?
Yes, anodizing adds 0.0005″ coating thickness per surface for Type II and 0.001-0.002″ per surface for Type III. Account for this buildup when tolerancing mating parts — a through-hole increases by twice the coating thickness (0.001″ total for Type II, 0.002-0.004″ for Type III).
Dimensional changes from anodizing are predictable when planned correctly. The process grows 50% outward and converts 50% of base aluminum into oxide, penetrating inward. External dimensions increase while internal features like holes get smaller.
Through our CNC machining and anodizing experience, we regularly measure parts before and after coating. A Ø6.00mm bearing bore becomes Ø5.50mm after Type II anodizing, requiring pre-machining to Ø6.25mm for final fit. M6x1.0 and smaller threads always require masking since coating buildup prevents proper thread engagement. Larger threads like M8x1.25 can accommodate Type II buildup with Class 2B threads.
Common design oversights include precision housings where mating parts no longer fit properly post-anodizing because clearances weren’t adjusted for coating thickness. Medical device components with O-ring grooves often show poor sealing when groove dimensions aren’t adjusted — the 0.0005″ per surface significantly reduces groove depth.
Thin-wall structures under 0.030″ thickness can distort during anodizing due to internal stresses. We recommend minimum 0.050″ wall thickness for reliable dimensional stability. Machining anodized surfaces removes protective coating, creating corrosion points, so specify critical surfaces for masking rather than post-machining.
Design Takeaway: Always increase hole diameters by coating thickness and decrease external fits by the same amount. Specify thread masking for M6 and smaller. Maintain minimum 0.050″ wall thickness to prevent warping. Plan critical assemblies around coating buildup rather than hoping to machine it away later.
Will anodized aluminum hold up under friction or outdoor use?
Type II anodizing provides 20+ years of outdoor corrosion protection and handles moderate wear applications. Type III hardcoat delivers exceptional wear resistance with 60-70 HRC surface hardness, suitable for high-friction sliding contacts and harsh outdoor environments.
Performance depends on anodizing type and application conditions. Type II excels in corrosion resistance, meeting ASTM B117 salt spray standards. Type III focuses on wear resistance with surface hardness comparable to tool steel but brittleness requiring careful application.
Industry testing data shows Type III hardcoat handles 50,000+ sliding cycles with minimal wear, while bare aluminum galls after 5,000 cycles. Type II maintains properties up to 500°F. However, point loads above 100,000 psi can chip Type III coating.
From our experience with custom parts across industries, properly sealed Type II shows excellent outdoor durability for decades, while unsealed coatings show chalking after 3-5 years. Marine applications consistently see better performance from anodized parts compared to painted alternatives. Industrial components using Type III maintain smooth operation in demanding environments, but chip when subjected to sharp impacts.
UV exposure breaks down anodizing sealer, not the oxide layer. In marine environments, incomplete sealing allows salt intrusion, causing pitting corrosion. Re-sealing every 5-7 years extends outdoor life significantly. Both types resist cleaning solvents and mild acids but degrade in strong caustics above pH 10.
Design Takeaway: Specify Type II with sealing for outdoor corrosion protection and moderate wear. Choose Type III for sliding contacts under distributed loading. Plan for re-sealing maintenance in harsh environments. Avoid Type III where sharp impact or point loading is expected.
Can I color-match anodized parts across multiple batches?
Yes, but it requires careful planning and realistic expectations. Specify color standards like RAL or Pantone with ±ΔE 2.0-3.0 tolerance ranges, use single-lot material when possible, and anodize color-critical parts simultaneously for best results.
Color consistency depends on multiple variables that change between batches. Alloy composition variations, even within the same grade, affect final appearance. The anodizing process itself — electrolyte age, temperature fluctuations, and current density — creates differences noticeable when parts are assembled together.
A ±ΔE 2.0 tolerance represents color differences barely perceptible when separated but noticeable in side-by-side comparison. Industry standards like AAMA 611 allow up to ±ΔE 5.0, while high-quality anodizers typically achieve ±ΔE 1.0-3.0 ranges.
From our experience with custom components, successful color-critical projects involve strategic planning rather than pursuing perfection. 6061 aluminum shows the most consistent results for color applications, while 7075 exhibits more variation. Black anodizing provides the most consistent results across batches.
Cost-effective workarounds often prove more practical than perfect matching. Design visual break lines between components where slight differences become less apparent. Single-lot material sourcing eliminates primary variation sources but requires advance planning.
Lighting conditions dramatically affect perception. Parts matching under fluorescent lighting can show differences under LED conditions. Sample approval processes provide color control for critical applications.
Design Takeaway: Specify measurable color tolerances based on application requirements. Consider design strategies that accommodate natural variation. Reserve tight control for truly critical applications.
Conclusion
Successful anodizing requires proper specification from drawing callouts to color tolerance planning. Most dimensional and performance issues stem from inadequate upfront planning rather than process limitations. Choose the right anodizing type for your application needs and design around natural process variations for optimal results. Contact us to explore manufacturing solutions tailored to your anodized aluminum requirements.
Frequently Asked Questions
No, anodizing will crack at those temperatures. The anodic coating begins to crack above 70°C (158°F) and will thermal craze when temperatures exceed 160°C (320°F). The aluminum substrate expands five times faster than the aluminum oxide coating, creating thermal stresses that crack the coating. For 200°C service, consider alternative coatings.
Maintain minimum 0.050″ wall thickness for reliable dimensional stability. Thin-wall tubes under 0.030″ thickness can distort during anodizing due to internal stresses from oxide formation. Sheet metal brackets under 0.020″ often bow after anodizing and may require fixture anodizing or design changes.
Likely batch-to-batch material variation or different anodizing loads. Specify color standards with measurable tolerances (±ΔE 2.0-3.0) rather than expecting perfect matches. Use single-lot material for color-critical assemblies and anodize all visible components simultaneously in the same tank.
Yes, but use 5356 filler rod instead of 4043. The 4043 rod is high in silicon and turns dark gray when anodized, creating visible “halo” effects around welds. 5356 has high magnesium content that anodizes to nearly the same color as 6061 base material, making weld lines virtually invisible.
Yes. Anodizing creates an electrical insulator, so specify masking for electrical contact areas. Threads M6x1.0 and smaller always require masking since coating buildup prevents proper engagement. Larger threads can accommodate Type II buildup but may need Class 2B specifications for clearance.
Increase hole diameters by coating thickness (0.001″ for Type II, 0.002-0.004″ for Type III) and decrease external fits by the same amount. For press-fit assemblies, machine bores 0.001″ oversize before Type II anodizing to maintain proper interference. Always specify which surfaces require masking to preserve critical fits.
