Anodizing is where many CNC parts go from good to scrap
I have rejected more production batches over anodizing issues than over machining dimensional errors. A part that is machined perfectly but comes out of the anodize tank with color inconsistency, white bloom, or pitted surfaces is a cosmetic reject - and cosmetic rejects kill programs when the parts are for consumer electronics, automotive trim, or visible medical device components. Anodizing quality varies widely between shops, and catching the issues requires knowing what to look for.
Common anodizing defects and what causes them
A good anodized finish on aluminum should be uniform in color, free of scratches and pits, with a consistent oxide layer thickness that meets specification. Here are the defects I see most often:
- Color inconsistency: batch-to-batch or even part-to-part color variation caused by inconsistent bath chemistry, anodizing time, temperature, or alloy composition variation
- Insufficient coating thickness: thin anodize offers less corrosion resistance and wears quickly. Type II should be 8-25 microns; Type III hard coat should be 25-100+ microns depending on specification
- Sealing failure (bloom or smut): white powdery residue on the surface indicates poor sealing after anodizing, leading to corrosion and dye bleed
- Pitting and burning: localized surface damage from improper racking, high current density, or contamination in the anodizing bath
- Scratches and handling damage: anodized parts are hard but not scratch-proof; poor rack removal, careless handling, or inadequate packaging mars the finish
- Uneven color in machined areas: areas machined with different tools or subjected to different cutting forces can anodize at slightly different rates, causing visible color shifts
How to specify anodizing properly on your drawing
Most anodizing problems start with ambiguous specifications. Do not just write anodize on the drawing. Specify:
- Anodizing type: Type II (sulfuric acid, conventional), Type III (hard coat), or chromic acid conversion (chem film) per MIL-A-8625 or ASTM B580
- Color: clear, black, or a specific color reference (RAL number or Pantone number - note that exact color matching on anodize is harder than paint)
- Coating thickness: call out a minimum and maximum thickness requirement (e.g., 12-20 microns for Type II)
- Sealing requirement: hot water seal, nickel acetate seal, or dichromate seal
- Surface finish before anodizing: anodizing does not hide machining marks; a 1.6 Ra machined surface will look different from a 0.8 Ra surface after anodizing
- Racking requirements: specify where rack marks are acceptable (usually non-cosmetic surfaces)
Incoming inspection checklist for anodized parts
When anodized parts arrive, inspect them before releasing to stock:
- Visual inspection under consistent lighting (daylight-equivalent LED at 500+ lux, viewed from 30-60 cm) for color uniformity, scratches, pits, and bloom
- Color comparison against an approved color standard or sample part (keep a signed-off golden sample at your receiving area)
- Coating thickness measurement using an eddy current thickness gage (e.g., Elcometer) on witness coupons or non-critical surfaces
- Adhesion test using the tape test per ASTM D3359 (cross-hatch or simple tape pull) for dyed coatings
- Seal quality test using the dye stain test or acid dissolution test per ASTM B136/B680 for critical corrosion applications
Pre-production approval prevents batch failures
For production orders where appearance matters, require the shop to send anodized sample parts (from the actual production setup, not special hand-tended samples) before running the full batch. Approve the color and finish in writing, and require the production run to match the approved sample. Hold the supplier accountable to the standard you approved, not to a verbal description or vague reference. Keep a reference sample from each approved run for future comparison.
Testing anodizing quality beyond visual inspection
For parts where corrosion resistance is critical (outdoor equipment, marine, automotive under-hood), consider salt spray testing per ASTM B117. A properly anodized and sealed Type II part should withstand hundreds of hours of salt spray without corrosion; hard coat Type III can last thousands of hours. Your anodizing vendor should be able to provide salt spray data for their process. If they cannot, they may not be running a controlled process.
Frequently Asked Questions
Can I expect perfect color matching between anodized batches?
Perfect color match between different batches of anodized parts is extremely difficult due to variations in alloy, bath chemistry, and processing parameters. For visible consumer products, plan to have all parts for a given assembly anodized in the same batch, or specify a color tolerance band with an approved standard.
Does hard anodizing add thickness to dimensions?
Yes. Type III hard coat builds up 50 percent of the coating thickness on each surface (so a 50-micron coating adds 25 microns per side, or 50 microns total on a diameter). Account for this in your machining dimensions. Type II conventional anodize adds 5-25 microns total depending on thickness specified.
Can anodizing defects be fixed?
Minor cosmetic issues on non-critical surfaces can sometimes be improved with touch-up processes, but anodize that is too thin, poorly sealed, or burned requires stripping and re-anodizing - and stripping changes part dimensions. Prevention through proper specification and process control is far better than rework.
Is black anodize harder to control than clear?
Black dye adds an additional variable. Improper sealing leads to black dye bleeding when parts get wet or are wiped with solvent. Specify a sealing requirement and do a simple solvent wipe test on incoming black-anodized parts.