Anodizing Aluminum: The Engineer's Guide to Process, Types, and Specifying
Type I/II/III per MIL-A-8625, process steps, dye colors, dimensional effects, masking, and how to call anodizing out on a drawing.
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 9 min read
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What Anodizing Actually Is
Anodizing aluminum is an electrochemical process that converts the surface of the metal into a controlled aluminum oxide layer — not a coating applied on top, but a ceramic-hard oxide grown from the base metal itself. That distinction matters: because the oxide is integral with the substrate, it cannot chip, peel, or flake the way paint or plating can. The anodized layer is hard, corrosion-resistant, electrically insulating, and porous enough (before sealing) to accept dyes, which is why anodized aluminum shows up everywhere from aircraft fittings to laptop enclosures. For machined parts, it is the single most common finish specified on aluminum — and one of the most frequently misspecified, which is what this guide is here to fix.
The Three Types Under MIL-A-8625
The dominant specification in North America is MIL-A-8625 (also written MIL-PRF-8625), which defines three anodizing types. Knowing which type your part needs is the first design decision.
| Type | Electrolyte | Typical thickness | Primary purpose |
|---|---|---|---|
| Type I | Chromic acid | Very thin, often under 0.0001 in (2.5 µm) | Corrosion protection on fatigue-critical and tight-tolerance aerospace parts; paint base |
| Type II | Sulfuric acid | ~0.0001–0.001 in (2.5–25 µm); commonly 0.0002–0.0007 in | General corrosion and cosmetic finish; the dyeable "decorative" anodize |
| Type III | Sulfuric acid, cold and high-current | Commonly 0.002 in (50 µm); roughly 0.0005–0.0045 in | Hard coat — wear resistance, abrasion, electrical insulation |
Type II is the default when a drawing just says "anodize." Type III (hard coat) is a different animal — thicker, harder, and dimensionally significant — and it gets its own treatment in our hard coat anodizing guide. The rest of this article focuses mostly on Type II, with notes where the types diverge.
The Process, Step by Step
A conventional Type II sulfuric anodize line runs the part through a sequence of tanks:
- Cleaning — alkaline or solvent cleaning removes machining oils, coolant residue, and shop handling contamination. Anything left on the surface prints through the final finish.
- Etching — a sodium hydroxide etch removes a thin layer of metal and the natural oxide, and produces the characteristic matte texture. Longer etch, duller finish.
- Desmutting — an acid bath dissolves the smut (alloying-element residue) left by etching, especially important on copper-bearing alloys.
- Anodizing — the part becomes the anode in a sulfuric acid bath; applied current grows the porous oxide layer. Thickness is a function of current density and time.
- Dyeing (optional) — while the pores are still open, the part is immersed in organic dye. Color lives in the pore structure, which is why anodized color is so durable.
- Sealing — hot water or nickel acetate hydration closes the pores, locking in color and completing the corrosion protection. Unsealed anodize stains and fingerprints easily.
Because the finish is grown in tanks, racking matters: parts hang on aluminum or titanium racks, and every part carries one or more small rack contact marks where current entered. A good drawing note tells the finisher where those marks are allowed.
Colors and Dyeing: Black Is the Default for a Reason
Clear (undyed) anodize shows the metal's natural gray-silver tone. Dyed Type II anodize can theoretically be almost any color, but in practice black dominates industrial work — black dyes are the most UV-stable and the most forgiving of alloy-to-alloy variation, and black hides rack marks and minor surface variation better than any saturated color. Red, blue, gold, and green are common; bright, color-critical cosmetic work should always specify a color standard or a limit-sample range rather than a name.
The alloy underneath drives the result. 6061 anodizes to a clean, consistent finish and is the reference alloy for cosmetic work. 7075 anodizes well functionally but cosmetic color — especially black — can vary with its zinc content. High-copper alloys like 2024 produce thinner, less protective coatings with muddy color response. Silicon-rich cast alloys anodize gray and uneven. If cosmetic appearance matters, machine the part from 6061 if the design allows; see aluminum grades for machining for the trade-offs.
Dimensional Effects: The Rule Designers Forget
Anodize grows both into the metal and outward from the original surface. The working rule of thumb is roughly fifty-fifty: about half the coating thickness penetrates below the original surface and half builds above it. So a 0.0005 in (13 µm) Type II coating adds only about 0.00025 in per surface — usually invisible to general tolerances, but very visible on a press-fit bore, a precision shaft, or a fine thread.
Practical consequences:
- Press fits and slip fits — state on the drawing whether dimensions apply before or after anodizing.
- Threads — thread pitch diameter grows by roughly four times the per-surface buildup; functional threads are usually masked or chased, or the buildup is engineered in.
- Electrical grounding — anodize is an insulator. Any surface that must conduct (grounding points, EMI contact lands) must be masked.
Type III coatings are thick enough that dimensions must be planned around them deliberately — that is covered in the hard coat guide.
Masking: Cheap at the Drawing, Expensive at the Tank
Masking is done with plugs, tapes, and lacquers before the part enters the line. Every masked feature adds labor, so mask deliberately: threaded holes that must gauge, bearing bores, grounding pads, and mating surfaces with tight tolerances. A clear masking note ("mask all threaded holes and datum B bore; rack marks permitted on back face") prevents the two classic failures — a thread that will no longer accept a fastener, and a ground contact that no longer conducts.
How to Call It Out on a Drawing
A complete anodize note answers five questions: what spec, what type, what class, how thick, and what to protect. A clean example:
ANODIZE PER MIL-A-8625, TYPE II, CLASS 2, BLACK. COATING THICKNESS 0.0004–0.0007 IN. MASK ALL THREADED HOLES. DIMENSIONS APPLY AFTER ANODIZING. RACK MARKS PERMITTED ON SURFACE MARKED "R".
Class 1 means undyed (clear); Class 2 means dyed, with the color stated. Add "dimensions apply after anodizing" whenever fits matter, and always name a racking-allowed surface on cosmetic parts. Ambiguous notes get clarified at quote review — unambiguous ones just get quoted. Our surface finish capabilities page summarizes the finishes we run, and the CNC machining tolerances guide covers how finishes interact with tolerance callouts.
Anodizing vs Other Finishes
Choose anodize when you want corrosion protection and wear resistance without meaningful dimensional change, a durable dyed color, or electrical insulation — and the substrate is aluminum, titanium, or magnesium. Choose something else when:
- Powder coating — you need a thick, impact-tolerant decorative coat, on any metal, at larger part sizes; see anodizing vs powder coating for the head-to-head.
- Electroless nickel or plating — you need conductivity plus corrosion protection, or the part is steel or brass.
- Chromate conversion (Alodine) — you need corrosion protection while keeping electrical conductivity, or as a paint primer on large weldments.
- Bead blast + anodize — not an alternative but a common pairing: blasting first gives a uniform matte texture under the anodize and hides machining marks on cosmetic faces.
Specifying It as Part of a Machining Order
The smoothest anodizing jobs are the ones where the machine shop owns the finish: machining allowances account for coating buildup, masking is planned with the fixture design, and one supplier is responsible for the finished part. That is how we run it — Type II clear and dyed anodize (black most common, other colors on request) and Type III hard coat are available as part of the same order as your CNC milling and turning work. Upload your drawing with the anodize callout and we will quote the finished part, flag any dimensional conflicts between the finish and your fits, and confirm color expectations against the alloy before anything ships.
Choosing a finish for machined aluminum? The surface finishes we offer run from as-machined through anodizing, plating, and paint — and CNC machining services
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