Anodizing is the most common finish on machined aluminum parts, and the one most often under-specified. A note reading “anodize clear” lets a shop deliver anything from a 3 µm decorative film to a 50 µm hard coating, sealed or not, with rack marks anywhere. This guide covers each type, the numbers behind thickness, hardness and growth, and the exact wording to put on a drawing.
What Anodizing Aluminum Actually Does
Anodizing grows an aluminum oxide film out of the base metal by making the part the anode in an acid bath. The film is integral, typically 5–25 µm for standard sulfuric anodize, hard, electrically insulating, and porous enough at the nanoscale to accept dye before sealing.
The part hangs on a rack while a rectifier drives direct current between it and lead or stainless cathodes. Oxygen released at the anode reacts with the aluminum to form Al2O3, so the oxide forms from the metal rather than sitting on it, and it cannot peel or flake the way paint can. About half the film converts metal below the original surface and half builds outward, which is why anodizing changes dimensions. A thin barrier layer sits against the metal; above it, a porous columnar structure of hexagonal cells with pores 10–25 nm wide takes the dye.
Type I, Type II and Type III: How They Differ
Type I is chromic acid anodize, thin and gentle on fatigue life. Type II is sulfuric acid anodize, the general-purpose protective and decorative finish. Type III is hard anodize, a colder, slower sulfuric process that builds a denser, thicker film. MIL-A-8625 assigns the numbers, and the type and class numbers belong on your drawing.
Type I produces the thinnest film, 2–5 µm, and is the traditional choice for fatigue-critical aerospace structure and parts that trap electrolyte. Its weakness is regulatory: the bath runs on hexavalent chromium, restricted in the EU. Type IB, a boric-sulfuric process, substitutes for it with comparable fatigue behavior and no Cr(VI). Type II is the workhorse, cheapest and most available, and takes dye cleanly. Type III runs near freezing at higher current density and much higher voltage; the film is thicker, denser and darker, and dyes read muted because the pores are smaller, so black is usually electrolytic rather than organic.
| Characteristic | Type I (chromic) | Type II (sulfuric) | Type III (hard) |
|---|---|---|---|
| Electrolyte | Chromic acid, 3–10% | Sulfuric acid, 15–20% | Sulfuric acid, 10–20% |
| Bath temperature | 32–40 °C (90–104 °F) | 18–22 °C (65–72 °F) | 0–5 °C (32–41 °F) |
| Current density | 1.5–3.0 A/dm² | 1.2–2.0 A/dm² | 2.5–3.5 A/dm² |
| Voltage | Ramped to about 40 V | 12–20 V | 40–100 V |
| Typical thickness | 2–5 µm (0.0001–0.0002 in) | 5–25 µm (0.0002–0.0010 in) | 25–50 µm (0.0010–0.0020 in) |
| Microhardness | 200–300 HV | 200–400 HV | 400–600 HV |
| Color range | Gray, limited dye uptake | Clear or any dye | Bronze-gray to black, muted dyes |
| Corrosion, sealed | Good, limited by thinness | Good to very good | Very good; 1,000 h salt spray at 50 µm |
| Wear resistance | Low to moderate | Moderate | High |
| Fatigue effect | Lowest debit of the three | Moderate | 30–60% loss common |
| Dimensional growth | About half film thickness | About half film thickness | About half, slightly more penetration |
| Relative cost | Moderate | Lowest | Moderate to high |
| Typical parts | Aerospace fatigue parts | Enclosures, handles, trim | Pistons, valves, tooling |
Bath Chemistry and Current Density
The bath sets film structure and current density sets growth rate. Type II runs 15–20% sulfuric acid at 18–22 °C and 1.2–2.0 A/dm². Type III uses similar acid at 0–5 °C and 2.5–3.5 A/dm²; the cold bath is what makes the film dense instead of soft and powdery.
Temperature is the most sensitive variable in hard anodize. Above about 10 °C the film softens, and at 25 °C the same bath produces a chalky, low-hardness coating, which is why Type III needs refrigeration and high pumping rates. Type II fails at the other end: a bath above 22 °C loses throwing power and colors unevenly. Current density is per unit of surface area, so rack design changes the effective value on the part. A small contact carrying the full load for a large part arcs and scorches the film. Growth runs roughly 0.3–0.5 µm per minute for Type II and 0.5–1 µm per minute for Type III. Dissolved aluminum is held to a limit, commonly 10–20 g/L, because a bath high in aluminum gives dull, brittle, poorly sealed coatings even at correct thickness.
Thickness by Type and Application
Specify thickness by what the surface has to survive. Interior decorative Type II runs 5–10 µm, exterior and handled Type II 15–20 µm, hard anodize for wear 25–50 µm, and heavy sliding or abrasive service 50–75 µm. Past about 50 µm the fatigue and brittleness penalties climb faster than wear resistance.
Most drawings specify a minimum, because the function is protection and thicker is not automatically better. A maximum matters when growth threatens a thread, a bore fit or a bearing seat, and both numbers belong on the drawing when the part is tolerance-critical.
| Application | Type | Thickness | Reason for the number |
|---|---|---|---|
| Decorative trim, badges, electronics enclosures | Type II, dyed | 5–10 µm | Lowest cost and growth; resists handling and cleaning |
| General exterior and handled parts | Type II, dyed | 15–20 µm | Outdoor barrier without visible dimensional change |
| Architectural exterior under AAMA 611 | Type II | 18–25 µm | Floor for warranted exterior color and corrosion |
| Abrasion surfaces, sliding contact, tooling | Type III | 25–50 µm | Wear life rises to about 50 µm, then flattens |
| Hydraulic and pneumatic bores and rods | Type III | 40–50 µm | Hard sealed film resists fluid and sliding wear |
| Heavy wear, abrasive food equipment, firearms | Type III | 50–75 µm | Margin for abrasion; beyond 75 µm the film crazes |
| Fatigue-critical aerospace structure | Type I or IB | 2–5 µm | Keeps the fatigue debit near zero |
| Assemblies where growth is unacceptable | Type II | 3–5 µm | Under 3 µm per surface after allowance |
Hardness, Wear and Corrosion Performance
Type III measures roughly 400–600 HV, often quoted as 55–70 HRC equivalent; Type II runs 200–400 HV. Wear life tracks thickness and seal quality more than headline hardness. Sealed Type II at 20 µm typically passes 336–500 hours of neutral salt spray, and 50 µm hard anodize commonly exceeds 1,000 hours.
Hardness is measured by micro-indentation on a polished cross-section, because the film is too thin for a direct Rockwell C test. Any “65 HRC” on a data sheet is a conversion, not a measurement, and hardness falls off near the porous outer surface. Anodized aluminum resists sliding abrasion and mild galling, which is why it suits piston bores, wear plates and food equipment. It does not resist point impact: a dropped part cracks the film and the crack becomes a corrosion site. Hard anodize also cuts fatigue strength, with reported losses commonly 30–60% on high-strength alloys, which is why fatigue-critical drawings call for Type I or IB. Corrosion performance depends on thickness, seal and alloy rather than color, and copper-bearing 2024 and 7075 pit more readily under the film than 6061.
Dyeing, Color and Sealing
Color goes in after anodizing because the film is porous. The part is immersed in dye that adsorbs into pores tens of nanometers wide, then sealed in hot deionized water, nickel acetate or a cold nickel-fluoride bath. Clear, black and bronze are the most repeatable; bright reds and blues fade fastest under UV.
Three coloring routes are in commercial use. Organic dyes give the widest range and the least UV stability. Electrolytic or two-step coloring deposits metal salts into the pores and produces bronze and black tones with excellent lightfastness, which is why exterior architectural work uses it. Integral color comes from alloying elements and gives bronze shades without dye. For exterior work the drawing must name the color system, because “black” can mean three processes with three fade lives.
Color matching has physical limits. Thicker films read darker, so 10 µm and 20 µm black will not match from the same tank. 6061 takes dye most predictably; 7075 and 2024 read darker and less uniform, and castings blotch where porosity interrupts the film. Specify a master sample or a color difference tolerance and expect a few units of lot-to-lot drift.
Sealing is the step most often skimped. Hot water at 95–100 °C for 20–30 minutes hydrates the oxide and swells the pores shut. Nickel acetate at 90–95 °C holds color better and is common on architectural work. Cold nickel-fluoride sealing runs at room temperature and uses less energy but needs tighter bath control. Dichromate sealing is a legacy corrosion treatment carrying the same Cr(VI) restriction as chromic anodize.
Masking, Racking and Contact Points
Every anodized part needs electrical contact, and every contact point is a bare spot. Rack marks cannot be eliminated, only placed. Mask threads, mating surfaces, grounding pads and bearing bores with silicone plugs, tape or peelable lacquer when the drawing requires those features uncoated.
Titanium racks last for years and leave small, clean contact points; aluminum racks are cheaper but get coated every cycle and must be stripped. Contact area must carry the part’s full current, or local burning replaces a uniform film. The practical control is a note: “rack marks permitted on surface B only” or “no rack marks on cosmetic face”. Without it, a rack mark can land mid-panel and scrap the part. Masking is hand labor, so a dozen masked threaded holes can cost more than the anodizing. Threads have three options: mask them, anodize and chase, or specify an allowance. Deep blind holes and narrow slots coat poorly for lack of electrolyte exchange.
Conductivity is the other functional trade. The film is a dielectric with high surface resistance, so an anodized enclosure will not ground through its mounting flange, and anodized busbars need masked contact pads. Where the part must conduct, chromate conversion coating keeps the surface conductive and anodizing is the wrong finish at any thickness.
Dimensional Growth and Tolerance Effects
Anodize changes dimensions. About half the film grows into the metal and half outward, so a 20 µm coating moves a surface roughly 10 µm. On a shaft that adds 20 µm to the diameter; in a hole it closes by 20 µm. Put the allowance in the tolerance stack before release.
A 25.000 mm shaft with 20 µm of Type II becomes 25.020 mm; a 10.000 mm bore becomes 9.980 mm. A 12.7 µm (0.0005 in) callout, the most common Type II note in North America, adds about 6 µm per surface. If the part is CNC machined to a ±0.005 mm tolerance, that growth exceeds the band, so either the pre-coating dimension is adjusted or the coated surface is excluded from the tolerance.
Hard anodize penetrates slightly more than it builds, so Type III growth is often under half the film. Do not rely on it; plan 50/50 and state the allowance. Threads cause the most scrap: coating on the flanks changes pitch diameter, and 15–20 µm is enough to stop a go gauge entering a class 2B nut. External corners also grow visibly faster because the film builds on two adjoining faces, which is why cosmetic parts specify a controlled edge radius.
Design Rules for Anodized Parts
Design for anodize the way you design for machining: allow for growth, avoid features that trap electrolyte, break edges, and keep cosmetic surfaces reachable by the rack and the current. Most failures found at incoming inspection trace to a feature never designed for the process.
- Specify five variables. Type, class, thickness, color and seal. A type number alone is not a finish specification.
- Break every edge to 0.3–0.5 mm. Sharp edges chip, and they show white on hard coat.
- Allow for hole closure of roughly one film thickness in diameter.
- Keep hole depth-to-diameter under about 5:1. Blind holes coat least at the bottom.
- Decide threads early: mask, chase after coating, or specify an allowance.
- Form before coating. Hard anodize on a bend under one material thickness crazes visibly.
- Flag welds. 4043 and 5356 filler anodize a different shade than 6061 base metal.
- Respect substrate finish. Type II clear reveals machining marks that Type III matte hides.
- Do not cosmetic-anodize A380 die castings. Use A356, or powder coating metal, which covers porosity instead of exposing it.
- Mask grounding pads. Anodize is a dielectric and will not carry a ground path.
- Avoid trapped assemblies: bearings, springs, dissimilar metals, sealed cavities.
- State cosmetic surfaces. “Surface A cosmetic, no rack marks” is enforceable; “good appearance” is not.
Which Aluminum Alloys Anodize Well
6061 is the most predictable alloy for anodizing and the default for machined parts, with 6063 standard for extrusions. 7075 and 2024 anodize acceptably but read darker and less uniform. 5xxx alloys coat well with visible stringer patterns, and high-silicon die castings finish poorly.
Silicon and copper inclusions do not anodize, so they interrupt the film and show as dark patches. A380 die casting at 8–12% silicon finishes mottled gray, while 6061 at low silicon and modest copper finishes clean. 7075 hard coats well for tooling and firearm parts but darkens the natural film. The materials selection guide covers alloy choice where the finish drives it. Copper alloys are not anodized at all and are picked when conductivity leads, as the brass vs copper comparison sets out.
Spec Callouts That Actually Control the Finish
A complete callout names the revision of the specification, the type, the class, the thickness, the color, the seal and the masking. MIL-A-8625 (now MIL-DTL-8625), AMS 2471 and 2472, ISO 7599 and ASTM B580 cover most global requirements, and they are not interchangeable.
The most common mistake is naming a specification with no type or class. “Anodize per MIL-A-8625” is ambiguous: the document defines four types and two classes, and a shop can satisfy the sentence with a 2 µm chromic film on a part needing 50 µm of hard coat. The second is mixing documents, such as ASTM B580 for the coating and AAMA 611 for color, without saying which governs when they disagree.
| Specification | What it covers | What you must add | Where it applies |
|---|---|---|---|
| MIL-A-8625 / MIL-DTL-8625 | Types I, IB, II, III; Class 1 undyed, Class 2 dyed | Type, class, thickness, color, seal, masking | North American defense and industrial work |
| AMS 2471 | Chromic acid anodize, aerospace | Thickness, class, masking | Fatigue-critical and bonded structure |
| AMS 2472 | Hard anodic coating | Thickness, sealing, alloy | Type III with aerospace process control |
| ISO 7599 | Decorative and protective sulfuric anodize, designation and tests | Designation, thickness, color, seal class | European and Asian supply chains |
| ISO 10074 | Hard anodize on aluminum alloys | Thickness, sealing | ISO counterpart to AMS 2472 |
| ASTM B580 | Anodic oxide coatings, general requirements | Type, thickness, appearance criteria | Commercial non-defense contracts |
| ASTM B244 | Eddy-current coating thickness | Accept limits | Non-destructive thickness test |
| ASTM B137 | Coating mass per unit area | Accept limits | Coupons and thickness disputes |
| ASTM B136, B680, B457 | Seal quality: dye stain, acid dissolution, admittance | Pass or fail limits | Verifying the seal |
| AAMA 611 | Architectural anodize for exterior use | Class, color, thickness | Building exteriors and warranties |
What to Write on the Drawing
Write a callout that answers six questions in one line: which specification and revision, which type and class, how thick, what color, how sealed, and what is masked. Anything less leaves a decision to the shop, and the shop will make the cheapest choice that satisfies the sentence as written.
- Decorative black: ANODIZE PER MIL-DTL-8625, TYPE II, CLASS 2, BLACK, 0.0007 IN (18 µm) MIN, SEALED. MASK THREADS AND GROUNDING PAD.
- Hard coat for wear: HARD ANODIZE PER MIL-A-8625, TYPE III, CLASS 1, 0.0015–0.0020 IN (38–50 µm), NATURAL. SEAL PER ISO 7599. EDGE BREAK 0.5 mm MAX.
- Fatigue-critical chromic: CHROMIC ACID ANODIZE PER AMS 2471, 0.0002 IN (5 µm), CLASS 1, NO DYE. RACK MARKS PERMITTED ON DATUM B FACE ONLY.
- Architectural exterior: ANODIZE PER ISO 7599, 20 µm MIN, ELECTROLYTICALLY COLORED BLACK, SEALED. COLOR TO APPROVED MASTER, AAMA 611 CLASS I.
Two habits prevent disputes: give a minimum or a range rather than a bare nominal, since thickness is measured and a nominal has no acceptance criterion, and put the masking note inside the coating callout, where it will not be dropped when a job is rushed.
How Anodize Is Inspected
A shop verifies anodize with four measurements: eddy-current thickness per ASTM B244, coating weight on a coupon per ASTM B137, seal quality by dye stain or admittance per ASTM B136 and B457, and salt spray on witness panels per ASTM B117. Visual checks catch rack burns, bare patches and color drift.
Eddy current is the everyday tool because it is fast and non-destructive. It must be calibrated on the same alloy, kept off edges and corners where the field distorts, and read on a flat area; within about 10% of nominal is normal. Cross-section microscopy per ASTM B487 settles disputes.
Coating weight is destructive and usually run on coupons from the same rack. The film is stripped in chromic-phosphoric acid and reported in mg/cm². Because film density runs 2.5–3.1 g/cm³, 10 µm corresponds to about 2.5–3 mg/cm², so the two methods cross-check. A part that passes eddy current but fails weight usually has a soft, porous film rather than a thin one.
Seal quality is the largest gap between what buyers assume and what they receive. Dye stain per ASTM B136 is pass-or-fail that the pores are closed; acid dissolution per ASTM B680 gives a number; admittance per ASTM B457 reads in the low microsiemens on a good seal. Without one of these on the purchase order, an unsealed coating passes every other check. Microhardness per ASTM E384 verifies hard anodize on a cross-section. A sound quality assurance process makes all of this standard for coated parts, with a certificate of conformance stating type, class, thickness and seal method.
Working With a Fabricator on Anodized Parts
Most anodizing problems are drawing problems, and the fix is cheapest before the first chip is cut. A shop that machines and finishes in-house can hold pre-coating dimensions, control edge breaks and rack where marks will not show, provided the drawing says what matters.
Surface finishing planned alongside the machining removes the finger-pointing that starts when a finisher blames the machinist. YC Prime Precision anodizes, powder coats, plates and passivates alongside CNC machining and sheet metal fabrication in Qingdao, so the finishing allowance is part of the process plan rather than a separate purchase order. Send the drawing with the callout and the tolerance stack for any coated surface, and the instant quote returns finish and machining priced together.
Frequently Asked Questions
What is the difference between Type II and Type III anodizing?
Type II is sulfuric anodize at 18–22 °C, typically 5–25 µm, and it takes dye in any color. Type III is hard anodize, run at 0–5 °C with higher current density to build a denser 25–50 µm film roughly twice as hard. Type III wears better but is darker, harder to dye and reduces fatigue strength.
How much does anodizing change part dimensions?
Plan on about half the coating thickness per surface. A 20 µm film moves a surface roughly 10 µm outward, so a shaft grows about 20 µm on diameter and a hole closes by the same amount. Add that allowance before machining. Tolerances tighter than about 0.005 mm should not run through an anodized surface.
Can 7075 aluminum be anodized?
Yes. 7075 anodizes and hard coats well, and it is common on tooling and firearm components. The copper and zinc content darkens the natural film and makes dyed color less uniform than on 6061, and thin films on 7075 pit more readily in corrosive service. Specify 6061 when color consistency matters.
Does anodizing prevent corrosion?
It improves corrosion resistance substantially when the film is thick enough and properly sealed, but it is not a guarantee. Sealed Type II at 20 µm typically passes 336–500 hours of neutral salt spray, while 50 µm hard anodize commonly exceeds 1,000 hours. Copper-bearing alloys and unsealed coatings perform far worse.
Can an anodized part be electrically grounded?
Only where the film has been removed. Anodic coating is a dielectric, so an anodized boss or pad will not carry a ground path. Mask grounding points before coating, or machine them after anodizing. If the whole part must conduct, a chromate conversion coating is the usual alternative because it stays conductive.
Why does black anodize fade or vary between lots?
Organic dyes fade under UV, and film thickness changes how dark the same dye reads. 6061, 7075 and castings all take dye differently, and every alloy batch varies slightly. For exterior parts use electrolytic coloring rather than organic dye, specify color against a master sample, and accept a few units of lot-to-lot color drift.
