Guide
Type II vs Type III anodize: which one should your drawing call for?
Type III anodize is the heavy, dense “hard coat” grown on aluminum for wear and abrasion resistance, while Type II is the conventional sulfuric-acid anodize used for corrosion protection, color and paint adhesion. Both come from the same US military specification and both turn the aluminum surface itself into oxide, but they are tested against different requirements, so the type you write on the drawing decides what the finisher has to prove.
By Saola Manufacturing engineering team · Published · Updated
This guide is for mechanical engineers and buyers who have to choose between the two types on a machined aluminum part, write the callout, and check what comes back from an overseas finisher. It works from the text of the specification itself: MIL-A-8625F, its 2003 amendment, and the 2019 notice that validated it. It covers what each type is tested for, hardness and wear, color, cost drivers, cast alloys and callout wording. How much each coating grows a bore or a thread is a separate subject, covered in the finish comparison guide linked at the end.
A note on the name. Many drawings say “MIL-PRF-8625”, but the document archived under that number is titled MIL-A-8625F, and its Notice 1 of 2019 states that the 2003 edition was reviewed and found valid for use in acquisition (MIL-A-8625F Notice 1). Whichever prefix your company uses, write the revision letter so both sides are reading the same text.
On this page
What is the difference between Type II and Type III anodize?
Type II is controlled by coating weight and judged mainly on corrosion resistance, while Type III is controlled by thickness and judged on abrasion resistance. Type II is sealed by default; Type III is left unsealed when wear is the main purpose.
MIL-A-8625F lists six types and two classes. Type II is “sulfuric acid anodizing, conventional coatings produced from sulfuric acid bath”, and Type III is simply “hard anodic coatings”, which the specification lets the processor produce by any process that gives a heavy, dense coating of the specified thickness. Class 1 means not dyed and Class 2 means dyed. The notes to the specification explain the intent: Types I, IB and II improve corrosion protection or act as a base for paint, and Type III provides wear- and abrasion-resistant surfaces with improved corrosion protection from its greater thickness and weight.
The practical consequence is that the two types fail inspection for different reasons. A Type II lot is proven by coating weight and a salt-spray panel. A Type III lot is proven by thickness and a Taber abrasion test, and by salt spray only when the order says the coating must be sealed. The table below puts the requirements side by side.
What does MIL-A-8625F require of each type?
The table lists the requirements the specification attaches to each type, with the paragraph number so you can check the wording. Anything the table does not list, such as a hardness number, is not a requirement of the specification.
| Requirement | Type II | Type III |
|---|---|---|
| Process (1.2.1, 3.4) | Sulfuric acid bath | Any process giving a heavy, dense coating |
| How the coating is controlled | Unsealed coating weight, 1000 mg/ft² minimum (Table I) | Thickness, nominal 0.002 in, allowed to vary by up to 20 percent for coatings up to 0.002 in (3.7.2.1); weight may be used instead (3.7.2.1.1) |
| Sealing default (3.8) | Completely sealed unless otherwise specified | Not sealed where maximum wear resistance is the aim; sealing must be ordered |
| Wear test (3.7.2.2, 4.5.5) | None | Taber wear index, max 3.5 mg per 1000 cycles on alloys with 2 percent copper or more, 1.5 mg on all others |
| Salt spray (3.7.1.2, 4.5.3) | 336 h, pit count limits | Only when sealing is specified (Amendment 1, Table II) |
| Alloy limits (3.4.3) | None stated for Type II | Not on nominal copper above 5 percent or silicon above 8.0 percent without approval |
| Dyeing (3.12) | Preferred type for dyed or colored parts | Class 2 allowed; not the preferred type for dyeing |
| Default if drawing names no type (6.2.2) | Supplier may furnish I, IB, IC, II or IIB | Never supplied by default; must be named |
Two rows deserve a second read. The last row means a finish note that only says “anodize per MIL-A-8625” lets the finisher choose among five conventional types and either class, per MIL-A-8625F paragraph 6.2.2; you get hard coat only if you ask for Type III by name. The sealing row means a hard coat that is also expected to resist corrosion outdoors has to say “sealed”, and the buyer has to accept some loss of wear resistance in exchange, which the specification states plainly in its note on Type III use.
For commercial drawings that do not cite military documents, ASTM B580 is the civilian specification for porous anodic coatings on aluminum (ASTM B580-79, reapproved 2019). It defines seven coating types in its own table and is written in SI units, so a MIL type number cannot be copied onto a B580 callout. Pick one specification and use its own designations.
Is Type III anodize harder than Type II, and how is that proven?
Yes, hard coat is the wear-resistant type, but MIL-A-8625F does not prove it with a hardness number. It proves it with a Taber abrasion test that measures how much coating a loaded abrasive wheel removes.
Per MIL-A-8625F paragraph 4.5.5, unsealed Type III test panels run under abrasive wheels with a 1000 g load for 10,000 cycles, and the weight lost divided by ten is the wear index. The limit depends on the alloy: alloys with a nominal copper content of 2 percent or more, which the specification says includes the whole 2xxx series plus 7050 and 7178, may lose up to 3.5 mg per 1000 cycles; all other alloys are held to 1.5 mg. Type II has no wear requirement at all.
That has three consequences for the drawing. First, a Rockwell or Vickers value copied from a data sheet is not something the specification asks the finisher to measure, so if you add one you are writing a new requirement and must also say how it will be tested. Second, per MIL-A-8625F paragraph 6.17, abrasion resistance falls as a hard coat approaches 3 mils and generally does not improve with added thickness, so thicker is not a route to more wear life. Third, sealing reduces wear resistance, so a sealed hard coat is a compromise, not an upgrade.
Which type gives better color and appearance?
Type II is the type for color. The specification names it as the preferred treatment for parts that will be dyed, while a hard coat carries its own natural tint that depends on the alloy and the coating thickness.
MIL-A-8625F paragraph 3.5 says the natural coloration produced by anodizing a given alloy is not counted as dyeing, so a Class 1 hard coat can legitimately come back noticeably darker than bare metal and still conform. Class 2 hard coat can be dyed, but the heavier, darker film limits the colors that read well; black and other dark shades are the typical choice. If appearance is part of acceptance, say so, and give a color reference and a sample to compare against. The specification points to FED-STD-595 only as an approximate guide, because those standards were written for paint.
Color on the same part can also vary for reasons outside the finisher’s control. Paragraph 3.13 excludes differences caused by the base metal, such as welds or a part with both cast and machined surfaces, from rejection unless the order says otherwise. When a cosmetic face matters, mark it on the drawing and say which faces are functional only, as the surface finishing page recommends for any finish.
Why does Type III usually cost more than Type II?
Hard coat typically costs more because the film is thicker and denser, the process has to be held tighter, and more of the part usually needs masking or post-finishing. The coating chemistry is only part of the price.
The cost drivers follow from the specification rather than from any one shop’s price list. A Type III lot is checked for thickness at not less than eight readings per specimen when an eddy-current gauge is used, per MIL-A-8625F paragraph 4.5.1, and it adds a monthly Taber panel to the process control plan. Hard coat on close-tolerance parts is often grown oversize and then lapped or honed back to size, which paragraph 6.7 describes, because the outer layer is rougher and less dense than the core. And because anodic coatings develop voids at sharp corners, paragraph 6.10.3 asks for edges and inside corners to be radiused before coating, which is machining time spent before the part ever reaches the tank.
The cheapest hard-coated part is therefore one where the hard coat is limited to the surfaces that actually slide or wear, the rest of the part is Type II or masked, and the machinist knows before cutting which faces are to be coated. Put that zoning on the drawing rather than in an email.
Can die-cast aluminum be hard anodized?
Usually not well. MIL-A-8625F bars Type III on alloys with nominal silicon above 8.0 percent unless the buyer approves it, and it warns that a hard coat of 2 mils or more is extremely difficult on high-silicon die castings such as 360, 380 and 383.
The common die-casting alloys sit at or above that silicon line. The Aluminum Association registers A380.0 at 7.5 to 9.5 percent silicon and 383.0 at 9.5 to 11.5 percent (Aluminum Association Pink Sheets, 2025), and ADC12, the JIS grade many Asian die casters quote, is a higher-silicon member of the same family. NADCA rates 380 and 383 at 3 of 5 for anodized appearance and 4 of 5 for anodized protection, where 1 is best (NADCA, Table 4.3). The specification adds that dyed castings may show bleed-out or uneven color from the porosity of the casting.
If a cast housing has one wear surface, the usual answers are to put a hardened insert or sleeve at that surface, to powder coat the casting and accept a different wear solution, or to make the worn part from a wrought alloy machined from bar or plate. If the drawing still calls Type III on a casting, get the approval in writing before tooling, as paragraph 3.4.3 requires, and agree the thickness the alloy can actually reach. The ADC12 vs A380 guide covers the alloy side, and the die casting page lists the two alloys cast on site.
How should a Type II or Type III callout be written?
Write the specification with its revision, the type, the class, the color if dyed, the sealing instruction, the Type III thickness if it is not the default, and the areas to mask. Paragraph 6.2 of MIL-A-8625F lists exactly these items as what an order should specify.
Three examples, adapted to typical machined parts:
- Dyed cosmetic housing: “Anodize per MIL-A-8625F, Type II, Class 2, black. Sealed. Color to match approved sample. Contact marks on the inside face only.”
- Sliding bore, wear first: “Hard anodize per MIL-A-8625F, Type III, Class 1, thickness per note 4, unsealed. Mask threads and datum face A.”
- Outdoor hard coat: “Hard anodize per MIL-A-8625F, Type III, Class 1, sealed. Coating thickness report required with each lot.”
The details that most often go missing are the contact-mark location, the masking, and whether the dimensions on the drawing apply before or after coating. Per MIL-A-8625F paragraph 3.9, parts must meet the drawing dimensions after coating unless the order says otherwise, so a note such as “dimensions apply after anodize” is the default rather than an extra. The RFQ preparation guide shows where these notes belong in the package.
When the machining and the anodize are sourced from Vietnam or anywhere else offshore, ask two questions with the quote. Is the anodize run by the machining supplier itself or by an outside finisher, and which plant does it? And will the finisher send the records MIL-A-8625F expects it to keep: a process control document, bath analysis, monthly process control tests and, for Type III, a thickness report and the Taber result? Parts machined on the CNC milling or CNC turning lines here can be quoted with the finish named on the drawing; the anodize is applied by a partner finishing plant we manage, and the quote names where each step is done.
Frequently asked
Is “hard anodize” the same as Type III?
In a MIL-A-8625F callout, yes: the specification defines Type III as hard anodic coatings. Write “Type III” rather than “hard anodize” alone, so the thickness, wear test and sealing rules of that type apply.
Should Type III be sealed?
Only if corrosion resistance matters more than maximum wear resistance. The specification leaves Type III unsealed by default for wear parts and says sealing reduces wear resistance.
Can Type II be used on a wear surface?
It will resist light handling, but the specification sets no wear requirement for Type II. If the surface slides, rubs or sees abrasive media, call Type III and test it.
Does hard coat affect fatigue life?
It can. MIL-A-8625F notes that hard coatings may reduce fatigue strength and that, as a general rule, the thicker the coating the greater the reduction, so review cyclically loaded parts before choosing Type III.
What should I send to get an anodized part quoted?
The drawing with the full callout, the areas to mask, the cosmetic faces and a color sample if the part is dyed. Send it for a quote or email rfq@saolamfg.com.
Sources
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, US Department of Defense, 10 September 1993
- MIL-A-8625F Amendment 1, US Department of Defense, 15 September 2003
- MIL-A-8625F Notice 1, notice of validation, US Department of Defense, 28 August 2019
- ASTM B580-79(2019), Standard Specification for Anodic Oxide Coatings on Aluminum (scope), ASTM International, 2019
- Designations and Chemical Composition Limits for Aluminum Alloys in the Form of Castings and Ingot (Pink Sheets), The Aluminum Association, revised August 2025
- Aluminum Alloys, including Table 4.3, NADCA Design, North American Die Casting Association, accessed September 2026