Guide
What do the MIL-A-8625 types and classes mean, and is MIL-PRF-8625 the same spec?
MIL-A-8625 is the US Department of Defense specification for anodic coatings on aluminum and aluminum alloys: it defines six coating types (I, IB, IC, II, IIB and III), which set the anodizing process and how heavy the coating is, and two classes (1 undyed, 2 dyed). Since 23 November 2020 the current document is MIL-PRF-8625F with Amendment 2, which supersedes MIL-A-8625F with Amendment 1 and keeps the same types and classes and most of the test requirements.
By Saola Manufacturing engineering team · Published · Updated
This guide is a decoder for the design engineer or buyer who has to read, write or quote an anodize note that cites the specification. It covers every type, the class and sealing rules, the tests a finisher has to run, the naming question (MIL-A, MIL-PRF and the cancelled SAE copy AMS-A-8625), and sample callouts. The choice between conventional sulfuric anodize and hard coat is covered in more depth in Type II vs Type III anodize, and how much each type grows a bore or thread is in anodizing dimensional change.
On this page
What do the six MIL-A-8625 types mean?
The type names the anodizing chemistry and coating weight: Types I and IB are chromic acid, Type IC is a non-chromic acid replacement for them, Type II is conventional sulfuric acid, Type IIB is thin sulfuric acid, and Type III is hard anodize. The type you write decides which coating weight, thickness and wear tests the finisher must pass.
The six types fall into three families. The chromic family (I, IB) and its two non-chromate replacements (IC, IIB) are thin coatings chosen for corrosion protection, paint adhesion and minimum fatigue penalty. Type II is the general-purpose sulfuric coating used for corrosion protection and color. Type III is the heavy, dense coating used where a surface slides or wears. Per MIL-PRF-8625F paragraph 6.21, today’s Type I is the same coating that the D revision called Type IA, so an old drawing that says “Type IA” maps to Type I.
Types I, IB, IC, II and IIB are controlled by unsealed coating weight, while Type III is controlled by thickness. The table gathers the process, the weight or thickness rule, dyeability, the fatigue note and typical use for each type, with the paragraph numbers of the 2020 text so each line can be checked.
| Type | Process | Unsealed coating weight (mg/ft²) | Typical thickness range (in) | Dyeability (Class 2) | Fatigue note | Typical use |
|---|---|---|---|---|---|---|
| I | Chromic acid, conventional | 200 minimum | 0.00002 to 0.0007 (0.5 to 18 µm) | Harder to dye; black may not be readily obtainable | Thin; preferred where anodize is needed on fatigue-critical parts | Corrosion protection, paint base, assemblies and recesses that can trap electrolyte |
| IB | Chromic acid, low voltage, 22 V, plus or minus 2 V | 200 minimum | 0.00002 to 0.0007 (0.5 to 18 µm) | Harder to dye; black may not be readily obtainable | Thin; preferred where anodize is needed on fatigue-critical parts | As Type I |
| IC | Non-chromic (mineral or mixed mineral/organic) acid | 200 minimum to 700 maximum | 0.00002 to 0.0007 (0.5 to 18 µm) | Harder to dye; black may not be readily obtainable | Maximum weight is set for fatigue purposes | Non-chromate alternative to I and IB; replaces a specified I or IB only with buyer approval |
| II | Sulfuric acid, conventional | 1000 minimum | 0.00007 to 0.0010 (1.8 to 25 µm) | Preferred type for dyed parts | Thicker than I/IB; thicker coatings reduce fatigue more | General corrosion protection, color, paint base |
| IIB | Thin sulfuric acid | 200 minimum to 1000 maximum | 0.00002 to 0.0007 (0.5 to 18 µm) | Harder to dye; black may not be readily obtainable | Maximum weight is set for fatigue purposes | Non-chromate alternative to I and IB; replaces a specified I or IB only with buyer approval |
| III | Hard anodic coating, any process giving a heavy, dense coating | 4320 minimum per 0.001 in of coating, if weight is used instead of thickness | 0.0005 to 0.0045 (13 to 114 µm); default nominal 0.002 (51 µm) | Allowed; Type II is the preferred type for dyeing | May reduce fatigue strength; review cyclically loaded parts | Valves, sliding parts, cams, gears, pistons, hinge mechanisms |
What is the difference between Class 1 and Class 2?
Class 1 is not dyed or pigmented, and Class 2 is dyed or pigmented. The natural tint that an alloy or the sealing process gives a coating does not count as dyeing, so a gray or bronze Class 1 part can still conform.
Paragraph 3.5 of the specification says natural coloration from the alloy and the characteristic color of the seal are both treated as undyed. That matters most for Type III, whose heavier coating carries a natural tint set by the alloy and the process, so the same callout can look different on two alloys. If appearance matters on a Class 1 part, the drawing has to say so; the specification does not.
Class 2 adds two obligations for the buyer. First, the color and the color uniformity have to be specified on the order or drawing, per paragraph 3.6.1, ideally against an approved sample, because the specification points to SAE AMS-STD-595 only as an approximate guide written for paint. Second, light fastness testing is done only when the order asks for it. When it is asked for, per MIL-PRF-8625F paragraphs 3.7.1.3 and 4.5.4 the dyed panels see 200 hours of continuous light exposure and must not fade beyond a Delta E of 3 unless the order sets another value. Cast alloys may show dye bleed-out or uneven color, and the acceptable degree has to be agreed with the buyer.
If the class is left off the drawing, paragraph 6.2.2 lets the supplier deliver either class. A finish note with no class is therefore an open choice, and it is a frequent reason two lots of the same part come back looking different.
Is MIL-PRF-8625 the same spec as MIL-A-8625?
Yes, it is the same document under a new designation: MIL-PRF-8625F with Amendment 2, dated 23 November 2020, supersedes MIL-A-8625F with Amendment 1 and keeps the same six types, two classes and core performance tests, with several referenced test standards updated. The “PRF” prefix marks it as a performance specification.
The cover page of the 2020 document states the supersession directly, and the type and class definitions in its paragraph 1.2 match the 1993 text (ASSIST QuickSearch, DLA). A drawing that still says MIL-A-8625 is not wrong, but it is ambiguous about edition, and the 2020 text changed several acceptance rules a buyer relies on. Writing the full designation with the revision letter and amendment removes the doubt.
AMS-A-8625 is not a separate live standard. SAE International published it in July 2000 as an SAE edition of MIL-A-8625F with editorial changes only, and in July 2003 issued AMS-A-8625A as a cancellation notice that sends users back to the latest MIL-A-8625 (SAE AMS-A-8625, SAE Mobilus). An old drawing that still cites AMS-A-8625 is therefore worked to the DoD document, and the cleaner fix is to update the note to MIL-PRF-8625F rather than mix MIL and SAE designations.
| Item | MIL-A-8625F (1993) | MIL-PRF-8625F w/Amendment 2 (2020) |
|---|---|---|
| Document type | Military specification | Performance specification |
| Types and classes | I, IB, IC, II, IIB, III; Class 1 and 2 | Unchanged |
| Dimensions after coating (3.9) | Parts must meet drawing dimensions after anodizing | Paragraph deleted; state on the drawing whether dimensions apply before or after coating |
| Lot sampling (4.4.2) | MIL-STD-105, Inspection Level II | MIL-STD-1916, Verification Level II; acceptance criteria set in the contract or order |
| Conformance inspection (4.4) | Visual and dimensional examination | Visual, color for Class 2, thickness for Type III; paint adhesion when ordered |
| Color guide (6.6) | FED-STD-595 | SAE AMS-STD-595 |
What is Type I chromic acid anodizing, and when should a drawing still call for it?
Type I is anodizing in a chromic acid bath, which gives a thin, gray coating with a small fatigue penalty and less risk than sulfuric types if electrolyte is trapped in a joint. Call it only when those properties are needed, because hexavalent chromium can make it harder to source than the non-chromate Types IC and IIB.
The specification gives Type I and IB two jobs that other types do poorly. Where anodizing of assemblies or complex shapes that trap electrolyte is authorized, paragraphs 3.3.1.2 and 3.3.1.3 say Type I or IB is used unless another type is named; paragraph 6.4 warns that solution retained in crevices, particularly from the sulfuric process, can cause severe attack. And where anodize is needed on fatigue-critical components, the notes in paragraph 6.1.1 point to Type I or IB because the coating is thin.
Type I also has alloy limits. Per MIL-PRF-8625F paragraph 3.4.1, it is not applied to alloys with nominal copper above 5.0 percent, nominal silicon above 7.0 percent, or total alloying elements above 7.5 percent unless the order allows it. For every type, heat-treatable alloys must already be in a heat-treated temper such as T4, T6 or T73 before anodizing (paragraphs 3.4.1, 3.4.2 and 3.4.3).
Types IC and IIB exist as non-chromate replacements, but they are not automatic substitutes. Paragraphs 3.4.1.1 and 3.4.2.1 prohibit putting IC or IIB on a part where Type I or IB is specified unless the buyer approves, and paragraph 6.1.2 warns that neither is a suitable replacement when electrolyte entrapment is the main concern. If your drawing says Type I and the finisher offers IIB, treat it as a deviation and approve or reject it in writing.
What sealing, dye and test rules come with each type?
Types I, IB, IC, II and IIB are sealed by default and are proven by coating weight and a salt spray test. Type III is left unsealed by default and is proven by thickness and a Taber abrasion test, with salt spray only when the order asks for a sealed hard coat.
Sealing. Per paragraph 3.8.1, all the thin and conventional types are completely sealed unless the drawing says otherwise; Class 2 parts are dyed first and then sealed. Per paragraph 3.8.2, Type III is not sealed where maximum wear resistance is the aim, and the order has to call for sealing when the hard coat must also resist corrosion outdoors. Sealing lowers wear resistance, and it can change how primers adhere, so a part that will be painted should say whether to seal.
Process control tests. The finisher runs these at least monthly on production parts or panels, not on every lot. Per MIL-PRF-8625F paragraph 4.5.3, corrosion panels see 336 hours of 5 percent neutral salt spray to ASTM B117, and per paragraph 3.7.1.2 they may show no more than 15 isolated pits in 150 square inches of test area, none larger than 0.031 inch across. Per MIL-PRF-8625F paragraph 4.5.5, Type III panels are abraded with CS-17 wheels under a 1000 g load for 10,000 cycles, and the wear index may not exceed 3.5 mg per 1000 cycles on alloys with 2 percent copper or more, or 1.5 mg on all other alloys.
Lot acceptance. What each lot gets is narrower: visual examination, a color check for Class 2, and a thickness check for Type III, with paint adhesion added only when ordered. Per MIL-PRF-8625F paragraph 4.5.1, Type III thickness is the average of at least eight eddy-current readings, with a metallographic cross section as the referee method. Because the 2020 text leaves the acceptance criteria to the contract, an order that names none gives the inspector no number to accept or reject against.
The thin types are harder to dye. Paragraph 6.12 notes that black on Types I, IB, IC and IIB may not be readily obtainable, so a black thin-film part is a question to ask before release, not after.
How do you write a MIL-PRF-8625 callout on a drawing?
Write the designation with revision, the type, the class, the color if Class 2, the sealing instruction, the Type III thickness if it is not the default, and any masking or contact-mark location. Paragraph 6.2 of the specification lists most of these as items an order should state; masking is not on that list but belongs on the drawing.
Three sample notes, written for typical machined aluminum parts:
- “Anodize per MIL-PRF-8625F, Type II, Class 2, black, sealed. Color to match approved sample. Contact marks on surface B only.”
- “Hard anodize per MIL-PRF-8625F, Type III, Class 1, thickness per note 5, unsealed. Mask threads and datum A. Dimensions apply after coating.”
- “Anodize per MIL-PRF-8625F, Type IIB, Class 1, sealed. Substitution of other types not permitted.”
The table turns the common gaps into a rule you can apply to any note before it goes out for quote.
| If the note… | What the specification then allows | Add to the drawing |
|---|---|---|
| Names no type | Supplier may furnish I, IB, IC, II or IIB; Type III is not among the options (6.2.2) | The type |
| Names no class | Supplier may furnish Class 1 or Class 2 (6.2.2) | Class 1 or Class 2 and the color |
| Says Type III with no thickness | Nominal 0.002 in (51 µm), plus or minus 20 percent (3.7.2.1) | Thickness, if the default does not suit the fit |
| Says nothing about sealing | Thin types sealed; Type III unsealed (3.8) | “Sealed” or “unsealed” when the default is wrong |
| Says nothing about dimensions | No rule in the 2020 text; each surface grows about half the coating thickness (6.10.1) | “Dimensions apply after coating” or machining and coated dimensions |
| Says nothing about acceptance | Sampling to MIL-STD-1916, criteria from the contract (4.4.2) | Acceptance criteria, or a reference to your inspection standard |
What should an RFQ for anodized machined parts include?
Send the drawing with the complete callout, the cosmetic and functional faces marked, the masking and contact-mark locations, and the inspection records you expect with each lot. Anything left to “finisher’s option” in the specification should be decided before quoting.
Most anodize problems on machined parts start before the tank. Sharp edges and inside corners leave voids in the coating, so the specification’s design notes ask for radiused edges, and per MIL-PRF-8625F Table III the radius grows with coating thickness, from about 1/32 inch for a 0.001 inch coating to about 1/8 inch for 0.004 inch. Threads, bores and press fits need either masking or a pre-coat size, because per paragraph 6.10.2 a 60-degree thread’s pitch diameter grows four times the per-surface growth. Those decisions belong to the part designer and the machinist, which is why they should be on the drawing that goes to CNC milling or CNC turning, not settled later in an email.
When we review a drawing with an anodize note, we check that the type, class, sealing and masking are all stated, that edges and corners are broken where the coating needs it, and whether the dimensions are meant before or after coating. We also ask who runs the anodize and which records come back with the parts, such as the process control document, bath analysis, monthly test results and, for hard coat, the thickness report. If a note is incomplete, the quote lists the open questions rather than guessing.
Frequently asked
Is MIL-A-8625 cancelled?
Not cancelled but superseded. MIL-PRF-8625F with Amendment 2 replaced MIL-A-8625F with Amendment 1 in November 2020 and keeps the same types and classes. Which edition applies is set by the contract or order, so confirm it on the purchase order when a drawing cites only MIL-A-8625.
What does “anodize per MIL-A-8625” with no type mean?
The finisher may choose Type I, IB, IC, II or IIB and either class. Hard coat is never supplied by default, so a wear surface needs Type III written out.
Is Type 1 anodizing the same as chromic acid anodizing?
Yes. Type I is conventional chromic acid anodizing and Type IB is the low-voltage chromic process. The D revision (1985) called the conventional process Type IA; editions before D called it Type I.
Can Type II be dyed any color?
Type II is the type the specification prefers for dyed parts, but the shade still depends on alloy and process. Specify the color with an approved sample and a tolerance, as covered in the anodize color guide linked below.
Does the specification require a hardness value for Type III?
No. Wear resistance is proven with a Taber abrasion test, not a Rockwell or Vickers number. A hardness value on the drawing is an extra requirement and needs its own test method.
What should I send to get an anodized part quoted?
The drawing with the full callout, masking, cosmetic faces and a color sample for dyed parts. Send it for a quote or email rfq@saolamfg.com.
Sources
- MIL-PRF-8625F w/Amendment 2, Anodic Coatings for Aluminum and Aluminum Alloy (ASSIST QuickSearch document record), US Department of Defense, 23 November 2020
- 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
- AMS-A-8625, Anodic Coatings for Aluminum and Aluminum Alloys, SAE International, issued July 2000; AMS-A-8625A cancellation notice July 2003
- ASTM B117-19, Standard Practice for Operating Salt Spray (Fog) Apparatus, ASTM International, 2019