Anodizing Aluminum to MIL-PRF-8625: Engineer's Reference

Updated October 12, 2026

Which document you are actually buying to

The governing document is MIL-PRF-8625F with Amendment 2, effective 23 November 2020. A trade article by two coating chemists describes the change from MIL-A to MIL-PRF as mostly administrative: test methods and referenced documents were updated, but how anodizing is performed and the performance requirements were not fundamentally changed. The numbers on this page come from the 1993 revision F text and its 2003 Amendment 1, which are public on DLA ASSIST mirrors. Treat them as the baseline and confirm any acceptance value against the current document before you write it into a contract.

Amendment 2 changed several things a buyer sees on certs. Coating thickness by eddy current is now cited to ASTM B244 and ASTM E376 instead of FED-STD-151, sampling moved from MIL-STD-105 to MIL-STD-1916, the abrasion test moved to ASTM D4060, and the 2024 test panel is now SAE AMS 4037. Ask the anodizer to cite "MIL-PRF-8625F (2)" on the certificate so nobody argues later about which revision was used. Our surface finishing overview covers the basic choice between anodize, chem film and passivation; this page goes into the anodize spec itself.

Types, classes and the related AMS specs

Pick the type for function, then the class for appearance. Chromic acid Types I and IB were the traditional choice for fatigue critical parts and crevices; Types IC and IIB exist as non-chromate substitutes for them; Type II is the general corrosion and color coating; Type III is the wear coating.

Type IC is defined as anodizing in a mineral or mixed mineral and organic acid bath that contains no chromic acid. The Aluminum Anodizers Council cites boric-sulfuric acid anodizing, developed by Boeing, as the example. Type IIB is a thin sulfuric film made in the ordinary sulfuric bath; a Products Finishing answer puts it at roughly 1 to 10 microns. Unless the procuring activity approves, the spec prohibits substituting IC or IIB where Type I or IB is called out, and its notes warn that neither may be a suitable replacement where electrolyte entrapment is the main concern.

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Coating weight and thickness verification

Types I, IB, IC, II and IIB are not accepted by thickness. They are accepted by coating weight, measured on undyed, unsealed parts or panels by stripping the oxide and weighing the difference. Type III is accepted by thickness, with coating weight allowed as a substitute at the procuring activity's option.

The weight test strips the panel in a phosphoric and chromic acid solution at about 212 °F, repeating five minute immersions until the weight stops changing, then divides the loss by area (ASTM B137 is the alternative method named). Type III thickness is the average of at least eight eddy current readings, with a metallographic cross section as the referee method in a dispute. The IC and IIB maximums are there for fatigue reasons, and the spec's notes say that if a higher IIB weight is acceptable you should call Type II instead.

The anodizer also runs process control tests at least monthly, with no more than 35 days between tests, and analyzes every tank in the line at least every two weeks, keeping bath records for at least a year after the order. You will not see those records on a cert, but you can ask for them on a source audit. Eddy current thickness on parts is covered on our nondestructive testing page.

Sealing, salt spray and what the seal does to paint

Types I through IIB are fully sealed unless the drawing or order says otherwise. Type III is left unsealed when wear is the purpose, and must be explicitly ordered sealed when it also needs corrosion resistance.

• Class 1 seals named in the spec include 5 percent sodium or potassium dichromate at pH 5.0 to 6.0 for 15 minutes at 194 to 212 °F, boiling deionized water, and cobalt or nickel acetate.
• Class 2 seals, applied after dyeing, include 0.5 percent nickel or cobalt acetate, boiling deionized water, or a duplex nickel acetate and dichromate seal.
• Type III seals include boiling water, hot 5 percent dichromate, or nickel or cobalt acetate. Sealing lowers wear resistance, and Amendment 1 made the Type III corrosion test apply only when sealing is specified.

Sealed coatings are tested for 336 hours in 5 percent neutral salt spray to ASTM B117 with the panel inclined 6 degrees from vertical. Acceptance is no more than 15 isolated pits across 150 square inches from five or more panels and no more than 5 pits in 30 square inches, none larger than 0.031 in, ignoring 0.062 in at edges and marks.

Two practical points. First, dichromate seals contain hexavalent chromium. On DoD deliverables, DFARS 252.223-7008 (January 2023) prohibits more than 0.1 percent hexavalent chromium by weight in any homogeneous material unless the contracting officer allows it, so a seal choice can become a contract question; see the chem film page for how that clause works. Second, the spec notes that sealing can strongly affect primer and adhesive adhesion, so if the part will be painted or bonded, state the seal (or no seal) the paint system needs. Primer systems are covered on our paint and primer page.

Type III growth, threads, holes and edges

Every anodic coating grows about half its thickness outward from the original surface, so plan the machined dimension, not just the finished one. The spec's design notes recommend putting both a pre-coat machining dimension and a coated dimension on the drawing.

• Surfaces: a 0.004 in Type III coating needs about 0.002 in per surface of pre-machining allowance on close tolerance features.
• 60 degree threads: major and minor diameters grow by twice the per-surface growth, and pitch diameter grows by four times the growth. A 0.002 in hard coat (about 0.001 in growth) can therefore move pitch diameter about 0.004 in, which is why threads are usually masked or cut oversize.
• Holes: small and tapped holes can range from no film to full thickness. The notes recommend anodizing holes over 1/4 in, and treating sized Type II surfaces with MIL-DTL-81706 material if they are lapped or honed after anodize.
• Edges: oxide develops voids at sharp edges. The suggested radii are about 1/32 in for a 0.001 in coating, 1/16 in for 0.002 in, 3/32 in for 0.003 in and 1/8 in for 0.004 in.

Type III can be processed oversize and lapped or honed to final size, since the outer layer is less dense than the core. The notes also warn that all anodic coatings are brittle and may crack and spall under force fits, and that Type III should not go on surfaces that will later need dimensional restoration by rework. The spec lists valves, sliding parts, hinges, cams, gears, swivel joints, pistons, rocket nozzles, insulation plates and blast shields as typical Type III uses.

Alloy effects and fatigue

Alloy chemistry limits which types can be applied and how good the result is. The revision F text sets defaults that only the order can override.

• Type I is not applied to alloys with nominal copper over 5.0 percent, silicon over 7.0 percent, or total nominal alloying over 7.5 percent.
• Type III is not applied to alloys with nominal copper over 5 percent or silicon over 8.0 percent without procuring activity approval.
• Wear index for Type III is a maximum of 3.5 mg per 1,000 cycles on alloys with 2 percent copper or more, and 1.5 mg per 1,000 cycles on everything else, so a 2024 hard coat is held to a looser wear limit than 6061 or 7075. Check the nominal copper of high copper alloys such as 2219 against the Type III limit before you order hard coat.
• Temper: heat treatable alloys must be in their final heat treated temper (T4, T6, T73 and similar) before anodize, and parts are anodized after machining, welding, forming and perforating.
• Mixed loads: alloys anodize at different rates, and the notes caution that running a time long enough for the slowest alloy can produce improper coatings on the fast ones.

On fatigue, the spec's position is qualitative but clear: Type I and IB are used on fatigue critical components because the coating is thin, the IC and IIB weight caps exist for fatigue, and hard coatings may reduce fatigue strength, which must be considered for cyclically loaded parts. Get the stress engineer's acceptance in writing before adding or upgrading anodize on a fatigue critical part. Alloy behavior in more detail is on our aluminum alloys page.

Dyes, color and touch-up

Class 2 is a dyed or pigmented coating, and the color and uniformity requirement must come from the drawing or order. The spec only requires that wrought parts be uniform; castings may show bleed-out or uneven color from porosity, and the acceptable degree is set by the buyer.

• Dye before sealing, without letting the coating dry, and do not leave dyed parts in rinse water more than 5 minutes before sealing. Type II is the preferred base for dyed work.
• Light fastness is tested only when ordered: 200 hours of UV exposure, with a fading limit of Delta E 3 unless the order sets another value.
• Paint color standards (FED-STD-595, now SAE AMS-STD-595) may be cited for anodize color only as an approximate comparison.
• Mechanical damage may be touched up with MIL-DTL-81706 conversion material, limited to 5 percent of the anodized area, and on Type III only where the surface is not subject to abrasion. Touch-up restores corrosion protection, not wear resistance.

What to call out on the drawing or PO

If the drawing does not state the type, the revision F notes allow the contractor to furnish Type I, IB, IC, II or IIB; if it does not state the class, either class may be supplied. That is rarely what the designer wanted, so fill in every field.

• Spec and revision: MIL-PRF-8625F (2), or the AMS spec your customer flows down
• Type and class; color and allowable color variation for Class 2, including castings
• Type III thickness and tolerance, and whether dimensions apply before or after coating
• Sealed or unsealed, and the seal chemistry if paint, adhesive or hexavalent chromium limits matter
• Masked areas on a view, plus thread, bore and grounding point treatment
• Coating weight in lieu of thickness for Type III, if you want it
• Light fastness and paint adhesion tests, if required
• Contact mark locations if they matter to function
• Prime approved source or Nadcap chemical processing requirement
• A cert per lot naming the spec, type, class, seal, thickness or weight result and the lot

Buyer guidance on approved sources and sequencing is on our special processes page, and what drives anodize pricing is on the special process cost page. Back to the coatings library.

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Sources

• DLA ASSIST copy via ibiblio: MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys (10 September 1993)
• DLA ASSIST copy via ibiblio: MIL-A-8625F Amendment 1 (15 September 2003)
• Finishing and Coating (March 2021): Rollout of MIL-PRF-8625 Rev F and implications for anodizers
• Aluminum Anodizers Council: military specification MIL-A-8625 summary
• Products Finishing: Type IIB anodize
• SAE Mobilus: AMS2471 anodic treatment, sulfuric acid process, undyed (revision history)
• SAE Mobilus: AMS2472 anodic treatment, sulfuric acid process, dyed (revision history)
• SAE Mobilus: AMS2469 hard coating treatment of aluminum alloys (revision history)
• SAE Mobilus: AMS2482F hard anodic coating, PTFE impregnated or codeposited
• SAE Mobilus: AMS2470 (revision history)
• Acquisition.gov: DFARS 252.223-7008 Prohibition of Hexavalent Chromium (January 2023)