MIL-PRF-8625 Anodizing: Type II vs Type III Hardcoat on Machined Parts

MIL-PRF-8625 Type III hardcoat anodize is a thick, hard aluminum oxide coating, typically about 0.002 in (50 µm) nominal, that grows roughly half outward and half into the metal. Type II is thinner and used for corrosion protection and colour. Type I is a thin chromic acid coating. The choice between them, and the way it is called out, decides whether bores still fit and threads still gauge after finishing. This guide covers types, classes, thickness, growth, masking, sealing, alloy behaviour and drawing callouts.

Types and classes in MIL-PRF-8625

MIL-PRF-8625 is a performance specification. It replaced MIL-A-8625, and the current revision is F with amendments; check which revision your contract names. It defines coatings by type (process) and class (colour).

• Type I: chromic acid anodize. Type IB: low-voltage chromic acid. Type IC: a non-chromic alternative to Type I and IB.
• Type II: conventional sulfuric acid anodize. Type IIB: thin sulfuric acid anodize, also used as a non-chromic alternative to Type I and IB.
• Type III: hard anodic coating, usually sulfuric acid at low temperature and higher current density.
• Class 1: non-dyed. Class 2: dyed or pigmented.

Commercial equivalents exist: SAE AMS 2469 for hard anodizing, and AMS 2471 (undyed) and AMS 2472 (dyed) for sulfuric acid anodizing. Substitution is a contract decision, not a shop decision.

Type I vs Type II vs Type III compared

Property	Type I / IB	Type II	Type III
Electrolyte	Chromic acid	Sulfuric acid, near room temperature	Sulfuric acid, cold (often near 0 °C)
Typical thickness	0.00002 to 0.0003 in (0.5 to 7.6 µm)	0.0002 to 0.001 in (5 to 25 µm)	0.0005 to 0.0045 in (13 to 114 µm), about 0.002 in (50 µm) nominal
Outward growth	Small	Roughly one third of thickness, process dependent	Roughly one half of thickness
Wear resistance	Low	Moderate	High; hardness often quoted near 60 to 70 HRC equivalent
Colour options	Gray, limited dyeing	Wide range, including black	Natural gray to bronze or dark gray; black dye possible, colour varies
Sealing	Sealed	Sealed by default	Unsealed unless specified
Fatigue effect	Smallest	Moderate	Largest reduction
Typical defense uses	Fatigue-sensitive aerospace structure	Housings, brackets, chassis, covers	Slides, rails, mounts, wear pads, insulating surfaces

Values are typical industry figures, not promises. The spec controls some types by coating weight rather than thickness, so check the current revision for the exact acceptance criteria. Type I uses hexavalent chromium, and many processors outside aerospace no longer run it; if your drawing calls Type I, ask whether Type IC or IIB is acceptable. Suppliers matched through milcnc.com arrange Type II and Type III through their finishing processors.

Thickness and dimensional growth

Anodizing converts the aluminum surface into oxide. Part of the oxide sits above the original surface and part replaces metal below it. For Type III, the usual rule of thumb is 50 percent outward and 50 percent inward. For Type II, a smaller share builds outward, often quoted near one third.

Growth applies to every coated surface, so diameters change by twice the outward growth.

Hypothetical example: a 12 mm H7 bore

A 12.000 mm bore toleranced H7 (0 to +0.018 mm) gets Type III at 0.002 ± 0.0002 in (50 ± 5 µm).

• Outward growth per side: 22.5 to 27.5 µm
• Bore diameter reduction: 45 to 55 µm
• Pre-anodize bore needed: about 12.055 to 12.063 mm to finish inside 12.000 to 12.018 mm

The 18 µm tolerance band shrinks to about 8 µm before anodize, because coating variation consumes the rest. That is achievable on a CNC with a good boring head, but it is a very different job from the drawing’s nominal H7. The usual alternatives are masking the bore or anodizing first and honing the coating to size.

Threads

On a 60 degree thread, pitch diameter changes by about four times the outward growth. A Type III coating that builds 25 µm outward moves pitch diameter by about 100 µm, far more than any 2A/2B or 3A/3B allowance. Threads are therefore masked in almost all Type III work. The alternatives are tapping oversize with the finish accounted for or fitting inserts after anodize. Never run a standard tap through a hardcoated thread to “clean it up”; it removes the coating and dulls the tap.

Where coating runs thin

Coating thickness drops inside deep bores and blind holes, and it grows perpendicular to the surface, which leaves a thin or void zone at sharp external corners. For Type III, break external edges with a radius of about 0.5 to 0.8 mm (0.020 to 0.030 in) where the design allows.

Masking

Mask what must stay dimensionally or electrically unchanged:

• Bearing bores, press fits and precision locating diameters
• Tapped holes and thread-milled threads
• Electrical grounding and bonding points (these usually get MIL-DTL-5541 Class 3 chem film instead)
• Mating faces with tight flatness where the coating would add variation

Processors mask with silicone plugs and caps, stop-off lacquer or vinyl tape. Show masked areas on the drawing with a hatched zone and a flag note, and allow a boundary tolerance (about 0.5 to 1 mm is typical for tape and lacquer). Type III also needs firm electrical contact during processing, which leaves bare rack marks. State where rack marks are permitted, otherwise the processor chooses.

Sealing

Sealing closes the pores in the oxide. Type II is sealed by default, commonly in hot deionized water, nickel acetate or dichromate. Dichromate gives good corrosion resistance and a yellow-green tint on Class 1, but it is a hexavalent process. The spec’s salt spray requirement for sealed Type II coatings is a common acceptance test (336 hours per ASTM B117 in recent revisions; check yours).

Type III is left unsealed unless the drawing or contract asks for it, because sealing softens the surface and reduces abrasion resistance. If a hardcoated part sits outdoors unpainted, for example a ground support fixture, specify the seal and accept some loss in wear performance.

6061 vs 7075 behaviour

6061-T6 (magnesium and silicon, 0.15 to 0.40 percent copper) anodizes evenly. Type II takes dye consistently and Type III comes out a uniform gray to dark gray, darker as thickness increases.

7075-T6 (5.1 to 6.1 percent zinc, 1.2 to 2.0 percent copper) produces a darker Type III coating that can be slightly less uniform. It is more prone to burning at sharp edges at high current density, so edge radii matter more. Clear Type II on 7075 often shows a faint yellow cast.

2024 (3.8 to 4.9 percent copper) is the difficult one; the spec applies a looser abrasion limit to alloys above 2 percent copper. High-silicon cast alloys give blotchy coatings.

Practical rules for any alloy: finish all machining and heat treatment before anodizing, never anodize assemblies containing steel inserts or dissimilar alloys and install helical coil inserts after finishing.

How to call out Type III on a drawing

A complete note removes most processor guesswork:

1. ANODIZE PER MIL-PRF-8625, TYPE III, CLASS 1.
   COATING THICKNESS 0.0020 ± 0.0005 IN (51 ± 13 µm).
   DO NOT SEAL.
2. MASK SURFACES INDICATED BY FLAG NOTE A.
   NO COATING PERMITTED IN THREADED HOLES.
3. RACK MARKS PERMITTED ONLY ON SURFACES INDICATED BY FLAG NOTE B.
4. DIMENSIONS APPLY AFTER ANODIZE UNLESS NOTED "PRE-FINISH".
5. PROCESSOR CERTIFICATE OF CONFORMANCE REQUIRED,
   INCLUDING THICKNESS READINGS.

Include the spec revision if the contract fixes it, the colour for Class 2 (a reference sample or FED-STD-595 number) and how thickness is verified: eddy current per ASTM B244 on production, microsection per ASTM B487 when disputes need settling. A tighter thickness tolerance such as ±0.0002 in is often possible on simple geometry, but agree it with the processor first.

Pre-anodize dimensions: who calculates them

There are three workable approaches:

1. Finished dimensions only. The drawing states after-anodize dimensions and the supplier calculates pre-finish sizes from the processor’s known growth. This is the most common approach and keeps the drawing clean.
2. Both sets of dimensions. Critical features show pre-finish and finished values. This removes ambiguity but must be kept consistent through revisions.
3. Masking. Critical fits are masked so their dimensions do not change.

Most buyers combine the first and third. Whichever you choose, ask the supplier to list pre-finish target dimensions in the DFM notes and to measure critical features before and after anodize on the first article. The AS9102 first article guide explains how to record this, and our quality page describes the inspection records matched suppliers provide. For how finish callouts fit alongside material, thread and marking requirements, see the mil-spec CNC machining guide.

If you already have a drawing with an anodize callout and want it reviewed for growth and masking before quoting, send it as an RFQ.

Sending an anodized part RFQ

Include the drawing (PDF, DWG or DXF) and model (STEP or IGES), the full finish note, alloy and temper, quantities and whether dimensions apply before or after finish. Mark masked areas and rack mark locations. The capabilities page lists the materials, tolerances and finishes routed, and parts must be non-controlled items such as housings, brackets, rails, mounts and enclosures.

We start with a mutual NDA, check the package for completeness and export sensitivity and match it with up to three qualified suppliers. Quotes arrive within 24 business hours of a complete RFQ and list the finishing processor, material certs, FAI plan and lead time. Submit your RFQ.