Your print says MIL-A-8625 Type II Class 2, or Type III Class 1. Here is the whole matrix: types, classes, thickness, sealing, growth and the callout.
MIL-A-8625 is the U.S. defense specification for anodic coatings on aluminum. Reclassified as a performance specification, it is now MIL-PRF-8625F, currently at Amendment 2, 23 November 2020, superseding MIL-A-8625F w/Amendment 1 of 2003.
Both numbers stay in circulation because drawings outlive specifications. A print calling MIL-A-8625 Type II Class 2 needs no revision; the type and class structure carried over unchanged. The specification controls what the coating must do, not bath chemistry or racking, which is why two compliant shops produce visibly different parts.
Thin chromic anodize; IB is the low-voltage variant, 22 ±2 V. For fatigue-sensitive parts and crevices that trap sulfuric acid. Minimum 200 mg/ft².
Non-chromate replacement for Type I and IB. 200 to 700 mg/ft².
What a print means when it just says “anodize”. Minimum 1000 mg/ft², porous enough to dye.
A second non-chromate alternative to Type I and IB. 200 to 1000 mg/ft².
Hardcoat: the only type controlled by a Taber abrasion requirement. Nominal 0.002 inch unless the drawing says otherwise.
Class is only color. Class 1 is non-dyed, Class 2 is dyed. Class 2 adds light-fastness: fading no greater than Delta E 3.
| Callout | Process | Spec thickness range | Color | Normally specified for |
|---|---|---|---|---|
| Type I Class 1 | Chromic acid | 0.00002 – 0.0007 in | Undyed; clear to light gray | Fatigue-critical airframe detail, faying surfaces, pre-paint base |
| Type I Class 2 | Chromic acid, dyed | 0.00002 – 0.0007 in | Dyed | Rare; thin chromic films hold dye poorly |
| Type IB Class 1 / 2 | Chromic, low voltage 22 ±2 V | 0.00002 – 0.0007 in | Undyed / dyed | As Type I, where voltage is restricted |
| Type IC Class 1 / 2 | Non-chromic acid | 0.00002 – 0.0007 in | Undyed / dyed | Chrome-free replacement for Type I |
| Type II Class 1 | Sulfuric acid, undyed | 0.00007 – 0.0010 in | Clear / natural | Corrosion protection, paint and adhesive base, insulation |
| Type II Class 2 | Sulfuric acid, dyed | 0.00007 – 0.0010 in | Black, red, blue, gold and the rest of the range | Color coding, identification, cosmetic hardware |
| Type IIB Class 1 / 2 | Thin sulfuric | 0.00002 – 0.0007 in | Undyed / dyed | Chrome-free thin film; fatigue and paint adhesion |
| Type III Class 1 | Hard anodize, undyed | 0.0005 – 0.0045 in (0.002 in nominal unless stated) | Natural — gray to dark bronze, alloy dependent | Wear surfaces, bores, pistons, valve bodies, actuation hardware |
| Type III Class 2 | Hard anodize, dyed | 0.0005 – 0.0045 in | Practically black; other colors are limited | Wear surface that also needs a dark finish |
Ranges the specification recognizes, not what a shop reaches on every alloy.
An as-formed coating is porous; sealing closes the pores. Type I, IB, IC, II and IIB coatings must be completely sealed unless the drawing says otherwise — by dichromate, hot deionised water, or nickel or cobalt acetate.
Type III is where prints go wrong: MIL-PRF-8625 states Type III shall not be sealed where the main function is maximum abrasion or wear resistance. Sealing a hardcoat buys corrosion performance and softens the surface: wear life traded for salt spray life.
“Type III Class 1, unsealed” and “Type III Class 1, sealed” are different parts with different service lives. The specification will not choose for you.
Anodize converts the substrate rather than plating onto it. Half the coating grows inward and half builds outward, so each surface grows by about half the coating thickness. On Type II that is noise; on Type III at 0.002 inch it is 0.001 inch per surface:
Mask threads, cut them after anodize, or open the pitch diameter first — and say which on the drawing. Our design guide covers masking callouts.
MIL-PRF-8625 states it plainly: the fatigue properties of aluminum alloys can be severely reduced by anodic coatings, and as a general rule the thicker the coating, the greater the reduction. The brittle layer cracks first under cyclic load and hands the substrate a stress riser.
That is why a structural drawing will call Type I or IB on a fatigue-critical detail and prohibit Type III outright, or confine hardcoat to a defined wear zone. Where no anodic layer is acceptable, chemical conversion coating gives corrosion protection and a paint base at negligible thickness.
That drives Class 2 color: dye sits in the pores and pore structure follows alloy and temper, so different alloys will not dye to an identical shade. Where color match matters, keep the assembly on one alloy and run it as a lot. A bright dip helps consistency but removes material.
The AMS set splits dyed from undyed into separate documents, which is why an AMS callout carries no class suffix.
Stated as the document states it, not as a performance claim. Types I, IB, IC, II and IIB are tested 336 hours in 5 percent neutral salt spray per ASTM B117, Class 1 and Class 2 alike — class does not change the corrosion requirement. Acceptance is by pitting: no more than 15 isolated pits, none larger than 0.031 inch. Type I and IB must also show no patchy dark gray areas.
Type III is tested only when specified as sealed. Unsealed Type III has no salt spray requirement; it is controlled by abrasion instead — a maximum Taber wear index of 1.5 mg/1000 cycles, or 3.5 at 2 percent copper or higher, on CS-17 wheels under a 1000 gram load for 10,000 cycles.
Name the specification, the type, the class and — for Type III or any non-default thickness — the thickness. Color and sealing go in when not the default.
Two omissions drive most quote-stage questions: a Type III callout with no thickness, and one with no sealing statement.
Gleco Plating has finished metal in Texas since 1979: family-owned, AS9100D and ISO 9001:2015 certified across three sites, ITAR Registered, DFARS and RoHS compliant. Anodize splits across our two plating plants.
Type II sulfuric anodize, Class 1 and Class 2, runs at our Rowlett plant. The full color range is there, so all Class 2 work is sourced there — the line behind anodizing in Dallas, TX.
Type III hard anodize runs at our McAllen plant. Hardcoat is a McAllen process, not a Rowlett one. Send the drawing with thickness and sealing condition; we will confirm growth allowance.
Type II vs Type III anodizing works that decision through; electroless nickel suits a wear surface that is not aluminum. Anodize sits in a broader anodizing services line with media blasting and powder coating — see under one roof. Most of it is aerospace and defense; certifications and approvals lists what we hold.
We are AS9100D and ISO 9001:2015 certified and ITAR registered. If your program flows down an approval we do not currently hold, tell us at RFQ and we will say so plainly rather than find out at first article.
Yes. About half the coating thickness builds outward on each surface, so a bore closes by roughly the full coating thickness — about 0.002 inch on a standard hardcoat. Machine oversize before coating, mask the bore, or dimension the print as after-coating and let us grind or lap to size.
Color, and only color. Class 1 is non-dyed, Class 2 is dyed. Thickness, corrosion requirement and process are identical. Class 2 adds a light-fastness requirement: no more than a Delta E of 3 shift on exposure.
Nothing, usually. MIL-A-8625 was reclassified as MIL-PRF-8625; the current issue is MIL-PRF-8625F w/Amendment 2. The type and class structure carried over unchanged, so a MIL-A-8625 Type II Class 2 callout is processed to MIL-PRF-8625 Type II Class 2. If your flow-down is specific about revision, state it on the purchase order.
Yes — that is Type III Class 2, and black is by far the most practical hardcoat color. Undyed Type III is already gray to dark bronze, so black dyes cleanly over it while lighter colors do not. Type III runs at our McAllen plant; colors other than black are Type II Class 2 work at Rowlett.
MIL-PRF-8625 defaults to a nominal 0.002 inch when the contract, purchase order or drawing is silent, with a tolerance of ±20 percent on coatings up to 2 mils. Type III is recognized from 0.0005 to 0.0045 inch, so if you want something else, put the number on the print.
MIL-PRF-8625 states Type III shall not be sealed where the main function is maximum abrasion or wear resistance, because sealing softens the surface. Seal it when corrosion protection matters more than wear life, and say so on the drawing. Type I, IB, IC, II and IIB are sealed by default.
2024 hardcoats, but copper makes the film less uniform; the specification relaxes the Taber limit at 2 percent copper or higher and restricts Type III above 5 percent. High-silicon castings such as 360, 380 and 383 are a real problem — the specification notes a 2 mil hardcoat is extremely difficult on them, and the finish is darker and mottled. Send us the alloy with the RFQ.
Not reliably across different alloys or tempers. Dye sits in the pores and pore structure follows the metallurgy, so 6061 and 7075 will not match in the same tank. If color match is a requirement, keep the assembly on one alloy, note it on the print, and let us run it as a single lot.
Type II and the full color range run at Rowlett. Type III hardcoat runs at McAllen. Send the drawing with type, class, thickness and sealing condition; we will confirm growth allowance and routing.
Precision metal finishing since 1979. Plating, anodizing, coatings and passivation from three Texas facilities.
Our paint, powder & Cerakote division: Gleco Paint & Powder Coating
Gleco Rowlett
2220 Grisham Drive
Rowlett, TX 75088
972-475-4300
Gleco McAllen
3800 W. Ursula Ave
McAllen, TX 78503
956-800-4069
