Type I, Type II, Type IIB and Type III hard anodize, to MIL-A-8625 and the AMS equivalents. The full color range runs at Rowlett; Type III hardcoat runs at McAllen. This page is the decision, not the brochure.
Plating puts a different metal on top of yours. Anodizing does not. The aluminum is made the anode in an acid electrolyte, and its own surface is converted electrolytically into aluminum oxide. Nothing is deposited. The oxide is grown out of the metal that was already there.
That one distinction drives almost every mistake engineers make with an anodize callout, because a converted layer consumes substrate as it forms. Roughly half the coating grows inward and half builds outward. Treat that as a design rule rather than a constant — the split moves with alloy, type and bath conditions — but budget your tolerance around it and you will not be surprised.
Work the arithmetic on a Type III callout at the 0.002 inch nominal thickness. Each surface gains about 0.001 inch:
At Type II thickness the same effect is real but usually below the noise floor of the tolerance. At Type III it decides whether the part is good. Machine oversize, mask the feature, cut threads after anodize, or dimension the drawing as after-coating — and say on the print which of those you chose. Our design guide covers masking and after-coating callouts; the full type-by-class matrix with specification thickness ranges is on the MIL-A-8625 type and class reference.
MIL-A-8625 — now issued as MIL-PRF-8625 — sorts anodic coatings by the acid and the coating built, not by the look of the part. A print that just says “anodize” almost always means Type II.
Type I, chromic acid. The thinnest of the family, run in chromic rather than sulfuric acid. It is chosen when the part cannot afford the fatigue debit of a heavier coating, or when the geometry has crevices, laps and blind features that will trap sulfuric acid and bleed it back out later to attack the part. Airframe detail parts and faying surfaces are the classic application. Type IB is the same coating at restricted voltage. Chromic anodize also holds dye poorly, so Class 2 in Type I is rare.
Type II, sulfuric acid. The general-purpose anodize and the reason most parts get anodized at all: corrosion protection, a sound paint and adhesive base, electrical insulation, and a coating porous enough to take dye. Run in sulfuric acid at around 70 °F, it lands in the region of 0.0001 to 0.0005 inch in normal production. Costs less than hardcoat, is faster through the shop, and takes color properly. Type II anodizing has its own page with thickness, sealing and specification detail; AMS 2471 covers the undyed version and AMS 2472 the dyed.
Type IIB, thin sulfuric. A thin coating produced in a standard sulfuric bath, added to the specification as a chrome-free substitute for Type I. It exists to solve two problems at once: it removes hexavalent chromium from the process, and because it is thin it costs the substrate far less fatigue life than a heavier anodic layer. It also outperforms a chromate conversion coating for corrosion. MIL-PRF-8625 controls it by coating weight rather than by thickness. If your print calls Type I and your program is driving chrome out of the supply chain, Type IIB is the conversation to have.
Type III, hard anodize. Run cold and hard to build a thick, dense oxide — typically 0.001 to 0.002 inch, with 0.002 inch the nominal unless the drawing states otherwise. It is the only type the specification controls by an abrasion requirement, which tells you what it is for: wear surfaces, bores, pistons, cams, valve bodies and actuation hardware. It can be ground or lapped back to size, and it can rebuild a worn aluminum surface. It is also the type that eats your tolerance, cuts fatigue life the most, and offers the narrowest color range. Type III hard anodizing covers it in full; AMS 2468 and AMS 2469 are the AMS counterparts. If you are choosing between the two workhorses, Type II vs Type III anodizing works the decision through side by side.
Class 1 is undyed. Class 2 is dyed. That is the whole distinction — class does not change the corrosion requirement, the thickness or the type.
Class 1 leaves the natural film: clear to light gray on Type II, and gray through bronze to near-black on Type III depending on alloy and thickness. Class 2 puts dye into the pores of the as-formed coating before sealing, which is why dye works on Type II and struggles on the thin chromic films.
We run undyed clear plus black, blue, brown, green, red, purple and yellow/gold. The engineering caveat matters more than the swatch: dye sits in pore structure, and pore structure follows alloy, temper, thickness and bath condition. Two lots of the same part number, run weeks apart, will be close but not identical. Two different alloys in the same assembly will not dye to the same shade at all, and no amount of process control removes that. Where color match across an assembly is a requirement, keep the parts on one alloy and run them as a single lot, and tell us at quote that they belong together. A bright dip before anodize improves consistency and brightness, at the cost of removing a little material.
If color is a functional requirement — identification, polarity, service-life coding — say so on the drawing. If it is cosmetic, say that too. They are quoted differently.
An anodic coating comes out of the tank porous. Sealing hydrates and closes those pores. It locks dye in, and it is most of what makes an anodized part survive salt exposure.
We seal in hot deionised water, nickel acetate, sodium dichromate, or a PTFE-impregnated seal where the part wants lubricity as well as closure. Each suits a different requirement, so name the one you want rather than leaving it to the shop.
The trade-off is the part engineers miss. Sealing softens the outermost surface of the coating. On Type II that is a good bargain almost every time and the specification requires the coating to be sealed unless the drawing says otherwise. On Type III it is a real choice, and MIL-PRF-8625 says the coating shall not be sealed where the primary function is maximum abrasion or wear resistance. That is why so many hardcoat prints read “unsealed”: the part is a bearing surface, and sealing would trade the property it was specified for.
“Type III Class 1, sealed” and “Type III Class 1, unsealed” are two different parts with two different service lives. The specification will not pick for you, and neither will we without a drawing that says so.
Aluminum oxide is a dielectric. An anodized surface does not conduct, and hardcoat — thicker and denser — insulates harder than Type II. This is a genuine feature when you want it and a field failure when you did not think about it.
The failure looks the same every time. A housing is anodized all over, then reaches assembly where a bonding strap, a chassis ground screw, an EMI gasket land or a connector shell needs metal-to-metal contact. There is none. The bond reading fails, and the fix at that point is abrading a finished part.
There are two clean solutions, and both belong on the drawing rather than in a phone call after first article:
Where the whole part must conduct, anodize is the wrong family and the answer is plating or conversion coating — see EMI shielding for the enclosure case, and chem film in Fort Worth, TX for local work. Anodize also insulates deliberately and well: insulation plates, standoffs and isolation washers are a normal Type II and Type III application.
Anodizing converts the substrate, so whatever is in the substrate ends up in the result. Alloying elements that do not convert to oxide stay behind in the film and show up as color variation, patchiness or a coating that will not build.
We anodize 1000 through 7000 series wrought aluminum plus sand and die castings. If the part is a casting or a high-copper alloy and the drawing carries a cosmetic requirement, raise it at RFQ so we can tell you what is realistic before you have a first article to argue about.
MIL-PRF-8625 states it directly: 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 oxide is hard and brittle. Under cyclic load it cracks before the aluminum does, and every crack hands the substrate a stress riser at the surface, exactly where a fatigue crack wants to start.
That is the reason a structural drawing on a fatigue-critical detail will call Type I or Type IB, confine Type III to a defined wear zone, or prohibit hardcoat in words. It is not conservatism for its own sake. If you inherit a print that says “Type III not permitted”, do not read it as a preference.
Where no anodic layer is acceptable at all, chem film gives corrosion protection and a paint base at negligible thickness and negligible fatigue cost. Where the requirement is wear rather than corrosion and the fatigue budget is spent, electroless nickel or a dry film lubricant over a thin anodize are the usual routes.
Anodize is not always the answer, and it is often not the only finish on the part. Read the row for the property that actually drives your requirement.
| Finish | Wear resistance | Corrosion | Conducts? | Build per surface | Color | Relative cost |
|---|---|---|---|---|---|---|
| Type II anodize | Moderate | Good, sealed | No — dielectric | About half the coating; usually within tolerance | Full dye range | Low |
| Type III hardcoat | Highest of the group | Good sealed, none required unsealed | No — strongest insulator here | About half the coating; ~0.001 in at nominal | Natural gray to bronze; black practical, others limited | Higher than Type II |
| Chem film (MIL-DTL-5541) | None — it is soft | Moderate; the paint base standard | Yes, and the low-resistance class is specified for it | Negligible, measured in millionths | Clear or gold; color is not a design tool | Lowest |
| Electroless nickel | High, and higher after heat treat | Good, if the deposit is continuous | Yes | Full deposit thickness on every surface, uniform in bores | Uniform matte gray; no color choice | Moderate to high |
| Powder coating | Impact and abrasion, not bearing wear | Good over a proper pretreatment | No | Thickest here — thousandths; mask tolerances | Full color and gloss control | Low to moderate |
| Cerakote | Good at very low film build | Good | No | Very low film build | Wide color range | Moderate |
Routing, in one line each. Bearing or sliding wear on aluminum: Type III, or electroless nickel if the fatigue debit or the growth is unacceptable. Corrosion plus conductivity: chem film. Corrosion plus paint: chem film under primer, or Type II where a tougher base is worth the growth — CARC and MIL-spec painting and powder coating both run at our Paint & Powder facility, as does Cerakote. To compare the whole field rather than the aluminum corner of it, start at types of metal plating and finishing, or at the metal coating and electroplating hubs.
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 is split across our two plating plants, and the split is not arbitrary — route the job to the right one and it moves.
The full anodize color range is at our Rowlett plant. Any Class 2 dyed work — black, blue, brown, green, red, purple, yellow/gold — is sourced there, along with Class 1 clear Type II. This is the line behind anodizing in Dallas, TX.
Type III hard anodize runs at our McAllen plant. Hardcoat is a McAllen process. If your print calls Type III with a dyed Class 2 requirement, say so at RFQ — color and hardcoat sit at different sites and the routing needs deciding before the part ships, not after.
Most anodize work here is aerospace, defense and electronics. Anodize also sits alongside media blasting for surface preparation and the rest of our capabilities, which is what under one roof means in practice: fewer vendors between the machine shop and the assembly line. Certifications and approvals lists exactly what we hold.
If your program flows down an approval we do not currently hold, tell us at RFQ. We will say so plainly rather than find out at first article.
Seven fields. A callout carrying all of them gets quoted; a callout missing two of them generates a phone call and a delay.
6061-T6 is not 2024-T3 is not A380. It sets color, hardcoat build and what is realistic.
Type I, II, IIB or III, and Class 1 undyed or Class 2 dyed. Name the specification and revision the drawing calls.
Especially on Type III, where the specification defaults to 0.002 inch nominal if you say nothing.
And whether it is functional or cosmetic — and whether parts must match each other across a lot.
Sealed, unsealed, or a named seal chemistry. On Type III this is a service-life decision.
Which pads, bores, threads and faying surfaces must stay bare or take chem film — dimensioned, not described.
State whether the dimensions on the drawing are pre- or post-anodize. This resolves the growth question in one line.
If the callout has a problem — a hardcoat thickness that will close a bore, a color requirement across two alloys, a ground path with no masked pad — we would rather flag it at quote than at receiving inspection.
About half the coating thickness per surface, because roughly half grows inward into the aluminum and half builds outward. At Type III’s 0.002 inch nominal that is about 0.001 inch per surface: an outside diameter grows about 0.002 inch, a bore closes about 0.002 inch, and thread pitch diameter moves about four times the per-surface growth — roughly 0.004 inch on a 60° thread, enough to fail a class 3A gauge on its own. At Type II thickness the same effect is usually inside the tolerance. Machine oversize, mask the feature, or dimension the print as after-coating.
Type II is sulfuric anodize in the region of 0.0001 to 0.0005 inch, chosen for corrosion protection, a paint base, insulation and dyed color. Type III is hard anodize, typically 0.001 to 0.002 inch, run colder to build a denser oxide and controlled by an abrasion requirement rather than by appearance. Type III wears far better and can be ground or lapped to size; it also costs more, takes more tolerance, cuts fatigue life more, and offers a much narrower color range. Type II vs Type III anodizing compares them property by property.
Black is the practical Class 2 color on hardcoat, and heavy Type III often reads dark gray to near-black in Class 1 with no dye at all. Beyond black the hardcoat color range is limited, and dyed hardcoat will not match a Type II part of the same nominal color. There is also a routing consequence at Gleco: the full color range is at Rowlett and Type III runs at McAllen, so a dyed hardcoat requirement needs discussing at RFQ rather than assumed.
No. Aluminum oxide is a dielectric, and hardcoat insulates more than Type II. If any surface on the part needs to carry a ground, take a bonding strap, seat an EMI gasket or make contact through a connector shell, that surface must be masked off during anodize or taken to chem film, which protects against corrosion while staying conductive. Put the masked zones on the drawing with dimensions.
No. MIL-A-8625 was reclassified as a performance specification and is now issued as MIL-PRF-8625, currently revision F with Amendment 2 dated 23 November 2020. The type and class structure carried over unchanged, so a drawing calling MIL-A-8625 Type II Class 2 is unambiguous and we process it. Both numbers stay in circulation because drawings outlive specifications. The MIL-A-8625 type and class reference lays out the full matrix.
Yes — we anodize sand and die castings as well as 1000 through 7000 series wrought aluminum — but set the expectation before the part runs. High-silicon casting alloys such as 356, 360, 380 and 383 contain free silicon that does not convert to oxide, so the film comes out darker, grayer and less uniform than on wrought material, and a heavy Type III build on the high-silicon die-cast alloys is difficult and sometimes not achievable to print. If the casting carries a cosmetic requirement, tell us at quote.
It depends which property the part was specified for. Sealing closes the pores and improves corrosion resistance, but it softens the outer surface and gives back some of the wear performance you paid for. MIL-PRF-8625 says Type III shall not be sealed where the primary function is maximum abrasion or wear resistance, which is why most bearing-surface hardcoat prints read “unsealed”. Type II is sealed by default unless the drawing says otherwise. State it either way — sealed and unsealed hardcoat are different parts.
Yes, and on electronics hardware it is the normal construction rather than an exception: anodize over the body of the part for corrosion and appearance, with bond pads, gasket lands and cover-screw bosses masked and taken to MIL-DTL-5541 chem film so they stay conductive. Masking threads, bores and interfaces is routine rack work. Define the masked zones explicitly and dimensionally on the drawing — “mask threads” alone leaves too much to interpretation.
Alloy, type, class, thickness, color, seal and masked surfaces. If any of those are missing or fighting each other, we will say so at quote.
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
