Black anodize on aluminum: how should you specify it so it stays black?

What is the difference between Type II and Type III black anodize?

Type II Class 2 black is a conventional dyed sulfuric-acid film and gives the most even, deepest cosmetic black on wrought alloys. Type III Class 2 black is a dyed hard coat: it wears far better, but its own gray-to-bronze tint and coarser surface usually make it read as a charcoal black that is harder to match from lot to lot.

The governing document is MIL-PRF-8625F with Amendment 2 (23 November 2020), which supersedes MIL-A-8625F with Amendment 1 and keeps the paragraph numbers quoted on this page; many drawings still say MIL-A-8625F. In it, the type names the anodizing process and the class says whether the film is dyed: Class 1 is not dyed and Class 2 is dyed. Paragraph 3.12 of the specification says parts to be dyed should preferably be given the Type II treatment, and note 6.12 warns that black may not be readily obtainable on Type I, IB, IC and IIB films, which are thinner than Type II or have a different pore structure and are harder to dye. So "black anodize" on a cosmetic part means Type II Class 2 unless there is a functional reason to ask for something else.

Type III is that functional reason. A hard coat is grown in a colder bath at higher current density into a much thicker, denser film, and the specification controls it by thickness (a nominal 0.002 in, held to plus or minus 20 percent per paragraph 3.7.2.1) and by a Taber wear test, not by color. Paragraph 3.5, which defines Class 1, says any natural coloration produced by anodizing a given alloy does not count as coloration, so an undyed hard coat can legitimately come back dark gray or bronze. Dyeing black over that tint gives a dark, durable part, but the black usually looks flatter and less uniform than a Type II black on the same alloy. If the part is a sliding or abraded component, choose Type III and accept a charcoal black. If it is a front panel that people look at, choose Type II. The wider comparison of the two types is in the Type II vs Type III anodize guide.

Why do 7075, 2024 and cast aluminum parts come out brown or blotchy instead of black?

Because the alloying elements do not anodize the way pure aluminum does. Copper in 2024 and 7075 tints and disrupts the film so the black reads brown or bronze, and silicon particles and porosity in castings leave gray, mottled or bleeding areas.

The film grows by converting the metal surface to oxide, so whatever is in the metal ends up in or under the film. Copper-rich particles in 2xxx and 7xxx alloys oxidize and dissolve differently from the surrounding aluminum, which leaves a thinner, more irregular film with a natural yellow-brown cast that shows through the dye. Silicon particles in casting alloys anodize very little and remain as gray particles in the film, and MIL-PRF-8625F paragraph 3.6 accepts that dyed castings may show dye bleed-out and a lack of color uniformity from their porosity, while paragraph 3.6.1.1 leaves the acceptable degree to the buyer. The same specification, in paragraph 3.13, excludes appearance differences caused by the base metal, such as welds or mixed cast and machined surfaces, from rejection unless the order says otherwise.

The table gives the composition limits that drive this, from the wrought alloy standard and the casting alloy registrations. The practical rule for a black part is to pick the alloy for appearance where you can. 6061 and 5052 give a clean, deep black. 6063 is used for extruded profiles and typically dyes well. 7075 gives an acceptable black that is usually a shade warmer, and it will not match a 6061 part next to it. 2024 and high-silicon castings are poor choices for a cosmetic black. If strength forces 7075 onto a visible part, make every visible part in the assembly from 7075 and anodize them together; the strength side of that decision is covered in the 6061 vs 7075 guide, and castings in the die-cast anodizing guide.

How do you specify gloss and color acceptance for a black anodized front panel?

Set the gloss through the surface prep, because anodize copies the texture underneath it, and set the color with a physical master sample plus light and dark limit samples on the production alloy. Name the cosmetic faces, the viewing light and any numeric limit you intend to measure.

An anodic film is thin and translucent, so a black anodized face shows every tool mark, scratch and blend line in the metal underneath; black makes them more visible, not less. The gloss you get is set before the part reaches the anodize tank. A glass-bead blast gives a uniform matte black that hides machining marks, a caustic etch gives a satin black, and a chemical bright dip or mechanical polish gives a glossy black that shows fingerprints and every flaw. Write the prep on the drawing, for example "bead blast, uniform matte, before anodize", and say which faces get it. If gloss is a requirement and not just a description, give a gloss range measured per ASTM D523 on a named geometry; the standard covers 20°, 60° and 85° geometries, and the 85° one is the usual choice for low-gloss surfaces such as most blasted black panels.

For color, the specification leaves dye color and uniformity to the drawing or purchase order (paragraph 3.6.1) and, in note 6.6, points to SAE AMS-STD-595 (formerly FED-STD-595) only as an approximate guide, because those chips are paint. So a black part needs its own reference. Blacks differ mainly in undertone: a blue-black, a neutral black and a brown-black can all be called black, and a black that looks fine alone looks wrong next to a different one. Approve a master sample and light and dark limit samples made from the production alloy and surface prep, mark the cosmetic faces on the drawing (often labeled A), and state that undertone is judged side by side with the samples under a daylight simulator. If both sides own a color meter, a Delta E limit per ASTM D2244 against the master can be added, but on a dark surface a lightness limit alone is often more useful than a total Delta E. The limit-sample method is set out in full in the color matching guide.

What should the black anodize callout on the drawing and RFQ say?

Name the specification, type, class, color and seal, then add the alloy, surface prep, color reference, cosmetic faces, light fastness for outdoor parts, masking and rack-mark locations, and whether dimensions apply before or after anodize. Anything left out is the finisher's choice.

An example to adapt, not to copy, for an indoor machined 6061 front panel:

• ANODIZE PER MIL-PRF-8625F, TYPE II, CLASS 2, COLOR BLACK, SEALED.
• Alloy 6061-T6; all visible parts of assembly 2210 to be the same alloy and anodized in one lot.
• Faces marked A are cosmetic: bead blast to a uniform matte before anodize, no visible tool marks, scratches or rack marks.
• Color and gloss to master sample 2210-M, between limit samples 2210-L and 2210-D, viewed under a D65 daylight simulator; no brown, red or blue cast on A faces.
• Mask grounding pad G and bore B; threads to gauge after anodize.
• Rack contact marks on face C only.
• Dimensions marked with an asterisk apply after anodize.

For an outdoor housing, add one line: light fastness required per MIL-PRF-8625F paragraph 3.7.1.3, with the Delta E limit of 3 the specification sets or a tighter one, or name an electrolytically colored black and approve samples made that way. For a wear part, change the type to Type III, Class 2, give the thickness, and accept that the black will be a charcoal shade.

The finish note only works if the metal underneath is right. Black faces show machining marks, so the milling strategy on a cosmetic face, the tool path direction, the step-over and where blends and tool changes land, matters as much as the anodize. When we review a drawing for CNC milling or CNC turning with a black anodize note, we check whether cosmetic faces are marked, whether the alloy can give the black the drawing expects, and whether masking and rack-mark locations are defined, and we ask before quoting if they are not. The anodizing itself is done by a finishing source to your callout, so the clearer the note, the less is left to interpretation. Send the drawing, the alloy and any color samples through the quote form.