Ano
Two Ways to Get Black Aluminum
When an engineer or buyer calls out a black anodized finish on a print, that one line can mean two different finishes depending on which MIL-A-8625 type and class the drawing names. Getting it wrong costs money in rework, rejected parts or a finish that does not hold up in service. This guide explains what separates Type II Class 2 black from Type III Class 2 black so you can specify, or check, the right finish the first time. For how we run black work day to day, see our black anodizing service page.
What Makes an Anodized Finish Black?
Anodizing is an electrochemical process that turns the surface of aluminum into a porous aluminum oxide layer. On its own that layer is clear. To make it black, the part goes through a dye bath after anodizing, the open pores take up the dye, and the pores are then sealed to lock the color in.
In MIL-A-8625 the Class tells you whether the coating is dyed: Class 1 is undyed (clear) and Class 2 is dyed. So any black anodized part, Type II or Type III, carries a Class 2 callout. The Type tells you how thick and how hard the oxide layer underneath is. Our plain-English explanation of Type II, Class 2 goes further on the callout itself.
Type II Class 2: Sulfuric Anodize with Black Dye
Type II is the conventional sulfuric acid anodize. A dyed Type II coating is commonly 0.0002 to 0.001 inch thick, and because the part grows outward by roughly half the coating thickness, it adds only about 0.0001 to 0.0005 inch per surface. It takes dye very well, which is why it gives the deep, even black most people picture.
- Thin coating with small dimensional change, friendly to close-tolerance features
- Strong dye uptake for a deep, consistent black
- Good corrosion protection for most commercial and industrial parts
- Called out as MIL-A-8625, Type II, Class 2
Type II black is the usual choice when appearance, reduced glare or moderate corrosion protection matter more than wear, and when tight tolerances have to hold. See Type II anodizing for the full service.
Type III Class 2: Hard Coat with Black Dye
Type III, usually called hard coat, is run in a colder bath at higher current to build a much denser and thicker oxide. Hard coat is commonly specified around 0.002 inch nominal, about four to ten times a typical Type II build, and the part grows about 0.001 inch per surface at that thickness. Hard coat can be dyed black, which is what makes it Class 2. Because the oxide is denser, the black can read slightly more matte or less saturated than a Type II black on the same alloy. The reason to choose it is wear resistance, not color depth.
- Much harder, more abrasion resistant surface than Type II
- Larger dimensional change, so bores, threads and mating faces need planning
- Black is available, with a harder, slightly matte look
- Called out as MIL-A-8625, Type III, Class 2
Type III black is specified for sliding surfaces, valve bodies, actuator parts and other aluminum parts that have to resist wear while presenting a dark finish. See Type III hard coat anodizing.
Side-by-Side Comparison
- Coating thickness: Type II about 0.0002 to 0.001 in (dyed) | Type III commonly 0.002 in nominal
- Growth per surface: Type II about 0.0001 to 0.0005 in | Type III about 0.001 in at 0.002 in nominal
- Wear resistance: Type II moderate | Type III high
- Look: Type II deep, even black | Type III black with a harder, slightly matte look
- Best fit: Type II for appearance, corrosion protection and close tolerances | Type III for wear surfaces and high-load parts
- Governing spec: both are MIL-A-8625; the Type and Class together define the finish
The numbers above come from our anodizing thickness and tolerances guide, which also shows how to allow for growth on bores and threads.
How to Read the Print Callout
A complete callout for black Type II reads: Anodize per MIL-A-8625, Type II, Class 2, black. For black hard coat: Anodize per MIL-A-8625, Type III, Class 2, black. If a print says only "black anodize" with no type or class, the callout is incomplete. A good anodizer flags it before parts go in the tank, and the right fix is to have the print owner confirm the type and class in writing. Our callout decoder walks through the other fields that belong on the print.
Alloy Affects the Black
Not every aluminum alloy anodizes to the same black. 6000-series alloys such as 6061 and 6063 generally take dye well and give a consistent black. High-copper and high-silicon alloys, including many 2000-series grades and castings, can come out with a different tone. If color match across parts or batches matters, talk through the alloy and the expected result with your anodizer before you lock the finish. We cover this in more detail in why black anodizing can look different between batches.
Choosing the Right Black Finish
- Will the surface see sliding contact, abrasion or repeated loading? If so, Type III is usually the fit.
- Are there threads, close-tolerance bores or press fits on the same part? Account for growth, or plan masking and plugging for Type III.
- Is the black mainly for appearance or to cut reflection? Type II Class 2 is usually the right and more economical choice.
- Does the drawing cite a military or aerospace specification? Follow the exact type and class on the print.
Questions About Black Anodizing for Your Parts?
AnoPRO runs both Type II Class 2 and Type III Class 2 black on aluminum parts at our Paoli, Indiana facility. If you have a print with a black anodize callout and want to confirm the right process before you ship parts, contact us or call 812-620-4646. We will review the callout and flag anything that needs clarifying before your parts go into the tank.