Anodizing Service for Small Aluminum Parts

Type II versus Type III hardcoat

Type II is the sulfuric anodize most aluminum parts get: a moderate layer, 0.00007 to 0.001 in thick (source 1), that raises resistance to corrosion and wear (source 1) and accepts dye (source 1). A print names the class as well as the type: Class 1 is non-dyed and Class 2 is dyed (source 2). Dye colors named in the reference include black, red and blue (source 1), and the shade varies with the base alloy (source 1).

Type III, hard anodize or hardcoat, is the same process taken further: a layer thicker than 0.001 in (source 1), grown in more concentrated solutions at lower temperatures with higher voltages and currents (source 1). Thickness adds wear resistance (source 1), so hardcoat is the callout for sliding faces and wear surfaces. The print names the type, class, color and thickness (source 2), and the quote follows the print.

Dimensional growth and what gets masked

Anodizing consumes some aluminum and builds some on top, so it changes dimensions. The Aluminum Anodizers Council’s rule of thumb is 2/3 in and 1/3 out for Type II and half in, half out for hardcoat (source 3). Its example: a 0.6 mil (0.0006 in) Type II coating reduces a hole diameter by 0.4 mil (0.0004 in), and a 1.5 mil (0.0015 in) Type III coating by 1.5 mil (source 3). A general practice on drawings is to state that dimensions apply after all surface finishes (source 1). Tight bores, ground features and press fits have to allow for that growth. In small threaded holes anodizing may make screws bind, so the holes may need to be chased with a tap to restore the original dimensions (source 1).

The anodic layer is insulative (source 1), so any face that needs grounding or electrical contact has to stay bare. Sections of a part can be masked; holes and bores can usually be masked without much difficulty, and flat areas can sometimes be harder (source 3). Mark those surfaces and any threads on the print, the same way you’d call out a tolerance, and the quote follows the print. If you coat parts yourself and need a machined mask that holds a boundary to a number, see masking fixtures.

Color, blasting, and alloy

Surface preparation shapes the final anodized look (source 5). Where a print calls for a bead blast before anodize, blasting can make minor machining lines less noticeable and gives a more uniform matte (source 5). If the look matters, say so up front. A finish called out on the print is quoted with the part. Because dyed colors vary with the base alloy (source 1), a 6061 part and a 7075 part dyed to the same color can read slightly different; one alloy across a matched assembly removes that variable.

On alloy: 6061 and 7075 are covered on their grade pages. The Aluminum Anodizers Council’s alloy reference lists 5xxx, 6xxx and 7xxx (zinc) alloys as clear with good protection and copper-bearing 2xxx as yellow with poor protection (source 6), and its FAQ calls the 2000 series generally the most difficult to anodize and the 5000 or 6000 series the easiest (source 3). Flag high-copper alloys early. This is a coating story for aluminum only: titanium ‘anodizing’ is a different process where color comes from oxide thickness set by voltage rather than dye (source 1), and that’s covered on the titanium material page.

Anodize or powder coat

Anodize is a film that grows into and out from the metal (source 3); powder coating is a separate opaque film applied over the metal. For aluminum brackets and housings, the choice usually comes down to appearance, wear and how much dimension the finish may use. If you want an opaque color coat and the tolerances can give a little, powder is the other route; see powder coating, and the full menu of options on CNC surface finishes. Either way the finish is specified with the part and quoted with it, so the finish, the masking and the tolerances are reviewed together.

Sources

All accessed 30 September 2026. Where a figure here differs from your print, the print governs.

1. Wikipedia, "Anodizing": en.wikipedia.org/wiki/Anodizing (Type II 1.8 to 25 μm, 0.00007 to 0.001 in, and Type III thicker than that, named for MIL-A-8625; the anodic layer is insulative; dye colors vary with the base alloy; harder films come from more concentrated solutions, lower temperatures and higher voltages and currents; thickness increases wear and corrosion resistance; threaded holes may need chasing; the general drawing practice that dimensions apply after all surface finishes; titanium color set by oxide thickness and voltage).

2. Aluminum Anodizers Council, "Military Specification MIL-A-8625": anodizing.org/military-specification-mil-a-8625 (types and two classes, Class 1 non-dyed and Class 2 dyed; coating thickness called out on the purchase document or part drawing; example designation giving type, class, color and thickness).

3. Aluminum Anodizers Council, "A Conclusive FAQ Guide for Anodized Aluminum": anodizing.org/anodized-aluminum-faq/, no date shown (2/3 in and 1/3 out for Type II and half in, half out for hard coat; hole diameter examples of 0.4 mil for a 0.6 mil Type II coating and 1.5 mil for a 1.5 mil Type III coating; sections of a part can be masked, holes and bores usually without much difficulty; copper-containing 2000 series generally the most difficult and 5000 or 6000 series the easiest; 1 mil = 0.001 in).

4. Products Finishing, Larry Chesterfield, "What Is the Correct Anodizing Specification?", 1 May 2010: pfonline.com/articles/what-is-the-correct-anodizing-specification (a drawing callout gives the specification, type, class and a minimum thickness).

5. ALCircle, Aranya Mondal, "Bead Blasting: The Finishing Process Behind High-Quality Aluminium Products", 15 June 2026: alcircle.com/news/bead-blasting-the-finishing-process-behind-high-quality-aluminium-products-119908 (bead blasting makes minor machining lines less noticeable and gives a more uniform finish; surface preparation quality influences the final anodized appearance).

6. Aluminum Anodizers Council, "Aluminum Alloy Reference for Anodizing" (Technical Bulletin 4-13, issued March 1994 as the page states): anodizing.org/alloys-suitable-for-anodizing/ (alloy series by coating character: 5XXX, 6XXX and 7XXX clear with good protection, 2XXX copper yellow with poor protection).