Surface Finishing
Anodizing grows into and out of the aluminum surface, which changes bores, shafts and threads. This guide covers Type II and Type III thickness, masking, color control and how to call out anodizing correctly.
Type II anodizing is a sulfuric acid process that produces a thin, dyeable oxide layer, typically 5–25 µm, used for corrosion protection and color on machined aluminum. Type III (hard) anodizing uses colder, more concentrated electrolyte and higher current density to grow a much harder layer, typically 25–75 µm, for wear resistance. Both coatings grow partly into and partly out of the surface — roughly half each way — so critical fits, threads and bores must be designed or masked with that growth in mind.
Anodizing is an electrochemical process that converts the surface of aluminum into aluminum oxide. The part is made the anode in an acid bath, and current drives oxygen into the surface. Unlike paint or plating, the coating is not deposited on top of the metal; it is grown from the metal itself, which is why it bonds so well and does not peel.
Freshly anodized oxide is porous. That porosity is what allows dyes to be absorbed, and sealing afterward closes the pores to lock in color and improve corrosion resistance.
The common reference specification is MIL-A-8625, which defines:
It also defines Class 1 (undyed, natural) and Class 2 (dyed). A typical callout reads "Anodize per MIL-A-8625, Type II, Class 2, black."
| Characteristic | Type II (sulfuric) | Type III (hard) |
|---|---|---|
| Typical thickness | 5–25 µm (0.0002–0.001 in) | 25–75 µm; 50 µm (0.002 in) is a common nominal |
| Bath temperature | Around room temperature | Near 0 °C |
| Hardness / wear | Moderate | Very high wear and abrasion resistance |
| Color options | Wide range of dyed colors | Natural color is gray to bronze; black and some dark colors are practical |
| Cosmetic quality | Excellent, uniform | More industrial look, color varies with thickness and alloy |
| Electrical insulation | Yes | Yes, higher dielectric strength due to thickness |
| Effect on fatigue | Small | Can reduce fatigue strength noticeably |
| Typical uses | Consumer products, enclosures, knobs, brackets, heat sinks | Pistons, slides, gears, valve bodies, wear surfaces |
| Relative cost | Lower | Higher (longer cycle, refrigeration, more power) |
Choose hard anodize when the surface slides, rubs or sees abrasive wear: guide rails, pneumatic cylinder bores, gear teeth, clamp jaws, tooling. It is also used when a thicker insulating layer is needed. For a part that only needs to look good and resist corrosion, Type II is cheaper and gives more consistent color.
As a working rule, about half the coating thickness penetrates into the original surface and about half builds up above it. That gives these practical effects:
Practical consequences:
Masking keeps the anodize off selected areas. Common reasons:
Methods include silicone plugs and caps for holes and threads, vinyl tapes for flat faces and liquid masking lacquers for irregular areas. Each masked area adds manual labor, and masking edges are never perfectly sharp, so tolerance the boundary of a masked zone generously (for example ±0.5 mm) and mark masked areas clearly on the drawing with a hatch or note.
A common alternative to masking for conductivity is to anodize the whole part, then machine or laser-remove the coating on contact areas. Laser removal is clean and repeatable for small pads.
Color is the most frequent source of disputes on anodized parts. Several variables shift the result even when the process is nominally the same:
To control color:
Black and clear (natural) are the most consistent colors. Bright reds, and especially light colors such as white, are difficult; true white is not achievable with standard dye anodizing.
Anodizing does not hide machining marks — it tends to reveal them. Tool marks, scratches and handling marks remain visible under the coating. Common pre-treatments:
Sharp edges anodize thinly and can chip. Break edges with a small chamfer or radius (around 0.2–0.5 mm) for better coating coverage, especially on Type III.
A complete callout includes:
Axisform provides Type II color and Type III hard anodizing together with sandblasting, brushing, polishing and laser marking, so machining allowances, masking and color can be agreed with one team. Send your drawing via our contact page for a review of finish-critical features.
Type II sulfuric acid anodizing is typically 5–25 µm (0.0002–0.001 in). Decorative parts often use around 10–15 µm, while parts needing more corrosion protection or deeper color use the upper end of the range.
Yes. Roughly half of the coating grows outward and half penetrates inward, so outer diameters grow and holes shrink by about the full coating thickness. Threads are affected most and are usually masked or machined with an allowance.
Not with standard dye anodizing. The oxide layer is translucent, so the metal underneath shows through and white comes out as a pale gray. Clear, black and dark colors are the most consistent options.
Hard anodize is naturally gray to dark bronze depending on alloy and thickness. Black is commonly available and some dark colors can be dyed, but bright colors are not practical because the coating is dense and dark.
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