What anodizing actually is
Anodizing is an electrochemical process that grows a layer of aluminum oxide out of the part's own surface. It is not paint and not plating. The part is the anode in an acid bath, and current converts the outer metal into a hard, porous oxide. The pores can absorb dye, and are then sealed to lock in color and improve corrosion resistance.
Because the coating is grown from the metal, roughly half of its thickness builds outward and about half penetrates inward. This rule of thumb matters for tolerances and is the root of many questions about anodized dimensions.
The two types most buyers meet
The US military specification MIL-A-8625 is the common reference for anodizing callouts, even on commercial parts. It defines several types; Type II and Type III cover most machined parts.
| Topic | Type II (sulfuric acid) | Type III (hard anodize) |
|---|---|---|
| Typical thickness | ~5–25 µm | ~25–75 µm, often ~50 µm |
| Bath conditions | Room temperature range | Near freezing, higher current density |
| Main purpose | Corrosion resistance, appearance, color | Wear and abrasion resistance, hardness |
| Color options | Wide range of dyes, bright colors | Natural gray to bronze; dark dyes such as black work best |
| Surface feel | Smooth, follows the machined finish | Slightly rougher, harder |
| Effect on fatigue | Small reduction | Larger reduction, should be considered on cyclic parts |
| Relative cost | Lower | Higher |
Type I, chromic acid anodizing, produces a very thin film and is used mainly in aerospace, often where fatigue is critical. It is less common today because of restrictions on hexavalent chromium.
Class 1 and Class 2
MIL-A-8625 also uses classes. Class 1 is non-dyed, the natural color of the oxide. Class 2 is dyed. A callout such as "Anodize per MIL-A-8625, Type II, Class 2, black" is clear and complete for most commercial parts.
Dimensional growth
The growth rule is the most important engineering point. For a Type II coating 12 µm thick, a surface grows by about 6 µm. A hole diameter shrinks by about twice that, around 12 µm, because both walls grow inward. A shaft diameter grows by about 12 µm.
For Type III at 50 µm, the effect is four times larger: about 25 µm per surface and around 50 µm on a diameter. That is enough to turn a slip fit into an interference fit.
There are three ways to deal with growth:
- Machine critical features undersize or oversize to allow for the coating
- Mask critical features so they stay bare
- State on the drawing whether dimensions apply before or after anodizing
Most commercial drawings state dimensions after finishing. If yours does not say, ask how the shop will interpret it.
Threads and anodizing
Threads are a frequent problem. Coating builds on both flanks, which can make tapped holes tight and external threads oversized, especially with Type III. Common solutions are to mask threads, to cut threads oversize for the coating, or to chase threads after anodizing, which removes coating in the threads. Helical inserts are often installed after anodizing so the insert itself carries the thread.
Masking
Masking keeps the coating off selected areas. Reasons include electrical grounding contact, bearing bores with tight fits, threads and sealing faces.
Plugs and caps
Silicone or vinyl plugs suit holes and threads. They are quick and repeatable.
Tapes and lacquer
Used for flat faces and irregular shapes. Hand masking takes labor, so large or complex masked areas add cost.
Rack contact points
Every part needs electrical contact with the rack. The contact spot is left bare or slightly marked. If certain faces are cosmetic, indicate where rack marks are acceptable. Show masked areas clearly on the drawing, with a hatched region and a note such as "Mask, no anodize". Ambiguous masking notes are a common cause of rework.
Color and appearance
Dyed Type II offers many colors, but color matching between batches and between alloys is not exact. 6061 tends to give an even, predictable color. 7075 and 2024 can look darker or less uniform. Cast alloys with high silicon content usually anodize poorly, giving gray or blotchy results.
The machined finish shows through the coating. Tool marks, scratches and flow lines from bar stock stay visible after anodizing. If appearance matters, consider bead blasting before anodizing for a uniform matte look, or specify a fine machined finish and careful handling.
For color-critical parts, agree on a physical sample or a color range. Photographs and screen colors are not reliable references.
Hard anodize for wear
Type III is used for pistons, guides, slides, gears and any surface exposed to abrasion. It is much harder than the aluminum underneath. However, it is brittle, and the substrate remains soft, so hard anodize does not resist heavy point loads that dent the aluminum.
Sealing reduces abrasion resistance slightly, so wear-critical Type III parts are sometimes left unsealed. A PTFE impregnated hard anodize can lower friction. Mention the function of the surface so the finisher can suggest the right options.
Electrical behavior
Anodized aluminum is an electrical insulator. Grounding paths, EMI gaskets and electrical contacts need bare areas, which is usually done by masking. Alternatively, a chemical conversion coating to MIL-DTL-5541 keeps conductivity while adding corrosion protection, but it is much less wear resistant.
Sharp edges and corners
The oxide grows perpendicular to the surface, so sharp outside corners can leave a thin or void area at the edge. A small edge break, such as 0.2–0.5 mm, gives a more uniform coating and fewer chips. This matters most with thicker Type III coatings.
Choosing between Type II and Type III
- Need color, appearance and corrosion protection: Type II
- Need wear resistance on sliding or abrasive surfaces: Type III
- Tight fits that cannot be masked: Type II, or machine allowance for Type III
- Fatigue-critical parts: discuss before choosing; thinner coatings affect fatigue less
- Electrical contact: mask, or use chemical conversion coating instead
Writing the callout
A complete callout includes the specification, type, class, color, thickness if it matters, seal requirements and masking. Example: "Anodize per MIL-A-8625 Type III, Class 1, 50 µm ±10 µm, unsealed, mask threads and bore A". State whether dimensions apply after finish.
Inspection of anodized parts
Coating thickness is normally measured with an eddy-current gauge on flat areas. Appearance is checked under agreed lighting. Some programs require a salt spray or seal-quality test on sample coupons. If any of these are needed for acceptance, put them on the purchase order so they are quoted and planned.
Questions to answer before you request a quote
- Which surfaces are cosmetic, and which are functional only?
- Which features must stay bare for electrical contact or tight fits?
- Do dimensions on the drawing apply before or after the coating?
- Is the color defined by a physical sample, a range, or simply a name such as black?
- Will parts in different alloys need to match each other?
- Is there a wear, corrosion or fatigue requirement that drives the coating type?
Answering these questions up front removes most of the back-and-forth that slows anodized orders. Even a short note in the quote request, such as "mask threads, black Type II, cosmetic on top face only", helps the reviewing engineer plan the work correctly.
Rework and stripping
Anodizing can be stripped chemically and reapplied, but stripping also removes a small amount of base metal and can etch the surface. A part that is stripped and recoated more than once may lose dimensions or show a different finish. It is better to settle color, masking and thickness on a first sample than to rely on rework.
Summary
Type II is the cosmetic and corrosion choice; Type III is the wear choice. Both change dimensions in a predictable way, and both need clear masking instructions. Most anodizing problems come from drawings that leave these details unstated, not from the process itself.