What is anodizing, and when should you spec it?
Anodizing explained for buyers: how it differs from paint and powder, Type II vs Type III (hard) anodize, colour, cost, and when it is the right finish for aluminum.
Quick answer: Anodizing is an electrochemical process that grows a hard, corrosion-resistant oxide layer on aluminum (and titanium). Unlike paint or powder, the finish is part of the metal, not a coating on top, so it does not chip or peel. Type II is decorative and can be dyed a colour; Type III (hard anodize) adds real wear resistance. Spec it to MIL-A-8625 for aluminum enclosures, machined parts and anything that slides or abrades.
It is grown, not sprayed
Paint and powder sit on top of the metal and add thickness. Anodizing converts the surface of the aluminum itself into a hard oxide, so it is dimensionally stable and cannot chip at an edge. That is the whole reason to choose it on a functional aluminum part.
| Type | What it is | Best for |
|---|---|---|
| Type II | Decorative, dyeable, ~0.0002 to 0.001 in | Enclosures, panels, cosmetic parts |
| Type III (hard) | Thick, wear-resistant, ~0.001 to 0.004 in | Sliding and wear parts, aerospace |
| Dye / colour | Colour absorbed into the pores, then sealed | Branding, part identification |
When to spec anodizing over powder coat
Anodize, do not powder coat, an aluminum part that mates, slides or seals. Powder adds thickness and chips at a sharp edge; anodize is integral and dimensional. For a heat sink, hard anodize also helps it shed heat. For a bracket that just needs to look painted and live indoors, powder is fine and often cheaper. The trade is in powder coat vs paint and the powder coating cost guide if the part is steel.
Spec the type, or get three different finishes
"Black anodized" on a drawing with no type and no spec is how you get three shops quoting three different finishes and one confused inspector. Call out the type (II or III), the spec (MIL-A-8625), the colour, and any surfaces to mask. Threads and bores that must hold tolerance need masking, because even a thin anodic layer changes the fit.
Cost and what to send
Anodizing is priced by surface area, rack space, type and colour. Hard anodize costs more than decorative; custom colours and heavy masking add labour. For a ballpark by part size and quantity, use the anodizing cost estimator. Send the part size and quantity, the base alloy, the type and colour, and masking needs, then request quotes from Canadian anodizers. Most aerospace anodizing capacity sits near the Montreal corridor.
FAQ
Can you anodize any metal?
No. Anodizing is for aluminum and titanium. It does not work on steel or stainless. For steel corrosion protection, look at powder coating, plating or galvanizing instead.
Is anodizing better than powder coating?
For aluminum parts that need wear resistance, tight tolerance or heat dissipation, yes. For a purely cosmetic finish on a part that never moves, powder coat is fine and can be cheaper. Match the finish to the job.
What is hard anodizing?
Type III anodize: a thicker, denser oxide layer (roughly 0.001 to 0.004 in) that resists wear and abrasion. It is the finish for pistons, valve bodies and sliding aluminum parts.
Does anodizing change part dimensions?
Slightly. The oxide layer grows partly into and partly out of the surface, so tight-tolerance features and threads are usually masked or accounted for. Tell the shop which dimensions are critical.
Can anodized aluminum be any colour?
Type II takes dye well across a wide colour range. Hard anodize (Type III) tends toward dark grey to black naturally and takes fewer colours. Specify the colour and type together.
Last reviewed: July 2026. Related reading: powder coat vs paint and what is passivation.
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