Type II vs Type III Anodizing: Which Spec

Both specs are anodizing — an electrochemically grown oxide layer that becomes part of the aluminium rather than a coating on it. The Roman numerals describe how far that growth is taken, and nearly every practical difference between them — thickness, wear behaviour, colour, masking, cost — flows from that one variable.

Reviewed by Tom, Senior Process Engineer·Dongguan source factory · 11+ years in precision sheet metal

anodized aluminum emblem — type ii vs type iii anodizing

What the two specs actually change

Both are sulphuric-acid anodizing per the MIL-A-8625 / MIL-PRF-8625 family of specifications. Type II runs a moderate layer — engineering references typically quote around 10–15 µm for decorative work — while Type III pushes a much thicker, denser coating, commonly in the 35–50 µm region depending on specification and application. Typical reference values only: actual thickness is specified per project and confirmed on the drawing.

Thickness drives everything downstream. The Type III layer is hard enough for wear surfaces and far more protective in abrasive duty, but it grows further into and out of the metal — dimensions move more, radii and threads close up more, and the thicker film takes dye less brightly. Type II stays cosmetic: vivid blacks, blues and clear satin finishes on enclosure faces that never see a wear path.

Side by side

Criterion	Type II	Type III (hard)
Typical layer	~10–15 µm (reference range)	~35–50 µm (reference range)
Colour range	Wide — clear, black, dyed colours read true	Muted — darker, less vivid dyes
Wear behaviour	Good for cosmetic surfaces	The reason the spec exists — abrasive duty
Corrosion protection	Strong, sealed	Stronger, longer
Dimensional growth	Small — usually tolerable on folded parts	Larger — mask or pre-size critical fits
Masking demands	Standard	Every critical dimension needs a decision
Cost direction	Reference	Higher — longer process, tighter control

Type II vs Type III — the practical differences

The decisions that follow in fabrication

Masking is where the specs diverge most in the workshop. Type II growth is small enough that most folded-enclosure features tolerate it; Type III on a formed part needs every tapped hole, mating surface and dimension-critical region masked or pre-sized, and the drawing should say which. Anodizing grows roughly half in and half out, so a bore that must hold size after Type III is a pre-machining conversation, not a discovery.

Colour is the other fork. Consumer-facing aluminium — front panels, visible faces — nearly always specifies Type II precisely because the dye range is wide and consistent. Type III colours skew dark and alloy-dependent; a "black anodized" structural bracket in Type III will not match a Type II front panel, and cross-spec colour matching is settled with a sample panel, not a hope.

One constant across both specs: the base alloy matters. 5052 and 6061 anodize reliably with slightly different film tone, which is why our grade pages call out anodizing behaviour per alloy and why mixed-alloy assemblies get their sample panels agreed at quotation.

How to choose, stated plainly

Visible faces, colour matters, no wear path → Type II
Wear surface, sliding contact, abrasive or harsh outdoor duty → Type III
One part doing both jobs (cosmetic + wear) → Type III with masked cosmetic areas, or split the parts
Tight post-coating tolerances on holes and fits → whichever spec, but the masking plan comes first
Unsure → send the drawing with the duty described; the quotation names the spec we would run

Frequently asked questions

Is Type III anodizing always better?

No — it is different. Type III wins on wear and protection but loses on colour range, dimensional growth, masking effort and cost. For a cosmetic enclosure face with no wear path, Type II is the correct specification and Type III would be over-specification with visible downsides.

How much does anodizing change part dimensions?

The oxide grows partly into the metal and partly outward — roughly half each way. Type II's growth is small enough that folded sheet parts rarely care; Type III's thicker layer moves surfaces more, which is why critical fits are masked or pre-sized. Exact growth per surface is a drawing-level conversation, confirmed per project.

Can Type III be dyed black?

Yes, but expect a darker, less vivid result than Type II black, with tone varying somewhat by alloy. Where two parts must match — say a Type III bracket beside a Type II panel — agree a sample panel at quotation rather than assuming a match.

Does anodizing work on steel or stainless?

No — anodizing is an aluminium process (the grade pages cover this too). Steel corrosion protection comes from zinc plating, galvanised sheet or coatings; stainless takes passivation and mechanical finishes instead.

Which alloys suit these specs best?

5052 and 6061 — the alloys we fabricate most — both anodize reliably in either spec, with 6061 giving a very consistent film and 5052 a neutral tone. The full behaviour is on each alloy's material page.