Both finishes protect aluminium, but they are engineered for different purposes. This guide compares them on colour, hardness, thickness and dimensional impact, with a clear rule for selecting the right one.
What anodizing is
Anodizing uses an electrochemical bath to grow a hard aluminium-oxide (Al₂O₃) layer into the metal, rather than laying a coating on top. Because the layer is part of the surface itself, it will not chip or peel. A single process delivers three functions: protection, decoration and electrical insulation.
Side by side
Representative values from our shop, confirmed for each part during DFM review; thickness ranges reflect how we typically run Type II and Type III in-house, and other shops may differ. Cost is shown on a relative scale.
Type II (sulfuric)
Type III (hard)
Thickness
3–5 µm
8–25 µm
Hardness
~300–400 HV
~400–600 HV
Colour
Any colour / RAL
Dark only (black / grey)
Wear
Moderate
High
Corrosion
Moderate
High
Cost
$$
$$$
Best for
Colour and cosmetic finishes
Wear surfaces, sliding fits, aerospace
Type II (standard anodize)
A thin film with good corrosion resistance and a wide colour range. It's the default choice for coloured and cosmetic parts where appearance matters.
Type III (hard anodize)
A thick, hard, wear-resistant layer for sliding surfaces, abrasion-prone parts and aerospace work. The trade-off is colour: only a few dark shades are available.
Account for dimensional growth: the oxide layer grows roughly half into the part and half outward, so a Type III hardcoat adds about 0.01 mm per face. On tight bores and fits, mark the critical dimensions on your drawing; we will either mask them off or machine the part slightly undersize to compensate.
From our shop floor: for an even colour, we bead-blast the part first so the dye takes uniformly. And because 6061 anodizes more evenly than 7075, it is the preferred choice for a cosmetic coloured part.
Three common mistakes
Writing "anodize" with no type — always specify Type II or Type III.
Ignoring Type III growth on tight bores — machine the bore undersize, or mask it off, to leave room for the coating.
Expecting a bright colour from Type III — it's available only in dark shades.
Selection rule
Colour or cosmetics → Type II. Wear or hardness → Type III. Threads and critical bores → mask them off.
