Type II vs Type III anodizing — decorative vs hard-coat.

Anodizing is an electrochemical process. The aluminum part is suspended in an acid electrolyte (typically sulfuric acid for Type II, cold sulfuric for Type III) and connected as the anode. A DC voltage drives oxygen to the aluminum surface, converting the top layer into aluminum oxide (Al₂O₃).

The oxide layer grows out of the aluminum — it's not applied like paint. About 50% of the oxide grows outward (adding thickness) and 50% grows inward (consuming metal). This is why anodized parts can stack up tolerance issues if you don't account for the coating thickness.

The oxide structure has a porous outer layer (where dye and sealant penetrate for decorative anodize) and a dense barrier layer closest to the metal.

Best for:

• Cosmetic parts — consumer electronics housings, architectural trim
• Color-coded assemblies (red, blue, black for identification)
• Corrosion protection in mild environments
• Electrical insulation (anodized coating is electrically insulating)
• Prep for paint or powder coat (anodize + paint is a common automotive stack)

Limitations:

• Wears through on high-friction surfaces within weeks to months
• Chipping at sharp edges if thickness >15 μm
• Dye fading under prolonged UV (specify UV-stable dyes for outdoor)

Best for:

• Wear surfaces — sliding bearings, piston sleeves, hydraulic bores
• Salt-spray exposure — marine, defense, aerospace
• Electrical insulation at higher voltages (thicker dielectric barrier)
• Dimensional buildup where you need additional size (careful — thickness matters)
• Parts that will see abrasive media (sand, dust, abrasive particles)

Limitations:

• Natural color is gray to black-green — poor for cosmetic use
• Brittle — sharp edges can chip; parts with thin walls can crack
• Dimensional buildup: 50 μm coating means total +50 μm on diameter (part grows)
• More expensive — slower process, tighter control

Anodize changes part dimensions. Thickness grows outward, but also a fraction inward (consumed from the metal). Rules of thumb:

For 50 μm Type III:

• External diameter grows by ~50 μm total (25 μm per side)
• Internal diameter shrinks by ~50 μm total (25 μm per side)
• Flat surfaces grow by ~25 μm per surface

For precision-fit parts, this matters. A Ø20 mm shaft with Type III 50 μm anodize becomes Ø20.05 mm (approximately). If it needs to slide in a Ø20.03 mm bore, you have a problem.

Solutions:

1. Machine the part undersize, then anodize to final dimension
2. Mask critical surfaces (tape, plugs) to prevent coating in those areas
3. Specify "post-anodize machine" for the critical feature (expensive, but accurate)

Type II at 15 μm has negligible dimensional effect (±8 μm) — often within tolerance band anyway. Type III at 50 μm always requires design consideration.

Short version: wrought aluminum alloys (6xxx, 5xxx, 7xxx) anodize well. Cast alloys (380, 383, 356) don't. If your part is a die casting, consider powder coat or plating instead of anodizing.