Can 6063 aluminum be anodized?
Yes. 6063 is the architectural anodizing alloy: it gives the clearest, most uniform and most colour-consistent finish of any common aluminum. Specify AAMA 611 or MIL-A-8625 Type II, the class, thickness and colour.
6063 exists largely because it anodizes so well. It is a low-alloy magnesium-silicide extrusion alloy with fewer and finer intermetallic particles than 6061 and none of the copper of 2024 or 7075, so the anodic oxide grows dense and transparent and the surface underneath shows through evenly. Window and curtain-wall extrusions, shower doors, LED heat-sink profiles, handrails, storefronts and consumer product housings are made from 6063 for this reason. The clear finish is a bright, slightly warm silver; dyed colours (bronze, black, champagne, gold) are uniform across a batch; and the electrolytically coloured bronze and black finishes used in architecture come out consistent from lot to lot.
The architectural spec is AAMA 611 (Voluntary Specification for Anodized Architectural Aluminum), which defines Class I at 18 µm (0.7 mil) minimum for exterior and high-traffic use and Class II at 10 µm (0.4 mil) for interior work, with tests for seal quality, abrasion resistance and colour. Industrial and defence drawings cite MIL-A-8625 Type II, Class 1 (clear) or Class 2 (dyed), typically 8-25 µm (0.0003-0.001"). Type III hardcoat works on 6063 too, but the alloy is soft (about 73 HB in T6 against 95 HB for 6061), so the hardcoat is a wear skin on a soft substrate; use 6061 for hardcoat parts that carry load at the surface.
Because 6063 is an extrusion alloy, the anodizer sees extrusion defects rather than machining marks. Die lines (fine longitudinal streaks), pick-up marks and structural streaking from the billet all show through a clear anodize, and the exposed face should be identified on the drawing so the extruder puts the best surface there. A brushed, blasted or caustic-etched pretreatment before anodizing hides most die lines; a bright-dip (chemical polish) before anodize gives the mirror finish used on trim. Machined faces on an anodized extrusion, such as end cuts and milled pockets, anodize slightly differently from the extruded skin and can look a shade darker.
Anodizing for 6063 parts: where to send them
What to specify
- Specification and class: AAMA 611 Class I (18 µm) or Class II (10 µm) for architectural, or MIL-A-8625 Type II Class 1 / Class 2 with colour for industrial
- Finish designation, for example the Aluminum Association AA-M12C22A41 style callout for a mechanical finish, chemical etch and anodic coating
- Exposed (significant) surface on the extrusion drawing, so the extruder and anodizer protect it and put the rack marks elsewhere
- Pretreatment: caustic etch (matte), bright dip (mirror) or mechanical brush, because it sets the look more than the anodize does
- Colour, colour tolerance (a range sample or a delta-E limit) and the colouring method: dye or two-step electrolytic for exterior bronze and black
- Seal method and a seal-quality test (ASTM B680 acid dissolution or dye stain) when the part is exterior
Pitfalls
- Die lines and structural streaks that were invisible on the mill-finish extrusion appear the moment it is clear anodized
- Colour drift between anodizing batches and between different anodizers; order all matching parts in one lot
- Dyed colours fade outdoors; for exterior bronze or black specify two-step electrolytic colouring, not organic dye
- Treating hardcoat on 6063 as a wear surface under load: the oxide is hard but the alloy under it is soft and dents
- Machined faces on an extrusion anodize a shade different from the extruded skin; keep machined faces off the show side or blast everything first
- Anodize on the inside of a hollow profile is thin and unsealed unless the anodizer racks and agitates for it; state it if it matters
Frequently asked questions
- Why is 6063 used for anodized architectural extrusions?
- Its low alloy content and fine intermetallics give the clearest, most uniform anodic film of any common aluminum, it extrudes into thin complex profiles easily, and its colour consistency lets an anodizer match window frames across a whole building.
- Does 6063 anodize better than 6061?
- Cosmetically, yes: a little brighter and more even, and better in bright-dip finishes. Functionally the two are close. 6061 is preferred for hardcoat and machined parts because it is stronger and harder; 6063 for extruded profiles that need to look good.
- How thick is architectural anodize on 6063?
- AAMA 611 Class I is 18 µm (0.7 mil) minimum for exterior use; Class II is 10 µm (0.4 mil) for interior. Industrial Type II coatings run 8-25 µm (0.0003-0.001"); about half of any of these grows outward from the original surface.
