Can 7075 aluminum be anodized?
Yes. 7075 takes Type II and Type III anodizing, but its copper and zinc content gives a darker, less uniform coating with a little less corrosion protection than 6061. Specify the type, class and thickness.
7075 anodizes by the same sulfuric-acid process as any wrought aluminum, and the coating works: it hardens the surface, takes dye and improves corrosion resistance. The difference from 6061 is in the alloy. 7075 carries 1.2-2.0% copper and 5-6% zinc, and copper-rich intermetallic particles dissolve during anodizing, leaving a coating that is slightly porous, greyer and less uniform in colour. Cosmetic parts show this as a duller, sometimes blotchy finish, especially in clear anodize; dark dyes hide it.
Type II (MIL-A-8625 Type II) is the common choice: about 8-25 µm (0.0003-0.001") of oxide, clear or dyed, for corrosion resistance and appearance. Type III hardcoat builds 25-75 µm (0.001-0.003") for wear surfaces; on 7075 the coating is a little softer and more prone to micro-cracking at edges than on 6061, so the anodizer will usually run it at a lower current density. Both types grow about half their thickness outward, which matters for fits and threads.
Because bare 7075 corrodes faster than 6061 and is susceptible to stress-corrosion cracking in the T6 temper, anodizing is often a requirement rather than a cosmetic choice on 7075 parts, and aerospace drawings pair it with a seal (nickel acetate or hot water) and sometimes a primer. For chromate conversion instead of anodizing, see chem film: it keeps electrical conductivity where anodize does not.
Anodizing for 7075 parts: where to send them
What to specify
- MIL-A-8625 Type II (decorative, corrosion) or Type III (hardcoat, wear), and Class 1 (undyed) or Class 2 with the colour
- Coating thickness, and whether dimensions on the drawing apply before or after anodizing
- Seal: hot-water or nickel-acetate seal for corrosion; unsealed only for a paint base or PTFE impregnation
- Mask threads, press fits, bearing bores and electrical contact surfaces; call out rack contact points if cosmetic
- Alloy and temper on the same drawing (7075-T6 or T73), because the anodizer adjusts the bath for it
- Edge radii of at least 0.8 mm (0.03") on hardcoat parts to stop the coating chipping at sharp corners
Pitfalls
- Colour match: clear-anodized 7075 looks darker than 6061 next to it; keep one alloy per visible assembly
- Hardcoat on 7075 is less hard and more brittle than on 6061; do not expect 6061 wear numbers
- Dimensional growth: a 50 µm (0.002") hardcoat grows a bore smaller by about 50 µm on diameter
- Smut and pitting on lots with heavy copper segregation; ask the anodizer for a first-piece sample on critical cosmetic parts
- Anodize is an insulator: mask grounding points, and specify chem film for EMI contact surfaces instead
- Stress-corrosion cracking in T6 is not fixed by anodizing; use T73 for short-transverse loaded parts
Frequently asked questions
- Does 7075 anodize as well as 6061?
- It anodizes fine functionally, but not as prettily. The copper in 7075 makes the coating darker and less uniform, so clear cosmetic parts are usually made from 6061 or 6063 and 7075 is anodized for protection or wear, often dyed black.
- Can 7075 be hardcoat anodized?
- Yes. Type III hardcoat works on 7075 and is common on tooling, firearm parts and aerospace fittings. The coating is slightly softer than on 6061 and the anodizer runs a gentler cycle; specify the thickness and radius the edges.
- How much does anodizing change 7075 dimensions?
- About half the coating thickness grows outward from the original surface. A 25 µm (0.001") Type II coat adds roughly 12 µm (0.0005") per surface; a 50 µm (0.002") hardcoat adds about 25 µm (0.001") per surface, or 50 µm on a diameter.
