The Anodized Part That Changed Color (Hard Anodize)
We anodized an aluminum bracket (a Type II anodize, the standard). On the floor, the part was a nice color (black). But a second batch (a different alloy, 6061 vs 7075) came out a different shade (grey). The problem: the anodize color depends on the alloy (6061 anodizes evenly, 7075 is darker/less even). We either used the same alloy (for color consistency) or accepted the variation (it was non-critical). We used 6061 (consistent color). The mistake was mixing alloys (the anodize color varied).
Anodizing (aluminum) — the process and the design. This article covers the basics.
What is Anodizing?
Anodizing is an electrochemical process (the aluminum part is the anode). It grows an oxide layer (Al₂O₃) on the surface. The layer is hard (wear-resistant) and corrosion-resistant. It can be dyed (a color, e.g., black).
Types
- Type II (standard): A thin layer (10–25 µm). Decorative (dyed). For a general bracket. Wear-resistant (not for a sliding surface).
- Type III (hard anodize): A thick layer (50–100 µm). Hard (wear). For a sliding surface (a wear pad). More expensive.
Step 1: The Alloy (Color Consistency)
The anodize color depends on the alloy. 6061 (the standard aluminum) anodizes evenly (consistent color). 7075 (a high-strength) anodizes unevenly (darker, blotchy). For a color-critical part (the visible surface), use 6061. For a non-critical (no color), any alloy.
The anodize rule: Use 6061 (consistent color) for a visible/dyed part. The part that changed color was 7075 (uneven). Use 6061. For a non-critical, any alloy. Type II (decorative) vs Type III (hard, for wear).
Step 2: The Threads (Masking)
Anodizing builds a layer (it adds 10–25 µm on each side). A tapped hole (after anodize) gets smaller (the layer fills it). Either tap the hole after anodizing (the tap cuts through the layer), or mask the hole (the anodize doesn’t cover it). Don’t anodize a tapped hole (it becomes undersized).
Step 3: The Color (Dye)
The anodize layer is porous (it absorbs dye). Dye it (black, clear, etc.). The dye is locked (after sealing, a hot-water dip). For a clear (natural), no dye. For a black, dye black. The color depends on the alloy (step 1).
Step 4: The Sealing
After anodizing (and dye), seal the part (a hot-water, or nickel acetate dip). The sealing closes the pores (the dye is locked, the layer is sealed). Without sealing, the dye washes out (and the layer is less corrosion-resistant).
| Type | Layer | Use |
|---|---|---|
| Type II (standard) | 10–25 µm | Decorative (dyed) |
| Type III (hard) | 50–100 µm | Wear (sliding surface) |
| 6061 alloy | Even color | Visible/dyed |
| Tapped hole | Mask (or tap after) | Undersized if anodized |
An Anodizing Checklist
- Is the part aluminum? (Not steel?)
- Is it Type II (decorative) or III (hard)?
- Is the alloy 6061? (Color?)
- Are the threads masked? (Or tap after?)
- Is the color right? (Dye?)
- Is it sealed? (After?)
- Is the layer right? (µm?)
- Is the wear OK? (Type III?)
- Is the corrosion OK? (Sealed?)
- Is the cost right? (Hard anodize?)
The Bottom Line
Anodizing (aluminum) is decorative and wear-resistant. The part that changed color was 7075 (uneven). Use 6061 (consistent). Mask the threads. Type II (decorative) vs III (wear). The part that was consistent (and sealed) wasn’t the cheapest finish — it was the right anodize.
