For a visible anodised part, the alloy and the way it was made decide the final look more than the anodising line does. Silicon, iron, copper and manganese all leave their mark in the oxide, and grain structure shows through as streaks or sparkle. This guide matches alloys to finishes (bright, mirror, natural, bead-blasted, brushed and black) and lists the material checks that prevent rejected batches.
Anodising does not add a coating on top of the metal; it converts the surface of the aluminium itself into aluminium oxide. Pure aluminium oxide is colourless and transparent, so anything else in the metal ends up inside the film or under it. Alloying elements and impurity particles either dissolve, stay in the film as inclusions, or leave tiny voids, and each of these changes how light passes through the oxide. That is why two parts anodised side by side in the same tank can look completely different.
For structural parts, designers choose an alloy for strength, then ask whether it can be anodised. For appearance parts the order reverses: start from the finish the customer will see, then find the strongest, most formable alloy that can deliver it consistently. Treating these as separate decisions is the most common source of rework.
| Element or feature | Effect on the anodised appearance |
|---|---|
| Silicon (free Si particles) | Grey to dark grey, cloudy film; strongest effect in cast and high-Si alloys |
| Iron (Al-Fe intermetallics) | Reduced clarity and brightness; a milky or dull look on bright finishes |
| Copper | Yellowish tint and softer film; above a few percent, poor anodising quality |
| Manganese | Grey or brownish tint, often uneven; why 3003 anodises poorly for cosmetics |
| Magnesium in solid solution | Little colour effect at low levels; above about 3 % films can turn yellowish in thick coatings |
| Chromium | Faint yellow tint in thicker clear films, noticeable in 5052 |
| Zinc (7xxx) | Generally acceptable colour, but film can vary with heat treatment |
| Coarse or mixed grain | Visible streaks, bands or "orange peel", most obvious after etching and bright dipping |
High-purity base metal with very low iron and silicon. Polishes and chemically brightens to a near-mirror and keeps that brightness through anodising. Strength is modest, so this route suits trim, cosmetic packaging, reflectors and decorative parts that carry little load.
Aluminium-magnesium alloys with controlled impurities. 5005 is the architectural "anodising quality" sheet that matches 6063 extrusions; 5252 is a high-purity bright-trim alloy with 5052-level strength and a clearer film.
Balanced strength, formability and machinability for enclosures, consumer electronics, appliance fronts and handles. Usually finished bead-blasted or brushed with natural or black anodising.
Used when a visible part must also carry load, such as frames, mid-frames and sports equipment. Requires the tightest control of heat treatment and machining, and the finish is chosen to hide what cannot be controlled.
| Alloy | Key chemistry (wt %) | Typical forms | Anodised look | Best for |
|---|---|---|---|---|
| 1050 / 1070 | Al ≥ 99.5 / 99.7 | Sheet, coil, slugs | Very clear, bright, silvery | Polished trim, cosmetic caps and tubes, reflectors |
| 5657 | Mg 0.6 – 1.0; Si ≤ 0.08, Fe ≤ 0.10 | Sheet, extrusions | Bright, high specular reflectance | Bright automotive and appliance trim |
| 5005 | Mg 0.5 – 1.1; Fe ≤ 0.7 | Sheet, coil | Clear silver, can be slightly grey | Architectural panels matched to 6063 |
| 5252 | Mg 2.2 – 2.8; Si ≤ 0.08, Fe ≤ 0.10 | Sheet, coil | Clearer and brighter than 5052 | Premium housings and bright trim needing strength |
| 5052 | Mg 2.2 – 2.8; Cr 0.15 – 0.35 | Sheet, coil, plate | Good; faint yellow in thick clear films | Mainstream housings, panels, enclosures |
| 6063 | Si 0.2 – 0.6, Mg 0.45 – 0.9 | Extrusions, tube, sheet | Uniform, clear; the reference extrusion alloy | Frames, trim, profiles, handles |
| 6463 | As 6063 with Fe ≤ 0.15 | Extrusions | Bright, chemically brightens well | Bright-dipped trim and decorative profiles |
| 6061 | Mg 0.8 – 1.2, Si 0.4 – 0.8, Cu 0.15 – 0.4 | Plate, bar, extrusions | Good, slightly greyer than 6063 | Machined housings and structural visible parts |
| 7075 | Zn 5.1 – 6.1, Mg, Cu 1.2 – 2.0 | Plate, bar | Acceptable; tint varies with Cu | High-strength parts where looks are secondary |
Chemistry ranges summarised from the Aluminum Association registrations. 5657, 5252 and 6463 are standard designations whose low iron and silicon limits exist specifically for bright anodised finishes.
Alloys to avoid for cosmetic anodising include 3003 and other manganese-bearing grades (grey, blotchy), 2xxx copper alloys (yellow, soft film), 4xxx silicon alloys (dark grey) and most die-casting alloys such as A380 and ADC12, whose high silicon gives a dark, uneven film. If a cast part must match wrought parts, consider painting instead.
| Finish | What it demands of the material | Suitable alloys |
|---|---|---|
| Mirror / bright anodised | Very low Fe and Si, fine grain, defect-free surface that survives polishing and chemical brightening | 1050, 1070, 1085, 5657, 5252, 6463 |
| Natural (clear) anodised | Consistent chemistry and grain across the batch; cleanliness, no roll marks | 5005, 5052, 6063, 6061 |
| Bead-blasted + anodised | Uniform grain and hardness so the blasted texture is even; the matte texture hides minor substrate variation | 5052, 5252, 6063, 6061, 6013 |
| Brushed + anodised | Grain direction consistent with the brush direction; no laminations that open under abrasion | 5052, 5005, 6063, 1050 |
| Black or dark dyed | Uniform film thickness and porosity; dark colours exaggerate differences in alloy and temper | 6063, 6061, 5052, 5252 |
| Light and pastel dyed | High clarity film; any grey from Si or Fe dulls the colour | 5252, 5657, 6463, 1050 |
Black anodising deserves special mention because it is unforgiving. A black part that looks perfect under the line lights can show bluish, reddish or greenish casts in daylight, and adjacent parts from different coils or extrusion billets may not match. Specify one alloy, one temper and, where possible, one material lot for parts that will sit next to each other in the assembly.
Laptop lids, tablet backs, speaker grilles, smart-home devices and SIM trays. Sheet housings are commonly 5052 or 5252; machined unibody parts use 6061, 6063 or 6013, and some premium phones use 7xxx alloys. Bead-blast plus natural, grey or black anodising is the usual finish.
Appliance fronts, elevator panels, furniture and automotive interior trim. 5005 and 5252 sheet for flat panels; 6063 and 6463 for profiles; brushed or natural finishes.
Lipstick cases, perfume caps, cosmetic tubes and bottle collars, often deep-drawn or impact-extruded. 1050, 1070 and high-purity 5xxx; polished or bright-dipped before coloured anodising.
Facade cladding, column covers and window trim where panels and extrusions must match. 5005 sheet paired with 6063 extrusions, natural or bronze anodised to AAMA 611 or Qualanod requirements.
Bicycle components, camera rigs, drone frames and tool housings. 6061-T6 and 7075-T6 machined parts, usually bead-blasted and dark anodised to hide machining variation.
High-purity 1085 to 1199 grades, or 5657, brightened and anodised for high reflectance and protection.
For extrusions, ask about billet quality as well as alloy. Homogenised billet with fine, uniform grain, extruded at controlled temperature and quenched evenly, is what makes 6063 anodise uniformly. Streaks along a profile usually trace back to die lines, coarse grain at the surface, or uneven quench, not to the anodising line.
Most appearance parts are finished with conventional sulphuric acid anodising (MIL-A-8625 Type II; ISO 7599 classes from about 5 to 25 µm). It accepts dyes in a full range of colours, keeps a bright or satin look, and is thick enough for indoor durability at 10 – 15 µm and for outdoor architectural use at 20 – 25 µm.
Hard anodising (Type III) builds a much thicker, harder film, but it is naturally grey to bronze or dark grey depending on alloy, takes dye less evenly and dulls a bright surface. Use it where wear resistance is the main requirement, and expect a functional rather than decorative look. Assuming that thicker or harder means "better looking" is a common mistake.
| Parameter | Type II (decorative) | Type III (hard) |
|---|---|---|
| Typical film thickness | About 5 – 25 µm | About 25 – 100 µm |
| Natural colour | Clear to silvery | Grey to dark bronze, alloy-dependent |
| Dyeing | Excellent, wide colour range | Limited, mostly dark colours |
| Brightness retention | Good on bright substrates | Poor |
| Main purpose | Appearance and corrosion protection | Wear and abrasion resistance |
| Defect | Typical material cause | Prevention |
|---|---|---|
| Streaks along an extrusion | Die lines, coarse surface grain, segregation in the billet | AQ billet, die maintenance, controlled extrusion speed and temperature |
| Banding on sheet | Variation in rolling or annealing across the coil width | AQ sheet, single-coil supply for matched parts |
| Grey or cloudy film | High Si or Fe, wrong alloy, or poor-quality recycled feedstock | Specify low-impurity alloys and verify chemistry on the certificate |
| Spots and pits | Inclusions, surface corrosion before anodising, embedded particles | Dry storage, protective film, clean machining coolant |
| Colour mismatch between parts | Different lots, tempers or alloys in one assembly | One alloy, one temper, one lot; boundary samples |
| Burnt or soft edges | Sharp edges, thin walls, or high copper content | Radius edges, design for uniform current density |
| Visible weld lines | Filler alloy differs from base metal | Use a filler that anodises close to the base (e.g. 5356 for 5xxx and 6xxx), or avoid welds on cosmetic faces |
Many of these issues are visible only after anodising, which makes them expensive to find. A short qualification run of finished parts from the production material lot, inspected under daylight-equivalent light, catches most problems before a full batch is committed. See our insight on anodising colouring defects for process-side causes.
For bright and mirror finishes, high-purity grades such as 1050, 1070, 5657 and 6463 give the clearest film. For housings that also need strength, 5252 and 5052 sheet or 6063 and 6061 extrusions and machined parts are the usual choices. There is no single best alloy; match it to the finish and the load.
6063, 6061, 5052 and 5252 all dye black well. The bigger challenge is consistency: keep one alloy, temper and material lot for parts that sit side by side, because black shows small differences in film and substrate.
Both contain about 2.2 – 2.8 % magnesium and have similar strength. 5252 has much lower iron and silicon and no chromium addition, so it anodises clearer and brighter. Choose 5252 for premium bright or light-coloured finishes and 5052 for bead-blasted or dark finishes where cost matters.
6063 anodises more uniformly and is the standard for visible profiles, trim and frames. 6061 is stronger and machines better but anodises slightly greyer. Use 6061 when the part carries significant load, and finish it bead-blasted or dark to hide variation.
Most die-casting alloys contain 7 – 12 % silicon, which produces a dark grey, blotchy film. They can be anodised for protection, but for cosmetic parts painting or powder coating is usually a better choice, or the part can be redesigned as a wrought extrusion or machined part.
Usually not. Type III hard anodising is thick and naturally grey to bronze, and it dulls bright surfaces. It is the right choice for wear resistance; for appearance parts, Type II sulphuric anodising is the normal starting point.
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