Anodizing is an electrochemical process that converts the aluminum surface into a durable, corrosion-resistant aluminum oxide layer. Unlike painting or powder coating, anodizing is not a coating applied on top of the metal — it actually grows from the aluminum substrate itself, creating an integral finish that cannot peel or flake. The workpiece is immersed in an electrolytic acid bath (typically sulfuric acid) and acts as the anode while a direct current passes through. Oxygen ions are released and bond with aluminum atoms at the surface, forming a dense aluminum oxide (Al₂O₃) layer with a characteristic micro-porous structure that can be sealed or dyed.
The process is widely used in architectural extrusions, automotive trim, consumer electronics, aerospace components, and lighting fixtures. For overseas buyers sourcing from China, anodizing quality consistency is one of the most common concerns — batch-to-batch color variation and uneven thickness are the top complaints we see from procurement professionals.
| Parameter | Type II (Decorative) | Type III (Hard Coat) |
|---|---|---|
| Coating Thickness | 5–25 μm | 25–150 μm |
| Bath Temperature | 18–22 °C | 0–5 °C (requires chiller) |
| Current Density | 1.0–1.5 A/dm² | 2.0–4.5 A/dm² |
| Processing Time | 20–40 min | 30–120 min |
| Voltage Range | 12–18 V DC | 15–100 V DC (ramped) |
| Hardness (HV) | 200–350 HV | 400–600 HV |
| Alloy Compatibility | 6061, 6063, 5052, 7075 | 6061, 2024, 7075, 2618 |
| Color Options | Clear, black, bronze, gold, red, blue (dyed) | Dark gray to black only |
| Sealing Requirement | Yes (hot water or nickel acetate) | Yes (PTFE optional for lubricity) |
China's largest anodizing hub. Hundreds of workshops in Nanhai district, Foshan serve the architectural aluminum extrusion industry. Over 60% of China's anodized aluminum extrusions come from this region. Shops here specialize in long extrusions up to 6 meters, offering competitive prices at $0.50–1.50/kg.
Strong in decorative anodizing for consumer electronics and lighting. Ningbo shops tend to have better quality control systems and color matching capabilities. Many are ISO 9001 certified and export directly to EU and North America. Price range $1.00–2.50/kg depending on color and finish grade.
Higher-end hard coat anodizing (Type III) is concentrated here, serving the automotive, semiconductor, and aerospace industries. These factories are more expensive ($2.50–5.00/kg) but meet Mil-A-8625 Type III standards. Many have Nadcap or AS9100 certification.
For small-batch precision parts, especially for electronics and prototyping. Quick turnaround (24–48 hour rush available), but per-piece costs are higher due to smaller tank sizes and more frequent racking changes.
Tip from experience: We've seen buyers save 30–40% on Type II anodizing by sourcing from Nanhai zone workshops. For Type III hard coat, Kunshan is the sweet spot for quality-to-price ratio. Always request a pilot batch (50–100 pcs) before committing full production — anodizing is one of those processes where sample quality often drifts in mass production.
Not all aluminum alloys anodize equally. 6061 and 6063 give excellent results with clear, bright finishes. 5052 is good but can show slight grey tint. 380 die-cast aluminum is notoriously difficult — high silicon content creates dark, blotchy anodized surfaces unsuitable for decorative applications. If your design requires 380 alloy, consider powder coating instead.
Anodizing color is electrochemical, not pigmented paint. Different batches of the same alloy, same dye bath, same time — and you may still see ΔE > 2.0 shift. Mitigation: specify at least one sealed master color chip per PO. For architectural work where panels sit side-by-side, request continuous-batch processing (all parts in one tank load). Expect a tolerance of ΔE ≤ 1.5 for standard colors, ΔE ≤ 1.0 for premium.
Every part needs contact points to carry current during anodizing. These racking marks (typically 3–8 mm diameter circles) will not have anodized finish. Discuss racking location with the supplier before tooling. For visible surfaces, ask about custom racking or do two-step processing and touch-up. This is especially important for decorative trim parts.
| Cost Component | % of Total | Notes |
|---|---|---|
| Chemical bath & electricity | 25–35% | Sulfuric acid + DI water, rectifier power. Batch size directly affects per-unit cost |
| Labor & racking | 20–30% | Manual racking/unracking is labor-intensive. Small complex parts cost more per kg |
| Dyes & color pigments | 10–20% | Custom colors cost more. Black is cheapest, red is most expensive (requires double dyeing) |
| Quality inspection | 10–15% | Thickness gauge, color spectrophotometer, salt spray tests |
| Sealing & final wash | 5–10% | Nickel acetate seal adds durability; hot water seal is cheaper but less effective |
| Packaging & logistics | 5–10% | Foam interleaving for soft anodized surfaces to prevent scratching |
Typical range: $0.50–3.00/kg for Type II, $2.00–6.00/kg for Type III hard coat. Minimum batch charges usually $50–150.
No — anodizing is specific to aluminum and its alloys. For stainless steel, ask about electropolishing, passivation, or PVD coating instead. Some suppliers may offer "black oxiding" for steel but that's a completely different chemical process (alkaline hot bath).
Most Chinese anodizing shops run batch tanks of 500–2,000 liters. Minimum dye bath volume for a custom color is typically 100–200 liters, which translates to about 50–200 kg per batch depending on part geometry. For small volume, expect 2–3x unit cost premium. Some Shenzhen shops offer mini-tank (20 L) services for prototyping at higher rates.
"Orange peel" is generally a pre-treatment issue — the aluminum surface must be properly etched and desmutted before anodizing. Different alloys etch at different rates. If the surface finish before anodizing is not uniform, the anodized layer will magnify these imperfections. Specify a desmut step in the process flow and request a surface finish Ra ≤ 0.8 μm before anodizing.
No — the ceramic oxide layer interferes with welding. If your design requires both welding and anodizing, weld first, then anodize the entire assembly. Alternatively, mask threaded holes and weld zones before anodizing — but this adds complexity and cost.
Type II anodizing grows a thick oxide layer (5–25 μm) with excellent wear and corrosion resistance. Chromate conversion (MIL-DTL-5541, also called Alodine or chem film) creates a very thin film (0.5–1.5 μm) primarily for corrosion protection and as a paint primer. Chromate has no significant abrasion resistance and is more conductive. Both have their place — choose based on your application's mechanical and electrical requirements.