Master anodizing for aluminum sheet metal parts. Learn about anodizing types, process steps, color options, and quality standards for durable aluminum finishes.
Introduction
Anodizing is the premier surface treatment for aluminum, providing corrosion resistance, wear resistance, and aesthetic appeal. At Fulei Metal, we provide anodizing services for aluminum components.
What is Anodizing?
Anodizing is an electrochemical process that converts the aluminum surface to aluminum oxide. Unlike plating, the coating is integral to the metal, formed by transforming the surface aluminum. The result is a hard, porous oxide layer that can be sealed and dyed.
Advantages
Hard surface (harder than base aluminum). Excellent corrosion resistance. Wear and abrasion resistance. Electrically insulating. Decorative (can be dyed any color). Does not peel or chip. Integral to the metal.
Anodizing Types
Type II: Sulfuric Acid Anodizing
Most common. Thickness: 10-25 microns. Porous layer absorbs dye. Can be clear or colored. Good general-purpose protection. Cost-effective.
Type III: Hard Anodizing
Thicker coating: 25-75 microns. Harder surface. Better wear resistance. Darker natural color (gray/brown). Limited color options. For demanding applications.
Type I: Chromic Acid Anodizing
Thin coating: 2-10 microns. Good paint base. Being phased out (chromium). Non-destructive testing use.
Process Steps
Cleaning
Degrease to remove oils. Etch in sodium hydroxide. Removes natural oxide and surface imperfections. Desmut in acid. Removes smut from etching. Surface must be clean and uniform.
Anodizing
Immerse in electrolyte (sulfuric acid for Type II/III). Apply electric current (12-24V for Type II). Aluminum is the anode. Oxygen forms at surface, creating aluminum oxide. Time: 30-60 minutes for Type II. Temperature: 20 degrees C (Type II), 0-5 degrees C (Type III).
Dyeing (Optional)
Porous oxide absorbs dye. Immersion in dye solution. Many colors available. Duration affects color intensity. Must be done before sealing.
Sealing
Closes pores in the oxide. Prevents staining and corrosion. Hot water sealing (95-100 degrees C). Nickel acetate sealing. Mid-temperature sealing. Essential for durability.
Color Options
Clear (natural aluminum color). Black, bronze, gold, red, blue, green, purple. Color matching is approximate (varies with alloy). Some colors are lightfast, others fade. Sample approval recommended.
Thickness Standards
| Application | Thickness |
|---|
| Indoor decorative | 5-10 microns |
| General outdoor | 15-20 microns |
| Architectural | 20-25 microns |
| Hard anodizing | 25-75 microns |
| Marine | 25+ microns |
Alloy Considerations
5005 and 5052: excellent anodizing, uniform color. 6061: good anodizing, slightly grayer. 6063: excellent for architectural. 2000 series: may have uneven appearance. 7000 series: variable results. Specify alloy for consistent appearance.
Quality Standards
ISO 7599: anodizing specifications. MIL-A-8625: US military standard. AAMA 611: architectural. Qualanod: European quality label.
Quality Testing
Thickness: eddy current gauge. Weight: gravimetric method. Sealing: dye spot test. Hardness: micro-hardness test. Appearance: visual inspection. Color: colorimeter comparison.
Common Defects
Uneven color: alloy variation or poor agitation. Pitting: from chloride contamination. Burning: excessive current density. Powdery coating: too-high temperature. Poor sealing: staining and corrosion. Die marks: from extrusion or forming.
Design Considerations
All aluminum surfaces can be anodized. Holes and recesses are coated. Sharp corners may have thinner coating. Welding discoloration may show. Rack marks are inevitable (contact points). Masking for selective areas is possible but adds cost.
Conclusion
Anodizing provides durable, attractive finishes for aluminum parts. At Fulei Metal, our anodizing capabilities ensure aluminum components meet quality and appearance requirements for international clients.
Anodizing Type, Coating Growth and What the Finish Cannot Hide
Anodizing grows an oxide out of the aluminium itself rather than adding a layer on top. That single fact explains most of its advantages and all of its limitations.
| Type and process | Typical coating thickness | What it is chosen for | What to expect |
|---|---|---|---|
| Type I, chromic acid | 2-7 µm | Thin coating where dimensional change must stay minimal, and for fatigue-critical parts | Lower wear resistance; less common now for environmental reasons |
| Type II, sulfuric acid, clear | 5-25 µm, commonly 8-12 µm | General corrosion and wear protection with a natural metallic appearance | Colour varies with alloy and with batch |
| Type II, sulfuric acid, dyed | 5-25 µm | Black and coloured finishes where the metallic look is wanted | Dyes fade under strong UV; darker colours show the effect more |
| Type III, hard anodize | 25-75 µm | Wear surfaces, sliding and abrasive duty | The part darkens, and the surface is less smooth than before |
| Phosphoric acid anodize | Thin | Adhesive bonding preparation rather than as a final finish | Used as a pre-treatment, not for appearance |
Two consequences follow from the coating being grown rather than applied, and both belong on the drawing. The first is dimensional: roughly half the coating thickness grows outward and half inward, so a 10 µm coating changes a dimension by about 5 µm per surface, which matters on close fits. The second is appearance: anodizing is transparent, so scratches, die lines, extrusion streaks and weld filler remain visible, and a weld on 6061 will usually tone differently from the parent material.
Frequently Asked Questions
Will anodizing hide a scratch or a weld mark?
No. The oxide is integral and transparent, so anything visible before anodizing is visible after it. If surface uniformity is the priority, powder coating is the finish that hides substrate variation; anodizing is the finish that preserves a metallic appearance.
How much does the coating change dimensions?
Approximately half the coating thickness per surface, because the oxide grows both outward and inward. On mating parts and threaded features, either allow for it or specify masked areas.
Does the alloy affect the result?
Yes, clearly. High-silicon casting alloys anodize to a darker grey, while 5xxx and 6xxx sheet anodize more cleanly. If colour consistency matters, use one alloy across the assembly and reference a master sample.
Questions about a specific part are usually faster to answer against the drawing — send it through the route below.
Practical next steps. Start with surface finishing service for the drawing review, read send the drawing for review for process context, and if you are choosing between routes, surface treatment for aluminium and coating thickness standards set out the alternatives side by side.