A Guide to Metal Surface Treatments
2025/9/25
Metal products often undergo surface treatments as a finishing process to prevent rust, improve durability, and enhance aesthetic appeal. Surface treatments are essential for aluminum alloys and iron-based materials, and this guide provides an in-depth overview of the most commonly applied treatments for metal parts.
Surface Treatments for Aluminum Alloys
The most common treatment for aluminum alloys is anodizing, which creates a protective anodic oxide film on the surface. This thick aluminum oxide layer improves corrosion resistance and wear resistance, while also allowing decorative finishes or additional functional properties.
Anodizing can be applied to aluminum series 1000 through 7000. The 1000 series is easiest to treat, whereas the 7000 series is the most difficult.
Since the anodic film forms inward from the original surface, re-anodizing will result in some material thinning.
Clear Anodizing
Also called “anodizing process,”clear anodizing retains the natural color of aluminum. Film thickness is 6–10 μm, providing corrosion protection and maintaining high dimensional accuracy.
Color Anodizing
This process adds decorative colors such as black, blue, red, or green. The film thickness is 15–30 μm. It is primarily used for visible parts and aesthetic enhancement.
Matte Black Anodizing
The surface is first treated with blasting or another pretreatment to create a matte finish, then black anodizing is applied. This reduces surface reflectivity and improves sliding properties.
Hard Anodizing
Hard anodizing produces a durable film with excellent wear resistance. It also provides electrical insulation. Film thickness is typically around 50 μm, and polishing can add a glossy finish. The final color may vary depending on the material and film thickness—for example, 30 μm on A2017 results in a greenish hue.
Aluminum Chemical Treatment (Alodine Treatment)
Alodine forms a chromate film directly on the aluminum surface rather than an oxide film. Two types are available: #1000 (colorless, suitable as a coating base) and #1200 (yellow, with better corrosion resistance). Electrical resistance can be adjusted by modifying treatment time.
Surface Treatments for Iron-Based Materials
Unichrome Plating
Originally a brand name for zinc plating with chromate treatment, chromate treatment is referred to as Unichrome plating. The finish is bluish silver and mainly used for decorative purposes. It has a film thickness of about 10 μm per side, making it unsuitable for products requiring high precision.
Compared to other chromate treatments, its corrosion resistance is lower, but it is a low-cost process, making it suitable for mass-produced items.
Electroless Nickel Plating
This plating method forms a nickel film through the oxidation of components in the plating solution, releasing electrons that generate the metal nickel coating. Since no electrical current is required, it can also be applied to insulating materials such as resins. Electroless nickel plating produces a uniform film thickness, making it suitable for high-precision parts or complex shapes.
Black Oxide (Fe₃O₄ Film)
Black oxide forms a thin, 1μm film and is mainly applied to iron-based materials. It provides corrosion protection while maintaining dimensional precision. The resulting glossy black surface is primarily functional rather than decorative.
Hard Chrome Plating
Also called industrial chrome plating, it is widely used for machine parts. The film thickness is 2 μm or more, with a hardness of HV 750 or higher. Among electroplating methods, hard chrome has the highest hardness and offers excellent wear and corrosion resistance.
Key Takeaways
Anodizing is the primary treatment for aluminum, enhancing corrosion and wear resistance and allowing decorative finishes.
Clear, color, matte, and hard anodizing provide different functional and aesthetic benefits.
Iron-based materials can be treated with Unichrome plating, electroless nickel plating, black oxide, or hard chrome plating to improve durability and appearance.
Step Up Your Project with Yumoto Electric
Yumoto Electric delivers high-quality components fast, worldwide. We provide precision machining for a wide range of metals using milling, turning, and specialized metal processing. We have experience with over 200 materials including aluminum alloys, stainless steel, molybdenum, and titanium.
If you’re unsure about material selection or machining methods, we offer complimentary consultations to help optimize your design and production. For any questions, please feel free to contact us.
