Materials #02: Alumite (Anodizing) Surface Treatment for Aluminum Parts

Alumite Surface Treatment (Anodizing)

What is Alumite?

Alumite, or anodizing, is an electrochemical treatment that forms an aluminum-oxide film on the surface of an aluminum part. This film is not an external coating layer sitting on top; it is formed from the base material's own surface. Therefore, with the right treatment conditions, Alumite can improve corrosion resistance, surface hardness, wear resistance and the appearance stability of the part.

In production-equipment design, Alumite is often seen on aluminum jigs, fixture plates, light covers, guide parts, brackets, parts that need weight reduction, or parts that need a clean appearance. For parts frequently touched by human hands, or exposed to humidity, cutting oil, metal dust or light friction, an Alumite treatment makes the surface more durable and easier to manage than bare aluminum.

When should you use Alumite?

Alumite is suitable when a part needs to:

Increase the corrosion resistance of aluminum.
Keep the surface clean and less prone to natural oxidation.
Improve surface hardness and light scratch resistance.
Create a color to distinguish models, fixture positions or design versions.
Stay light in weight while still needing a stable surface.
Serve as a plate, jig, base or cover in automation equipment.

However, Alumite is not a good choice for every case. If a part takes heavy impact, heavy wear, requires very tight fitting tolerances, or needs conductivity at the contact surface, check carefully before specifying it.

Common types of Alumite

1. Standard Alumite

This is the type commonly used for ordinary mechanical parts. Its main goals are corrosion resistance, appearance stability and surface protection. The color can be natural silver-white, black, or other specified colors depending on the processor.

2. Hard Alumite

Hard Alumite forms a thicker and harder film, suitable for parts with friction, light sliding, or a need for higher surface durability. However, the thicker film also affects the dimensions after treatment more, so you must state clearly which areas need masking or which areas must keep their tolerance.

3. Black Alumite

Often used when you need to reduce light reflection, distinguish parts, add a finished feel, or use it in a machine assembly with a camera/vision system. For image-inspection systems, black can help reduce noise from reflection, but you still need to test with the actual light source in use.

Effect on dimensions and tolerances

A point often overlooked is that Alumite can change the surface dimensions. The oxide film grows partly into the base material and partly out from the surface. So for pin-fitting holes, bushing holes, sliding surfaces, keyways or bearing-mounting positions, you should not just write "Alumite all over" if the tolerances matter.

On the drawing, you should consider:

Which area is treated.
Which area is not treated.
Whether masking is needed for threads, precision holes or electrical-contact surfaces.
Whether the tolerance is measured before or after treatment.
Whether color and appearance requirements are inspection criteria.

If it is not stated clearly, the processor may treat it according to their internal standard, resulting in a part that does not fit after treatment or has to be reworked.

Notes for drawing callouts

A practical callout could be along these lines:

Material: A5052 / Surface treatment: Black anodizing / Mask precision holes and threaded holes / Critical dimensions after surface treatment.

For a Japanese drawing, you can write:

材質: A5052 / 表面処理: 黒アルマイト / 精度穴・ねじ部マスキング / 重要寸法は処理後寸法

Not every drawing needs to be written this long, but for important parts, stating it clearly from the start reduces the risk of back-and-forth between design, the processor and assembly.

Alumite ordering checklist

Is the aluminum material A5052, A6061, A7075 or another type?
Do you need standard Alumite or Hard Alumite?
Is there a specific color requirement?
Are there threads, precision holes or mating faces that need masking?
Are critical tolerances inspected before or after treatment?
Does any surface of the part need conductivity?
Is there an appearance standard for clamp marks, hanging marks or color variation?

Conclusion

Alumite is a very useful surface treatment for aluminum parts, especially in jigs, automation equipment and light mechanical parts. However, to use it effectively, the designer must treat Alumite as part of the technical requirements, not just a note line at the bottom of the drawing.

For parts with critical tolerances, define clearly the treated area, the masked area and the post-treatment inspection conditions. This is a small point but it reduces assembly errors, reduces rework, and makes communication with the processor clearer.

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