Anodizing Aluminum: Process, Colors, and Cost

Two identical aluminum housings from the same order come out of the anodizing line with a visibly different shade, even though both were specified as "anodized, silver." The cause almost always comes down to details missing from the order itself: alloy, layer thickness, or a binding color reference.

In brief: Anodizing (in German, Eloxieren) uses anodic oxidation to grow a hard oxide layer directly out of the aluminum surface, protecting it from corrosion and allowing it to be colored. Not every aluminum alloy anodizes equally well, and cost depends mainly on layer thickness, color, and batch size. Buyers sourcing anodized parts should pin these down themselves in the spec sheet, since leaving them open lets the supplier decide.

What Is Anodizing?

Anodizing is a surface treatment process in which aluminum is connected as the anode in an electrolyte bath. This causes an oxide layer to form directly on its surface. Unlike painting or powder coating, no separate layer is applied on top. The oxide layer grows out of the base material itself and is correspondingly firmly bonded to it.

Chemically, the part acting as the anode undergoes the reaction 2 Al + 3 H₂O → Al₂O₃ + 3 H₂. Hydrogen forms at the counter electrode. The resulting aluminum oxide is considerably harder and more corrosion-resistant than the bare metal underneath. Standard anodizing typically produces a layer thickness of 5 to 25 micrometers with a hardness of roughly 200 to 350 HV.

How Does Hard Anodizing Differ from Standard Anodizing?

Alongside classic, decorative anodizing, hard anodizing (Harteloxal) exists for parts under heavier mechanical load. The difference lies in the process itself. Hard anodizing runs in cooled acid baths at higher voltage, as WOMag describes in its analysis of hard anodizing, producing a noticeably thicker and harder layer:

In our sourcing projects, the choice between standard and hard anodizing is often made too late in practice. Usually, it only happens once a part shows premature wear in the field. Anyone designing a moving or mechanically loaded part should clarify the required layer hardness with the manufacturing partner during the design phase. That avoids problems surfacing at incoming inspection of the first production batch.

Which Aluminum Alloys Can Be Anodized?

Not every aluminum alloy takes an anodized layer equally well. Pure aluminum and the unalloyed to lightly alloyed series produce the most even, clearest results. More heavily alloyed materials, though, can develop blotching or an uneven color tone depending on the alloying element, as the Wikipedia entry on the anodizing process explains:

The purer the material, the more even the result. With copper- and zinc-containing alloys such as EN AW-7075, the alloying elements partially redissolve during anodizing. That noticeably reduces coating quality.

For cast parts made from more heavily alloyed aluminum, such as die-cast aluminum, results are often less consistent than for wrought alloys. A higher share of alloying elements like silicon disrupts even oxide layer formation. Anyone designing a part specifically for anodizing should factor the alloy into material selection from the start, to avoid costly adjustments after production.

What Colors Are Possible with Anodizing?

The classic color range runs from EV1 (natural, silvery) through various bronze tones to EV6 (black). Color is produced by a metal salt in the electrolyte bath. These EV designations remain common in practice. Drawings and spec sheets, though, are increasingly shifting toward the internationally aligned C color codes of the European anodizers' association EURAS (e.g. C-0 for natural tone through C-35 for black). The technical delivery conditions for anodized aluminum itself, independent of color designation, are set out in the current DIN 17611 standard.

For sourcing, what matters most is that color tone can vary slightly between batches. A verbal color description alone isn't precise enough for this. The German association for aluminum surface finishing (VOA) therefore recommends agreeing on color limit samples with the supplier that later deliveries are checked against.

What Does Anodizing Cost?

The cost of anodizing is driven by several factors that can reinforce each other.

In our sourcing projects, batch size is the cost driver customers underestimate most often. An order volume that's economical for manufacturing the part itself is often too small for a separate anodizing run at the finishing partner. As a trading company, we pool such finishing orders across multiple customers where possible into a batch size that's economical for the anodizer. That's similar to how negotiating minimum order quantities works in the underlying manufacturing itself.

What Should Buyers Watch for When Sourcing Anodized Parts?

Anyone sourcing anodized aluminum parts should specify the alloy and the desired color by EV or VOA designation at minimum. The spec sheet should also fix the layer thickness or standard versus hard anodizing, and the acceptable color tolerance. Without these details, the supplier decides at their own discretion, which can lead to complaints on visible parts.

Choosing the right manufacturing partner matters just as much. Some finishers regularly run large batches for the required color and layer thickness; others only serve that specification occasionally. The former tend to deliver more consistent results. A structured supplier evaluation helps assess that track record before the first production order, catching potential problems early.

The extra cost of hard anodizing or a tighter color tolerance only pays off if it's actually needed. As with choosing between different steel grades, the total cost of ownership should factor in how much stress the part will actually be under. That way, the finish matches the actual load.

Frequently Asked Questions About Anodizing

What Is the Difference Between Eloxieren and Anodizing?

None, in substance. Eloxieren is simply the German-language term for the same electrochemical process known internationally as anodizing.

Can Any Aluminum Be Anodized?

Not with the same result. Pure aluminum and lightly alloyed series like the 3000, 5000, and 6000 series produce even, clear results. Copper- or zinc-containing alloys like the 2000 and 7000 series, though, can develop an uneven color tone.

Can Stainless Steel or Steel Be Anodized?

No, not in the true sense. Anodizing depends on forming an aluminum oxide layer, which only happens with aluminum and, through a different process, titanium and magnesium. Stainless steel instead forms a thin passive layer of chromium oxide on its own. That layer works differently and can't be created or reinforced through anodizing.

Which Is More Expensive, Anodizing or Powder Coating?

It depends on the part. For small to medium batches in a standard color, anodizing is often similarly priced to powder coating. Special colors or hard anodizing can increase that cost. Powder coating covers a wider color range. It doesn't bond the color layer to the base material as firmly, however, as an anodized layer that grows out of the metal itself.

Conclusion: Specify Anodizing Requirements Clearly

Anodizing reliably protects aluminum parts from corrosion and opens up a wide range of colors. It only delivers a dependable result, though, when alloy, color, layer thickness, and color tolerance are defined upfront. Leaving these open hands the supplier a decision instead. That decision should really be based on how the part will actually be used and how it needs to look.

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