What is Anodized Aluminum?

What is Anodized Aluminum

The use of anodized finishes has helped make aluminum one of the most commonly used materials in the world today. You’ll find anodized aluminum in thousands of consumer, commercial, and industrial products.

Aluminum is used because it’s lightweight, resistant to corrosion, and easily malleable. You can cast aluminum into any shape, and easily machine complex aluminum parts. When aluminum is anodized, it maintains the lightweight properties and machinability while adding strength and durability. Aluminum is the most common metal that’s anodized, although other nonferrous metals such as magnesium and titanium can be anodized.

In this article, we’ll explain how the anodizing process works, as well as the benefits and uses of anodized aluminum. 

How is aluminum anodized?

The main reason aluminum is anodized is to protect it from corrosion and wear. The process also allows manufacturers to add a layer of color to the aluminum, which enhances its aesthetic value even as it protects the metal.

Anodizing aluminum is an electrochemical process, which simply means it involves a mixture electricity and chemical reactions. The process actually changes the aluminum rather than adding a layer of paint or protective spray that wears off. Because it is integrated into the metal rather than just applied to the surface, it cannot peel or chip.

Before the aluminum is anodized, the parts are throughly cleaned and rinsed. Then the parts are immersed in a conductive solution with lots of positive and negative ions floating around. Then, a positive electric charge is applied to the aluminum, making it the “anode”, while a negative charge is applied to plates suspended in the solution. The electric current causes positive ions to be attracted to the negative plates, and negative ions to attach to the aluminum part.

When the part is dipped in the acid bath, the high electrical currents that cause surface oxides to “grow”.  This oxide growth is called anodize. 

The anodizing process adds a layer of oxidized aluminum so that its surface is no longer reactive. Anodized aluminum is still very conductive, but the surface is much harder than un-anodized aluminum. The layer of oxidation offers mechanical protection, ie. protection from scratches and mechanical wear, as well as chemical protection from water and oxygen. 

Here is a graphic of what anodized aluminum looks like:

Anodized Aluminum

Does anodizing wear off?

Anodizing provides a thin aluminum oxide layer, which will deteriorate over time. Depending on the thickness and quality of the anodization, the surface will last between 10-20 years

Because of its longevity, anodizing is one of the best ways to improve the durability of aluminum. The anodized layer can be removed in a Sodium Hydroxide acid wash. You could also remove the anodizing with rough sandpaper if you want to get back to the unfinished aluminum. 

Does anodizing add thickness?

When aluminum is anodized it adds a small amount of material to the part. In the graphic below, you can see how some of the thickness of the coating is ingress into the material and some of the thickness is added to the material. For most alloys, the anodized coating is approximately 50% ingressed and 50% growth.

Anodized Aluminum

How much thickness is added? Anodized coating thickness may vary depending on the aluminum alloy and the specific anodizing process.

Typical coating thicknesses are as follows:

  • Up to 5µm for chromic acid anodizing
  • 5-15µm for clear (natural) sulphuric acid anodizing
  • 10-25µm for dyed (colored) sulphuric acid anodizing
  • 20-70µm for hard anodizing, with thicknesses up to 100µm on some alloys.
100µm is 0.1 millimeters, so the added thickness would be unnoticeable to the human eye. And when you’re ordering parts, the anodization process would be factored into the dimensions. 

Types of anodizing

There are a few different types of anodizing that each have different uses and purposes. The end result of each process will look similar. The main differences will be the thickness of the oxidation layer.

Chromic acid anodizing

Chromic acid was used in the early 1900s to anodize parts. The process adds a thin layer of oxidation to the parts.

Sulfuric acid anodizing

The Sulfuric acid anodizing process looks the same as the Chromic acid, other than a different acid is used. Sulfuric acid adds a thicker layer of oxidation and is more suitable for adding color, so it’s used more often than Chromic acid.

Hard anodizing

Hard anodizing also uses a sulfuric acid, but produces a thicker anodized layer that is also suitable for adding color.

Should every part be anodized?

Aluminum doesn’t always need to be anodized, but we find that in most cases it’s the best choice. The cost of anodizing aluminum is similar to painting the parts, and anodizing offers greater durability and long-lasting color. 

Spex offers anodizing as an additional service when you order precision aluminum parts. 

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