What is anodizing?

In surface treatment technology, anodizing refers to a group of coating processes, where oxide layers are produced or strengthened on metallic objects by means of electrolysis. During anodizing the metal is immersed in an electrolytic solution (e.g. sulfuric acid) and functions as the anode (positive pole) - in contrast to galvanic surface treatment processes, where the workpiece is the cathode. The electrical current causes an oxide layer to form on the surface of the part, the thickness of which depends on the voltage level. Unlike other coating processes for example, during anodizing no other material is added, instead the surface layer results from the conversion of the base material itself. Anodizing is mainly used to improve the corrosion protection of metals. The process is used for light metals such as aluminum, magnesium, titanium, tantalum and niobium.

Diagram of the anodizing process
Anodizing of aluminum

Anodizing process (electrolytic oxidation)

By far the most widespread version of anodizing is the electrolytic oxidation of aluminum.

During this anodizing process, the aluminum part is immersed in an electrolyte bath - usually diluted sulfuric acid - and it is allowed to function as the anode within a DC circuit (DC sulfuric acid process). Through electrolysis the water molecules are split into oxygen and hydrogen, and the oxygen reacts with the aluminum. This anodic oxidation produces a controlled conversion of the surface into aluminum oxide (Al2O3). The 5 to 25 µm thick oxide layer produced in this way contains micropores directly after formation, due to the process. These pores can be compacted or sealed by an aluminum hydroxide layer, produced using boiling demineralized water.

Before compaction, there is also the option to embed a colored dye and subsequently lock it into the material, by dipping into a dye bath. In this way, parts can be produced in almost all colors. The desired colors are usually specified using RAL color codes.

Benefits and application area

In particular colored anodizing is often used for decorative purposes. Transparent anodizing without a dye also often fulfils a visual function. Here, thin anodizing layers up to 20 µm are usually sufficient. 

Thicker layers (20–25 µm) are mainly used if the resistance of the workpiece to corrosion and wear needs to be increased, for example if the workpiece will be used outdoors and/or is exposed to salt water. The oxide layer increases the corrosion resistance of the aluminum part, as long as it is intact and does not contain any gaps e.g. due to mechanical damage such as scratches and scrapes. A further version of the anodizing process, where a particularly thick layer is produced, is hard anodizing.

An additional - often desirable - effect is that due to anodizing the natural electrical conductivity of the aluminum is suppressed and the oxide layer can therefore be used as an insulator. 


Blue hook made by color anodization

Which type of aluminum should be used for anodizing?

Whilst the differences during functional anodizing are minor, the visual appearance and quality of the surface can vary tremendously depending on the alloy used. Low-alloyed aluminum such as EN AW-1050A (Al99.5), as well as the magnesium-containing alloy EN AW-5754 (AlMg3) are particularly suitable for decorative anodizing.

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