Oxidize aluminum

Metals oxidize, as does aluminum. It is to be distinguished between the natural and an artificially created oxide layer. In this guide, we explain both methods for oxidizing aluminum.

This is how oxide forms naturally

Aluminum reacts to contact with oxygen, forming an oxide layer. Depending on the subsequent conditions of use of the component component made of aluminum, this may be desirable or disturbing. This oxide layer can be artificially generated by different methods, and the natural oxidation can be adjusted. Basically, you can differentiate the following procedures:

  • natural oxidation in the dry air
  • natural oxidation in moist air
  • natural oxidation in water
  • artificial oxidation by anodic oxidation

Properties of the oxidation layer

An oxide layer is quite stable and resistant in the pH range 4 to 8. However, alkaline solutions and acids can remove or destroy the oxidation layer. The controlled use of such chemical removal is also referred to as pickling of the aluminum.

In addition, cement and lime also destroy an oxide layer. If aluminum comes in contact with limescale or cement leaching on a facade, the oxide layer becomes unstable. However, the melting temperature of oxide is between 1,600 and 2,100 degrees Celsius, that of aluminum depending on the alloy between 580 and 680 degrees. This must be considered when welding or aluminum brazing.

Natural oxidation in dry air

In dry air, the oxide layer grows several millionths of a millimeter a day. By raising the temperature, the oxidation can be accelerated. Up to a temperature of about 500 degrees, the state of the oxide layer is amorphous. Above that, the aluminum is crystalline and can only grow very badly.

Natural oxidation in moist air

In moist air, the oxide layer will grow to a thousandth of a millimeter. In addition, two different oxide layers grow here. The first one is very dense and thus virtually without pores, which is why it is also called a barrier layer.

This layer contains moisture and is referred to as trihydroxide. Since this process can also be observed outdoors and dirt particles are trapped here, this layer is clearly recognizable by its greyish discoloration.

Natural oxidation in the water

Also in the water, two oxide layers are formed. However, water can be heavy metal contaminated. In such a case, there is a risk that corresponding ions penetrate. If copper ions penetrate, electroplating occurs and the aluminum is destroyed. Colloquially, this is also referred to as pitting. Therefore, cooling water in an aluminum engine, for example, must be filled with glycol in the summer.

The anodic or electrolytic oxidation

The aluminum enters an acid bath and is then energized. This also creates an oxide layer. This process is also known as anodizing. Salts are mixed with color pigments, which are deposited in the pores. This method is called electrolytic coloring. Almost all color variants are possible.

In the electrolytic coloring, different shades are formed ranging from black to bronze and brown. Light and weather-resistant oxidation layers are applied in the so-called GS process and can not be subsequently colored.

Tips & Tricks

In addition to this complex coloring, which is usually primarily a protective layer, you can also burnish or matt aluminum. The latter, however, is a mechanical process.

Video Board: Aging Aluminum Cold Water Process