Real rust does not exist on aluminum

From a purely chemical point of view, aluminum can not rust, because it behaves differently in the oxidation than iron. The contact with air causes the surface to close itself. However, so-called contact corrosion can "skip" due to individual alloying partners or direct contact with ferrous metals.

Clouding possible, but no rust

One of the great advantages of aluminum is the specific nature of the oxidation. An aluminum oxide layer several micrometers thick forms on the surface, which prevents rusting of the metal. Unlike iron, aluminum does not need to be protected against rust because it does not produce iron oxide that is rusting.

A rust-like process can be caused by the constant violation of the oxide layer of aluminum. The "injured" layer always puts new oxidation processes into effect, which correspond to corrosion in the interior. In this case, discoloration and unevenness of aluminum may occur. Clouds of this kind are not called rust.

Ferrous alloys

Aluminum components and materials almost never consist of pure metal, but are alloyed with other metals. A problem in the production of the alloys is that the alloying partners with particularly stabilizing properties for the soft metal aluminum disturb the formation of the oxide.

Therefore, there are aluminum types that can rust in the usual and apparent sense. The iron oxide is formed exclusively by the foreign metals. Typical alloying partners of aluminum are:

  • beryllium
  • chrome
  • iron
  • copper
  • magnesium
  • zinc

Another cause of apparent rust on aluminum may be due to contact corrosion. For example, if bare unprotected aluminum directly touches stainless steel, it can take on the electromagnetic role of an anode. As electrons move to the opposite cathode, aluminum loses physical substance. In addition, the natural self-oxidation is disturbed, leading to the assumption that aluminum rusts.

When anodizing, oxidation is controlled

For example, to give corrosion-tolerant alloying partners no chance of forming iron oxide, aluminum is oxidized. In this anodizing, the sealing oxide layer is artificially induced by different electromagnetic methods. The anodized surface protects alloyed aluminum from the reaction with moisture and water and closes airtight.

Chemical corrosion

Often there is talk of great corrosion resistance of aluminum in the pH range between four and nine. Even when leaching acids with pH values ​​below or above this range, rusting is an inaccurate designation of the onset of reaction. The reaction of some aggressive chemicals disturbs the formation of oxide or prevent it completely. This effect leads to metal irritation, which causes, for example, the rust-like pitting corrosion.

Tips & Tricks

When purchasing materials and components made of aluminum that are used outdoors, look for the type of alloy. The alloying partners should be as free of corrosion as possible or the surfaces should be sealed by anodizing.

Video Board: Aluminum and Mercury