Outdoor use of untreated aluminium
When using untreated aluminium outdoors, there are a number of factors one must take into consideration. On this page you will find information about the natural formation of oxide layer on aluminium that increases the corrosion resistance of the metal, as well as information about numerous conditions that can have a negative impact on its corrosion resistance.
The oxide layer makes untreated aluminium more corrosion-resistant
When using untreated aluminium outdoors, a thin oxide layer is formed on the surface. When aluminium comes in contact with oxygen, an oxide layer forms, which is strongly attached to the aluminium surface. The layer protects the metal against further attacks, thereby increasing its resistance to corrosion.
If there is oxygen in the surroundings where aluminium is located, the oxide layer will be restored should the aluminium be scratched, for example. At the same time, the layer is insoluble in water and resistant to numerous chemicals. Finally, the hardness of the oxide layer is equivalent to the hardness of glass. Because of the oxide layer formation, it is important to note that untreated aluminium does not retain its original, shiny surface.
Factors that should be avoided when using untreated aluminium in outdoor areas
If untreated aluminium is used in outdoor areas, it is important to know that in spite of the relatively high corrosion resistance of aluminium, certain factors will affect the resistance of the metal to corrosion. For this reason, it is recommended that direct contact between aluminium and the following be avoided:
- Other metals, such as copper, lead and iron (especially under humid conditions)
- Inorganic acids (for example, hydrochloric acid and sulphuric acid)
- Formic acid, oxalic acid and chlorinated solvents
- Mercury and its salts
- Sea water and chloride solutions
- Water containing heavy metals
- Damp wood types and wood impregnated with copper-containing salts
- Alkaline building materials, such as fresh concrete
However, there are various alloys and treatment methods that have the ability to reduce the corrosion that occurs on the aluminium surface due to some of the above factors.
Soiling reduces corrosion resistance
Dirt build-up on the untreated aluminium surface can also reduce the corrosion resistance of the metal. Corrosion resistance is often reduced as a consequence of the soiling which keeps the surface moist for a longer period time. It could be avoided by cleaning the soiled surfaces 1-2 times a year depending on the degree of contamination.
The following options can be used for cleaning soiled surfaces, and are listed here in order of least to most harsh on the untreated surface. Plain water is mildest to the raw metal, whereas abrasive cleaners are very harsh on the surface.
- Plain water.
- Mild soap/detergent.
- Solvents, such as kerosene white spirit or mineral turpentine.
- Non-corrosive, chemical cleansers.
- Polishing with wax.
- Grinding with wax.
- Abrasive cleaning agent.