3.Acids and bases
The optimal pH value for the oxide layer is between 4 and 9. This means that acidic and alkaline environments outside this range degrade the oxide layer and cause pitting in the exposed aluminium surface.
Bases break down aluminium faster than acids. For example, concentrated sodium hydroxide reacts so violently with aluminium that it can start to boil.
Wet concrete in a construction site is a typical risk factor to be aware of. Concrete has a pH value of 12.5-13.5, far above the tolerance of the oxide layer. To avoid corrosion, the aluminium should be kept away from wet concrete or covered with a protective material while the concrete is wet. Anodising cannot prevent this type of corrosion.
4.Risk factors for untreated aluminium
Due to the oxide layer, aluminium can often be used as is without surface treatment, even outdoors. However, as described above, there are limits to what the oxide layer can withstand.
By choosing the right alloy for the specific environment and avoiding risk factors, such as strong acids and bases, you increase the lifetime of the metal.
5.Risk factors
As a rule of thumb, you should avoid exposing untreated aluminium to direct contact with the following substances and materials:
Other metals, e.g. copper, lead and iron. This is especially important in humid conditions.
Inorganic acids, e.g. hydrochloric acid and sulphuric acid.
Formic acid, oxalic acid and chlorinated solvents.
Bases
Mercury and mercury salts.
Chloride solutions.
Water containing heavy metals.
Acidic wood, damp wood and wood impregnated with copper-containing salts.
Alkaline building materials, e.g. fresh concrete.
However, with the right alloy or surface treatment, you can minimise the risk from most of the above factors. You might not avoid corrosion altogether, but you can minimise the damage.
