Durability
Aluminium alloys have excellent durability and corrosion resistance, but their behaviour can be influenced by the way in which they are used.
Aluminium’s natural affinity with oxygen results in the formation of a transparent oxide film when aluminium is exposed to air. This film is extremely hard, chemically stable, corrosion resistant, and adheres strongly to the parent metal surface. Once formed it prevents further oxidation, and, if damaged in any way, will reform if sufficient oxygen is available.
If the surface is pitted by air-borne pollutants found in industrial or marine atmospheres (e.g. sulphuric acid, sodium chloride) the resulting chemical reaction produces a larger volume of powdered corrosion product than the volume of original pit. This seals the surface and inhibits further corrosion.
Ongoing protection against corrosion in this way requires oxygen to be available at the surface. Therefore aluminium alloys can suffer corrosion when used in ways that prevent this, e.g. continuously wet environments like sheet end laps or under accumulated debris.
Contact with Other Materials
In general, direct chemical attack of aluminium only occurs when the pH is below 5 or above 8.
Contact with other metals can result in corrosion due to galvanic reactions. Copper, steel, stainless steel and lead will all cause attack of the aluminium alloy. Contact with zinc will cause the zinc to be attacked.
Entrapment of moisture between adjacent aluminium surfaces will cause water stains. The stain is a superficial condition, and the mechanical properties of the aluminium alloy are not affected. The degree of staining may be judged by the relative roughness of the stained area. If the surface is reasonably smooth the stain appearance may be able to be improved by abrading with steel wool and oil.