The corrosion behavior of Aluminium & its alloys is controlled by the fact that like stainless steel it is a passive metal with a protective air-formed film forming naturally on the surface. This naturally forming film is made up of two layers which normally have a total thickness of about 7 nanometers. The lowest layer is termed the barrier layer & it is amorphous and has no structure. The layer that grows on top of this is hydrated & less compact. It tends to grow at high humidities or with contact with water.
The air formed passive layers are very stable & result in very low corrosion rates in neutral environments. However the film dissolves in both acid & alkaline conditions. High corrosion rates can be expected below pH3 & above pH9.
Common Aluminium Alloys
Aluminium alloys are generally separated into 8 distinct series which are designated 1xxx through 8xxx. The classification is as set out below:-
- 1000 Series Aluminium alloys are variations on pure aluminium. They are unalloyed & are generally more than 99% pure & have a combination of good electrical conductivity & chemical resistance.
- 2000 Series Aluminium alloys contain copper as the main alloying ingredient. They have moderately high strength but tend to be brittle & exhibit poorer corrosion resistance than other alloy series.
- 3000 Series Aluminium alloys containing silicon in combination with copper or magnesium. They can be age hardened to improve their mechanical properties. They have fairly good corrosion resistance but are particularly resistant to pitting corrosion.
- 4000 Series Aluminium alloys also contain silicon as the main alloying ingredient but do not contain copper or magnesium. Their main use stems from their lower melting point which confers good casting qualities.
- 5000 Series Aluminium alloys are based around magnesium. They have good resistance to sea water & find use in marine applications as they have both moderate strength & ductility. They are easy to weld & anodise well.
- 6000 Series Aluminium alloys contain both magnesium & silicon. They have good resistance to atmospheric corrosion and form an attractive film after anodising. They are relatively easy to roll, extrude and forge.
- 7000 Series Aluminium alloys contain zinc as the majority alloying metal but may also contain additions of copper, magnesium, chromium or zirconium. Some of the copper-containing variants have a strong tenancy to exfoliation corrosion in the heat-affected zones of welds. However alloys such as 7075 have good resistance to aggressive atmospheres.
- The 8000 series Aluminium alloys may contain tin, iron or silicon. They are relatively uncommon but can be rolled to thin sheets which gives them some use as fins for heat exchangers & as dishes or thin foils.