The general corrosion behavior of aluminium alloys basically depends on three factors:
- The stability of the oxide film
- The alloying elements, which can influence the oxide coating
- The environment
Regarding stability, we normally say the oxide film is stable in solutions where the base-to-acid levels defined by pH are between 4.5 and 8.5. However, and this is a bit technical, it is influenced by the presence of certain anions and cations including H+ and OH-ions, and aluminium can be attacked in neutral solutions.
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Greater purity, greater resistance
It is generally true that the greater the level of aluminium purity, the greater its corrosion resistance. That said, some elements can be alloyed with aluminium without reducing the corrosion resistance of the metal – and in some cases, the resistance can be improved.
The alloying elements that have little or no effect include Sb, Bi, Pb, Si, Ti, Zn. Those that have a detrimental effect include Cu, Fe and Ni. The influence of these elements depends on the amount present, the metallurgical condition of the alloy, the state of the addition in the alloy structure, and the environment to which the material is exposed.
Corrosion resistance by aluminium alloy group
As I’ve said, in general, aluminium alloys have good corrosion resistance in most environments and with many chemicals. Now I will specify the general corrosion behavior of each alloy group:
- 2xxx-series alloys. These have relatively poor corrosion resistance and require surface protection when used in corrosive environments. Wrought alloys are protected by cladding, and in this form they are most resistant to atmospheric and marine corrosion.
- 3xxx-series alloys. These have good corrosion resistance and may be better than the 1100 alloy in marine environments and cooking utensils because of a reduced effect from the Fe in these alloys.
- 4xxx-series alloys. The alloys that do not contain copper as an alloying element are corrosion resistant.
- 5xxx-series alloys. These are as corrosion-resistant as 1xxx-series alloys, and even more resistant to salt water and some alkaline solutions. This series offers the best combination of strength and corrosion resistance of all aluminium alloys, making them suitable for bridge decks subject to road salts.
- 6xxx-series alloys. These have good resistance to atmospheric corrosion, but in general, they offer slightly lower resistance than Al-Mg alloys. They can be used unprotected in most atmospheres and waters.
- 7xxx-series alloys. This series is considered to have three main sub-groups:
- Al-Zn binary alloys are highly anodic and used as sacrificial cladding for alloys such as 3003, 6061 and 7075. Their corrosion resistance is comparable to that of 1100 alloys.
- Al-Zn-Mg alloys have good corrosion resistance to normal atmospheres and fresh water. With seawater, their corrosion performance is a bit more questionable. They tend to have greater resistance to alkalis and less to acids, than other aluminium alloys.
- Al-Zn-Mg-Cu alloys require protection in corrosive environments and are comparable to 2xxx-series alloys.
Good corrosion resistance with clad alloys
I would like to conclude with some words about clad aluminium alloys.
Clad alloys are those clad with high-purity classing alloys such as 1230, 4004, 6003 or 7072, which are often called “alcladding” alloys. These are composite wrought products comprised of an aluminium alloy core with a thin layer of corrosion-resistant cladding alloy that has been metallurgically bonded to one or both surfaces. The cladding is anodic to the core and thus protects the core.
Consequently, as a class, clad alloys have an extremely high resistance to corrosion.
