Corrosion resistance of Al Alloys 2

[MetalEng]

Why do Al-Cu alloys in as-cast or heat treated conditions show weaker corrosion resistance in comparison with pure
Aluminum or Al-Si alloys?

 

[kenvlach]

The corrosion resistance of aluminum is due to a very thin, highly protective layer of amorphous alumina (&[ignore]gamma[/ignore];-Al₂O₃) formed whenever the bare metal is exposed to air or water. Corrosion resistance is best for pure aluminum, and is reduced by any alloying, even in solid solution. However, certain alloying elements such as Cu and Zn are worse than others such as Mg and Mn. Precipitate phases within the alloy especially lower the corrosion resistance by interfering with the continuity of the protective alumina film. Cu-containing precipitates such as CuAl₂ and (Fe,Cu)Al₆ are generally worst, since these particles are cathodic to the aluminum solid solution phase, forming galvanic cells when exposed at the surface. Any oxidized Cu is reduced by adjoining Al, thus creating defects in the oxide film. Contrary to this, Mg₂Si has almost the same electrode potential as an Al solid solution saturated with Mg₂Si, which accounts for the generally good corrosion resistance of 6xxx alloys cf. 2xxx alloys. A similar potential situation for MnAl₆ explains the good corrosion resistance of 3xxx alloys. Corrosion-prone Al-Cu alloys such as 2024 are often used with a cladding of a purer Al alloy, i.e., ‘Alclad.’.
Among casting alloys, corrosion resistance is best for 5xx series (Al-Mg) followed by some of the 3xx series (those low in Cu, such as 356 and 360).
Although Si itself is very corrosion resistant due to forming a similarly protective amorphous SiO₂ film, it can cause corrosion as Si is cathodic to Al. It also makes it difficult to increase the oxide thickness via the anodization process.

The effect of alloying on the corrosion resistance of aluminum alloys is more complex than this brief description. For further details, see
Corrosion of Aluminum and Aluminum Alloys, J.R. Davis, ed., ASM Int. (1999),

http://www.asminternational.org/Tem...=Ecommerce/ProductDisplay.cfm&ProductID=10655

or various books on aluminum metallurgy and corrosion.

 

[MetalEng]

thanks a lot, kenvlach