Aluminium in Aqueous solution

Question

Aluminium in Aqueous solution

Biggadike (Mechanical)

(OP)

14 Aug 02 03:06

I'm trying to get some concrete information on the behaviour of aluminium and other normally passivate-forming metals in aqueous solution.
Whilst these metals will readily corrode in an aqueous solution under the influence of a raised potential, looking at the Pourbaix diagrams I can find, it seems to me that they all have a level of pH where no corrosion will take place. It also seems to me (and this may be a statement of the obvious) that it is the extremes of pH attacking the passivating oxide film which enables the corrosion to take place.
I wonder, but can find no information, whether there is in fact a potential at which corrosion will spontaniously begin even at neutral pH?
Can anyone enlighten me?

kclim (Materials) · Answer 1 · 2002-09-20 23:14:46

I imagine at extremely high positive potentials, it may be possible to break down the aluminium oxide layer and cause corrosion regardless of pH, but at high potentials, other competing anodic reactions would become significant, such as the oxidation of water and these would have to be taken into consideration.

In practice, impurities in solution will be a major factor in determining the corrosion behaviour of aluminium. For example, chloride ions aid the dissolution of the 'protective' oxide layer, thus hindering passivation at lower pH. This is not only isolated to aluminium, but also many other metals (steels).

Biggadike (Mechanical) · Answer 2 · 2002-09-23 02:54:29

Thanks for your reply kclim,
A mechanical engineer/product designer by trade, I find that the text books I have on these matters stop short at the level of detail required to fully explain many real events.
I'll pop off and get myself a chemistry degree. In the mean time, your explanation has made the situation a lot clearer.

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Aluminium in Aqueous solution