How much does aluminium expand when oxidizing?

[Klikoos]

Hello everyone,

I was wondering how much certain metals or more specifically aluminum expand when they oxidize.

On site we encountered a large pit on a submerged aluminum pipe. The pit was about 7.06 cm^3 in volume (if you were to approximated the volume as a hemisphere), after removing the corrosion the only thing remaining was a dimple of about 0.26 cm^3. This would give the corrosion an expansion ratio of over 27 which seems a bit high to me.

Cheers,

Alex

 

[Chumpes]

hi
these look like easy calculations from oxydes stoechiometry, material balance and densities, considering compact corrosion products
i leave it to you !

but porosity migth also play a role... see if you need evaluation after initial calculation above.

regards

 

[Klikoos]

I did a calculation with moles and densities which does give a 31% increase in volume when it oxides but still a long way from the 27x that we observed.

 

[btrueblood]

did your calculation assume a hydrated oxide of aluminum, i.e. (Al2O3)3H2O? Sp. gravity for a crystal of the hydrate is around 2.4 - 2.5 g/cc. Problem is that the actual corrosion deposit is likely amorphous, not crystalline, which raises its volume quite a bit.

 

[EdStainless]

I was going to say at least 10x based on experience, but 20 or 30x doesn't surprise me.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube