corrosion of aluminium -cause?

[dragstrut]

i was out at a building recently. i practice in structural engineering of aluminium components, though i am not a specialist expert in corrosion related matters.

I saw aluminium corroding like the below images.

it appears the stone immediately adjacent to the aluminium is attacking the aluminium?

this was approx 500metres from the sea. A sheltered harbour, behind some rolling hills, not exactly "inland", but nowhere near breaking surf or too much airborne salts i would imagine...

this worries me a bit as in my practice, we often mount ceramics on aluminium rails, even sometimes consider mounting natural stone near aluminium, i never believed corrosion was an issue for aluminium in my applications, this has me second guessing things..

 

[EdStainless]

500m is considered near the shore.
Salt mist and fog is carried in, it condenses on your structure and then runs down into the stone.
I would wager that if you picked up some of those stones, rinsed them with DI water and then analyzed for Cl you would see big numbers.
It looks like the stone has damaged the surface coating on the Al.

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P.E. Metallurgy, consulting work welcomed

 

[NorthCivil]

Thanks for the response @EdStainless

would this be the combination of the chlorides, with the alkalinity of the stone?

as Aluminium is typically fine in salt laden environments. in my experience, in saline environments, even uncoated aluminium will usually form a surface rust (patina) which safeguards against further corrosion

 

[weldstan]

Al alloys do not like to sit on alkaline concrete and stone in moist conditions. Add on the Cl from its proximity to the ocean and conceentration thereof from alternate evaporation/condensation cycles and you have the perfect recipe for the corrosion observed.

 

[NorthCivil]

I have found this paper - which describes the chemical process quite well:

https://citeseerx.ist.psu.edu/docum...ion damages through,of a building mate- rial.

Particularily pages 71-74