I have a spare set of brakes that I put in my parts box in the shed

I guess they’ve been there a year

Opened the box yesterday and the alloy is corroding. I thought anodising protected against this?

Is it the damp that causes alloy to corrode?

That will be the shed pixies.Not only do they steal small parts before they hit the ground but have a habit of peeing on bike parts in the night.
It’s acidic you know. 😀

Yer will be the damp over a log period.

How clean were they when you put them in storage?

Just takes a bit of salty residue/muck plus moisture from condensation over winter and any aluminium alloy will corrode given time…