[Conor]

Ainsley, you have touched on what may be the most likely source. That a previous user had sucked all the air out letting moisture into the cylinder and the powder was some oxidation product. Or some moisture had entered through the compressor of course.


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On the subject of Ali corrosion, look at the following
From a purely thermodynamical point of view aluminium is active. However, in oxygen containing
environment (air, water), aluminium is rapidly covered with a dense oxide layer [1]. The oxide layer
is essentially inert, and prevents corrosion. The thickness of the layer may vary as a function of
temperature, environment and alloy elements. Oxide films formed in air at room temperature are 2-3
nm thick on pure aluminium. Heating to 425°C may give films up to 20 nm [2]. If the oxide film is
damaged, e.g. by a scratch, new oxide will immediately form on the bare metal [3].

So, yes it does corrode, but no, not very much.