I doubt they are aluminium, You'll probably find they are steel with some plating or galvanised coating. You could brush some CL grease or rust proofing wax on them to stop further corrosion.

Thanks. So the white deposit is actually corroded zinc galvanising? I'll try to put something on to stop or slow down the corrosion.

I don't know what the plastic coating is for. In my opinion it makes corosion worse by trapping water under a damaged coating.