Aluminium work hardens a lot more than copper does, and yes with time you get intermetallic corrosion as well, which creates thin layers of insulating oxide through the cable. Thicker cables this takes a lot longer. You will find the reason the wire is copper coated before insulation in motors and transformers is to both allow it to be soldered, and also to provide a barrier to oxygen penetrating, though I have seen wire down to 36SWG be copper coated aluminium now, before it was only things that were 18SWG or lower that were CCA, but now you see very thin CCA wire, especially in things like network cable, where you get 7 strands in 22AWG wire in the sheath, and those do sort of hold up, though they do not solder at all well, and will hold up in a crimp for a while. Gel filled Scotchloks though do work well on them.