[kneekap]

Yes, the preservative in pressure treated plywood is bad for aluminum.
Pine plywood always checks badly outdoors. Fir plywood would be
better. Better is Hydrotek or Aquatek or Okume marine ply.

The very best thing would be polymer sheeting. That stuff is great.
Easy to work with too.

http://www.american-plasticlumber.com/sheet.html