Quoted:
Random brown corrugated cardboard may or may not be recycled, and if it is it may or may not be high in acid or sulfur. The cardboard used in GI ammo packing is new material and meets some GI standard - it does not contribute to corrosion by itself. However, ammo plants are maintained in a relatively stable environmental state, with controlled temperature and humidity, so when the ammo cans are packed, the air inside is dry enough to not be a problem.
If you're worried about contact with cardboard contacting your brass, you can line it with wax paper, which is neutral for just about anything.
This +1 - If you have known cardboard that is used for ammunition, you are
usually safe. If you use random cardboard from unknown sources, you may/may not be safe.
I do not know how you'd verify which is which. I do know I have a bit of WWII ammo that is still in it's origonl boxes (45 & 30-06) and all of it looks pristine.
I have some 30-06AP that was from the Korean war era that shows corrosion, I dunno if this was from the cardboard or improper storage.
I would
ALMOST bet that any cardboard that is used in ammo boxes is neutral, have heard some is not.
It still should be kept dry. YMMV
Edit for edit
needs