I found some older brass ammo that had started to show almost like rust (assuming corrosion) spots on the casings. Been stored in a basement in original boxes in a wooden cabinet for probably a decade.
Anyway, im looking to mitigate this going forward... I remember a prior thread on ammo storage, someone mentioned they use a plastic moisture barrier bag inside a metal ammo can. Unfortunately I can't remember the specifics of the bag or find the old thread...
Anybody remember.
Best I've been able to find are VCI bags cut for ammo cans, but they're $12 each right now. That's a little out of my current budget.

Not sure about the plastic barrier, but you can get those little bags of desiccant and toss a couple into each ammo can. Of course they don't last forever.

All my long term storage ammo gets unboxed and placed in vacuum sealer plastic bags, vacuum sealed, and then housed in ammo cans.
I write the ammo amount on each bag.

Sorry, I have no experience storing ammo long term. I normally shoot the majority of what I have and then load more when I need it. With the exception of 50 BMG, I do not currently load that.

I would be inclined to vacuum seal it a couple times. I think anything that keeps oxygen out will prohibit corrosion.

Vacuum sealing seams like a good idea i haven't thought of.
I will have to look into a sealer setup. Would be nice to have for food as well...

All of my ammo is stored indoors and I've never had a corrosion issue ever .

Paper boxes will absorb moisture from the air and lead to corrosion over time, more so in a higher humidity environment. Humidity will almost always be lower indoors than in someplace like a garage or shed. Of course I have no idea what the humidity might be in Wolfe's basement.

Vacuum sealing should help, especially if you include a couple of desiccant packets. With rifle ammo especially though (pointy bullets), I'd worry about the ammo poking holes in the bags.

A better solution, IMO, would be to get some ammo cans with known good O-rings and dump the ammo into them along with the desiccant packets. Label the outside of the cans so you know what's in them a year later without having to open them to see.

ETA: I recommend using the .30 cal cans. The .50's get really heavy when you fill them up with bullets.

^^^
This is what I do. Surplus or new ammo cans. Desiccant packs are cheap at Amazon; toss a couple in there. I use a stencil to spray the caliber on front top and sides with yellow paint. I use a white wax pencil to mark the count and type on the front of the can (example: 147 gr. FMJ or 124 gr. LRN). It can be wiped off with your hand or a cloth and rewritten if you change anything. It is stored indoors but no corrosion. Outdoor might be a different issue but the military has used cans forever.