Corrosion is evident by the presence of green verdigris on the brass cartridge cases. Also, ammo that has been submerged for a sufficient time at a sufficient depth can allow water to seep into the primer. This damage cannot be undone. Using steel wool or the like to shine up the cases doesn't eliminate the weakened spot caused by the corrosion. Dessicant packs will not undo water or humidity damage.
Preventive measures are all that's effective to keep ammo usable for many decades.
There are a number of commercial chemicals available to seal cartridges. The best is spray lacquer. Allow them to dry. A bottle of clear nail polish, with its handy brush inside the cap, can be used to paint a line around the juncture of the case neck and bullet.