So many people ask us this question every day and I will attempt to answer for new comers to shooting surplus ammo.
Corrosive ammunition is ammunition that uses a primer that has chemicals that when ignited leave a residue of corrosive salts. Most often these primers have potassium chlorate, or sodium petrochlorate which, when burned, decompose into potassium chloride or sodium chloride.
So basically when you use corrosive ammo it leaves salt or other element that needs to be cleaned from your gun.
Hope this helps
Al