Rod, common
An arrow with a green crystal as a head. If the arrowhead is broken, it will deal acid damage and dissolve any armor the target is wearing over 1d4 rounds. If the target is not wearing armor, it must succeed a DC12 constitution save or take 1d8 damage over 1d4 rounds.
Am I missing something? Why is this so low rated?
It dissolves any armor at best within 1 round, reducing any creature which is wearing armor from an armor class of 18 (20) down to 10 + Dex-mod (+ shield). Meaning, shoot it, let it dissolve the armor, even if it takes 4 rounds. During that time you'll try to maintain distance. After that you rush over it. Boss-fight over.
Considering that a lot of mobs are already not that high with their armor class and an average attack bonus, even at a low level, you basically make any mob have an effective AC of 6 - 8. That's just uncool for any DM trying to put anything in front of the group that has at least some kind of toughness.
So, I'd remove the first part where it dissolves any armor, increase the CON DC to 15 and leave the rest as it is. That'd be a cool item.