So first the good news. If you knock a monster into another one with PISORF (would think normal knockback also works the same), both get also the corrosion damage

But the damage calculation is really strange:
Used a gnome wizard with a prepped spoon -> so base damage is at 6, 50% base damage from PISORF -> 3 physical PISORF/knockback damage for each monster. With WEYWUT to put two level3 goblin adjacent to each other.
First started with 2 layers of corrosion. And i did 4 damage (i think). So first thought, the corrosion damage gets also halved.
Then used 3 layers, and i would have assumed, that the damage will get rounded down. But i made 5 damage.
Now I'm at 4 layers -> 6 damage
5 layers -> 7 damage
7 layers -> 9 damage
Don't understand the math. It seems that the first layer of corrosion seems to be missing.
Will make a second run and report the damage for only one layer (for this run i started directly with two layers).