I was playing around in my test world. I noticed that protection enchant reduces potion damage, but the armor loses no durability.
Steps to reproduce:
1. Enchant armor with protection(higher levels give more noticeable effects)
2. Get a splash potion of harming.
3. Hit yourself with the potion without the armour.
4. Noe hit yourself with potion while wearing armour.Compare results.
5. Note how the armour doesn't take damage.