In the description of the artifact it says that when attuned it causes damage (1d12 for each permanent corruption points PC have) and then all permanent corruption is removed.
My players came up with a question which I don't know how to answer. They want to attune to an item that gives them 2 points of permanent corruption, then attune to the Water of Dusk, get rid of the permanent corruption that way, attune to another item, disattune to the water of dusk, attune to it again, sustain damage. And by using that - have 2 magical items and no points of corruption.
Technically this is "rules-as-written" as I can't find any mentions that permanent corruption from items anyhow different from other permanent corruption, but I have a feeling that it goes against the actual meaning behind rules. I have a feeling that for example if you unattune to the flask, you'll regain all permanent corruption from your items.
And actually same question about other means of getting rid of corruption (e.x. drinking water of the dusk's Drop of the Sun) but there as it is a rare consumable it is better to allow them get rid of permanent corruption points that way.