The water they’re evaporating is not the same water flowing through servers. There would be a heat exchanger so that all the anti-corrosion additives stay in the closed loop side.

Any additives at all in the evaporative side would just get concentrated more and more as water evaporates. Pure distilled water would be ideal since there would be no residue to periodically clean, but at the scale they need it, they basically have to use lightly filtered river water as far as I know.