RO water is aggressive because there is very little of anything in it mineral, or buffer wise. Roughly, waters are either scaling or corrosive. If they contain little to nothing, they will dissolve many things they come in contact with. Metal or mineral. Water is always trying to reach equalibrium by dissolving or depositing something. It won't create acid, but some RO water is on the acid side, say 6.0-6.5 depending on how far it is processed.