Why is RO water more corrosive?
Water passing RO does not have the same chloride concentration.
The raw RO water contains almost no ions, aside of those that passed by leakages of the osmotic membrane.
This disbalances the water natural $\ce{CO2/HCO3-}$ pH buffer, leaving the ionic alkaline buffer part before membrane, so water becomes more acidic with corrosion aggressive $\ce{CO2(aq)}$.