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Recently I installed a whole house reverse osmosis (RO) system. It was working great for a couple of weeks, unfortunately I didn't do all my research about RO and its affects on metal pipes.

Recently my water started having a brown tinge and metallic smell to it. Looking into I found that RO water will corrode metal piping. So I immediately disconnected the RO system.

My understanding is that there is a zinc coating on the pipes to help protect from corrosion, and I fear the first few weeks of nice looking RO water was my zinc coating protecting the iron, but the zinc has been corroded away and now the iron is exposed. The pipes were already pretty old, I estimate 70 years or when the house was built. Is this an accurate understanding of galvanized piping? Is the zinc coating gone, and the pipes will fail sooner than later now?

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  • Does this RO system have a pH adjustment system before the RO membrane? Is that why the pipes corroded?
    – Jim Stewart
    Commented Nov 4, 2021 at 17:02
  • Not sure the ph before the ro filter but it was softened well water with tds around 150 in pex pipe. After the RO filter the ph was around 7.5 and tds was around 5-10 ppm with galvinized piping going into a slab.
    – dmoody256
    Commented Nov 4, 2021 at 17:36
  • I ask because I did not think that RO itself would make output water corrosive, but thought that a system for changing pH before RO membrane might make the output water more corrosive. Without researching the matter it would seem to me that a water softener could make water more corrosive to galvanized pipes. Doesn't a water softener replace Ca and Mg ions with Na? Ca and Mg ions might act to "passivate" galvanized pipes and so prevent corrosion. Is it recommended to soften water before RO? I have never heard of galvanized steel pipes penetrating and under a slab. US this common?
    – Jim Stewart
    Commented Nov 4, 2021 at 19:12
  • yeah it is recommended to soften the water before RO. RO water is corrosive to metal because it has very low tds, so it has lots of polar H2O molecules with very few + or - ions for the H2O to make weak ionic bonds with. This means it can be sticky to polar molecules that would be in metal pipes, even with a neutral PH. It basically becomes a more powerful solvent. The RO filter removes pretty much all the Na and you have almost pure water in the pipes. Really I just want to know if the zinc on the pipes is gone, does that mean the metal pipes wont last much longer?
    – dmoody256
    Commented Nov 4, 2021 at 20:53
  • You generally don't want to run RO water through metal pipes. It's probably picking up a lot of mineral deposits that have coated the insides of the pipes over the years, and could be attacking the pipes as well. I'm not sure that softening it first would make a difference. I'm hoping @blacksmith37 will see this and chime in.
    – Mark
    Commented Nov 5, 2021 at 0:40

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I'd just rip out the galvanized and replace it with PEX

Galvanized piping has a rather poor longevity record in plain old tap water applications, never mind RO water, which will eat it in no time, as you found out. So, I'd rip out all the galvanized and replumb in PEX, which won't react with the ROed water.

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  • Unfortunate, but a whole house RO system just seems like a bad idea. Different studies also show that PEX potentially leeches other chemicals into drinking water, but PEX (and other plastic piping) are really the only common piping methods to not significantly corrode from RO water. Something of a catch 22: Remove a bunch of impurities only to potentially introduce new ones with any piping option used to get it to your mouth.
    – statueuphemism
    Commented Nov 6, 2021 at 2:15
  • @statueuphemism -- I'm not keen on whole-house anything when it come to water supplies because it rather limits what you can do when it comes to retrofitting fire sprinklers into your house
    – ThreePhaseEel
    Commented Nov 6, 2021 at 2:18
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    It's very disappointing that a water treatment company would sell a whole-house RO system without addressing the impacts it will have. But if you choose to replumb in PEX (which will solve the corrosion issues), you should consider activated carbon filters at the drinking water outlets.
    – Mark
    Commented Nov 6, 2021 at 4:42

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