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I encountered a corrosion problem with a reverse osmosis (RO) unit years ago. A cooling water system in a remote gas compression plant used RO-processed river water to supply makeup water. The cooling tower blowdown (outlet from the cooling water system) was sent to a WAG injection system. The problem was high levels of corrosion throughout carbon-steel portions of the cooling water piping system. Tests showed low pH and low amounts of dissolved solids.

I had heard of this problem in university courses. My mental bookmark was:

RO permeate water could be corrosive because Le Chatelier's principle says the lack of dissolved solids causes the equilibrium of 'solids dissolving versus solids depositing' to shift towards the 'solids dissolving' end of the balance.

In the cooling water piping system, I guessed the solids that could be dissolved might be the passive layer protecting the parent metal of the carbon steel pipe.

The solution ended up being: open up a bypass valve to permitting some of the RO's inlet water to bypass the RO elements and mix with the permeate in order to bring the conductivity of the water up to a satisfactory level.

However, I would like to know more about the RO permeate corrosion phenomenon. What is the exact chemical mechanism at work when the inner surface of carbon steel pipe corrodes?

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  • $\begingroup$ Why would you not expect corrosion ? Natural waters tend to be less corrosive because of calcium and neutral pH. You removed Ca and made it acidic with CO2. $\endgroup$
    – blacksmith37
    Apr 27, 2021 at 14:06
  • $\begingroup$ Water piping in oil production tends to be lined with cement or polymers to reduce corrosion. $\endgroup$
    – blacksmith37
    Apr 27, 2021 at 14:09

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The absolute worst corrosion condition for mild steel is acidic water with dissolved oxygen in it. A corrosion text will give you the exact chemical equation (I cannot remember it).

When RO'ed water is exposed to air, two things happen: 1) it absorbs CO2, which makes it acidic, and 2) it absorbs O2, which facilitates acidic attack of mild steel.

The usual solution is to buffer the water into alkalinity and deoxygenate it.

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