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I have a question regarding the use of NaCl as a water softener.

Recently i was asked to comment on a series of test results from a water softening device were a lot of corrosion had occurred post softening. Just to be sure we are on the same page, NaCl was added to the water to dissolve* the CaCO3, Ca(CO3)2 & CaSO4.

*Ion exchange is not my field of expertise, so I'm not quite sure if this is the correct wording

Now, according to the folks who ran the test, there were no signs of ‘Chlorides’ in the water sample, what they mean by this is still unclear. I was also informed that no precipitate was collected. Now to the actual question: they wanted to know why the pipes used, post softening, had crevice corrosion inside the pipe. My first intuition was that there were still $Cl^-$ ions in the water which is causing the steel to corrode. However, the folks running the test assured me that the chloride had bonded with the calcium ions.

I’ve been trying to figure out what in the world could cause the pipes to corrode, assuming that they are correct, and the chloride have formed CaCl2.

Have you guys got any ideas? Have I misted something, or am i right to assume that the folks who ran the test don't know what they are talking about.

If you need any more information, feel free to ask. As stated earlier, this is not my field of expertise so if I'm wrong about something please correct me.

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A traditional softener has a zeolite resin that "holds" positive ions. When it is charged the Na ions fill the resin. Then in service , the positive ions - Ca , Mg , etc , exchange places with the Na ions, putting Na in the water . In theory the back flush of the softener after charging, removes the Cl ions. So that in service the only Cl present would be what is in your raw water as the Cl is not affected by the softening. Water corrosion results from conductivity ( any ions) and oxygen; Although Cl ions are especially active. Generally "waters" leave a CaCO3 type scale that forma a protective layer ( scaling ) that reduces corrosion. Serious evaluation of water corrosion looks at the water "scaling index" to get a initial idea of how corrosive it may be. Softening water reduces the scaling index and promotes corrosion . I removed a softener from one house I owned for that reason; However it was Lake Michigan water which has a very good scale index - enough to be protective but not enough to restrict water flow. I also had to replace a couple sections of copper pipe that had corroded through in the 20+ years the softener had been in service. On the other hand , softeners are useful in very hard natural waters.

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  • $\begingroup$ I'll try and ask a few more question of the folks who ran the test based on your answer. So far I've learned that the pipes were made of 'en 1.4404 stainless steel' and that the hardness of the water is around 28 (dGH/°dH). I don't know if they are using zeolites as a medium, the pictures I've seen so far does not seem to indicate that they do. I will return with more info when I have it. $\endgroup$
    – Christian S. O.
    Dec 2 '19 at 11:05
  • $\begingroup$ Okay, I've acquired the product data sheet from the company who operates the equipment. Rough translation of the description: "Salt-pellets for use in chemical industry as well as water treatment and food production". I believe this means that they aren't in fact using not using zeolites with NaCl added, and simply ads a bunch of NaCl to the solution to soften the water. $\endgroup$
    – Christian S. O.
    Dec 2 '19 at 20:10
  • $\begingroup$ I am not a water specialist , however, hardness is similar to scale index but not quite the same , but I never used dH. I expect the stainless is in the softener , not the house piping. The softener has resin ( maybe another name ) in it. $\endgroup$
    – blacksmith37
    Dec 3 '19 at 14:57

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