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I have a water that is saturated with Calcium Magnesium Carbonate $\ce{CaMg(CO3)_2}$ (Dolomite). This precipitates in particularly metal pipes and whenever the water is heated. Instead of ion exchange (Sodium for the group 2 metals), reverse osmosis or deionisation, how about reducing the pH to say $\pu{6.6 - 6.9}$ with an organic acid such as Acetic acid or Citric acid? Would that corrode the $\ce{Fe/Zn/C}$ containing surfaces whilst the more electropositive elements are still present?

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  • $\begingroup$ Reducing the pH of tap water to $6.6$ or $6.9$ will not corrode pieces of Zinc or Iron metals. Even pH $5$ is not sufficient. $\endgroup$
    – Maurice
    Jun 2 '21 at 10:34
  • $\begingroup$ There are no more electropositive elements here. $\endgroup$ Jun 2 '21 at 13:39
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    $\begingroup$ You got it backwards: increase the pH and the hydroxide salts $\ce{Mg(OH)_2}$ and $\ce{Ca(OH)_2}$ - which are insoluble in water - precipitate. Wash away with circum-neutral pH water. This is one of the ways it's handled on a commercial or city-wide scale. $\endgroup$
    – Todd Minehardt
    Jun 2 '21 at 15:16
  • $\begingroup$ A water softener should prevent future scale deposits . It will replace Ca and Mg with Na. $\endgroup$ Jun 2 '21 at 16:05
  • $\begingroup$ @Todd Minehardt: increasing the pH will ppt the carbonates, since the soluble species in the hard water are bicarbonates. Probably a good way would be to add Mg(OH)2, which is essentially insoluble to begin with. it would convert itself (and the bicarbonates) to carbonate (which is actually more soluble!), but would than not ppt when heated. The resultant pH might be a bit bitter (~9.5-10, not quite 10.5, because of dissolved MgCO3). $\endgroup$ Nov 1 '21 at 13:16
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$6.6$ is neutral for pretty much all purposes. So is $6.0$. So is $5.0$. You need to go way lower than that. Table vinegar may have $\rm{pH}\approx2.5\dots3.0$, and that's what you want to use to hear that gentle fizz of dissolving carbonates.

Yes, this is a viable method. No, it will not corrode steel pipes to any significant extent, provided that you apply it for minutes, not days, and flush the system with water afterwards.

So it goes.

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If by "metal", you mean steel: Our water guy recommended sulfamic acid for cleaning. I don't know the chemistry but this is old, standard technology. Water treatment companies like Nalco could have information and supplies. NACE ( national association of corrosion engineers) would have a variety of references ; for a price. We used citric acid and ultrasound for cleaning steel and causing no damage to the steel.

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