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Cleaning products producers advise against using "home remedies" instead of their products. This advice against using citric acid to remove limescale was especially surprising for me:

Citric acid reacts unfavorably with limescale. In the case of very heavy deposits, the citric acid surrounds and seals the limescale like a protective coating. This results in insoluble deposits inside the appliances, which even descalers can no longer remove.

Surely the citric acid itself cannot "seal" anything as it is water-soluble, so I expect there to be some sort of "passivation layer" appearing on top of the limescale. Is there a known reaction that fits the description, and which one is it?

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    $\begingroup$ Calcium citrate is not very soluble in water : $0.9$ g in $100$ mL at room temperature. So it may happen that using citric acid on heavy limescale deposits may change this insoluble substance into another insoluble deposit. It is not a passivation effect. $\endgroup$
    – Maurice
    Nov 25, 2021 at 21:56
  • $\begingroup$ Acetic acid is a much better bet - the solubility of calcium acetate is 34.7g/100ml $\endgroup$
    – Waylander
    Nov 25, 2021 at 22:06
  • $\begingroup$ @Maurice I think 0.9g/100mL is still easily rinsed with water, and certainly not something I would call an "insoluble deposit". $\endgroup$ Nov 26, 2021 at 0:05
  • $\begingroup$ @Waylander In the same page I have linked to they discuss acetic acid as well: it is said to dissolve plastifiers from plastics, making them brittle. $\endgroup$ Nov 26, 2021 at 0:13
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    $\begingroup$ The low solubility of calcium citrate may result in precipitation of calcium citrate within crevices of the scale and reduce the surface available for dissolution, especially if the concentration of citric acid is low and becomes depleted. On the other hand, high citric acid concentrations will certainly have a strong corrosive effect on the scale, and still not (very) corrosive toward the metal. Citrate also chelates calcium, so high concentrations of the acid will take away the scale nicely. $\endgroup$ Nov 26, 2021 at 17:17

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There is no cure-all solution for descaling and no "reaction" because scale is not a standard compound. Its composition will vary with water quality. So it is a bad idea by these companies to spread rumors that citric acid passivates limescale.

One has to know what the scale is and the construction material, to begin with descaling. I think it all depends on the application. For example, citric acid is ideal for descaling dishwashers and washing machines. It is not corrosive to steel and forms complexes with iron, if there are rust stains. Note that descaling is done with citric acid (citric acid is in excess) in these machines.

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