[ThomasMonk]

I recently introduced some calcium hypochlorite into a water cooling system to kill off some fungal growth. I dissolved the calcium hypochlorite powder in warm water and then poured the solution into the cold water circulation of the system. This caused a white precipitation to solidify out of the solution. This precipitation is now lining my pipework. I have no idea as to the chemical composition of this precipitation but I need to find something to run through the pipes to dissolve it. Any ideas?

I have a sample of the precipitation on which to conduct tests.

I should point out that the precipitation was caused by the change in temperature from hot to cold, and was not due to any other chemical in the water system (which was reasonably pure).

[ThomasMonk]

To answer my own question: the residue is most probably calcium hydroxide (slaked lime) and/or calcium carbonate (chalk) which are dissolved by hydrochloric acid. Hmm, I'm not sure I should be adding hydrochloric acid to my cooling system. That would give me calcium chloride which should at least dissolve in the water.

Okay, that seemed to do the trick, but I just can't add 10 litres of hydrochloric acid to my cooling system... it would rot the seals. Can anyone suggest something less dramatic that will dissolve the slaked lime?

[Hunter2]

What about citric acid or acetic acid. The same stuff is used  to remove lime in a coffee machine.

[ThomasMonk]

I've tried citric, acetic, & formic acids and they of course dissolve any calcium carbonate, but they don't seem to touch the slaked lime. Something that does work is the oxygen-based washing powders (sodium carbonate peroxyhydrate). These seem to convert the slaked lime into a fluffy residue that is then dissolved by the aforementioned acids (at least formic acid). However, these oxygen-based washing powders advise against metal contact for the same reason as hydrochloric acid. Although the copper in my cooling system would be okay, the aluminium parts would be attacked. I'll test to see just how corrosive these oxygen washing powders are to aluminium.

[ThomasMonk]

My tests seem to suggest that this Vanish Oxi-Action washing powder isn't as corrosive to aluminium as hydrochloric acid. So flushing my system through with this might work. The new residue created should be suspended in the water rather than stuck to any surfaces and can therefore be flushed out with water. So I shouldn't need to flush through with a descaler. I will however need to connect in some sort of expansion vessel to account for the gas produced by the reaction.

So what bright spark can tell me the products of reacting calcium hydroxide Ca(OH)₂ and sodium carbonate peroxyhydrate 2Na₂CO₃.3H₂O₂?

[Borek]

What you wrote so far makes me think you have not just a typical scale present, but it is contaminated with something organic, that is getting destroyed by the hydrogen peroxide. As far as I can tell hydrogen peroxide should not react with calcium hydroxide - that is, hydrogen peroxide always decomposes to water and oxygen, and could be the decomposition is accelerated by the calcium hydroxide presence, but technically it is not equivalent to these two reacting.

[ajkoer]

OK, 2Na2CO3.3H2O2 will, in time (hours), dissolve Aluminum foil (it forms an insoluble white fluffy Al(OH)3), and I would suspect, a similar reaction time with Copper. As such, you may be able to quickly treat the problem and then rinse, rinse,.. with some possible metal discoloration, but no significant erosion (at least, in my opinion). If possible, perform a test.