dann2 - 14-9-2008 at 11:54
Hi,
I have a kettle made from Copper that needs the Lime removed from it. (the Lime has build up from hard water deposits).
I have seen Citric acid and Formic acid used for descaling Kettles but these attack the Copper.
Can anyone recommend something which will attack the Lime but not the Copper.
One (perhaps barmy) idea I have had was to add Sulphuric acid to the Kettle and using a Graphite rod connect a power supply to the Kettle + Grahpite
thus cathodically procecting the Kettle. You could heat the acid to make it work faster. The Lime deposits are very hard and stubborn.
Dann2
DJF90 - 14-9-2008 at 12:16
Lime (CaCO3) does not react well with sulphuric acid, as in my experience a protective layer of insoluble calcium sulphate stops the rest of the lime
reacting. HCl would likely be a better choice, as it should not react with the copper metal (as copper is below hydrogen in the reactivity series, and
there is no oxidiser present).
kclo4 - 14-9-2008 at 13:07
Last time I checked Vinegar doesn't react with copper metal, It will take off any oxide making it look bright pink.
Calcium, and Magnesium Acetate are also very soluble so you wouldn't have the problem H2SO4 gives you.
I would make sure it isn't touching any other metal however. I mixed vinegar with salt and it started to dissolve some copper around the edge of a
penny where the zinc was exposed. I'm pretty sure this was some sort of electrochemical reaction.
chemkid - 14-9-2008 at 13:44
HCl pretty simple...barely reacts with copper and will take only a few seconds to work
12AX7 - 14-9-2008 at 15:27
Let's see here...
HCl: complexes copper. Dang. (Probably not an issue at dilute strength though.)
Acetic acid: complexes copper. Dang.
Sulfuric acid: leaves copper alone, but doesn't dissolve calcium real great. Dang.
Nitric acid: strong oxidizer...but only when strong. Dilute HNO3 (5%?) should work perfectly, no? 
Does formic acid complex transition metals? I see a few journal hits for "formatocopper", so I guess it exists under some conditions at least.
Smooth off the copper with a scrub pad when you're done. A little acid won't hurt anything, but it will leave a matte surface prone to corrosion.
Oh, and a dash of oil rubbed on when you're done to keep it shiny. Then hang it on the wall, because copper is almost too beautiful to use. 
Tim
chloric1 - 14-9-2008 at 16:24
I am going for a stretch here but....how about NO acid?! I am thinking chelating agents here. A couple suggestions:
Tetrasodium EDTA or other EDTA
Sodium Citrate
Tetrapotassium pyrophosphate
Sodium Tripolyphosphate
Sodium gluconate with a little NaOH
All easily had from ebay and will not oxidize copper metal.
12AX7 - 14-9-2008 at 16:36
EDTA crossed my mind, but how well does it complex calcium, and how quickly? Could be soaking for a while. In the mean time, oxygen is going to work
on the copper. I think the solution will inevitably turn green (or blue or whatever), it's just to what degree.
Tim
dann2 - 14-9-2008 at 16:44
Thanks alot for the responses.
I used 12% HCl. Builders store for cleaning cement. Worked very fast and I got no blue colour so it did not attack the Copper.
Formic acid (I tried it) does give a blue colour. It was 100% stuff though.
Dann2
The_Davster - 14-9-2008 at 16:53
I think this is being overcomplicated
HCl
It takes time for enough oxygen to dissolve in HCl to even notice colouring of the solution with green CuCl4(2-). And if you are still worried add a
little sulfite or thiosulfite as a reducing agent to the solution.
Just fill the kettle, and stopper to prevent air from entering.
After lime has dissolved rinse well and oil
ScienceSquirrel - 14-9-2008 at 17:16
Sulphamic acid is pretty juicy stuff.
It is the basis of a lot of descaling materials.
watson.fawkes - 15-9-2008 at 12:07
@dann2: I like the idea of trying galvanic protection. You can test whether it works with a piece of copper wire in your cleaning solution. Look for
a mass difference in the test piece before or after.
As for a chemical, just try a commercial vendor. One I use is CLR Calcium-Lime-Rust Remover. Its MSDS lists its active ingredients. Here's the
manufacturer Jelmar's MSDS page. It's currently listed acids are lactic and gluconic.
(Aside @ScienceSquirrel. I believe CLR used to contain sulphamic acid.)
I also might point out that regardless of your cleaning method, you can always plate new copper on. Right after cleaning, that base surface should be
right ready to go. Since you're only cleaning the inside of a vessel, you can use that same vessel to hold your plating solution. In other words,
since you don't need to plate the outside, don't try.
chloric1 - 15-9-2008 at 13:11
I don't necessarily think I am trying to overcomplicate things just wanted to offer a nonaggresive form of lime removal. The chelating agents I
referred to, especially polyphosphate, have specific activity towards calcium and magnesium. I was thinking that dann2 would be able to use reagents
I mentioned without comprising any natural patina that might be on the pot. Some copper items are better off brown, green or whatever.
argyrium - 16-9-2008 at 11:13
chloric1 is correct in his post concerning Na polyphosphate for a non aggressive/conservative treatment that would do little damage to the patina on
Cu (or bronze/brass).
The commercially available product, Calgon (NaPO3)x at 5 - 15% in water can do pretty amazing things to calcareous deposits if allowed to soak -
sometimes for days.
Alkaline Rochelle salts and alkaline (NaOH) glycerine can also be quite effective, but slower still.