Removing old green solder flux residue off Cu

[jim]

Is there a chemical which cleans (very) old green solder flux residue off copper tube & brass fittings?

TIA for suggestions.

[Malcolm]

Possibly corroded copper fron an acidic flux. Try wire wool on a small patch

Malcolm

[The Natural Philosopher]

definitely. The fluxes are mostly water soluble, but the copper salts may not be.

Green is copper carbonate or copper chloride I think.

There seems to be a reaction path that ends up with most copper exposed to air going towards copper carbonate eventually especially in the presence of acids and water.

sure that dissolves in a weak acid, and the resultant new salt can be washed off.

Or use proprietary copper cleaning products. Like Brasso.

[Malcolm]

Speaking as a chemist - probably basic copper carbonate. It would dissolve in something like vinegar - brick acid would probably be too much unless very highly diluted. Using acid would mean cleanig very carefully with dilute sodium becarbonate and then copious amounts of water. I would prefer a machanical method such as fine wire wool followed by brasso to polish the surface. It may be worthwhile to apply a lacquer to the clean surface to preserve the bright copper finish as it will quickly acquire a dull coating of copper oxide.

Malcolm

[The Natural Philosopher]

isn't stage one the reddish cuprous oxide?

I have a copper firehood and that starts off bright, then goes a deep orange-brown..it would I think go black, but in the absence of damp or weak acids, never green.

[Andy Dingley]

No. Use mechanical cleaning. Steel wool is OK but scratchy, plastic Webrax or 3M pads leave a better finish.

Be careful with the residue too. Copper acetate, which can certainly form from fluxes, is significantly toxic.

Also be careful with chlorides around bronzes and some brasses (although not a problem on pure copper) as it can trigger "bronze disease" (JFGI) that becomes self-perpetuating.

[NT]

+1.

The only thing to add is that flux desposits, depending on the flux, often wont be washed off by whatever removes the green compound. One more reason to go mechanical with wire wool.

NT

[Rick Hughes]

"Using acid would mean cleanig very carefully with dilute sodium becarbonate "

I'm not a chemist ... but when I did my A-Level chemistry Sodium Bicarbonate was an Alkali not an acid ?

Or did you mean neutralize with Sodium Bicarbonate 'after' cleaning with acid.

[Gib Bogle]

His sentence did start "Using acid".