Green deposit on copper pipe at fittings

Resues of acid/active flux, I believe. The active flux contains hydrochloric acid which removes the surface oxide layer & 'cleans'/pickles the pipe and fitting when heated, prior to the solder melting & running.

I don't know what the green compound is, I'm sure a chemist will be along shortly. If left inside a heating system, the acidic flux residues can cause serious problems with corrosion.

It comes from the use of acid flux, and not cleaning any excess off afterwards. Shouldn't do any harm, though.

Water and wire wool or any other suitable abrasive. Car paint wet or dry paper if you have any lying around. Wash off with clean water afterwards.

You normally wipe the joint with a wet rag once the solder has set.

Maybe a copper dihydrate? Starts off as a copper chloride CuCl2, a yellow-brown solid which slowly absorbs moisture to form a blue-green dihydrate (according to Wikipedia).

Doesn't it get diluted by the water to the extent it can't do anything? It also seems to stop corroding the outside after a while - the reaction becomes used up.

Unless flushed out &/or neutralised, I believe it makes the water acidic (pH

I'd say ots PROBABLY copper carbonate...sort of natural end point of any copper type oxide and CO2..

Ni. The reaon its around the joints is they they have a flux residue usually. And have been heated. If you WASH the joint and then wool it up, it stays good, usually

Darned good washing and some wire wool