Originally Posted by thekctermite
Hopefully some other folks will have some other tips or scenarios to make this thread more informative.
I'll take The KCTermite up on that.
1. The purpose of sanding the outside of the copper pipe is not to clean it of oils. The purpose is to remove the copper oxide that forms on copper when it's exposed to air. That oxide film forms slowly at room temperatures, but almost instantaneously at soldering temperatures (or from about 350 to 450 deg. F, depending on the solder you're using.)
Copper is an orangy/gold colour, but Copper oxide is brown in colour, and it's the formation of a copper oxide film over the copper that causes new pennies to turn brown in time. Ditto for copper piping and everything else made of copper.
Oxygen atoms hold onto their outer electrons very tightly. Metal atoms, on the other hand are very loose with their outer electrons, and it's this difference that prevents metals from having a high affinity for oxygen or even metal oxides. Liquid metals, like molten solder, have a very high affinity for bare copper metal, but little to no affinity for copper oxide. In order to get the solder to stick properly to the copper of the pipe and socket, it's necessary to remove all the copper oxide from both AND prevent it from forming during the soldering process. Only then will there be enough capillary pressure drawing the heavy liquid solder into the joint to fill it completely. Any copper oxide film on the pipe or socket will prevent the molten solder from sticking to the copper oxide, resulting in a leaking soldered joint.
2. Soldering flux consists of petroleum jelly (which is what "Vaseline" is) with zinc cloride powder mixed into it. The purpose of the flux is threefold:
A) it prevents an oxide layer from forming on the bare copper
B) it cleans the bare copper of any remaining oxide on it, and
C) it promotes good flow of the solder by keeping the copper metal bare until it is displaced by the solder.
3. The petroleum jelly's job is to prevent oxygen from the air from coming into contact with the bare copper metal while the joint is being soldered. So, in that respect it merely acts as a physical barrier between the copper and the surrounding air. So, you can solder using Vaseline as flux, but the solder will only flow into the joint. It'll beed up on the surface of the copper pipe because the Vaseline outside the joint quickly burns off and exposes the bare copper metal to oxygen in the air.
To prove this to yourself, next time you solder a joint, sand an entire foot (12 inches) from the end of the pipe and flux that entire foot. After you solder, allow everything to cool and clean off the residual flux from that entire foot, you'll find that the entire 12 inches before the joint is clean and bare, just like it was before fluxing and soldering. The flux on the copper pipe prevented the pipe from oxidizing and discolouring as usually happens on sanded copper piping near the solder joint (or source of heat).
4. The zinc cloride acts like an acid. At soldering temperatures it dissolves any residual copper oxide in the joint that wasn't removed by sanding. It dissolves the copper oxide much more aggressively than the copper metal.
5. When you solder a joint, what actually happens is that the molten solder has a greater affinity for the bare copper metal than the flux does, and so it's CAPILLARY PRESSURE that draws the molten solder into the joint to displace the flux inside it. Once the molten solder is in the joint, something called "amalgamation" occurs in a very thin film at the solder/copper interface. That thin film consists of an alloy of the copper and solder that results when the two dissolve in one another. Some of the tin and antimony (or lead) atoms dissolve in the solid copper and some copper atoms dissolve in the molten solder right at the surface of the copper.
(The following was told to me by my metallurgy prof in University. I tried to find a website to confirm in Google, but was unable to.)
If you ever unsolder a joint, you'll find that you cannot completely remove the old solder from the end of the pipe by heating with a torch and wiping. That's because the melting point of the alloy formed at the amalgamation plane varies from that of solder on it's outside to that of copper on it's inside. So, the "solder" doesn't wipe off because it's not molten because it's no longer pure solder with the melting point of pure solder.
6. Everything from sanding the copper pipes to adding zinc chloride to the flux is designed to prevent copper oxide from forming inside the joint. At soldering temperatures, that oxide layer will form instantaneously. So, if for some reason (like not allowing a path for the heated air inside the pipe to escape) the solder isn't drawn all the way around the joint, then adding flux and solder when the joint is hot will (at best) just plug up the leak in the solder joint. It won't end up filling the joint with solder. The reason why is that while you are soldering, the flux won't burn because there isn't any oxygen inside the joint for it to react with. If air inside the pipe expands and leaks out through the fluxed joint, the flux exposed to air will burn off, exposing the bare copper metal that will immediately form an oxide film over it. After that, the molten flux will NEVER bond properly to that oxide film because oxygen hordes it's electrons. The best that you can hope for is that you can plug the END of the leak in the joint with solder. But, that's just a leak waiting to happen.
7. So, how does this affect what you do? Understanding the process allows you to understand why it's seldom necessary to remove old solder from old pipes or sockets. If you unsolder a copper pipe from a socket, you're best bet is to NOT remove the solder from the socket or the end of the pipe. That solder in the socket or on the end of the pipe will protect the underlying bronze, brass or copper from oxidation. Just leave the solder on the end of the pipe or in the socket until you need the pipe or valve. Then, when you need that piece of pipe, just heat the end of the pipe and wipe the old solder off. If it's a valve, heat the socket of the valve and insert a piece of sanded and fluxed copper pipe into it until it goes into the socket, and then pull it out, thereby removing the old solder from the socket. (Repeat if necessary) Then clean the end of that pipe with steel wool or a piece of sandpaper to remove any oxides that may have formed while the pipe was cooling down. Brush out the socket. Flux the pipe end and the ID of the socket and fit them together (even if both appear to be still covered by solder). Now just heat and add enough solder to ensure the joint is full (by the formation of a drop of solder on the bottom of the joint as discussed by KCTermite). That solder joint will be just as strong and last just as long as had you gone to the trouble of using a new pipe and sanding the old solder out of the sockets of the old valve. In fact, it'd prolly last longer because by not sanding the old solder out of the sockets, you're not enlarging the ID of the socket.
To learn more about soldering and brazing copper, brass and bronze, go to the Copper Development Association's web site at:
and click on the "Publications" link,
then click on the "Publications List" link
then RIGHT CLICK on the "Soldering/Brazing/Welding" link on the list on the LEFT column of links, and chose "Open in new Window".
Then download everything you want.