I have a question I run into in my trade, and I was wondering if someone here might have some insight into how Copper pipe might change character---or something.

I have noticed, in the HVAC and boiler piping (Both steam and water) that frequently when I'm called on to replace coils, or fix a leak in copper piping used for steam that it is more difficult to clean the old copper pipe sufficient enough to get the soft solder to flow correctly. It really seems to require an aggressive cleaning, beyond what is required to just "shine" it up and remove the surface oxidation layer.

Might anyone have a guess as to what happens to copper that might cause this?

I have run a couple thousand feet of copper pipe in my day and I have found the same thing except with new copper as well on the last job I did. I had a new roll of solder (lead free) and a tin of about 1 year old flux. Out of about 100 joints I had 3 that would not take. Sometimes if you get the pipe too hot, that will do it. Not the case this time, everything was spotless clean and fluxed on both parts. I don't get it!!! Only things I can figure is impurities swet to the surface with the heat and cause the joint to fail or the flux went bad. Another thing I have found to cause fits- Is not cleaning the pipe well beyond the end of the fitting, seems that it acts like a dam for the solder or drags junk into the joint with the solder.

Those would be my guess'

Many metals have grain that soaks up impurities and can cause trouble. One of the reasons to sweat steel before painting it.

Tim; I have run into this phenomena before. I believe that the copper gets a deep level of oxidation/corrosion that will not allow the solder to flow. Higher heat plus more aggressive cleaning help but I will also replace the old copper wherever it is not too difficult to do. Sometimes it is cheaper to do that than to fight with it. The real old tubing is the toughest. Tubing only a few years old is only slightly more difficult to solder than new. Tubing in basements and crawl spaces is often harder to solder than the stuff under the sinks. The hardest joints are where you have to lie on your back and reach around stuff and then make the solder flow uphill while fighting off hordes of brown recluse spiders! If you use straight propane switch to Mapp gas for a hotter flame. The higher heat will help you get good joints fast (and speed is important because copper oxidizes quickly).

To clean old copper for soldering, brush acid flux on the area to be soldered, heat with the torch till fluxed area is dry, then sand.

This is simply a case of space madness... View said Ren & Stimpy episode for details.. Basically in layman terms its a issue caused by contract deliberations between the Martians and the Napkin makers union....

No really I think the other responses are on the nose, we make assumptions that what we "see" is all there is to the situation. Most likely there is a contaminate that is keeping the materials involved from doing there job... Anytime I have issues such has this I start over changing only one variable at a time until desired results are archived

Excellent advice Larry!
BTW-Solving the problem by changing only one variable at a time is what`s commonly known as following a"Logic tree".One thing at a time lets you pinpoint the problem and the proper fix so you are not left guessing next time which of the changes was the one that actually worked.

If my enquiring mind is working overtime,I cut the old out,bring it home and play with in my spare time to see if I can outsmart it by devious means.Mostly the spare time is taken up by other stuff,it goes in the recycle pile and knowing it`s fixed and I got paid is enough :)

Another important consideration is lead free solder, it is harder to make a joint than the old 50/50, half lead half tin. To overcome this, use solder with silver in it, even less than a % will allow the solder to flow well. A bit expensive but well worth it IMHO. You will need more heat than for 50/50 and this is where the mapp gas shines. Just be carefull to not overheat the pipe and burn out or dry the flux. The way I do this is to rub the solder on the oposite side of the joint from where the flame is playing, as soon as it starts to flow, it is there.

A friend of mine is a general contractor and as such he sometimes feels that he can do anything. He recently had a similar problem with making up some copper pipe joins, they just wouldn't hold, potable water supply. He kept getting call backs for pin hole leaks. I went with him on one of his call backs and the problem was wrong flux, wrong solder, well one of the problems. He was using the new lead free solder with an ancient can of flux, no longer even had a label. Second big problem was just trying to fix joins without cleaning all of the old lead/tin solder off the join, yes he was making it bright but it don't mix with the new lead free stuff. I had him cut back the pipe to free himself of lead contaminated joins, getting all new fitting free of contamination and get the right flux and solder and use no more heat than necessary to get the solder to flow, cherry red is a tad hot. These were homes that had been built in the early fifties so there was also a heavy layer of calcium on the inside of the pipe that needed to be cleaned off. A lot of guys forget to clean the inside of the pipe on water systems with hard water.