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Old 11-17-2015, 12:27 PM   #1
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Bad solder joints, fixable?


Random thought of the day.

Most of my solder joints are good but in the past I've had some solder joints that failed when I turned the water on. I pull them apart and re clean everything and try again and it will usually take and be good. Not sure what exactly I did wrong, the bad ones usually the solder just doesn't suck in so maybe not clean good enough or not even heat or something.

But after you discover that one of your solder joints is no good what are your options? Like I said usually I pull it apart and retry and it works the second time. But was wondering if it's possible to fix without taking apart? Is it possible to add some flux and heat the joint back up and add some solder? Or is that a waste of time and just pull it apart and redo it? I've never tried to just add solder, usually short cuts don't work out well for me so I've always just pulled it apart and started over. But have always wondered.

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Old 11-17-2015, 01:57 PM   #2
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Depends on what the issue was. If lack of solder then adding more will fix. If poor cleaning then dismantling and cleaning is needed. Since you can't be sure, dismantle and clean is the best option.

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Old 11-17-2015, 02:10 PM   #3
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If the joint failed--these are the usual reasons-
Improper flux technique
improper heating (to little or to much)
poor cleaning
misfit connections.

Often when one joint fails--draining, reheating--brushing on fresh flux--then soldering will fix the issue--Mike-
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Old 11-17-2015, 04:38 PM   #4
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I suggest playing the flame on and off the joint so the inside pipe gets flame half the time.

Otherwise it will take longer for the inside pipe (male surface) to get hot enough and the flux may boil away (it lasts only a finite time) and the male surface oxidizes (literally re-uncleans itself) before the solder bonds in which case the solder might fail to bond in numerous places.

After finding out that a joint failed, when you brush more flux on and resolder without disassembling, you are hoping that the flux got sucked in to re-clean the mating surfaces. If the flux only got sucked in a little then only the outer edge will get treated and essentially you are putting on a solder patch around the edge as opposed to making a full solder bond. Now, that might hold okay.

To help assure you used enough solder, keep adding solder until is just starts to overflow. You don't have to add so much solder that a a droplet forms and falls off.
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Last edited by AllanJ; 11-17-2015 at 04:53 PM.
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Old 11-17-2015, 05:06 PM   #5
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Ponch:

There are lots of reasons for bad solder joints, but one of the most common reasons is that the person doing the work doesn't allow a path for the hot air inside the copper piping to escape. You need to understand that it's the relatively weak force of capillary pressure that draws solder into the joint, and if the flow of solder into the joint is opposed by the pressure of the hot air inside the pipe, you're joints won't take solder. So, ALWAYS ALWAYS ALWAYS think to yourself: "When I start heating up this joint, does the hot air inside the pipe have a way to escape?" And, if the answer is NO, then you're about to make another leaking solder joint.

The reason why you need to take a leaking solder joint apart and resolder it becomes apparent once you understand exactly what soldering flux does.

Essentially, soldering flux's job description is as follows: "Act as a physical barrier between the bare copper metal inside the solder joint and the oxygen in the air for as long as it takes for the pipe to heat up and capillary pressure to draw molten solder into the joint."

That's because copper rusts. But, the oxide layer it forms is completely different from the rust that forms on steel. Copper oxide forms a film that bonds tenaciously to copper metal, AND is highly impermeable to oxygen molecules. So, as the copper oxide film grows in thickness, it better and better protects the underlying copper metal from further oxidation. This is why the brown copper oxide layer that has formed on 80 year old copper pipe isn't very much thicker than the copper oxide layer on 10 year old copper pipe, and that stuff isn't very much thicker than the copper oxide layer on 2 year old copper pipe. And that's why the copper plumbing pipes inside a house will, in general, last longer than the house.

Now, the reason why that copper oxide layer is super important is because molten solder will bond with copper metal, but NOT with copper oxide. I don't know why that is, but the best explanation I've heard so far is because of something called "electronegativity", which is really just a measure of how strongly atomic nucleii will hoard their electrons about themselves. Oxygen is the second most electronegative element on the periodic table; only Fluoride has greater electronegativity. Metals, on the other hand, have very low electronegativity. Metals, in fact, will allow their outer electrons to wander around the neighborhood so that a metal can best be thought of as metal atoms suspended in a cloud of electrons. The problem is that as soon as you introduce oxygen into the system, it hoards it's electrons to itself, and interferes with the free movement of electrons throughout the copper, and throughout the metal solder and between the copper and the metal solder. And, in that way, the difference in electronegativity prevents the solder from bonding to the copper oxide because the electrons in the solder won't flow through that copper oxide film into the copper, and vice versa.

So, the whole purpose in using flux is to act as a physical barrier between the bare copper metal inside the joint and the oxygen in the air outside the joint until the joint is hot enough that the flux melts and capillary pressure draws molten solder into the joint (thereby pushing the molten flux out of the joint), and at no time exposing the bare copper metal to any oxygen, thereby preventing the formation of copper oxide on the copper inside the joint. Copper oxide forms relatively slowly at room temperatures, but it's almost instantaneous at soldering temperatures, so as you heat the joint it's imperative that the flux fills the joint until molten solder can be sucked in by capillary pressure. After that, it doesn't matter what happens because the solder coats the bare copper and prevents any formation of copper oxide inside the joint.

Now, if what I'm saying is correct, then you should theoretically be able to use ordinary Vaseline as flux on a solder joint. The Vaseline will coat the bare copper so that oxygen can't get at it, but will melt at soldering temperatures so that capillary pressure can draw the molten solder into the joint. And, I have personally used Vaseline for this purpose just out of curiosity, and while it doesn't work nearly as well as flux does, it does give you a solid solder filled joint that doesn't leak.

Now, in a case where you make a leaking solder joint, by the time you turn on your water and realize that you have a leak, the water pressure has already pushed whatever remaining flux there was in that joint out. It was suggested that you could now try to "fix" that joint by:

"Often when one joint fails--draining, reheating--brushing on fresh flux --then soldering will fix the issue."

No, most times it won't. That's because the water has already pushed the flux out of the joint, and washed the bare copper clean of flux. The drop might form slowly on the underside of the joint, but water is flowing quickly through the tiny path that's open to it between the pipe and the fitting, and that quickly flowing water will clean the flux off both the pipe and the inside of the fitting. Now, when you heat that joint, an oxide layer is going to form on the bare copper long before you get the copper pipe hot enough to melt solder. And, once that copper oxide layer forms on the bare copper, NOTHING will remove it except taking the joint apart, sanding or brushing any oxide off, and soldering a second time.

What "draining, reheating--brushing on fresh flux --then soldering" will do is possibly form a solder "plug" around the end of the fitting. That is, water can still get into the tiny annular ring between the fitting and the pipe, it just can't leak out the end of that ring, so you think you've fixed the problem. But, give that solder joint a good shake, and the chances are good that the solder plug will leak.

Now, the reason why you can do that draining, heating, brushing and soldering is because soldering flux contains a chemical called zinc chloride. What's zinc chloride? Zinc chloride is a metallic salt that behaves very much like an acid at soldering temperatures. When it's hot, it dissolves copper oxide far more aggressively than it dissolves copper metal. It's added to soldering flux so that if you didn't remove all of the copper oxide by sanding or brushing, the zinc chloride will normally remove the rest, thereby hopefully resulting in a good solder joint.

When you try to "fix" a leaking solder joint by draining, heating brushing on some flux and soldering, all you're doing is using that zinc chloride in the flux to dissolve the copper oxide on the copper surfaces where the pipe goes into the fitting. Then, when you apply the solder, the solder melts and forms a plug where the pipe goes into the fitting. And, that plug might prevent leakage, but you don't want to rely on that plug, especially if this joint is going to be covered up with drywall.

Also, it's that supposed acidity of the zinc chloride that the reason why they'll tell you to clean off any flux on the piping after you finish soldering. While that's "officially" correct, the zinc chloride isn't acidic enough to do any harm to copper piping unless it's hot. So, the only time, in my view, it's important to clean the solder off the pipe after soldering is if you're soldering copper piping for a hot water heating system where water temperatures will often be close to 200 degrees F. You don't come anywhere near that with hot water from a water heater, which might typically be around 130 degrees F.

So, if you're having trouble getting a joint to take solder, the first thing you should do is check the two obvious causes:
1. Am I getting the joint hot enough?
2. Am I providing a path for the hot air inside the pipe to escape (other than through the joint I'm trying to solder).

If you're meeting those two criteria, then the best thing you can do is keep applying flux to the hot pipe/fitting joint to keep that joint full of flux. When the joint finally gets hot enough to take solder, it will be a good solder joint because you've done the most important thing by preventing any oxygen from the surrounding air from getting to the bare copper metal inside the joint.

And, finally, as you are now fully aware, the key to good soldering is to ensure that the bare copper metal never comes in contact with oxygen from the air. That means:

A. That you should flux your pipe ends and the insides of your fittings as soon as possible after sanding or brushing them.

B. It is NOT necessary to remove cold solder from the end of a pipe or the inside of a fitting. That layer of solder will prevent oxygen from the air from getting at the copper better than a coating of flux will, so treat that solder as the new surface of the pipe and/or fitting. I just use a dry rag to wipe the end of the pipe, and even that thin layer of solder on the end of the pipe is enough to protect the underlying copper from coming into contact with the oxygen in the air. But, you can sand the solder on the pipe just enough to remove any oxidized tin or lead and brush out the fitting to remove same. Then flux both the tinned end of the pipe and interior of the fitting socket. Now, heat the two and the pipe will slide into the fitting once the solder melts. At that point, touch your solder to the joint just to ensure the joint is completely full of solder. And, that will be result in a GOOD solder joint, just as good as if you'd gone to the trouble of removing all the old solder and fluxing the bare copper metal. (Because you used the existing solder as a flux.)

Hope this helps.
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Last edited by Nestor_Kelebay; 11-17-2015 at 05:29 PM.
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Old 11-17-2015, 05:49 PM   #6
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If anyone is interested in the process by which flux serves to produce a good solder joint, one of the best references available is from the Copper Development Association.

The Copper Development Association publishes "The Copper Tube Handbook" which is a vertable cornucopia of technical information about the kind of copper tubing used in residential plumbing. In that handbook will be a section on soldering and brazing, which my last post was based on. The URL for the handbook is here:

www.copper.org/publications/pub_list/pdf/copper_tube_handbook.pdf

Copy and paste that URL into your browser's location bar, and it should load onto your computer so that you can save it as a file.
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Old 11-17-2015, 07:36 PM   #7
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I didn't think just heating it up and adding flux and solder was a good practice. Why I've never even tried it. I don't do a ton of solder joints and most of mine turn out alright but every once in a while I get a bad one, usually when it's a repair and in a tricky place.

Thanks for all the advice. Especially Nestor for the very detailed learning lesson and the link. I will spend some time reading that tonight.
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Old 11-17-2015, 09:04 PM   #8
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Soldering is a pretty straight forward task, but it needs to be done in a certain sequence. There are books and videos you can read/look at to get the information you need. After that, all it takes is a little practice.
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Old 11-17-2015, 09:37 PM   #9
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I need lots of practice.

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Old 11-17-2015, 11:20 PM   #10
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A really interesting fun fact that you'll find in that Copper Tube Handbook is that the Ancient Egyptian symbol for copper was the Ankh:



Which, just as we have homynyms for words that sound the same but have different meanings, was also the symbol the Egyptians used for immortality, or everlasting life.

This is almost certainly not a coincidence, but proof that the ancient Egyptians were aware of coppers excellent natural resistance to corrosion. They wouldn't have been aware that it was caused by the formation of a highly impermeable oxide film over the copper, but they would have been aware that copper didn't corrode the way other metals, like silver for example, did.

I just thought that was really interesting.
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Old 11-18-2015, 04:36 AM   #11
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I have had the odd joint fail with paste flux, but I can't remember EVER having a failed joint with Griffon S-39
if you take an S-39 joint apart you will see full coverage every single time.



http://www.jcwhitlam.com/Product/3/234/154#

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