Although I'm not a plumber I have done a fair amount of solver soldering with copper pipes, from a small repairs to a complete house re-plumbs in copper and have never had a problem until now.

The project involved solver-soldering a small internally threaded mild steel lug (7mm diam and 10mm long) onto a small length of 20mm diam mild steel tube. Knowing how important cleanliness is I ensured both mating parts were bright clean - applied a small coating of silver solder flux (same stuff I use for copper) and proceeded to heat it (oxy/acetylene) to the point where the silver started to melt - the result was a black joint with almost no silver solder flowing/adhereing to the joint, it looked like a classic example of bird sh*t welding. I tried ordinary 5% silver solder some of the more expensive stuff which I think may be 15% and also some 45% I had on hand - same result with each. I eventually had to throw the pieces out and ended up machining it from solid to get the project finished.

I have heard that there is a flux called BLACK FLUX - has anyone heard of this - is it available locally ? is this what I need or do I need to rethink my technique ?


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