Soldering Flux should be Applied only to
the Male End of a Copper Pipe Joint

By TK NG

            Soldering flux is used to dissolve oxides present on copper pipe and fitting surface, facilitate wetting by molten solder, and coat the hot surface to form an oxygen barrier for preventing its oxidation.

2.         Applying soldering flux to both the male and female ends of a copper pipe joint would unavoidably result in the excess flux being pushed into and out of the pipe joint during the assembling process (see Fig 1).

Figure 1

3.         If soldering flux is applied to the male end only, excess flux would only be pushed to the outside of the pipe joint where it can be easily removed (Fig 2).

Figure 2

4.         If excess flux remains on the inside surface of copper pipe or fitting, it would be difficult, if not impossible, to remove after soldering as there is no or only limited access.  As soldering flux is corrosive in nature, it can attack copper pipe to form bluish/greenish salts (e.g. copper chlorides) if excess flux is not removed after soldering.  Soldering flux should therefore only be applied sparingly to the outside surface of the male end of a copper pipe joint.

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