Use a flux that will dissolve and remove traces of oxide from the cleaned surfaces to be joined, protect the cleaned surfaces from reoxidation during heating, and promote wetting of the surfaces by the solder metal, as recommended in the general requirements of ASTM B 813. Apply a thin even coating of flux with a brush to both tube and fitting as soon as possible after cleaning ( Figures 7.9 and 7.10).
Figure 7.9. Fluxing: Tube
Figure 7.10. Fluxing: FittingWARNING: Do not apply with fingers. Chemicals in the flux can be harmful if carried to the eyes, mouth or open cuts.
Use care in applying flux. Careless workmanship can cause problems long after the system has been installed. If excessive amounts of flux are used, the flux residue can cause corrosion. In extreme cases, such flux corrosion could perforate the wall of the tube, fitting or both.