Flux is a chemical compound, with a base of potassium tetraborate or potassium fluoride. It can be found in the form of a paste or powder, and it is used to:

1) Facilitate the wetting of the filler metal

2) Dissolve metal oxides that form during heating

3) Protect the joint surfaces from oxidation.

A flux should possess other qualities according to its composition, namely, it should:

- Start to melt at a lower temperature than the solidus of the filler metal used

- Attach itself evenly over the surfaces of the joint

- Not decompose at the maximum brazing temperature. It should be easily removed with water or by brushing.

Type / Description Range Fluidity Activities Duration Available in
Powder Paste
No. 1 - FLUX 1 F 530 - 750 Excellent Excellent Average Yes Yes
No. 2 - FLUX 1100 550-800 Good Good Very long Yes Yes
No. 3 - BLACK FLUX 600 - 1000 Good Good Very long No Yes
No. 4 - DRY FLUX 550 - 850 Good High Very long Yes Yes
No. 5 - FLUX 751 550-800 Good High Average No Yes
FLUX 707 180 - 350 Good High Average No Yes

Here are some solutions that can generally be used depending on the base metal: Copper and its alloys: solutions of sulphuric acid at 10-15%, ferrous metals, nickel and its alloys: solutions of hydrochloric acid at 10-15%. After the chemical treatment, the workpiece must be immediately rinsed with running water and dried. The duration of the treatment will depend on the residue layer and the wetting temperature.

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