The operation of brazing, soldering and welding has brought out the use of fluxes in service. Flux is a chemical substance, which is made for cleaning the surface of the job and preventing oxidation in operations, such as welding, soldering and brazing. It is rather difficult to have satisfactorily brazed or soldered joints without the use of fluxes. Therefore it is very essential to use flux in soldering, brazing and welding. Soldering flux may be defined as a substance (solid or liquid) applied to a metal to make solder flow readily and gives a permanent joint. The success of a solder depends to a large extent upon the proper choice of flux.
The soldering fluxes are needed due to the following reasons:
(a) To clean the surface of the base metal during heating.
(b) To eliminate impurities appearing on metal surface.
(c) To breakdown the surface tension of molten solder which helps the easy flow.
(d) To prevent the formation of fresh oxides by forming a protective layer on the solder.
Qualities of a Good Soldering Flux
The requirements of a good flux are:
(a) It must remain liquid at the soldering temperature.
(b) In its liquid state it must act as a cover over the joint and exclude the air.
(c) It must dissolve any oxide film present on the surface being joined.
(d) It should be readily displaced from the joint surfaces by the molten solder.
Action of Flux
A – Flux solution lying above oxidized metal surface
B – Boiling flux solution removing the film of oxide (as Chloride)
C – Bare metal in contact with fused flux
D – Liquid solder displacing fused flux
E – Tin reacting with the base metal to form compound
F – Solder solidifying
Types of Fluxes
Soft soldering fluxes can be divided into two categories:
(a) Active or Chloride Fluxes
(b) Safety or Protective Fluxes
Active or Chloride Fluxes
Chloride fluxes are used in most of the soldering works. These fluxes quickly dissolve the oxide, rust and grease from the base metal and at the same time act as a barrier to prevent the further oxidation. Unfortunately, all active fluxes have a corrosive residue, so the joint must be carefully washed with hot water after completion of soldering. Otherwise residue of such flux may cause corrosion. The complex type of soldered joint should be immersed in a solution of 1 to 2% (by volume) of hydrochloric acid (HCl) in water then washed carefully in water.
Because of difficulty of avoiding residual corrosion, the use of active fluxes is banned in electrical and other works which cannot be washed effectively. These fluxes are made in various forms as paste, salt or fluid.
Safety or Protective Fluxes
These fluxes are less corrosive in action as compared to the chloride fluxes and less active in removing surface oxides. These prevent oxidation during soldering but will not remove oxide which is already present on the joint surfaces. So surface must be cleaned properly before soldering. These types of flux are especially used on electrical, radio and computer assemblies or other works where complete freedom from acid or corrosion is necessary. These are supplied in four forms as follows:
(a) Paste / powder flux
(b) Soldering fluid
(c) A soldering cream
(d) Cored solder wire
Specific Fluxes for Different Metals
|Zinc and galvanised work||Dilute hydrochloric acid|
|Brass||Zinc chloride (Killed spirits) or resin|
Different Types of Fluxes and Uses
|Nomenclature of Flux||Type||Uses|
|Compound tinning||Active||Tinning big-end bearings|
|Safety||Soldering of stainless steel conductors of ignition cables|
|Paste, soldering||Safety||All metals, except stainless steel: where a non-corrosive joint is required|
|Solution, soldering||Active||For general purpose|
|Hydrochloric acid(HCl)||Active||For general purpose, zinc and galvanised iron|
|Tallow||Safety||Soldering of lead and zinc metals|
|Olive oil||Safety||Pewter (Brittania metal)|
|Flux, non-corrosive||Safety||For use in affixing identification labels to steel tubing|
|Ammonium chloride or Sal-ammoniac||Active||For tinning large surfaces, cooking tins, tinning soldering iron|
|Phosphoric acid||Active||Stainless steel pipes|
|Zinc chloride or D.T.D. 81 or Killed Spirit||Active||General engineering work (i.e. sheet metal work)|