The most important element in the repair of any equipment is soldering. Solder, fluxes, solder paste - all this must be taken into account when performing repair work. If everything is relatively clear with solder - usually tin-lead solders with different melting points (depending on the composition of the alloy) are used for this, then what about flux? What is it for?
The main purpose of the flux is the removal of oxides from the surface, as well as the reduction of surface tension in order to improve the spreading of solder. In addition, the soldering flux protects the junction from environmental influences.
What are the fluxes
According to their effect on metal surfaces, fluxes are of the following types.
- Active (also called acidic). They contain hydrochloric acid, fluoride and chloride metals. Active soldering flux intensively dissolves the oxide film formed on the metal surface, thereby ensuring high mechanical strength of the future compound. It should be remembered that the active flux is not suitable for the repair of electrical equipment, since its residues erode the soldering place over time.
- Acid free . These include rosin, as well as fluxes prepared on the basis of rosin with the addition of turpentine, alcohol or glycerol. The acid-free soldering flux not only cleans the metal surface from oxides, but also protects it from further oxidation. In addition, the use of rosin does not cause surface corrosion. It is usually used for soldering copper, bronze and brass products.
- Activated . Prepared from rosin, to which phosphoric or hydrochloric acid aniline, salicylic acid and diethylamine hydrochloride are added in small amounts. Activated soldering flux can be used for a wide variety of metals and alloys (iron, copper, nickel, bronze, nichrome, silver, steel). It can also be used to solder oxidized parts made of copper alloys without first stripping them.
- Corrosion resistant . These fluxes are made from phosphoric acid with the addition of various solvents and organic compounds. In addition, some of these fluxes may include organic acids. Corrosion-resistant flux is used for brazing copper and its alloys, as well as silver, platinum and constantan.
- Protective . These include olive oil, wax, petroleum jelly, powdered sugar. Protective fluxes do not have a chemical effect on the metal, and also protect the cleaned surface from oxidation.
In addition, for the soldering of cast iron products, carbon steels, as well as copper and its alloys, borax (sodium tetraborate) is used, which is a white crystalline powder having a melting point of 741 ° C.
Also, borax (more precisely, its mixture with boric acid in a ratio of 1: 1) is used for brazing stainless steel and hard heat-resistant alloys.
As a flux for brass products, a mixture consisting of equal parts of sodium chloride and calcium chloride is used.
Soldering aluminum products requires a flux having a low melting point. Typically , aluminum brazing flux contains from 30 to 50% potassium chloride.
Soldering flux may be in the form of a powder, liquid or paste. In addition, there are special solder pastes in which solder particles are already contained with the flux.
What is important to know when performing soldering
When choosing a flux for brazing, not only the material from which the parts to be brazed are made are taken into account, but also which solder is used. The melting point of the flux should not exceed the melting point of the solder.
Regardless of the type of flux used, the place of soldering after completion of work must be wiped with a cloth moistened with acetone or rectified alcohol. Then clean this area with a brush or brush moistened with any solvent to remove flux residues. This is especially true for active flux, since its decomposition products not only pollute the soldering site, but also are a hotbed of corrosion.