Brazing Fluxes

Brazing fluxes are the chemical substance that that accelerates brazing, welding and soldering by chemically cleaning the joining metals. All process of joining of metals are carried out at high temperature. Flux is mainly used to prevent the base metals and filler materials from oxidation. At room temperature, flux remains inert, but becomes highly reducing at higher temperatures and protect them from getting oxidized.

• Types of Fluxes
• Selection of Brazing Flux
• Composition of Flux
• Flux Faqs

Function of Brazing Flux
One of the major function of brazing flux is to encourage wetting of the base metal by the filler metal. The efficiency of wetting can be expressed as Brazeability. These fluxes are capable of dissolving any oxides leftover on the base metal surface and also on the liquid filler metal. Fluxes aren't meant for removing oil, grease, dirt, etc.

Brazing flux remain active throughout the brazing process. The coating of the flux should be uniformly distributed all over the base metal to protect them till the brazing temperature is attained. At the brazing temperature, the filler metal should displace the flux from the joint. The resulting fluxes should be used only on the materials for which they are designed.

Composition of Flux
Flux is mainly consists of a base and an activator. The base may be either in liquid or in solid form, and acts as a carrier for active constituents like activators, solvents, detergents, and other additives. Besides that, it also carry away the residues of fluxing. It creates a protective coating over the cleaned surface to preserve it from further oxidation. The primary action of fluxing is performed by the activators, while the base executes mild actions.

• Base
  Base materials are also known as vehicles. These vehicles can be obtained either from pine tre sap in a natural manner, or synthetically derived from resins or rosins. Other types of base materials like alcohols, glycerols, and glycols are water soluble. It forms a base for water soluble fluxes enabling the residues to be washed away with water.
  
  
• Activators
  Activators may vary from organic salts (mild active substance) to inorganic acids (very corrosive chemicals). Some of the organic salts are fatty acids and compounds containing amine and carboxylic groups. Some strong fluxes consists of halide salts of amine like amine hydrochloride. Hydrochloric acid (HCl), hydrobromic acid (HBr), and hydrophosphoric acid are some of the most corrosive acting agents.

Action of Flux
Flux is basically used as a cleaning agent to remove oxidation during brazing. Heating a metal like iron leads to rapid oxidation. Using of flux cleans the metal which we are explaining in the figure below. As the iron is moved in the direction given below, the boiling flux cleans away the oxide film.