Composition of Flux

Fluxes are the smoothening agents, which facilitates the brazing processes. Their primary function is to protect the base metals from being oxidized during the brazing and wetting the base metals by the filler materials. Flux is a chemical compound and are found in many forms, such as paste, powder, etc.


Broadly composed of a base and an activator. The base of a flux can be either liquid or solid. This is the carrier of the active constituents of the flux, like activators, detergents, solvents and other additives. The base also acts as a solvent to carry away the residues of fluxing, forming a protective coating over the cleaned surface. This helps in preventing oxidation. While the base performs mild action, the fluxing action is performed mainly by the activators found in the flux. Activators range from mild active substances like as organic salts to very corrosive chemicals like as inorganic acids. The type of activators to be used is known by the surfaces to be soldered or brazed.

Constituents of Flux
A number of chemical substances are used in the manufacturing of fluxes. When flux is heated to brazing temperature, these chemical constituents react with each other forming new compounds. These compounds are quite different physically and chemically from the unreacted constituents. For example, if fluoborate is constituent of a flux then after heating fluorides may be formed.

With the increase in temperature, the reaction rate of the flux with oxygen, filler metals, base metals, and foreign materials also increases. The composition of flux must suit all the factors of the brazing cycle. Halides like fluorides and chlorides are necessary in fluxes for the alloys comprising of aluminum and other highly electro-positive metals. Some of the most common ingredients of chemical fluxes are:

• Fused Borax
  Fused borax is a high temperature melting material used in lower temperature brazing processes.
  
  
• Borates
  Borates have good oxide dissolving power and provide protection against oxidation for longer periods. They are useful in formulating fluxes that melt at higher temperatures. Most borates melt around 760°C and are very effective at this temperature. In the molten condition, they have relatively high viscosity and therefore mixed with other salts to increase fluidity.
  
  
• Elemental Boron
  Boron powder is used to increase the overall fluxing action. Silver brazing fluxes contain elemental boron offering improved protection on carbides and refractory oxides, like nickel, chromium, cobalt, etc.
  
  
• Boric Acid
  It is one of the primary constituents of brazing fluxes because it helps in the removal of glass like flux residues left after brazing. The melting point is higher than fluorides but lower than borates. Silver brazing fluxes are constituted of potassium borates and boric acid, combined with potassium fluoborate and fluoride compounds.
  
  
• Fluoborates
  Fluoborates act similar to the borates in several respects. They don't provide protection against oxidation to the same extent as borates do. However they tend to flow better in the molten state and show great oxide dissolving properties.
  
  
• Chlorides
  Chlorides have a similar function to fluorides but the temperature range isn't so effective. They should be used with a little bit of caution because at lower temperatures they may depress the melting points of fluoride based fluxes.
  
  
• Fluorides
  Fluorides are readily used in fluxes as cleaning agents because they react readily with most metallic oxides at higher temperatures. They are useful when confronted with chromium and aluminum oxides. They help increase the fluidity of molten borates thus helping their displacement and improving the capillary action of molten brazing filler metal. Fluorides can generate harmful fumes and so requires strict attention in use.
  
  
• Alkalies
  Alkalies are used to elevate the useful working temperature of the flux. Potassium and sodium hydroxides are the main alkalies to be used however they are used in a small amount. Their major disadvantage is that they are highly deliquescent. Its presence in other flux agents, even in small quantity can create problems in humid weather and can harshly reduce the storage life of the flux.
  
  
• Wetting Agents
  Wetting agents are primarily used in liquid and paste fluxes to help the flow and spreading of the flux on the base metals before brazing.
  
  
• Water
  Water in fluxes can be present in two ways. Either it can be present as water of hydration in the chemicals used in formulating the fluxes or as a separate addition for making paste or liquid.