15 January
Brazing paste flux brushed on pipe

Brazing Flux 101

Bob Henson

A chemical flux is commonly used with many braze filler metals. Proper fluxing is important because the flux absorbs oxides formed during heating and promotes the flow of filler metal. Fluxes are chemical mixtures that contain varying chemical components. The mixture is blended to ensure smooth application and adherence.

Ever wonder about the different flux types and where each is used? Here's a crash course that explores the types of silver brazing flux we manufacture:

Stay-Silv® White Brazing Flux
This is a white paste flux that's used for 90% of silver brazing applications. White flux is useful for brazing copper, brass, steel, stainless steel, and nickel alloys. It has an active temperature range of 1050 -1600°F (565 - 870°C). Use it with the Safety-Silv® high silver brazing filler metals (AWS BAg series) on the above base metals, and the Stay-Silv® phos-copper-silver (AWS BCuP series) on copper to brass.

White flux conforms to American Welding Society (AWS) specification A5.31; class FB3-A, and Aerospace Material Specification (AMS) 3410.

Stay-Silv® Powder Brazing Flux

Hot-rodding a brazing rod with powder flux
Hot-rodding a brazing rod with powder flux

This is similar to the white flux except in a powder form. Because it's a powder it will not separate or harden during storage (keep lid tightly closed as the flux will draw moisture from the air). The flux can be used in its powder form by heating the end of the brazing rod or wire and dipping it in the flux. This is sometimes called "hot rodding". The flux will adhere to the heated rod. The flux can also be mixed as needed with water or alcohol to form a paste, which can then be applied to the part prior to brazing.

This flux conforms to AWS A5.31 class FB3-F and has a temperature range of 1200 - 1600°F (650 - 870°C).

Dynaflow® Auto Dispensing Flux
Flux application can be automated on brazing machines using dispensing equipment. For automated flux dispensing mixture, particles must be held below a minimum size to prevent clogging orifices and lines. Dynaflow flux was originally developed as a "smoother" flux to facilitate automated feeding. This consistency is developed by introducing an additional step during the manufacturing process. The smoother consistency also reduces fluid separation inside unused containers.

Dynaflow Flux is used for similar brazing applications as the standard white flux.

Stay-Silv® Black Brazing Flux

Braze joint with Harris Stay-Silv Black Flux
Pipe fitting with Stay-Silv® Black Brazing Flux

The black flux formula includes powdered boron which gives the flux a black color (really more brown – but "brown" brazing flux doesn’t have the same panache). This ingredient provides two features:

  1. Flux absorbs oxide during heating. There is a maximum amount of oxide absorption the flux provides, after which it becomes saturated and is no longer effective. This formulation allows the flux to withstand longer heating cycles without breaking down. These applications may include specific types of concentrated localized heating (induction, for example), or may occur during torch heating of large parts. In these cases the black brazing flux is a good choice.
  2. Some metal oxides are more difficult to flux. These include stainless steel, nickel alloys, and tungsten carbide. The black flux helps dissolve these oxides and assists in filler metal wetting.

Black flux is primarily used with the Safety-Silv high silver brazing filler metals (you’ll often find it used with the nickel containing 40Ni2 or 50N products). It conforms to AWS A5.31 class FB3-C and AMS 3411 and has an active temperature range of about 1050 - 1700°F (565 - 925°C).

ECO SMART® Flux

Braze joint with ECO SMART Green Brazing Flux
ECO SMART® Green Brazing Flux applied to copper pipe

Boric acid is a component of many brazing flux mixtures. The European Union’s REACH (Registration, Evaluation, Authorization, and Restriction of Chemicals) regulation notes that boric acid is categorized as a "Substance of Very High Concern" based on test data indicating it may be a reproductive toxin.

ECO SMART Flux formulation has the following benefits:

  1. Provides a boric acid free flux formulation to remove the boric acid component and promote use of a safer brazing flux.
  2. A green color that turns clear or transparent close to brazing temperature. This makes it easier for the operator to gauge when to apply the braze filler metal.
  3. Is available in several formulations to meet every brazing need:
    1. Green Paste – for standard brazing applications (similar to Stay-Silv white flux and used with the same filler metals and applications).
    2. Green Powder – in powder form.
    3. Black Paste – boron modified flux for longer heating cycles and difficult to braze base metals (similar to Stay-Silv black flux and used with the same filler metals and applications.).
    4. Black Powder – boron modified flux in powder form.

Bob Henson's photo

Bob Henson

Bob Henson is the Technical Director for The Harris Products Group and has over 40 years of metal joining experience. He has authored or co-authored several patents and has had numerous published articles.

Bob is active in many industry organizations and committees. He is a Life Member of the American Welding Society, (AWS), and chairs the A5H committee that writes brazing filler metal and flux specifications. Bob is also a member of the AWS Brazing Manufacturers Committee, the US Technical Activities Group that reviews International ISO brazing documents, and the AWS A5 Committee on Filler Metals that reviews specifications for arc welding electrodes, gas welding rods and other filler metals, covering both ferrous and nonferrous materials. Bob serves on the National Skills USA HVACR Technical Committee and is the event chair for the Skills HVACR brazing competition. He is an RSES member and part of the RSES Manufacturer’s Service Advisory Council.


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