Brazing practice — commonly used flux Which flux is most commonly used in brazing to dissolve surface oxides and promote wetting of the filler metal?

Introduction / Context:
Brazing joins metals using a filler that melts above soldering temperatures but below the base metal melting point. Flux selection is crucial because clean, oxide-free surfaces are required for good wetting and capillary action.

Given Data / Assumptions:

• General-purpose brazing on steels and copper alloys.
• Conventional torch or furnace brazing environment.
• Goal is reliable oxide removal and protection during heating.

Concept / Approach:
Borax (sodium tetraborate) and borax-based mixtures are the standard fluxes for brazing. They dissolve metal oxides, protect the surfaces from further oxidation at elevated temperatures, and improve flow of the filler. Zinc chloride and ammonium chloride are more common in soft soldering as “killed spirits” or active fluxes. Rosin in alcohol is a mild, non-corrosive soldering flux for electronics, not for high-temperature brazing.

Step-by-Step Solution:
Identify the joining method: brazing (high temperature, capillary flow).Match flux chemistry to temperature/oxides: borax-based fluxes withstand brazing temperatures and dissolve oxides.Select “borax”.

Verification / Alternative check:
Materials handbooks and filler metal datasheets list borax/boric acid blends as the most widely used brazing fluxes.

Why Other Options Are Wrong:

• Zinc chloride / Ammonium chloride: primarily soldering fluxes; too active/corrosive for many brazing applications.
• Rosin with alcohol: for electronics soldering at low temperatures.
• Caustic soda: not a brazing flux.

Common Pitfalls:
Using soldering fluxes in brazing leads to residues and poor joint wetting at high temperatures.

Final Answer:
borax