When brazing stainless steel, which flux characteristics are important?

• A. Flux should be highly acidic and leave residues that promote oxide formation.
• B. Flux is not necessary for stainless steel brazing.
• C. Fluoride-based fluxes are avoided due to poor wetting on stainless steel.
• D. Flux should effectively remove and prevent stainless oxide formation, typically fluoride-based fluxes known for active stainless steel wetting.

Answer: (D)

Stainless steel brazing hinges on removing the chromium oxide layer and preventing it from reforming as heat is applied. The flux for stainless steel must clean that oxide quickly and then shield the surface from re-oxidizing, allowing the filler metal to wet and capillary into the joint. Fluoride-based fluxes are used because they actively dissolve the stainless oxide and promote wetting, which leads to a reliable, well-filled joint. After cooling, residues should be cleaned because they can be corrosive if left. An acidic flux that leaves residues promoting oxide formation would counteract wetting. Saying flux isn’t needed ignores the oxide barrier. Fluoride-based fluxes aren’t avoided for poor wetting; they actually enhance wetting on stainless steel.