Under what circumstance should flux residues be removed after brazing?

• A. Always remove flux residues regardless.
• B. When residues are corrosive or in service environments where corrosion may occur; otherwise many rosin-based flux residues can be left, depending on spec.
• C. Never remove flux residues.
• D. Only remove if flux is water-soluble.

Answer: (B)

Flux residues can be a source of corrosion and contamination if left in service. After brazing, residues that are corrosive or that can promote moisture retention or chemical attack at the joint pose a risk, especially in humid or aggressive environments. That’s why you should remove flux residues when they are corrosive or when the service conditions could lead to corrosion. Rosin-based flux residues, on the other hand, are relatively inert in many cases, and many specifications allow leaving them in place if the service environment and cleanliness requirements permit. So the practical rule is: clean residues that are corrosive or in environments where corrosion could occur; otherwise, residues may be left if the spec allows it. Flux type and service conditions, not just whether the flux is water-soluble, determine the cleaning need.