When installing copper pipe, what is the purpose of flux, and what could happen if flux is not removed after soldering?

• A. Flux is used to seal the joint permanently.
• B. Flux is a decorative coating.
• C. Flux is used to heat the pipe without damage.
• D. Flux cleans surfaces to enable proper soldering; residual flux can cause corrosion or staining if not cleaned.

Answer: (D)

Flux cleans and prepares the copper surface so solder can wet the joint and flow properly. It removes oxides and oils and prevents oxidation while you heat the joint, helping solder to spread by capillary action. It isn’t a sealant, a decorative coating, or something that heats the pipe. If flux residues are left after soldering, they can be corrosive or staining, especially with acidic flux, and over time can promote corrosion at the joint or lead to leaks. In potable systems, leftover flux can also affect water quality, so the joint should be cleaned and flushed after soldering.