If excess flux is used when applying it to a tube, what issue can arise from the residue inside the tube?

Using excess flux when soldering or welding pipes can lead to corrosion issues due to the residue that may remain inside the tube. Flux is used as a cleaning agent to prepare surfaces for soldering by removing oxidation and impurities, but when too much is applied, it can be challenging to completely clean it out after the process.

The residue left behind can attract moisture and create an environment conducive to corrosion, especially if the pipe carries water or other fluids. Over time, this corrosion can weaken the pipe, lead to leaks, and potentially cause failure in the system. Therefore, it is crucial to use the appropriate amount of flux to ensure effective soldering while minimizing the risk of residue that could result in corrosion internally.

In contrast, while issues like blockages, oxidation, and burrs can arise from improper techniques or excess material used in other contexts, they are not primarily caused by the residue of flux within a tube, making corrosion the more relevant concern in this scenario.