Final answer:
Fluid leaking through a newly brazed joint may be due to corrosion caused by the reduction of oxygen at the Pb/Fe joint, where lead acted as a cathode and promoted the oxidation of iron.
Step-by-step explanation:
During testing, if fluid leaks through a newly brazed joint, it may be because of corrosion at the Pb/Fe joint. The less active lead (Pb) could have acted as the cathode for the reduction of oxygen (O₂), which promotes the oxidation of the more active iron (Fe) nearby. This process can lead to corrosion that weakens the structural integrity of the brazed joint, resulting in leaks.