Final answer:
An inspector checking threaded joints before seal welding must examine Pb/Fe joints for potential iron pipe corrosion, as lead can catalyze iron oxidation.
Step-by-step explanation:
When an inspector is checking threaded joints prior to seal welding them, an important item to check is the potential for corrosion at Pb/Fe joints. It is crucial to examine existing Pb/Fe joints for corrosion of the iron pipes due to the galvanic cell formation where the less active Pb acts as the cathode and the more active Fe serves as the anode.
This leads to the reduction of O₂ at the Pb, thus promoting the oxidation of the iron nearby.