Final answer:
Lead in solders can cause lead poisoning, and mixing metals like copper, iron, and lead in plumbing can result in rapid corrosion due to galvanic reactions.
Step-by-step explanation:
Using tin-lead solders on water piping is discouraged due to health and corrosion concerns. Lead in solders can lead to lead poisoning, which is a serious health hazard, particularly in drinking water systems. This issue has gained attention following incidents like the Flint Michigan Water Crisis. Moreover, the corrosion aspect is explained by galvanic reactions between different metals. When copper is used to replace lead pipes but is joined to iron pipes using tin-lead solder, there could be an issue of rapid corrosion at the Cu/Fe pipe joints. In the presence of an electrolyte (such as water), the less active lead in the solder acts as the cathode, while the more active iron acts as the anode, leading to corrosion of the iron pipes.