This question is based on the "Roman Bath" video. What coated the inside of Roman lead water pipes and is generally credited with preventing lead poisoning?

What coated the insides of Roman water pipes was mineral deposits.

Explanation:

Back in ancient Rome the water was rich in minerals, that would create a crust inside the aqueducts by deposition, coating the pipes and protecting the water from lead contamination.

Over time, the build-up of mineral deposits can clog the pipes, which requires periodic maintenance, in many cases the coating of the pipes could be accidentally removed.