Galvanized pipe
Older steel water pipe coated in zinc that corrodes and clogs over time.
Galvanized steel was common for water supply lines in homes built before the 1960s. The protective zinc coating eventually wears away inside, letting the steel rust and the pipe slowly clog with corrosion — reducing water pressure and discoloring water. If you have galvanized supply pipes, gradually replacing them with copper or PEX is a common older-home upgrade.