I’ve got a clogged sink, and have been reading the labels on a bunch of different products lately. I’d always assumed that they were a strong acid of sorts, which would dissolve whatever sort of material was built up in the pipes. But I’ve noticed that a lot of these products specify that they’re non-corrosive, so I’m not sure what other sort of mechanism may be at play here.
Just to add, alkaline substances turn fats into soap, making them not only dissolve in water, but also help dissolving the surrounding fat.
Acids are much more harmful to metals, and wouldn’t have this effect.