Black Iron Pipe vs. Carbon Steel Pipe

What is the difference between black iron pipe and carbon steel pipe, or are there any?

Sure are. Black iron pipe is cast from several grades of ductile or malleable iron whereas carbon steel pipe is generally wrought welded or seamless. Cast iron pipe is used for underground or submerged applications and for main steam pipes and branches that are subjected to acids. It was also common to use cast iron pipe and fittings for municipal cold water lines 4″ diameter and above. Commercial die casting is unsuitable for lines subjected to expansion strains, contractions, and vibration unless the pipe is very heavy. It is not suitable for superheated steam or for temperatures above 575 degrees F. Cast iron pipe in underground applications (such as sewer lines) usually has bell and spigot ends whereas exposed pipe usually has flanged ends.

Ductile or malleable iron [DMI] can be used for potable water supply whereas carbon steel [CS] cannot; it rusts from within and sheds particles. DMI cannot be welded whereas CS can be. DMI cannot be threaded whereas CI can be; different horses for different courses.

In addition to all the above you can join stainless steel tank (threaded) with threaded copper adaptors directly whereas you cannot join galvanized pipe and copper. That will corrode unless you use special connectors. I’m forgetting what they call them. They are inert so you don’t get the corrosion. I’m sure someone else can help with the name. They sell them in the plumbing supply houses. I’ve never seen them in home depot.

In fact you shouldn’t even mix black and galvanized in the same runs. Give them enough time and they will corrode and leak at the joints. They didn’t know that when they ran the gas lines in my house and mixed in a few galvanized fittings about a hundred years ago. Or they knew but figured they would be dead and buried by the time the pressure washer started leaking. I had to run all new black pipe.

If you go to Home Depot and ask for schedule 40 (or 80) “black iron” pipe, you will get steel pipe, easily threaded and welded. Galvanized schedule 40 (or 80) pipe is the same stuff, but galvanized, of course, so you wouldn’t want to weld it.

I know you can use packing machine for natural gas lines, but at Home Depot they told me I could not use the galvanized pipe for gas.

I’d always assumed the black coating was carbonized oil (as on a black iron frying pan) but I read recently that it is simply lacquer.

Apparently, the problem with galvanized power tool for gas plumbing is that particles or flakes of zinc can get into valve orifices, etc. I’d think little particles of rust or lacquer would do the same, but evidently not.