A.Mike Casey, a licensed plumber in Connecticut and California and co-author of Code Check Plumbing, responds: Although black iron pipe is a bit less expensive, galvanized pipe offers better corrosion resistance. In my experience, both types of piping work fine, and the IRC and the National Fuel Gas Code allow both — along with corrugated stainless steel tubing (CSST) and copper tubing (with some limitations) — to be used for natural gas distribution piping.
But you’re right: Some municipalities, particularly in the Northeast, still allow only black iron for natural gas piping. This is probably because the quality of natural gas once varied more widely than it does now, and local contaminants like hydrogen sulfide could cause problems with certain types of piping.
For example, natural gas with a significant amount of hydrogen sulfide and moisture in it can react with copper tubing, leading to black flaking within the system. Or, sulfur in the gas can react with the zinc coating in galvanized pipe, causing it to flake off. Because these flakes can clog the small openings in gas-burning appliances, some model codes have prohibited the use of galvanized and copper gas lines.
But natural gas is much cleaner than it was 40 years ago, and nearly all fuel gas goes through a sulfur recovery unit before it enters the distribution system these days. Also, fuel-gas piping installations now require that drip legs or sediment traps be installed at the horizontal connection to the appliance to collect any moisture or debris flakes flowing with the fuel gas.
Still, even though fuel gas has been cleaned up, some states haven’t changed their codes to permit galvanized pipe or copper tubing. I recommend that you contact the people at your local building and safety department and use the piping they specify for your specific fuel-gas installation.