Short definition
Steel pipe is iron alloy with controlled carbon content (typically under 0.3%) formed into seamless or welded tube. In residential plumbing, “steel pipe” usually means galvanized steel (zinc-coated, used for legacy water supply) or black steel / black iron (uncoated, used for natural gas distribution). Carbon steel and stainless steel show up in industrial and specialty applications.
What it is
Two characteristics distinguish residential steel pipe varieties:
- Galvanization. Galvanized pipe is dipped in molten zinc for corrosion resistance — the silver-gray finish. Black iron is uncoated and shows the dark mill-scale finish.
- Application. Galvanized was the pre-1970 water-supply standard; black iron is the standard rigid material for indoor gas distribution.
Both are joined with malleable-iron threaded fittings, sealed with pipe dope or PTFE tape — galvanized for water, gas-rated yellow PTFE for black iron gas service.
Why it matters to a homeowner
In an older Washington home, “steel pipe” likely means one of two things:
- Galvanized supply — repipe target. Tuberculation chokes flow over decades.
- Black iron gas — code-compliant for gas, requires a licensed gas fitter for modifications.
Inspection reports that say “steel pipe” without specifying are ambiguous. If you’re unsure, look at the finish (silver-gray vs. dark mill scale), and ask what service it’s carrying.
Common variants and what steel pipe is not
- Steel vs. cast iron. Cast iron has 3 to 4 percent carbon and is brittle. Steel has under 0.3 percent and is ductile.
- Steel vs. ductile iron. Ductile iron has magnesium added for fracture resistance — used for modern utility mains.
- Galvanized vs. black steel. Zinc-coated vs. uncoated.
- Stainless steel pipe — alloyed with 10%+ chromium for corrosion resistance.