Pipe Fitting Material Guide: Brass vs. Stainless vs. Black Iron vs. Plastic

Most pipe fitting mistakes are material mistakes. The thread is right, the size is right, and eighteen months later there is a green crust on a joint that should have been stainless, or a galvanized nipple has eaten itself in a compressed air line.

Here is how the common materials actually differ, and which job each one belongs on.

Brass

Use it for: potable water, compressed air, fuel oil, low-pressure hydraulics, instrumentation, general plumbing.

Brass is the default for a reason. It machines cleanly so threads seal well, it resists corrosion in water service, it handles moderate pressure and temperature, and it costs less than stainless. For most shop air lines and most water work, brass is the correct answer and anything more exotic is money you did not need to spend.

Where it fails: dezincification in aggressive or high-chloride water, which leaves that pink, porous, crumbling surface. Ammonia will stress-crack it. And it is soft enough to gall or deform if you overtighten.

Stainless steel

Use it for: food and beverage, chemical, marine and coastal, washdown environments, high temperature, anywhere hygiene or corrosion is the governing concern.

304 covers most industrial work. 316 adds molybdenum and is the one you want around chlorides — salt air, pool chemistry, food brine, de-icing. If the environment is coastal or the media has chlorides in it, the upcharge from 304 to 316 is cheap compared with replacing a corroded run.

Where it costs you: price, and galling. Stainless-on-stainless threads seize under torque — use an anti-seize or PTFE tape and do not power-tighten.

Black iron (malleable iron)

Use it for: natural gas and propane, fire sprinkler, steam, high-pressure structural runs, and increasingly for furniture and shelving frames.

Strong, cheap, and the accepted standard for gas. Malleable iron takes threading and abuse that would deform brass.

Where it fails: water. Black iron rusts, and rust scale sheds into the line and clogs valves and screens downstream. It is not for potable water and it does not belong outdoors unpainted.

A trap worth naming: galvanized fittings in compressed air. The zinc coating flakes over time and the flakes travel into tools and regulators. Galvanized is for water; it is not a general-purpose upgrade to black iron.

Bronze

Use it for: marine service, pumps, valve bodies, bearing-adjacent applications, seawater.

Bronze handles salt water far better than brass and resists the dezincification that kills brass in aggressive water. Costlier and less common in standard fitting sizes, so it tends to show up where the application specifically calls for it.

Copper

Use it for: refrigeration and HVAC lines, potable water, medical gas.

Naturally antimicrobial and excellent thermal conductivity. Mostly sweated rather than threaded, so copper fittings live alongside a different install method than the rest of this list.

Aluminum

Use it for: weight-sensitive work, modular compressed-air systems, pneumatics, some hydraulic returns.

Light and corrosion-resistant. Softer than brass, so thread damage is easy and torque specs matter more than they do elsewhere. Watch for galvanic corrosion where aluminum meets steel or brass in a wet environment — dissimilar metals plus moisture is a battery.

Plastic — PVC, CPVC, PEX, polypropylene, nylon

Use it for: drainage, irrigation, chemical lines, cold water, low-pressure air, anywhere corrosion resistance beats mechanical strength.

• PVC — cold water and drainage. Not for hot water and not for compressed air; it fails brittle and it fails suddenly.
• CPVC — the hot-water version of PVC, rated roughly to 200°F.
• PEX — flexible potable water, freeze-tolerant, fewer joints per run.
• Polypropylene and nylon — chemical resistance and pneumatics, where the media would attack metal.

Schedule 40 vs. Schedule 80: same outside diameter, thicker wall on 80, so higher pressure rating and smaller bore. Schedule 80 is usually grey rather than white.

The five questions that pick the material for you

1. What is the media? Potable water, gas, compressed air, hydraulic oil, a chemical, a food product — this eliminates most of the list immediately.
2. What pressure and temperature? And is either one cycling? Cycling is harder on a joint than a steady high number.
3. What is the environment? Indoors dry, outdoors, coastal, washdown, chemical exposure. Chlorides are the single biggest driver toward 316.
4. What is it connecting to? Dissimilar metals in a wet environment corrode galvanically. Match materials or isolate them.
5. Is there a code? Gas, potable water, medical and fire suppression all have requirements that override preference.

Thread types, briefly

NPT is tapered — the seal comes from thread deformation, so it needs tape or dope. NPS is straight and seals on a gasket or O-ring. BSPT and BSPP are the British equivalents and are not interchangeable with NPT despite looking close. A BSPT fitting in an NPT port will start and then leak.

Where Supply Tie fits

We stock roughly 3,000 fittings across brass, steel and iron, stainless, plastic, rubber and hose, bronze, aluminum, copper and tube stock — so the material recommendation is not steered by what happens to be on our shelf.

Fittings and hydraulics ship free on orders over $100.

Not sure which material the job calls for? Tell us the media, the pressure and the environment, and we will tell you what belongs there.

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