SS304 vs Mild Steel vs Aluminised: What Should You Actually Build Your Exhaust Out Of?

If you've priced up tube for a full exhaust or turbo pipework and felt your stomach drop, you're not alone. It's usually the first fork in the road for anyone about to start fabricating — and the internet is full of conflicting opinions. So let's cut through it.

There are really three contenders for a home or pro fab job: mild steel, aluminised steel, and 304 stainless. Each has a place. None of them is "the best" in every situation — it depends on your budget, how long you want the system to last, and whether it's going to be seen.

Mild Steel: cheap, easy, and disposable

Mild steel is the cheapest option by a wide margin, and it welds like a dream — forgiving heat range, easy to source, easy to grind and finish. If you're building a one-off exhaust for a track car that gets binned every season, or you're still learning to weld and don't want to torch $300 worth of stainless practising your technique, mild steel makes sense.

The catch is obvious the first wet winter: it rusts. Not "might rust eventually" — it will, and usually starting from the inside out, so you don't see it coming until there's a hole blowing exhaust note into your cabin. For anything that needs to survive more than a couple of years, or anything exposed to WA's coastal salt air, mild steel is a false economy once you factor in redoing the job.

Aluminised Steel: the middle ground that's disappearing

Aluminised steel gets brought up a lot in older exhaust threads — it's mild steel with an aluminium-silicon coating that buys you some corrosion resistance without the stainless price tag. It's a genuine option for daily-driver mufflers and OEM-style systems.

The problem for fabrication specifically: once you cut, weld or bend aluminised tube, you compromise the coating right at the exact points that matter most (welds and bends), so you lose a chunk of the corrosion benefit exactly where you need it. It's fine for straight factory-style runs; it's a poor choice for custom mandrel-bent systems with lots of joins.

304 Stainless: the standard for a reason

This is where most serious builds land, and for good reason. SS304 is an austenitic stainless with enough chromium and nickel content to resist heat, corrosion, and scaling under real boost and real heat cycles — not just look shiny on day one.

What actually matters when you're buying it:

• Wall thickness consistency. Cheap stainless can vary in wall thickness around the bend, which means uneven heat absorption and a higher chance of blowing through when you weld it. Look for tube and elbows sold with a stated wall thickness (1.6mm+ brushed finish tube and 1.65mm wall elbows are a solid, weldable spec for most turbo and exhaust applications) rather than "approx" numbers.
• Brushed vs polished finish. Brushed hides handling marks and blends cleanly with other brushed components — genuinely useful if you're building something that'll be photographed or judged. Polished shows every fingerprint and scratch but looks incredible fresh. Pick based on how much upkeep you actually want to do.
• Weldability. SS304 responds predictably to TIG heat, which is what makes "stack of dimes" welds achievable for hobbyists, not just pros with years of practice. It doesn't warp or discolour as unpredictably as some cheaper stainless grades can.

So which one do you actually pick?

Quick gut check:

• Track-only, budget build, might get replaced anyway → mild steel, no shame in it.
• Daily driver, OEM-style system, minimal custom bends → aluminised is fine, but check what you're actually saving once you factor in labour.
• Custom turbo piping, intercooler runs, show engine bay, or anything you want to still look good in five years → SS304, every time. It costs more up front and pays for itself by not doing the job twice.

A tip most people skip

Buy your tube, elbows and reducers from the same batch/spec if you can. Mixing wall thicknesses between components in the same system means uneven heat behaviour across joins — you'll notice it the moment you start welding a thin-wall piece to a thick-wall piece, because one side burns through before the other even colours up. If you're planning a multi-stage build (say, an intercooler pipe now, exhaust later), it's worth sourcing everything from a supplier that keeps a consistent spec across their whole SS304 range rather than piecing it together from wherever's cheapest that week.

At the end of the day, material choice is the one decision in a fabrication project you can't undo after the welds go in. Get it right at the start and the rest of the build gets a lot less stressful.