Exhaust Materials: Mild Steel, Stainless, Titanium and What Lasts
The alloy an exhaust is made from decides how long it survives salt and condensation, how much it weighs, and a large part of what it costs.
304 stainless is the aftermarket default for good reason. Titanium buys weight saving at a steep price, and on any material the weld and bend quality matter more than the grade.
Exhausts live in the worst environment on the car: hot gas and acidic condensation inside, road salt and stone chips outside. Material choice is mostly a corrosion decision.
Mild and aluminised steel
Plain mild steel is the cheapest option and the shortest lived, because it rusts from both sides and the inside is where it usually starts. Aluminised steel is mild steel with an aluminium silicon coating that resists that corrosion far better, and it is common on original equipment systems for exactly that reason. It is still steel underneath, so once the coating is cut, chipped or welded through, the exposed area corrodes normally. Both are fine on a car with a defined remaining life, and neither is a sensible choice for a system you intend to keep.
The stainless grades
409 stainless is ferritic, cheap by stainless standards, and widely used on original systems. It resists perforation well but surface rusts to a dull brown, which matters if the tips are visible. 304 is the austenitic grade most aftermarket systems use: strongly corrosion resistant, polishable, and it stays bright. 316 adds molybdenum for better resistance to chlorides, which is the case for cars living near the coast or on heavily salted roads. The practical difference between 304 and 316 on a normal road car is small, and 304 is where most of the market sensibly sits.
Titanium, and what actually decides longevity
Titanium is the weight play. A titanium system can be roughly 40 percent lighter than the equivalent 304 one, which is real weight removed from behind the axle line. It also colours attractively with heat, is expensive to buy, and demands careful welding with proper gas shielding. Whatever the material, construction is what kills most systems first. Mandrel bends keep the full bore through a corner where crush bends restrict it, and welds that were not back purged corrode from inside the pipe long before the parent metal does. Ask about bends and welds before you ask about the alloy.
The conditions that matter
- ✓304 stainless is the sensible default for a system you plan to keep.
- !Aluminised steel corrodes normally once its coating is breached.
- !Mandrel bends and properly purged welds affect flow and life more than the grade does.
Sources
These are the provisions the road-legality rules linked above rest on.
- Road Vehicles (Construction and Use) Regulations 1986, reg. 54
- MOT inspection manual: 8. Nuisance (GOV.UK)
- Catalytic converters and diesel particulate filters (GOV.UK)
General guidance only. Details vary by exact vehicle and change over time; confirm with a reputable specialist before modifying.