All exhaust systems on all cars sold in the United States since the early 80’s have been stainless. That’s because of government emissions requirements mandating that the entire exhaust system last for at least 8 years/80,000 miles.
“Stainless” doesn’t mean stain proof. All components contain at least 11% chromium to improve high temperature oxidation and corrosion resistance.
There are two sections of an exahsut system; the “hot end” (the resonator & converter up to the manifold) and the “cold end” (the exhaust pipe after the resonator, the muffler and tailpipe). The cold end is produced from stainless grades similar to 409. I say “similar” because in Japan they don’t produce 409. This is fully stabilized ferritic stainless steel that is weldable (with no chromium carbide precipitation in the heat affected zone or weld nugget), therefore weld integrity is very good. That doesn’t mean that welds can’t fail.
The hot end is primarily made of grades similar to 439, 18CrCb and 18SR. These have 18% chromium, therefore much better high temp oxidation resistance.
The general failure mechanism of the hot end is usually high temperature oxidation and/or fatigue. High temps caused by lengthy high engine speeds and rich fuel mixtures will accelerate oxidation of stainless. The higher the temperature, the longer the time at temperature and the more cycyles of high temperature, the more oxidation occurs. This isn’t “rust” per se, it’s more like a gray scale that forms. As the hot end oxidizes, there is less structural steel remaining, and fatigue can set in. Keep in mind that the strength (and fatigue resistance) of stainless steel decreases dramatically at high temps.
The cold end uses less chromium because it doesn’t see the temperatures of the hot end. If you have a lot of short trips, moisture remains in the muffler along with dilute sulfuric and carbonic acid (products of combustion). Over time, they tend to rust out the muffler and tailpipe from the inside. The exhaust pipe (between the resonator & muffler) may fail from either corrosion or oxidation.
Lots of highway miles will allow an exhaust system to last much longer since this keeps moisture out when the car is not running.
BTW: I’m a Metallurgical Engineer, and a former Market & Product Development Engineer for a stainless steel company. I’m no longer involved with these products, but for a time I was very involved with the stainless exhaust systems of all domestic auto manufacturers.
Anyone Seen a Rusted Out OEM Exhaust System? - Page 2 - S2KI Honda S2000 Forums