Exhaust system temperature decreases along the flow:

- Manifold and near-engine section: severe high-temperature oxidation; 441 or 444 ferritic stainless steel is commonly used, with Nb/Ti stabilization. - Mid-section: 409/436, balancing cost and corrosion resistance. - Tail section and muffler: 409 or 436; condensate corrosion is the main risk.

304 can also be used in some sections, but has high cost and large thermal expansion.

Key indicators: high-temperature oxidation resistance, thermal fatigue, condensate corrosion resistance (Cl- and pH).

Procurement note: ferritic steels have poor weldability and are sensitive to grain growth; welding process must be controlled.