Steel Grades for Exhaust System

The exhaust system is one of the most heavily stressed components of a car. During operation, it is simultaneously exposed to high temperatures, moisture, and aggressive chemical substances. What steel grades are best suited for exhaust systems?

Corrosion of Exhaust Systems

Exhaust systems are exposed to two types of corrosion:

• External corrosion, caused by water splashed from the road surface, especially water containing road salt used for de-icing.
• Internal corrosion, caused by condensated gases formed in the cold sections of the exhaust system.

In addition, exhaust systems are welded, which can expose stainless steel to intergranular corrosion.

Carbon Steel Exhaust Systems

Regular carbon steel is the cheapest option, but it also has the lowest corrosion resistance. An exhaust system made from this material may need replacement every 18 months, whereas the expected service life of the engine and drivetrain is around 10 years. Given current steel and labor costs, this is not the best solution.

Aluminized Steel Exhaust Systems

Carbon steel can be coated with a layer of aluminum. Aluminum is significantly more corrosion-resistant, and in practice this extends the service life of such exhaust systems to approximately 30 months. However, the harsh operating conditions of underbody components, especially the exhaust system, still limit durability. This solution is more viable than carbon steel.

Corrosion-Resistant Steel Exhaust Systems

The composition of steel 1.4512 (or AISI 409) is approximately 0.02% C, 0.6% Si, 0.3% Mn, 11.4% Cr and 0.4% Ti. Chromium significantly increases resistance to both external and internal corrosion, while titanium stabilizes and maintains a ferritic microstructure. This makes the material easier to cold-form and more weldable, while also reducing susceptibility to intergranular corrosion. The service life of exhaust systems made from this steel is 4–5 years, and its cost is only about 1.5 times higher than that of ordinary uncoated carbon steel, making it more cost-efficient.

An alternative is 304 stainless steel (1.4301), which contains a higher chromium content and an alloying addition of nickel, but no titanium.

Steel for Catalytic Converters

Catalytic converters use steel containing 11.5–19% chromium and stabilized with niobium, zirconium, and titanium in various combinations. Chromium provides corrosion resistance. Niobium, zirconium, and titanium, which have a strong affinity for carbon, protect against intergranular corrosion. Titanium and zirconium counteract undesirable changes during welding. Niobium improves the toughness of the heat-affected zone and increases creep resistance. Examples of alloys with these characteristics include steel 441 and steel 444.