 Everything you need to know about steel
From best to worst, here is a breakdown of the 4 most common types of steel used in auto exhaust components today.
T-304 Stainless (or 18/8 stainless) is the most versatile and most widely used stainless steel, available in a wider range of products, forms and finishes than any other. It has excellent forming and welding characteristics. Excellent corrosion resistance under most conditions.
T-409 Stainless is a titanium stabilized ferritic stainless steel. Although regarded as a general-purpose chromium stainless steel the primary application for T-409 is automotive exhaust systems. It resists corrosion but a light surface rust will form under most conditions; this rust retards further corrosion but makes the surface undesirable for decorative applications. Less expensive than T-304 stainless
Aluminized steel is cold rolled steel sheet that has been hot-dip coated both sides with aluminum-silicon alloy by the continuous method. This process assures a tight metallurgical bond between the steel sheet and its aluminum coating, producing a material with the properties of both metals. Excellent corrosion resistance.
Mild steel is the most common form of steel as its price is relatively low while it provides material properties that are acceptable for many applications. Mild steel has a low carbon content (up to 0.3%) and is therefore neither extremely brittle nor ductile. Mild steel will rust under most conditions. | 
