Stainless steel has chromium along with other elements. This makes the metal harder, stronger, and resistant to corrosion compared to ferrous metal.

Due to the nature of 304 and 409 stainless it is widely used for vehicle exhaust systems.

The difference between the 304 and 409 stainless steel is the amount of chromium and nickel found in each. 304 stainless steel contains a much higher percentage of chromium and nickel.

This makes it much more resistant to rust. 409 stainless steel is still more rust-resistant than Mild Steel However, neither type of metal is 100% rust-proof.

Detail

409 Stainless Steel
Chromium 10.5% - 11.75%
Nickel 0.5%

304 Stainless Steel
Chromium 18% - 20%
Nickel 8% - 10%

409 stainless steel can contain up to 90% iron, meaning a magnet will stick to it and is prone to rust.

304 stainless steel is the highest quality material used for exhaust parts and has a much lower quantity of iron. A magnet will not stick to it. Once it has been heated, manipulated during manufacturing it could become magnetic. 304 is a much harder material and difficult to bend, cut etc.