Q: What is the difference between 304 and 409 stainless steel?

A: All stainless steel contains chromium and other elements. The added elements make the metal harder, stronger, and resist corrosion.

The two main types of stainless steel used for exhaust system components are 304 and 409.

The difference between each type of stainless steel is the amount of chromium and nickel found in each.

304 stainless steel exhaust systems contain a much higher percentage of chromium and nickel, which makes them much more resistant to rust.

409 stainless steel exhaust systems are still more rust-resistant than aluminized steel.

However, neither type of metal is 100-percent rust-proof.

409 Stainless Steel304 Stainless Steel

How can you tell the difference?

409 stainless steel can be almost 90-percent iron, which means a magnet will stick to it. It will also develop surface rust over time, especially after being exposed to heat.

304 stainless steel has a much lower concentration of iron. Magnets will not stick to it. It will also develop a golden color over time, after being exposed to heat.

Important Note: Welded or bent areas of 304 stainless steel could be magnetic. If you do check it with a magnet, check a straight section that hasn’t been welded.

What are the pros and cons?

(Image/Summit Racing)

In general, stainless steel is more difficult to bend and weld than mild steel. In some cases, working with stainless steel could require heat treatment.

This relieves the internal stress caused by bending and welding. Stainless steel is more likely to crack, due to this internal stress.

Most modern factory exhaust systems are made from 409 stainless steel. It is strong and will last eight to 10 years in a moderate climate. 409 stainless steel will rust on the surface. However, this isn’t a concern for most people because it is hidden under the vehicle. Surface rust can be cleaned with a wire brush.

304 stainless is the highest quality material used for exhaust parts. It will last 10 years or more in a moderate climate. However, it is the most expensive. It will not rust for many years. But, it will turn a golden brown color after being exposed to heat.

an exhaust system under an old muscle car
(Image/Wayne Scraba)

This is another in a series of weekly Q&A Mailbag sessions with Summit Racings tech department, in which there are hundreds more. Click here to see them all.