304 is an austenitic stainless steel high in chromium and nickel with very high corrosion resistance. It is one of the most commonly produced and used stainless steel grades. 409 is a ferritic stainless steel with less chromium and very little nickel. It less expensive to produce than 304, but also provides less corrosion resistance.
These two alloys the most common stainless steels used for car exhaust systems. They are useful in these applications because of their superior ability to handle the heat of the car exhaust. However, 409 stainless will tend to show surface rust sooner than 304, even while it maintains structural integrity.
409 is more corrosion-prone than 304 stainless steel, but it provides a longer use-life than common OEM aluminized steel exhaust systems. It is the next step up in corrosion protection.
Aluminized steel is regular carbon steel that has been hot-dipped into an aluminum alloy. There is no chromium or nickel in the carbon steel. Instead, the hot-dip aluminum bonds to the outer layers of the steel and works as a sealant against rust. Carbon steel is less expensive than stainless steel, and the aluminum is rust-protective. However, dings and scrapes can expose the steel beneath and start the process of rusting. These pipes and mufflers can rust through and need replacing.
Aftermarket upgrade to stainless steel is often the choice for car enthusiasts and performance specialists. Anything can be replaced in the system: catalytic converters, pipes, mufflers, or tips. There are two reasons people commonly choose stainless: First, stainless steel is rust-resistant and more likely to last the life of the car. Second, non-OEM parts are often designed to enhance the car’s power or mileage. They are designed to help airflow and thermal venting to increase the efficiency of the engine. These augmentations are usually made in a metal that will last the life of the car.
409 stainless is often all that is needed for the boost to longevity or performance. 304 stainless steel adds an aesthetic component. This metal is unlikely to develop rust speckles in regular conditions. That said, salt can affect 304. For those wanting to maintain a pristine finish, all salt deposits in visible areas should be wiped clean as soon as possible.
Where 304 stainless steel is exposed to high heat, it develops a golden-brown, brass-like look. If the color is not desired, commercial metal polish and a cloth often are all that is needed to remove the browning. However, over time, browning will return. Some exhausts are ceramic coated to prevent this.
When cleaning stainless, it is important not to use any steel brushes or scrub pads. Any iron deposits left on the surface will change the surface chemistry and therefore the rust-resistance of the metal.
In extreme conditions, where there is a lot of exposure to road de-icing chemicals, 316 stainless steel exhaust tubing is available. This is a premium austenitic alloy. It costs substantially more than 304 versions but resists corrosion in saltwater due to added molybdenum.
304 is the most common commercial grade stainless steel. It is also known as 18/8 to indicate it has (around) 18% chromium and (around) 8% nickel. It’s the grade most used in food preparation—from utensils to food manufacturing equipment—because it tends to provide the most value. It does not rust easily in wet conditions, is easy to sterilize, and maintains toughness even in hot and freezing temperatures. Due to being weldable and formable, it is easier to manufacture than some other grades.
409 stainless is one of the cheaper stainless steels, and more vulnerable to surface rust than 304. Speckles of rust mean a metal can’t be sterilized, so this grade is not used for food. However, it has excellent strength and thermal properties. In manufacturing contexts, it can be used in dry-only or non-food-facing components. Springs and other small components used in electronics might be made of 409, where its thermal abilities are needed, and dry conditions are guaranteed.
304 or 18/8 stainless
Gets a golden-brown look when exposed repeatedly to high heat, but experiences very limited corrosion, except in extremely harsh chemical environments. Used in decorative applications.
Excellent weldability but may need annealing afterward.
Many applications, including most commercial kitchenware, cookware, indoor architectural detailing, food manufacturing, and stainless steel fasteners.
Light surface corrosion can show up in chemically challenging environments, but functionally 409 is much more resistant than aluminized steel.
Must be preheated and worked at low weld temperatures.
Primarily automotive: exhaust, catalytic converters, and mufflers. However, this alloy is being used more often in manufacturing and construction, in places where surface rust is acceptable. It is an inexpensive substitution where heat is an issue, but chemically accelerated corrosion is not.
There are a wide range of metals used for exhaust systems, not just the common ones listed above. For many, rust is part of the natural lifecycle, even if it doesn’t look great. Choices are made on performance, longevity, and aesthetic grounds.