There are so many different options with aftermarket exhausts that it is almost a mind-numbing subject.
One of the biggest rifts is the selection of aluminized steel or stainless. Let’s first back up and take a look at the molecular level of both metals and what properties they have.
Aluminized Steel is just plain old steel that has been hot-dipped on all surfaces by an aluminum-silicon alloy. By dipping the steel in this molten alloy, it forms a tight bond on the metallurgical level with its host metal and produces a metal that carries the unique properties of both metals. So, it retains its strength at temperatures lower than 1500 degrees F, yet carries a higher corrosion resistance than steel, thanks to its aluminum surface. It also maintains a high level of heat reflectivity, being able to reflect up to 80 percent of the heat projected onto it. While the aluminum surface maintains a high level of corrosion resistance, if the aluminum layer is broken and the steel below exposed, corrosion is likely, especially in an exhaust system where water is vaporized into steam.
Stainless Steel is an alloyed substance, in that during the smelting process it was mixed with other metals. In particular, to be considered stainless, there has to be a minimum content of at least 10.5 percent chromium by mass. This chromium content bonds on a molecular level with the atoms of the steel (and sometimes the addition of molybdenum, as well) and creates a metal that is super high in corrosion resistance and staining, and is low maintenance. In normal oxidation scenarios with carbon steel, when exposed to moisture and oxygen, steel will start to rust. The rust layer that forms is molecularly larger than the steel it used to be, and the resulting expansion causes the flaking away that we are all familiar with, which exposes the steel below to another oxidation attack.
Stainless, on the other hand, oxidizes in a fashion similar to aluminum. A film microscopically thin film of chromium oxide forms on the surface, and prevents further corrosion by blocking oxygen and moisture access to the steel molecules.
Both metals sound like pretty good materials to make an exhaust system from, right? Well, it depends. Aluminized steel is cheap, and is readily assembled as it is essentially just low carbon steel with a thin aluminum layer. This means that any exhaust shop with a welder set up for mild steel can weld together an aluminized exhaust. It is worth noting, however, that when welding or cutting aluminized steel, the aluminum surface is disrupted and you have introduced an entry point for corrosion in the entire system.
Stainless steel exhausts are normally manufactured from 304 Stainless, which is the most common stainless produced. Despite its availability, 304 Stainless is still considerably more expensive than aluminized steel, and requires special wire and expensive shielding gasses to weld due to the high chromium content in the metal. This means that specialized exhaust shops are your primary stainless steel fabricators.
So, when you are putting aftermarket exhausts together, make sure you take note of the construction of the kit you are considering to purchase, or the muffler’s material construction. Some companies construct mufflers from aluminized steel, where manufacturers like Gibson or Magnaflow exclusively use stainless steel.
What material works best for you? Only you can decide! If you live in the desert where you have to try to make things rust, the budget-friendly aluminized steel is your friend. Live in Michigan where you are not sure if that is a pile of snow, or a pile of road salt? Invest in stainless.