Aluminized steel is mild steel that has been coated with an aluminum-silicon alloy. The silicon helps the aluminum stick to the steel. The aluminum then resists corrosion.
How is it identified?
Aluminized steel will be light gray. It may even look shiny compared to mild steel. However, it is not shiny like stainless steel or chrome.
Aluminized steel is still magnetic. It can be checked with a magnet.
What are the pros and cons?
Aluminized steel is more expensive than mild steel. But, compared to stainless, it is very affordable. It is also easier to work with than stainless steel.
The aluminized coating will protect the metal from rusting. However, if it is scratched or damaged, it will allow the metal to rust. Areas that scrape over a curb or get dinged by rocks will rust just like mild steel. Welded joints will rust as well.
A common problem with aluminized steel exhaust is rusting from the inside out. Moisture collects inside the exhaust when it's cold. If you only take short trips, the exhaust never gets hot enough to evaporate all of that moisture. That moisture causes the inside of the system to rust.
In a moderate climate, aluminized steel might last 3-5 years on a daily driver. In a dry climate, an aluminized exhaust system could last 8 years or more.