Westsidebilly

you're almost right...

Steel needs atleast 12% chromium to be Stainless Steel.

The chromium in the steel combines with oxygen to form a thin, invisible layer of chrome-containing oxide. what you called a "coating"...which prevents rusting...

If the metal is cut or scratched, more oxide will quickly form and recover the exposed surface...so it will still resist rust.
The film ofcourse needs oxygen to self-repair, so stainless steels have poor corrosion resistance in low-oxygen and poor circulation environments... Which is not the case for an exhaust.

Soy you can't exactly remove that layer...its innate to the metal.