Surface rusting is not really a matter of which series of stainless is used. In fact, type 409 (not a 300 series) is typically used in automotive exhaust systems.
From
The Home Machinist's Handbook by Doug Briney:
quote: Stainless steels do not require any special coating to protect them, but unless stainless steel is
passivated after machining, it is subject to corrosion and rust. The passivation process simply removes all of the ferrous material from the surface, leaving only the chrome and nickel exposed.
From
http://www.stebro.net/stwc.htm:
Why 409 type Stainless Steel pipes and shells?
409 type Stainless Steel is specially designed for automotive exhaust systems.
409 type Stainless Steel resists atmospheric and automotive exhaust gas corrosion.
409 type Stainless Steel developes a light surface discoloring which will retard progressive corrosion. [/list]
Here is a link that will tell you more than you ever wanted to know about the common grades of stainless steel.