[s-cars] How much rust should one expect to appear on a stainless steel exhaust?

[Postupack, Jeff]

OK-

I am happy to report the "Magnet test for stainless material identification"  is no longer supported by the SAE.

Called Scott Taylor @ Stromung to get the story straight on material type in the Gen #4 Stromung exhaust.
I also consulted the S-car List metallurgist, Mike Bess, who is President of METALS MARKETING & TECHNOLOGY
Thanks for listening and reading 
Jeff Posto 
_________________________________-
Turns out I did not know all the facts either.
Stromung for the C4 chassis Audi's
Muffler, Resonator internals and tips are Type 304 stainless
304 has a higher nickel content. 
All 3" pipes, braces and misc tubing is Type 439 Stainless.
Resonators , Muffler Shell and Cats are all type 439.
Welds are TIG using stainless feed wire.

Known as Automotive stainless..widely used in the industry.
Cost comparison of #304 to #439 
$4.95/ ft (304) to $11 / foot (439) 
Moving 304 material is now 2x cost that it was 2 years ago.
Chinese last year drove prices higher , now nickel procurement is
exorbitant.
__________________

Mike Bess wrote the following also for The List education.
You'll probably wished you never got me involved in this ;-)
304 stainless (18% Chromium, 8% Nickel) is the standard first choice for corrosion resistance in many applications. I was not up on 439 so I need to do some research (citations below). A lot of OEM auto manufactures started using 409 stainless (11% Chromium) a number of years ago for improved corrosion resistance over plain carbon steel. Then they started to use aluminized 409 (think of hot dip galvanizing galvanizing but using aluminum) to get more corrosion resistance. Then there was the development of 439 (18% Chromium, 0.4% Titanium) as an improvement over 409 with better corrosion resistance. 
Costs have indeed skyrocketed as you have noted, both due to Chinese appetite for materials as well as other international demand and supply side constraints (be thankful you do not need Titanium or aerospace quality maraging steels). 
439 is actually a good choice for the pipes. It expands less than 304, dissipates heat faster, is readily welded and has good high temperature oxidation resistance. Thus its use for both pipes and the cat and resonator is ideal.
These systems should last as long as mechanical issues are not a problem. From a corrosion perspective (both high temperature and road salt related) 439 will work just fine. We could always consider a Titanium exhaust system like used on the Corvette (even lighter weight).
My metallurgical opinion.....
METALS MARKETING & TECHNOLOGY
Michael Bess
President
A good pdf brochure which shows some corrosion data for 409, 439 and 304 stainless.
www.aksteel.com/pdf/markets_products/carbon/Aluminized_409-439_Data_Bulletin.pdf
More information
Comparative study of high temperature oxidation behaviour in AISI 304 and AISI 439 stainless steels
ABSTRACT
This work deals with a comparison of high temperature oxidation behaviour in AISI 304 austenitic and AISI 439 ferritic stainless steels. The oxidation experiments were performed between 850 and 950 °C, in oxygen and Ar (100 vpm H2). In most cases, it was formed a Cr2O3 protective scale, whose growth kinetics follows a parabolic law. The exception was for the the AISI 304 steel, at 950 °C, in oxygen atmosphere, which forms an iron oxide external layer. The oxidation resistance of the AISI 439 does not depend on the atmosphere. The AISI 304 has the same oxidation resistance in both atmospheres, at 850 °C, but at higher temperatures, its oxidation rate strongly increases in oxygen atmosphere. Concerning the performance of these steels under oxidation, our results show that the AISI 439 steel has higher oxidation resistance in oxidizing atmosphere, above 850 °C, while, in low pO2 atmosphere, the AISI 304 steel has higher oxidation resistance than the AISI 439, in all the temperature range investigated.
More information.
304 Stainless
This grade of stainless steel can be used where the tube length is less than three metres. The coefficient of thermal expansion for 304 is 1.8×10-2 mm/m/°C which is substantial more than that of carbon steel. When the vessel is hot the thermal stresses in the tubes will be high. Tubes of 304 stainless steel should always be annealed after welding.
439 Stainless Steel
ASTM TP439 is a titanium stabilised ferritic grade of stainless steel (17-19% Cr) which is recommended for long evaporator or pan tubes (in excess of 5m long)
Advantages of grade 439
* fully ferritic metallurgical structure (ensured by the titanium stabilisation)
* very good weldability and ductility;
* inter-crystalline corrosion resistance;
* pitting corrosion resistance;
* full immunity to stress corrosion.
* Coefficient of thermal expansion (in the range 0°C - 100°C) is 1.02×10-2 mm/m/°C
Stress corrosion cracking
This type of corrosion occurs when
*	A susceptible material is subject to 
*	mechanical stress in a 
*	corrosive environment 
In an evaporator, pan or juice heater under the above conditions the result will be cracks leading to breakage in the area near the tube plate.
The danger of stress corrosion cracking exists in virtually all evaporators. The risk will be higher if tubes over 7 metres in length (some designs of continuous pans, falling-film evaporators and Kestner evaporators). Ferritic stainless steels are immune to stress corrosion cracking
Heat transfer
Thermal conductivity of ferritic stainless material is 40% higher than that of austenitic grades (like 304, 304L, 316 or 316L) i.e.: 26 vs 15 watt/metre/°C.