[s-cars] How much rust should one expect to appear

[pkrasusky at ups.com]

Sir Posto Compiled a Research Paper / Thesis and further Op Cit'd to
resolve the matter:


<<<Date: Thu, 31 May 2007 19:57:07 -0400
From: "Postupack, Jeff" <Jeff.Postupack at analog.com>
Subject: Re: [s-cars] How much rust should one expect to appear on a
	stainless steel exhaust?

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_Bull
etin.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 temperat
 ure range investigated.SNIIIIIIIIIIIIIIIIIIIIIIIIIIIPPPPPPPPPPP>>>





Man...

It's the things like this that make it great to stay here on der List as
most do whenst their UrS's go byebye.  Too funny.  Learned more here
than I do in a week of life's daily routine.  Sad.

Thanks Jeff / Mike!  

-Paul