Re[2]: Header Wrap and Stress Corrosion Cracking

[Robert_A_Brady@domain.elided (Robert A Brady)]

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Greg wrote:

"Typically, so far as I know, intergranular stress corrosion cracking is a 
problem which goes with most 300 series stsinless steels at elevated 
temperatures. I think with carbon steels at the sort of temps involved, the 
whole part goes away!"

Correct.  The austentic stainlesses are most prone to external stress 
corrosion cracking, although the phenomenon has been observed in alloys 
ranging from brass (cartridge casings) through carbon steels.  The mechanism, 
an initial crack in the passivated oxide coating, which sets up a potential 
between the oxide layer and the unoxidized alloy creates a source of ions to 
form and lead to further corrosion of the crack.  Apparantly, the specific 
mechanism (after the intial crack and resulting potential) differs among the 
alloys and is anything but conclusive for the austentic stainlesses.  AFAIK, 
many exhaust headers are fabricated of carbon steel and work reliably.  
External stress corrosion cracking (ESCC) of stainless is not a "high" 
temperature phenomenon.  The mechanism requires water to be present, as a 
source of hydrogen ions, and is found typically at operating temperatures 
between 55 - 120 C.  I suppose that at higher temperatures, liquid water 
doesn't hang around (unless of course, the pressure is high enough to keep it 
liquid).  The most commonly given explanation for ESCC also requires other 
ions to be present, most typically chlorine.  Any coastal environment has 
enough chlorine in the ambient moisture to supply sufficient quantities. (The 
type of insulation has also been attributed as a source of chlorine). The 
insulation provides a trap for the chloride containing moisture.  Thus, 
uninsulated stainless surfaces need not worry about ESCC.  

"Type 321 and type 347 SS are immune to the problem--they are what is used 
for things like tubular exhaust manifolds upstream of turbochargers (when 
things are done RIGHT) and for exhaust manifolds on aero engines (where 
weight and reliability  are truly serious concerns)."

They may be immune to another problem, but not ESCC. 

"The SAE Handbook lists these two as being meant for exhaust manifold 
applications. And--insulating a mainfold made of either of them, even 
upstream of a turbocharger turbine--will not elevate their temperature to a 
high enough level to cause corrosion or cracking problems."

Again, ESCC is not a high temperature phenomenon.  Perhaps the problems 
they're trying to avoid are different in nature.  

"BUT--watch out for thermal expansion--there will be enough thermal 
expansion/contraction that real allowances for it, including expansion 
joints, will be necessary."

Yes, of course.

If you've managed to read to this point, there is probably one item to note.  
ESCC is probably not much of an issue with exhaust manifolds.  They don't 
spend much time in the right temperature range for it to be a concern.  
Secondly, if the header wrap is fiberglas, there's probably not much concern 
with the insulation supplying chlorine.  

As Greg pointed out, the real problems with exhaust manifolds are high 
temperature related (thermal expansion, cyclic stress cracking, and perhaps 
high temperature corrosion from sulfer containing fuels.... although that's 
probably a stretch).  Wrapping a stainless header isn't likely going to lead 
to increased corrosion or ESCC.  Wrapping a carbon steel header is likely to 
make that item rust faster, as the wrap will act like a sponge for water when 
the car is not in use.

bob brady
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Date: Mon, 26 Jul 1999 07:27:22 -0600
To: Robert_A_Brady@domain.elided (Robert A Brady)
From: bearbvd@domain.elided (Greg Hermann)
Subject: Re: Header Wrap and Stress Corrosion Cracking
Cc: alfa@domain.elided
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