Let me apologize if this is too off topic for this group but I do read
you guys talking about carbon content, etc.

I build water cooled exhaust manifold systems for small 2 - 4 cyl sail
boat engines. The prevailing practice is to not build exhaust systems
from stainless steel. However, I believe that stainless would both
last longer and look better, the latter IS important, and am in doubt
as to the prevailing practice.

The operating conditions are as follows:

The actual manifold consists of the necessary exhaust passages welded
up from stainless pipe and encased inside a water jacket with engine
cooling water circulated through it. A short "mixing elbow" is
attached to the outlet of the water cooled manifold and sea water is
injected at this point to cool the exhaust gasses for passage through
a rubber hose to the hull outlet. Water and exhaust gas exiting the
hull outlet are relatively cool.

Estimated temperatures a exhaust gas within the water cooled
manifold approximately 3-400 degrees F.

Gasses entering the mixing elbow - probably a little lower due to the
water cooled manifold.

Gasses and water exiting the elbow - warm to touch but can hold hand
on elbow below water injection point indefinitely. Gas and water
exiting outlet warm but not too hot to hold hand in exhaust stream.

Now my question. Since I believe that the prevailing "knowledge" is
based on the fact that stainless will absorb carbon at high
temperatures and become hard and consequently be inclined to crack or
break with vibration, at what temperature will stainless, say 316L,
start to absorb carbon from the exhaust gasses. In short, would a
stainless exhaust system absorb sufficient carbon over say, a 10 year
period to become brittle?

Your comments, or a pointer to existing data, will be highly
appreciated.
Bruce-in-Bangkok
(Note:remove underscores
from address for reply)