Oi. Mine was meant to be scientific. ;-) The manifold itself has little in
the way of cooling and exhaust gasses can make parts of it reach very high
temperatures. But the engine castings are basically restricted to coolant
temp so the fixings at that end of the manifold will be kept cooler by
conduction.

Actually the head is cooled or rather kept at a reasonable acceptable
temperature - hopefully by the "cooling system" whereas the downpipe is where
ALL the exhaust gasses meet and remember they are still expanding thus slowing
and the heat remains there longer.  If that area gets anywhere near "red hot"
which it can easily do then you have an oxygenating situation.  When anything
burns it must combine with oxygen - when metal or rather steel (iron) burns if
creates Ferrous Oxide that is commonly referred to as RUST.  Eventually the rust
or FeO2 expands and meets the other members of its clan on the flange, studs,
washers or whatever might be about that contains Fe (iron).

When we used to do turbo conversions we always used Stainless studs and Aircraft
locking stainless nuts (the type with the pinched washer type locking
arrangements) DO NOT make the mistake of using Ny-Locks as the plastic WILL melt
and the nuts fall off.

FORD downpipe nuts used to be made of BRONZE - albeit not high quality but
bronze all the same which was a step in the right direction and these 17mm nuts
were self locking too as were the studs that were, strangely, made to X-thread
to lock into the manifold and or cyl heads.

Also remember that the downpipe connection is lower that the head flange thus
susceptible to water splashes and condensation from short journeys where the
temperature doesn't get high enough. Another thought is that unleaded fuel burns
at a higher temperature than leaded and more vigorously so the temperature is
naturally going to be hotter too.

Exhaust systems are usually aluminium coated these days and some are SS but not
many.  The exhaust is designed to run HOT to enable the cat to start working
ASAP so high temps again at the downpipe (cats cause restrictions that slow the
gas thus heat is held in place for longer). Hot exhausts mean less water vapour
and cats mean less acids etc so we now have exhausts that last for ten years or
more rather than the old days when they needed swapping every two years due to
rust and acid eating through the silencers (mufflers).

The original cats and exhaust were as good as new when I sold my E38 740i two
months ago - 13 years old with 198K miles.

There's my 2p or 2c worth but someone out there will disagree..................!