Paul wrote:
 >>>

Except for one thing Al and I am not saying this because I necessarily
think its true. I am just saying it is a possibility.  Steel and SS tends
to oxidize at high temp. If you can keep the oxygen away from the
surface they might not corrode so badly and perhaps last longer. Mild
steel flakes off in large chunks when used for rotary engine exhaust.

Bill Schertz wrote:

 >>

There is a way that such a thin coating can work, although I have no way
of knowing whether they are doing it that way. If the coating has a LOW
EMISSIVITY the radiated heat will be much reduced. This is the only
effect that could cause a significant temperature drop by a thin coating.
emissivity = absorptivity = (1- reflectivity) so a low emissivity coating
is a highly reflective coating in the appropriate wave length (infra red for
exhaust manifold). A shiny SS surface has low emissivity, however it
quickly oxidizes to a high emissivity surface.  if the ceramic coating
prevents that, it COULD be effective. The emissivity as a function of
wavelength needs to be measured in order to determine whether it is
effective or not.

Al Gietzen wrote:

I'd guess that with the incredible turbulance that the convective/conductive
heating on the inside would be dominant.  Lowering the emissivity on the
outside would increase the wall temp.

Fred Osborn wrote:

Don't discount the radiation from exhaust gas to the inside
of a corroded SS
pipe. If the thin inside coating can maintain a high inside
reflectivity it
could be very effective in reducing the SS pipe temp.

 
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