On Sat, Jan 02, 2016 at 12:15:00AM +0100, Knapp wrote:
> What sort of corrosion would you have in space?
> Would a sheet of iron rust in a near earth orbit?

Yes, eventually.  In near Earth orbits, there are traces of oxygen in
the form of highly reactive individual atoms instead of the usual
bound molecular pairs.  The concentrations are extremely low, but an
orbiting object sweeps through a huge volume over the course of a few
years.  The relative speeds also contribute plenty of energy to the
reactions that occur.

> If so how long would it take compared to at ocean level on the
> earth?

That's sort of a "how long is a piece of string" question, because it
depends greatly on lots of variables that can each change the results
by large factors.

The single most important factor would be the height of the orbit,
with solar activity the second most important.  The density of the
oxygen drops by a factor of about 100,000 from around 200 km altitude
up to 1000 km.  Solar activity affects lower altitudes by up to about
a factor of 2, and high altitudes by a factor of about 100 either way.

For real satellites, not very much corrosion can occur before the drag
drops the orbit into the atmosphere.  Enough to cause problems, but
nothing like the bulk corrosion you see in metals on the surface.

Traveller satellites could actively keep station essentially forever
though.  They could orbit low enough to encounter a significant mass
of atomic oxygen in only a few weeks, using thrusters to maintain
speed where a more realistic satellite would quickly run out of
reaction mass and be dragged down into the bulk of the atmosphere.  In
that sort of scenario, you would want a hull material capable of
withstanding such a sustained chemical attack.  You would not want to
use iron.

- Tim