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Re: [Phys-l] Magnetism and rust



It's tempting to propose electro-chemical corrosion, a potent effect when dissimilar metals are in contact in moist conditions: but I notice the surface resistivity of a ceramic magnet I tested, read very high - a strike against this mechanism

Brian W



\\On 12/5/2010 1:24 AM, William Robertson wrote:
A guess. Rust begins to form when iron (usually in a pitted part of
the metal) oxidizes, or loses electrons in an interaction with the
water and the oxygen in the water that come in contact with the
surface. This adds excess electrons to the metal. The iron ions that
are in the water on the surface lose another electron in the water
(They become Fe3+) and then are attracted to the extra free electrons
in the metal, where rust forms via a complete circuit. In general,
this happens randomly as you get concentrations of extra electrons in
the metal near the pitted part and still covered by water. Now if you
have a magnetic field present, that would concentrate the extra
electrons as they move in circular paths. That would concentrate the
rust in the area of the magnetic field.

Of course, it might just be a concentration of water as others have
suggested.

Bill




On Dec 4, 2010, at 11:37 AM, Peter Schoch wrote:

As I was cleaning up this morning -- putting things away for winter
-- I discovered something unusual that I can't explain using any
Physics I know...

I have a steel router table. On it I have a magnetic feather-board
-- a strong, ceramic magnet in a holder that then has 'fins' on it
to keep the work being routed tight to the fence. The table,
despite it being cold and damp was rust free -- EXCEPT under where
the magnet was! As I lifted off the feather-board, I found a
perfect rust outline of the magnet inside the feather-board!

Now, why would that be? If all the other surfaces are rust-free,
what would cause the magnet to induce rust? (I'm always looking for
practical examples of physics to show/describe to my students.)

Any suggestions are appreciated,
Peter