On Mon, 28 Sep 2009 16:44:13 -0700, "Michael Koblic"
wrote:


"Terry" wrote in message
.. .

The rust isn't being *transferred* in electrolytic derusting.

In any electrochemical cell, such as the cell constructed for
derusting, something at the cathode (to be derusted) is being
"reduced" which means "gains electrons". If the cathode is a rusty
object, the oxygen of the rust (Fe2O3) is being converted to water or
hydroxide ion, depending. (If it's an inert piece of metal or
graphite, water is converted to hydrogen gas.) Rust-free iron metal
is the product. Clean it up and oil it quickly! The iron surface
usually has a fair bit of surface area and will rust again very
quickly.

Something at the anode is being oxidized (which, chemically speaking,
means "loses electrons". If the anode is a piece of iron, the oxygen
in the water is converted to hydroxide ions, and the result is iron
hydroxide or hydrated iron oxide.


No argument with the principles. It is the details that puzzle me.

If you hook up iron as anode and stainless steel/copper as cathode in a
saturated solution of NaCl you get H2 coming out and a Fe(OH) sediment
forming at the cathode. At the anode you get Fe entering the solution as
Fe2+ ions, effectively dissolving the anode:

http://electrochem.cwru.edu/encycl/a...-machining.htm
http://school.mech.uwa.edu.au/~james...-chem/ecm.html

It works, 'cos I done it. What I do not understand how is the de-rusting
process different in that it dissolves the iron oxides preferrentially. Is
the underlying iron spared? I would have thought that the process is not
dissimilar from the above and there will be some dissolution of the
non-rusted iron but my chemistry does not stretch that far.

The cathode is where the electrons are going. Whatever is at the
cathode will be reduced----if it *can* be reduced. Iron can be
reduced (accept electrons) if it's in an ionic state such as +2 or +3,
but once it's in the metallic state (oxidation number of zero) it
can't accept electrons anymore. (Metals don't form negative ions
easily at all.) So instead, the hydrogen ion near the electrode is
reduced to hydrogen gas.

Terry