On Mon, 17 May 2004 07:10:31 -0500, Unknown
posted:
On Mon, 17 May 2004 04:44:41 GMT, "Dan White"
wrote:
,;
,;"Unknown" wrote in message
m...
,; On Fri, 14 May 2004 16:30:02 GMT, "Dan White"
,; wrote:
,;
,; ,;
,; ,;"Charles Erskine" wrote in message
,; e.com...
,; ,; More than you probably wanted to know:
,; ,;
,; ,; http://yarchive.net/metal/rust_remove.html
,; ,;
,; ,; snip
,; ,;
,; ,;This is the post I pasted in this thread originally to try and answer
,;the
,; ,;original question.
,;
,; OK I missed your original post. I wrote the article you refer to. I am
,; a chemist and do know what is happening in this procedure so let's
,; start fresh.
,;
,; What is it that needs more clarification on this topic? You ask the
,; questions and I will try to give a reasonable explanation.
,;
,;I had a feeling you were the same person as the original post I pasted.
,;Sandy had the original question and I was trying to help. The only thing I
,;still don't understand is exactly what the "complex" is that drives the
,;Fe203 to dissociate and reform this complex with Cl-. Is it some kind of
,;hydrated Fe complexed with Cl-?
If the rust is present as Fe203 then the iron is trivalent and will go
into the salt-acetic acid solution predominately as the FeCl6(-3)
complex. Note that there is no change in oxidation state of the iron.
That iron complex is a central trivalent iron ion (with a +3 charge)
which usually has a coordination number of six. That means there will
be six positions around that iron ion that are occupied by some
species. Since the concentration of chloride is so high relative to
the other anions the predominate species will be FeCl6(-3). The net
charge of the complex is -3 because each chloride carries a -1 charge
and the iron has a +3 charge. There certainly will be some anions
where an acetate ion and/or a hydroxyl ion will replace one or more of
the chlorides. For this discussion the exact composition of that iron
complex is not only unknown but is of no particular interest. Just
remember that if too many of the chlorides in that complex are
replaced by hydroxyls or acetates then the iron will precipitate as a
hydrous oxide or basic acetate and that is why one uses such a high
concentration of chloride.
So with excess acetic acid present, "hydrous oxides or basic acetates"
will be precipitated? Surely they will be redissolved by the excess
acetic acid as soluble acetates?
It is the stability of that iron chloride complex and the whopping
excess of chloride ions that drives this reaction.
What is formed from the stable soluble ferric acetate that is taken
out of the process?
If all this chloride is "taken out of the scene" what happens to all
those lonely sodium ions?
The high solubility
of iron chlorides prevents the reaction from coming to a screeching
halt due to precipitated of hydrous iron oxides and basic acetates.
Surely excess acetic acid does the same thing?
There is no oxidation or reduction reaction at this point.
That iron complex will oxidize iron metal. The simplified net reaction
is
2Fe+3 + Fe(0) -- 3Fe+2
Only when exposed to atmospheric oxygen, No?
This reaction is one that you don't want to happen because that Fe(0)
is the iron metal you presumably are trying to recover rust free. This
also should suggest to you why you should keep oxygen (air) out of the
solution.
Yep.
Fe+2 is oxidized to Fe+3 by oxygen and the reaction takes place
readily because of the stability of the iron(III)-chloride complex. If
you allow air into the process you will be producing more Fe+3 which
in turn reacts with the iron metal (see the above reaction). If you
bubbled air through the solution this process will continue until you
run out of iron metal or the process gets bogged down by
precipitation. It is this reaction which will give you an etch line at
the liquid surface. Why?...because that is where the oxygen is.
Yep.
This process works well with Fe203, less well with Fe304.
Magnetite is quite resistant to any attack.
It usually just falls to the bottom as a black sludge IME.
Since the concern about using chloride has been mentioned I will
address that issue as well. The chloride will be pretty well rinsed
off of the surface. The freshly cleaned iron surface is quite reactive
and needs some type of protection. If you keep iron dry it won't rust.
WD-40 is not a good option as it will pick up water.
WD-40 surely repels water. I suggest that heavily pitted iron which
has had salt solution soaking into the pits will be quite difficult to
rinse clean.
Chloride can remain in microscopic cracks where it can accelerate
stress corrosion.
That's what I said.
I thought you were contradicting this above.
If you are cleaning up an I-beam for a bridge this
process might not be a good choice. If you are cleaning a wrench e.g.
that is not an antique I wouldn't worry about stress corrosion.
Further corrosion? I would certainly not want that.
Wrenches in my hands suffer extreme stress. I've broken several
If I
am cleaning some tool that the kids left out in the rain I probably
will dunk it in a 5% solution of muriatic acid, rinse it and apply a
light wax.
Hooley Dooley, how long will that last? What's wrong with a quick
squirt of CRC or WD 40 and a wipe with a rag?
If you have a valuable antique get some advice from someone else as
cleaning may not be appropriate.
And certainly don't use hydrochloric acid (shudder) or salt and
vinegar. Save the latter for your fish and chips
Why not use vinegar without the chloride? Vinegar is ~5% acetic acid.
Now for a little of the requested math. The dissociation constant of
HAc (acetic acid) is ~10^-5. (Ten to the minus five) In simplified
terms the H+ concentration in solution times the Ac-1 equals ten to
the minus five. Since the H+ = Ac-1 the acetate and the hydrogen ion
concentration in 5% acetic acid will equal the square root of 10^-5 or
somewhere around .02 molar. If someone wants to be picky it actually
calculates to be a tad more concentrated. That concentration of
hydrogen ions won't keep trivalent iron in solution.
So you're saying that acetic acid won't dissolve Fe2O3?
It does for me. The label of my vinegar does not list sodium chloride.
This thread has stirred up some interest. I will stick with it at
least for a while. Let's see if this answers some of the questions or
if I have everyone more confused.
I'm still struggling with your claim that ferric acetate in excess
acetic acid is unstable.
If you say that the sodium chloride keeps it in solution and stops it
precipitaing out as ~ ferric hydroxide, then that is not my
experience. I end up with a dirty yellow solution when I soak rust in
vinegar. I have never used salt in the vinegar, but next time I have
need to clean some rust, I will try with and without, to see if the
claimed speed increase occurs, or you claim of precipitation of ferric
hydroxide occurs and does not occur.
Museums soak very rusty items in sodium hydroxide solutions.
I believe this just stops further corrosion until the rust can be
removed by other means.