"Sandy" wrote in message
...
On Wed, 12 May 2004 13:43:42 -0700, Larry Blanchard
posted:

In article , Sandy
says...
I'm wondering why the salt. It surely does nothing but perhaps cause
problems later on.

See your friendly local chemistry professor :-).

Do you not know what it does?
I've got a fairly reasonable handle on chemistry, and I can't see what
the salt is there for. The acetic acid will dissolve rust, and slowly
dissolve iron. No need for the salt unless you know of a good reason.
Please expound.


A quick search turned up this. This guy says it pretty well. The bottom
line is that the dissolution of the rust is prompted by the presence of
acetate and chloride ions. Iron would rather form a molecule with acetate
or chloride ions, which are soluble in water than remain as iron oxide,
which is insoluble. It is easy to introduce chloride in the form of salt,
and greatly increase the rate of reaction. The acid component of the
solution apparently does not take part in the dissolution of the rust (but
does dissolve the pure iron). It prevents the iron ions from reforming into
iron hydroxides, which are insoluble as is the original iron oxide (rust).

dwhite
From: (Don Wilkins)
Newsgroups: rec.crafts.metalworking
Subject: Rust
Date: Sat, 03 Apr 1999 01:35:37 GMT

On 2 Apr 1999 13:49:14 GMT, (James W.
Swonger) wrote:

In article ,
Grant Erwin wrote:
Muriatic acid works fine on rusty steel. It attacks the rust a lot
faster than
the steel.

As a chemist my first choice is hydrochloric acid as a solvent for
rust. Muriatic acid is readily available from the local hardware
stores. Yes it will also dissolve iron so you don't want to dump the
parts in and come back next week to see if the rust is dissolved.

...
Vinegar alone is just a slower and milder form of the above. Commercial
pickling (submersion in a hot bath of a combination of strong acids) is
a more extreme version of the above.

I guess I want to mildly disagree with this. Vinegar is about 5%
acetic acid and also contains various organic stuff to provide some
taste but which are not particularly essential to the derusting.
Acetic acid is a weak acid and is going to behave differently than
hydrochloric acid.

The acetate ion forms soluble complexes with iron in solution which
aids in the dissolution. Since acetic acid is a weak acid there will
be a lot of the acetate ion tied up as undissociated acetic acid.

Hydrochloric acid is a strong acid and will be essentially completely
dissociated. The chloride ion forms a stable complex with trivalent
iron. It is the stability of this complex which aids in dissolving the
iron oxide (rust).

The addition of sodium chloride to the vinegar provides the extra
complexing power of the chloride to this mixture.


I'm not sure what happens chemically with salt and vinegar. I've only
used it to clean coppers, brasses and bronzes where if it works it is
like magic.

The chloride and/or the acetate ions form stable complexes with iron
ions which aids in the dissolution of rust. The trivalent iron forms
much more stable complexes which would lead one to believe that the
solutions would be more effective with e.g. Fe2O3 than with lower
oxides. Those tenacious black oxides are more dense and usually
contain divalent iron oxide.

The combination gives you a mild hydrochloric acid with other junk
floating in it.

H+
Na+
Cl-
CH3COO- (acetate)

Well sort of but there will also be a fair amount of undissociated
acetic acid.

The acetate might give you some buffering effect, perhaps.
I've seen acetic acid as an additive in a couple of electroplating
recipes.

Yes but the buffering is not important in this case.

The active principle is the HCl. For the copper based materials,
it converts the insoluble oxides to soluble chlorides. I expect
the same activity is responsible for iron cleaning.

In principle it is the same but it is the complexing power of the
anions (chloride and acetate) which drive the reaction. It needs to be
acidic enough to prevent the precipitation of the very insoluble iron
hydroxides. Other than that the hydrogen ion does not participate in
the dissolution of the rust. The hydrogen ion concentration does
contribute to the speed at which the metallic iron disappears.

There have been extensive discussions of an electrolytic method in the
past which I have stayed clear of. In that case it appears that the
rust removal is accomplished by creating hydrogen gas underneath the
rust coating and 'blasting" it off. This would explain why some of
those black dense oxide coatings are not removed.

The electrolytic method should not remove much metallic iron but on
the other hand what ever is on the surface as rust and gets "blasted"
off is not likely to be redeposited as metallic iron back on the piece
from the same location from which it originated.

In other words what is turned to rust ain't gonna get put back where
it once was.



Search for Google's copy of this article
----------------------------------------------------------------------------
----

From: (Don Wilkins)
Newsgroups: rec.crafts.metalworking
Subject: Rust
Date: Sun, 04 Apr 1999 18:25:07 GMT

On Sat, 3 Apr 1999 12:04:33 -0600, "Don Foreman"
wrote:


. It needs to be
acidic enough to prevent the precipitation of the very insoluble iron
hydroxides. Other than that the hydrogen ion does not participate in
the dissolution of the rust. The hydrogen ion concentration does
contribute to the speed at which the metallic iron disappears.

Does this mean that using stronger acetic acid probably would not speed
rust
removal but may increase rate of attack on steel?

I probably should say I don't know the answer to this question. My
opinion not backed by any research is that the 5% concentration of
acetic acid in vinegar probably isn't the optimum concentration. I ran
through some equations below to show why increasing the acetic acid
concentration doesn't increase the hydrogen ion concentration at the
same rate as occurs with hydrochloric acid. At 5% acetic acid I doubt
if the hydrogen ion concentration is high enough to dissolve iron.


Rust is already oxidized iron so when it is dissolved there is no
requirement for a change in oxidation state. When you dissolve the
metal (any metal) into an aqueous solution the metal is oxidized. If
the metal is oxidized then something must be reduced. If there is
nothing there to be reduced then the metal won't be oxidized.

In the mixtures under discussion (hydrochloric acid solutions or
vinegar-salt) there are three things present in the solution to serve
as oxidizing agents. These are the hydrogen ions from the acid, the
oxygen dissolved in the solution, and ferric iron.

So if you increase the hydrogen ion concentration then there will be
an increase in the dissolution of the metal (in this case iron). This
will be very apparent when a strong acid is used such as hydrochloric
acid because all of the acid is ionized.

Acetic acid however is a weak acid so an increase in the concentration
of this acid does not increase the hydrogen ion concentration
proportionally. If you look at the equation for the ionization of
acetic acid

HAc ---- (H+) + (Ac-) one finds that there is an equilibrium
constant for that dissociation

(H+) (Ac-)/ (HAc) = ~10^-5

since (H+) = (Ac-)

(H+) = square root of (HAc)*10^-5

With that it is obvious that increasing the acetic acid concentration
is not going to cause a rapid increase in the dissolution of iron
metal.

Who ever came up with the addition of chloride ion did so to provide
something to form a complex with the dissolved iron among other
things. If you look at that equation above for the dissociation it is
obvious that if they had added acetate ion (e.g. sodium acetate) it
would have reduced the hydrogen ion concentration probably causing
precipitation of iron compounds. Clever idea for kitchen chemicals.

On the other hand there are other reactions which will come into play
once the process is started.

As most of you know ferric chloride can be quite corrosive and is a
pretty good etching agent. The ferric chloride produced from the
dissolved rust is going to look at that freshly produced iron metal
surface as a nice place to do some etching (dissolve iron metal).

Of course there will be some divalent iron present as well. This will
come either from the rust or by the reaction of the ferric chloride
with the metallic iron.

Oxygen (from air) dissolved in the solution will oxidize the ferrous
ions up to ferric. This ferric can then react again with metallic
iron. If you don't want to dissolve iron metal then you should remove
air from the procedure. A little bit of ferric chloride in a sodium
chloride solution with a continuous supply of oxygen can dissolve one
hell of a lot of iron.

I hope this hasn't confused the issue more than it helped. I suspect
that one could make a better rust removal solution using glacial
acetic acid, water, and sodium chloride. I believe the formula was
developed because of the ready availability of vinegar and salt. I
would vary the chloride concentration as well as the acetic acid
concentration. If you don't want to dissolve iron metal keep oxygen
out of the process.

If you are inclined to do this be careful with glacial acetic acid. It
doesn't burn or cause immediate discomfort on the skin but if not
removed promptly will cause large thick patches of skin to come off
leaving tender exposed meat. This is a type of chemical burn that you
are not likely to get more than once.

As I read and reread this post I am not entirely satisfied but there
is some very complex chemistry taking place and it is not easy to
describe it in a short note.