Salt and vinegar for rust removal

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On Wed, 12 May 2004 13:43:42 -0700, Larry Blanchard

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.
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wrote:

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.

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.

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.

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

Yes but the buffering is not important in this case.

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.
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From: REMOVE snipped-for-privacy@means.net (Don Wilkins)
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On Thu, 13 May 2004 02:26:20 GMT, "Dan White"

Slowly (very)

Hydrogen ions?

Which are not supplied by the salt.

Yep.
Yep
As with everything.....

Yep. Godawful slow.

Of course.

Well simply a salt of a strong acid and a weak base.

H+ + Fe --->

H+ + Fe + O2 ---> Rust :)

Fine, but what does the chloride contribute?

Of course, but as vinegar is so harmless and cheaply available...

I just want to know the rationale for the addition of neutral chloride ions. They don't increase the dissociation of the acetic acid, I would have thought. Iron chlorides are no more soluble than iron acetates.
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Sandy

Sandy - I provided that rather long post that you responded to yesterday. I deleted it because of its size, and most of it is unrelated to your question. Also, you should realize (if you didn't) that I was passing on an article written by someone and didn't add any of my own thoughts, since I have forgotten so much chemistry I didn't want to dust off the brain cells. However, after reading your correct responses to his article, I had some thoughts that might possibly help, although I don't have a proven answer to your question. :(
Let me say that I agree that the driving force in the reaction to dissolve the iron oxide is the hydronium ion, and formation of water on the right side of the equilibrium. I think the answer to why you have to add salt to the mix is that the salt increases the ionic strength of the solution. This improves the conductivity of the solution, and might have the effect of improving the rate of reaction by allowing an easier transfer of charges among the reacting ions. I know this is a bit vague, but I don't know if anybody has a really good answer on this.
On the other hand, I recall that the equilibrium constant is really based on the activity of the ions in solution, and not their concentration. At higher ionic strengths (due to salt addition), I believe the activity, and therefore, the equilibrium constant, is lower than it would be without the salt. It seems to me this would tend to lower the dissociation of the acetic acid, and bind up even more H+ than without the salt. I think if this is correct, that it isn't the prevailing factor as I believe the salt does increase the reaction rate.
I think the case of cleaning your copper pots is similar to the rust issue. In that case, if you pour vinegar on the copper pot with the oxides on it, you won't see much if anything happen. When you sprinkle salt on the surface wetted with vinegar, you quickly see the oxides disappear from the spots where the salt is dissolving. It is very clear that salt does speed the rate of reaction in the case of copper oxides, and I have to think it does the same with iron oxides.
If you don't believe salt does anything, it is simple enough to test for yourself on two equal rusty spots. As to "why" it works, I have to think it is because of improved migration of electrons through solution due to the higher ionic strength of the solution. You might find, for example, that it takes two weeks to do the job with acetic acid alone, and one day with the salt added. Whatever problems you say the salt may cause later on may be outweighted by the time factor. Of course it is also possible (probable?) that the acid alone will not only take longer, but might in fact not remove as much oxide in the end.
I had one other thought, improbable as it may be: I'd say the Fe3+ ion is relatively large compared to the Cl- ion. On the other hand, the acetate ion is of course a molecule and not a single ion. Maybe there is also a bit of steric hindrance going on as well. With no Cl- present, all the Fe3+ has to bond with the acetate molecule (3 of them). If there is some difficulty fitting 3 of these molecules on one Fe ion, the reaction could be inhibited. If free Cl- is present, it could more quickly and easily neutralize the Fe ion and move the molecule away from the reacting area.
regards, dwhite
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single ion.
Minor correction: it is a single ion, just not a single atom.
dwhite
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On Thu, 13 May 2004 08:21:20 GMT, "Dan White"

Yep, it's big, but this surely only matters when it has to associate with a cation. Crystalisation.
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But steric hindrance is not a phenomenon reserved only to the crystalization of compounds. There are several kinds of hindrance, and they don't all have to do with crystallization. I don't think it really matters anyway because the acetate ion probably isn't nearly large enough. It was just a thought.
dwhite
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On Thu, 13 May 2004 07:31:37 GMT, "Dan White"

Interesting thoughts, Dan. This discussion has certainly disturbed some old cobwebs in my attic :) Gotta be over 40 years since that happened, although my daughter is doing some elementary chem at night school and my helping her is also raising the dust.
I don't disbelieve that it helps matters, but I just can't quite understand why, although you have given me some food for thought. I have never tested it, although as it happens, I had a rusty steel band on a hose fiting that I wanted to remove and so stuck it in a glass of white vinegar overnight. It cleaned the rust off enough to show me that there was so much metal left that it needed the grinder to get it off.
I can't really buy the conductivity argument, as what needs to happen is the migration of H+ to the rust and the migration away from it of iron ions.
I also can't buy the spatial problem of the size of acetate ions, coz the stuff is all in solution, and not forming crystals.
BTW, the problems later with soaking a piece of rusty steel in salt solution is that the salt gets trapped in the fine pits of the rusted surface and later attracts moisture and THEN the conductivity of this solution exposed to oxygen continues the corrosion at a pace.
Anyways, thanks for the stimulating discussion :)
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Sandy

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Ditto! All I can say is that the presence of ions in solution, ionic strength, does definitely affect how species in solution react. Maybe there is some physical chemistry website or ng you can visit and ask this question. I'd be interested to know, too!
dwhite
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snipped-for-privacy@optonline.net says...

Wow! I didn't mean to start such a learned discussion :-). My knowledge of chemistry is limited to making various explosive compounds, learned long ago in my juvenile days. And lately, I think I've forgotten most of that - CRS seting in :-).
But what I meant by "ask a chemist" is that a friend of mine who is a chemist said that the vinegar and salt combined to form a weak hydrochloric acid. I took his word for it.
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Where ARE those Iraqi WMDs?

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I think Sandy's point on this was that the H+ ions are already in solution from the acetic acid. The presence of Cl- ions doesn't change anything related to acid strength. For instance, it does not change the dissociation constant for acetic acid, thereby increasing the H+ concentration. It is the H+ concentration that determines how strong the acid is. My point was that the activity of the ions in solution is decreased by the addition of a lot of ions like Cl- and might actually lower the acidity. This does not seem to be the predominant outcome, however, because the salt does increase the speed of the reaction.
dwhite
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On Thu, 13 May 2004 09:04:26 -0700, Larry Blanchard

Yep it makes what might be regarded as a "weak" hydrochloric acid. This doesn't mean "dilute" BTW. And guess what? Acetic acid is "weak" so there is effectively no difference on this count. If you could make acetic acid "strong" you would have the equivalent of hydrochloric acid. Strong and Weak wrt acids means that they dissociate forming the H+ and anions to a greater or lesser extent.
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On Thu, 13 May 2004 09:04:26 -0700, Larry Blanchard

You shouldn't have as he was wrong.
Vinegar is approximately 5% acetic acid plus some other goodies to provide some taste. The hydrogen ion concentration is not sufficient to react with metallic iron and therein lies one of the keys to the process. The other key is the fact that chloride ions form a stable complex with iron ions in solution. The iron chloride complex is strong enough so that iron oxide will dissolve and form that complex. Since there is no oxidant strong enough to react with iron metal the net result is that the iron oxide goes into solution as the chloride but the iron metal does not react.
It is essential that the solution be kept oxygen free or the metal will dissolve. This is particularly noticeable if you allow the metal to be "derusted" to stick out of the solution into air e.g. you will find that there has been a dissolution of iron metal at the air liquid interface.
The role of the acetic acid is to keep the solution acidic enough to prevent the precipitation of iron oxide but low enough so that iron metal does not react with hydrogen ions. It is the high concentration of chloride that removes the rust not a "weak hydrochloric acid".
If one used a concentrated salt solution without the acetic acid then one would get a preciptate of hydrous iron oxide at the surface. This would slow the reaction to a crawl.
A weak acid such as acetic acid allows one to put a lot of acid in the solution but maintain a relatively low hydrogen concentration.
The solution if kept covered can be used repeatedly until the amount of dissolved iron reaches a point where the hydrous oxide begins to precipitate. If the used solutions are left open to the air then it will accumulate ferric chloride as a result of air oxidation. That ferric chloride is an oxidizing agent strong enough to react with iron metal which is the reason one gets an "etch line" at the liquid surface.
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says...

Maybe there

Can you explain exactly what this iron-chloride complex is? Are you saying that the iron oxide (rust) is preferentially breaking it's molecular bonds and is reforming as some kind of complex, or as iron chloride? I take it that it is not iron chloride because you say below that if oxygen is introduced, then ferric chloride will form. Second question: What is the reaction that transforms this "iron chloride complex" into ferric chloride?
Your mechanisms sound interesting, but it's hard to know if this is the actual path without knowing the driving forces mathematically.
dwhite

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On Fri, 14 May 2004 03:56:58 GMT, "Dan White"

Both ferrous ions and ferric ions form stable chloride complexes. Stable enough so that the rust does dissolve in the strong chloride solution by breaking iron-oxygen bonds. When the rust dissolves in a chloride solution one will get a solution which contains both species. In the presence of metallic iron the ferric chloride (from dissolution of iron III in rust) will be reduced to ferrous chloride so when the reaction is done we have a ferrous chloride in solution. If one adds oxygen (from air) then the ferrous is oxidized to ferric and this ferric immediately reacts with the metallic iron. The result is that one dissolves more metallic iron than is necessary and probably more than one wants.
In general you don't usually find three different oxidation states of an element present in solution at the same time. The highest oxidation state (ferric in this case) tends to react with the lowest (iron metal) to equilibrate with the one in the middle (ferrous). If you keep adding more air to form more ferric it should be obvious that the above reaction will continue until you run out of iron or oxygen. You can run out of oxygen by keeping the pot covered and the piece immersed.
Usually the quantity of rust dissolved is small in comparisons to the mass of iron metal so one doesn't notice the fact that some iron metal is sacrificed in this procedure. If you allow air into the mix the effect of dissolved oxygen can be very apparent. You can demonstrate this by letting a piece of iron be partially immersed in the solution. You will get an etch line at the liquid surface. If it is some antique you are restoring this etch line will not be a pleasant sight and will be almost impossible to fix.

It is nice that the correct chemistry is also interesting.
If you need the math lookup some coordination chemistry text books at a technical library. The topic is probably not of that much interest for this group. Google "coordination chemistry" with the quotes will give you a good start. Probably more than you ever wanted to know.

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wrote:

OK, thanks for the follow up.
dwhite
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posted:

Much like stable acetate complexes?

Just like in acetic acid solutions?

Where does this ever happen? Surely the rust dissolves in a solution of H+ with a non-precipitating anion?

Yep, the acidic solutioss speed up the atmospheric corrosion process.

Yep. Basic measures to stop atmospheric corrosion.

Which is why I personally would prefer mechanical derusting procedures in ethe medium of non-polar solvents that tend to prevent further corrosion.

Yep! Sad!

Isn't the correct chemistry always the most interesting? :)

Ayup.
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More than you probably wanted to know:
http://yarchive.net/metal/rust_remove.html
<snip>
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This is the post I pasted in this thread originally to try and answer the original question.
thanks, dwhite
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On Fri, 14 May 2004 16:30:02 GMT, "Dan White"

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.
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