HSMs with a penchant for collecting and restoring old tools are always
interested in ways to remove rust.  Abrasion, acid, vinegar and salt, and
naval jelly (phosphoric acid) are ok for something that consists of surfaces
that can be reached easily.  But for stuff with pockets, recesses or
depressions that can't be reached, electrolytic techniques are the way to go.
RCM has discussed this subject at length on a number of occasions, but I
thought the writeup below was one of the better written treatises on the
subject so I captured it for the newsletter.

-----------------------------

I wrote this up for the EAIA Chronicle a few years back. I stuck it on
rec.ww a couple of months back. It sounds like it's time to post it here.

Electrolysis is a standard technique in the artifact restoration business.
I wrote this up for the Chronicle of the Early American Industries
Association a few years back. Most of the tool collectors around here use
it:

A plastic tub; a stainless steel or iron electrode, water and washing
soda (NOT baking soda!!) and a battery charger. About a tablespoon of soda
to a gallon of water. If you have trouble locating the washing soda,
household lye will work just fine. It's a tad more nasty--always wear eye
protection and be sure to add the lye to the water (NOT water to lye!!!)
The solution is weak, and is not harmful, though you might want to wear
gloves.

[ Washing soda is sodium carbonate.  Baking soda is sodium bicarbonate.  The
two are *not* interchangeable.  Look for washing soda in the detergent aisle
in the supermarket.  Washing soda is very basic with a pH of around 11.  I'd
be very careful to keep it (or lye) from coming in contact with any aluminum
parts, which are attacked by strong bases. - MWK]

The iron electrode works best if it surrounds the object to be cleaned,
since the cleaning is "line of sight" to a certain extent. The iron
electode will be eaten away with time. Stainless steel has the advantage
(some alloys, but not all) that it is not eaten away. The electrode is
connected to the positive (red) terminal and the object being cleaned, to
the negative. Submerge the object, making sure you have good contact, which
can be difficult with heavily rusted objects.

Turn on the power. If your charger has a meter, be sure come current is
flowing. Again, good electrical contact may be hard to make-it is
essential. Fine bubbles will rise from the object. Go away and come back in
a few hours. Rub the object under running water with a plastic pot
scrubber. Depending on the amount of original rust, you may have to
re-treat. The clean object will acquire surface rust very quickly, so wipe
it dry and dry further in a warm oven or with a hair dryer.

The polarity is important!! The surface rust is being converted to metallic
iron, so the process is totally self limiting. I have left things (by
mistake) for several days: the water was largly gone, by electrolysis, but
the object was fine. Reverse the polarity and your object is being eaten
away!!! The rust will go along with it, but that's not what you had in
mind, is it??

There are lots of variants: suspending an electrode inside to clean a
cavity in an object; using a sponge soaked in the electrolyte with a
backing electrode to clean spots on large objects or things that shouldn't
be submerged (like with lots of wood)

The surface is left black. Rusted pits are still pits. Shiny unrusted metal
is untouched. The method will cope with any degree of rust, from surface to
heavily scaled.

Use plastic and junk iron for electrodes. For electrodes, I buy cheap
stainless spoons at the flea market for treating small stuff in a dishpan
and large iron things as electrodes in my trashcan bath. The bath will last
until it gets so disgusting that you decide it is time for a fresh one.
There is nothing especially nasty about it-it's mildly basic-so disposal is
not a concern, except you may not want all the crud in your drains.

One caution: Painted surfaces *may* be damaged.

On antique tools, I generally treat immediately with a hard paste wax,
applied with the tool hot enough to melt the wax.( the oven or a heat gun
is handy here)

Try it--it beats any other method, especially for antique tools, where that
pickled look that acid gives totally destroys the value.

Ted Kinsey
Private replies: kinsey@u...

============================================================== ==============
Newsgroups: rec.crafts.metalworking
Subject: Re: Cleaning rust off tools with black oxide finish?
From: dwilkins@m... (Don Wilkins)

On Tue, 18 Jun 2002 08:15:13 GMT, "Harold & Susan Vordos"
<vordos@t.> wrote:

>Don,
>
>I'm sorry to say that I didn't read your corrected information, and have had
>no luck finding it.   I went back to the "What makes salt and vinegar work"
>thread and didn't find your name there.   If you would be so kind, please
>tell me where I can find it, I'd like to have a better understanding of how
>the process works.    I have a few issues  to deal with when I use the
>process in the future and knowing what I'm dealing with might be helpful.
>
>As to your second statement, I'd enjoy knowing just what part you don't
>agree with regards hydrogen embrittlement.  As far as I know, it's a known
>phenomenon, is well documented.     Could you please elaborate on your
>position so I can better understand it?

At the risk of getting flamed I will repost my explanation. I believe
this is the third time I have gone through this exercise.

***********
Vinegar is approximately 5% acetic acid plus other minor ingredients
which enhance its taste but not its effectiveness in this application.
Acetic acid is a weak acid with a pKa of ~5. If my calculations are
correct the acid concentration is about 0.1 moles per liter with a
hydrogen ion concentration around 0.001 moles per liter (the acetate
ion concentration would also be ~ 0.001).

At  a pH of ~3 the hydrogen ions are not concentrated enough to
dissolve metallic iron with the release of hydrogen gas. The acetate
anion does form a complex with ferric iron and to a lesser extent with
ferrous iron but at such a low concentration in the presence of a
saturated solution of chloride ions the acetate anion is going to be a
minor player. The result is that this witches brew is going to
dissolve iron oxides due to the complexing power of the chloride ions
but not attack metallic iron due to the lack of a strong enough
oxidizing agent.

My suggestion is that the purpose of the acetic acid is to keep the
solution acidic enough to prevent the precipitation of hydrous oxides
of iron. Now if you look back at the calculations (above) you will see
that most of the acetic acid is not dissociated into hydrogen ions and
acetate ions. As hydrogen or acetate ions are "consumed" there is a
reservoir of undissociated acetic acid available for replenishment (in
effect a buffer of sorts).

This solution is not going to last forever. Eventually there will be
so much dissolved oxides present so that precipitation occurs and if
it occurs on the oxide surface (which it will) then the reaction slows
to a disgustingly slow rate.

Other things can follow one of Murphy's laws or a corollary to one of
his laws to foul things up.

One thing that has already been discussed is allowing the piece to
stick out of the solution. This would not be a problem if the
atmosphere above the surface is inert. Oxygen in air provides enough
oxidizing power to attack metallic iron and you will see a region at
the interface where iron has dissolved.

If a well used solution has been stored with access to air then it not
only will become saturated with oxygen but all of the soluble iron
(from previous use) will be oxidized to ferric iron. You now have a
solution of ferric chloride which will attack metallic iron.

If one has a piece where removal of even a small amount of iron is
undesirable one should put a "practice piece" in a fresh batch of brew
to remove oxidants prior to use.

Chemistry in concentrated solutions can be very complex but this
explanation should provide a general idea of what is happening and how
to avoid problems.
**********

Perhaps I should explain "practice piece" here. I am referring to
adding a piece of iron without the oxide in order to remove oxidants
such as dissolved oxygen or ferric chloride.

 

submitted by Marv Klotz