Electrolytic Removal of Rust

by Kevin Chamberlain, Member HTPAA ( published in Tool Chest #63, February 2002)

Introduction

At our last Tool Conference, there was a Panel Discussion dealing with the cleaning and restoration of old hand tools. This is a key concern for anyone interested in collecting old tools, since one rarely finds tools in mint condition in an original box. It is much more common to find tools showing the combined effects of correct usage, abuse, neglect and simple ageing. The primary question is: should cleaning and/or restoration be attempted at all?

The answers are conflicting; some aim to restore the tool to a brand-new appearance, eliminating all evidence of its life history. Others think any attempt at restoration will destroy the character and value of an antique tool.

Most think some intervention is desirable to remedy the effects of abuse and neglect, while still retraining as much patina and character as possible. There is an important caveat - "when in doubt, do nothing". Cleaning and restoration of antique tools should be done in a careful and meditative mood - otherwise rash actions may cause loss of valuable information and/or further damage.

Removal of rust from old tools or other artefacts may or may not be a desirable goal. In some cases (museum specimens) stabilization to prevent further deterioration may be all that is necessary. Rust may be regarded as undesirable by tool users or collectors because it obscures identifying marks, causes seizure of moving parts, creates an unpleasant rough texture to the touch, stains the hands and/or the workpiece, or is simply unsightly. I believe rust removal is often justified, but the method chosen should not alter the surface in other ways and should result in a reasonably attractive surface appearance.

The electrolytic method of rust removal meets these aims very well and is also a gentle, cheap and adaptable method. Electrolysis is now being used by a wide range of artefact restorers, from tool collectors to marine archaeologists.

In this article, I mainly aim to outline the practical aspects of the electrolytic method so that members can try it for themselves. Several HTPAA members have been using the method for some years and find it very useful

What is Rust?
Rust arises from the surface oxidation of iron or steel in the presence of atmospheric oxygen and moisture. As the rust forms, the surface of the iron is eaten away, sometimes evenly, but often deep local pitting occurs beneath wart-like protuberances. Rust occupies more volume than the iron it replaces, and so moving parts will tend to seize as they rust.
Chemically, most red rust is hydrous ferric oxide FeO(OH). If water enters this chemical structure, yellow-brown limonite may form FeO(OH).nH₂O. Quite often, black iron oxides are also present usually magnetite (Fe₃O₄), an iron oxide which both conducts electricity and can be magnetised. The rusting process, and the conditions which promote, inhibit or stabilise it, are all important topics which are beyond the limits of the present article.

Methods of Rust Removal
Rust usually binds strongly to the underlying iron or steel. Various methods have been used to remove rust. Mechanical methods include the use of emery paper, sandpaper or wire brushes (either manual or powered). More aggressive methods include sand blasting or shot blasting. Plastic bead blasting is a more recent and gentler variant. Clearly these methods will scratch the metal surface if the abrasives are harder than the metal. Wire brushes, for example, do not appear to scratch hard steel surfaces but they can be quite damaging to soft iron surfaces.

Gas flames are sometimes used to dislodge rust, relying on the different expansion rates of the oxides and the metal in response to heat. Simple boiling in water is also used to loosen rust. Here the thousands of tiny bubbles which form and grow at the metal surface may act to mechanically dislodge the rust layer.

In industry, chemical methods of de-rusting include the use of strong acids under carefully controlled conditions. These acids are too dangerous for home use. Vinegar or dilute phosphoric can be used successfully at home, although the latter leaves a uniform grey appearance which some find undesirable. Acids attack the iron oxides directly, but also etch the mental surface to some extent, generating hydrogen gas in the process.
Other chemical agents include the commercial product "Corro Dip" (from Liquid Engineering International Pty Ltd) and the traditional molasses solution. The molasses method usually takes weeks and may work because of acids formed in the molasses solution by fermentation. I have been told that iron or steel objects left too long in molasses solution will eventually be eaten away, supporting the idea of an acid etching progress. However other chemical reactions may also be involved in this old method.

The Electrolytic Method
The electrolytic method is a cheap, gentle and effective method which causes minimal alteration to the metal surface. It may seem complex, but is actually easy to set up and use. It is quite safe, provided certain sensible precautions are taken (see below). The electrolytic method involves immersing the rusty object in an electrically-conducting solution of washing soda (sodium carbonate). The negative lead (black) from a battery charger is attached to the object, and the positive (red)lead is attached to a stee1 electrode dipping into the solution. When the current is turned on, electrochemical reduction reactions occur at the metal/oxide interface on the object's surface. These reductions loosen the rust layer, allowing it to be easily brushed off. These reactions may involve the direct reduction of iron oxides to finely divided iron. However, it is clear from observation that a major reaction at the negative electrode involves the production of thousands of tiny bubbles of hydrogen from the electrochemical reduction of water. The hydrogen may in turn react chemically with the iron oxides, or it may simply act to physically dislodge the rust layer. Whatever the mechanism, the process does not appear to cause etching or deposition on the metal surface. Of course, removal of rust will reveal any damage to the surface (such as pitting) which has already occurred.

Important Safety Precautions
1. Since pure water is a poor conductor of electricity, a soluble salt, (called an electrolyte) has to be added to make an electrically conducting solution. The best salt to use is sodium carbonate (washing soda). A packet can be bought in supermarkets for a few dollars. Washing soda solutions are alkaline and will irritate the skin and (especially) the eyes. Always use eye protection and gloves and wash off any spills immediately. Do not try to use other salts - no better results will be obtained and dangerous effects may occur. Caustic soda, for example, is far too corrosive, and even strong solutions of ordinary table salt will generate toxic chlorine gas at the positive electrode.

2. The battery charger is attached to the mains and must be completely shielded from the solutions which conduct electricity. Make sure no spills can touch the battery charger - especially when it is unattended. The 6/12 volt leads from the charger are relatively safe, but you may still get an unpleasant shock if you put your hands in the solution or touch the electrodes while the current is running. Turn off the current before making adjustments to the electrolysis bath.

3. A major reaction occurring in the bath is the splitting of water into hydrogen gas (at the negative electrode) and oxygen at the positive electrode). Hydrogen will combine explosively with oxygen or air if ignited. (Remember the Hindenburg!) All flames (including cigarettes) must be removed from the area, and sparks caused by touching the leads together must be avoided. The work should be well ventilated to dilute and remove these gases safely. Do not use this method in a confined, poorly ventilated area.

Preparation of the Electrolyte Solution
A sufficient concentration of washing soda is about 10 gram/litre (about 1 teaspoon per pint). The concentration may be increased somewhat but the results will not change greatly. Make sure all the crystals have dissolved before using the solution.

The Electrolytic Bath

De-rusting of small pick head by electrolysis

The simplest variant of this method requires a non conducting inert plastic container (plastic dish, box, bath, bin etc.). Some ingenuity is needed to find containers deep or long enough for items such as long saw blades or leg vices. After removing any wooden handles, brass fittings, etc. from the object, sufficient washing soda solution is added to completely submerge it.
A stainless steel strip is recommended for the positive electrode or anode (e.g. a piece about 2-3" wide and long enough to emerge from the solution). Ordinary scrap iron or steel can be used, but the surface will quickly clog up with corrosion. Do not use copper or other metals, as these will be rapidly eaten away.
The reactions at the anode involve the production of bubbles of oxygen gas from oxidation of water, plus the direct oxidation of the metal electrode. Stainless steel is most resistant to the latter process, but even it may show some minor pitting after a while. The anode should be brushed clean at intervals. The red lead from the battery charger should be clipped to the anode where it emerges from the solution. If this attachment clip dips under the surface, it will be eaten away. The negative lead (black) is attached to the rusty object. In this case, the attachment clip may be submerged under the solution - corrosion does not occur at the negative electrode (cathode). It is very important to have good contact at the attachment point, so these should be cleaned by wire brush or emery paper. The object and the positive electrode should be separated by at least a few inches. If they are allowed to touch, a short circuit will occur and the battery charger may be damaged.

The Battery Charger
Any 6 or 12-volt battery charger will work, provided it can produce a few amps of direct (DC) current. A current of about two amps at 12 volts is typical (a charger with a current meter is useful as it shows you what is happening). If several objects are attached in parallel, or a very large object is attached, the current may rise. Be careful not to exceed the capacity of your charger. The current may be reduced by increasing the separation between the object and the anode or by diluting the solution with water. A car battery would also work as a DC power source.

The Process

Hydrogen bubbles forming on the object's surface during electrolysis

Once the circuit is completed, tiny bubbles will stream from both electrodes. The time required for effective de-rusting will vary from 30 minutes for small objects (such as auger bits) to many hours for large objects such as a leg vice. Overnight operation is common. No harm is done by leaving the circuit on for long periods, as long as the charger does not overheat or gases do not build up in an unventilated area. After a time the object should be rotated to avoid "shadow" effects (uneven de-rusting). If part of the objet has been left projecting above the solution, the object should also be inverted to de-rust the exposed part. As time passes, some of the rust will fall off and sink to the bottom of the container. When enough time has elapsed (learned mainly by experience), turn off the charger, remove the object from the bath and rinse off the electrolyte with water.

The residual rust will now appear as a dark surface sludge which can be easily removed with a hand wire brush or plastic scourer. This is less messy when done under water (e.g. in a water-filled bucket). After rinsing and thorough drying, the object will now appear free of red rust, but there may still be a thin layer of closely-adherent black oxide.

For certain antique artifacts, this grey-black appearance may be quite attractive. However, brief power brushing will quickly remove this thin layer and give a progressively shiny and burnished appearance (appropriate for items such as plane blades).

Baking in an oven for an hour or two at about 150°C (300°F) is an option which will give the objects an attractive bronze-yellow patina, deter further rusting and protect against hydrogen embrittlement (see below). Alternatively a rust inhibitor such as RP-7 may be applied, or the object simply oiled or waxed to deter future rusting. If only part of the objet was submerged there will usually be a faint "tidal mark" where the object emerged from the solution. This is one reason to seek containers large enough to submerge the whole object at once. I find that intact japanning is not usually damaged by electrolysis, but loose paint of any type will be stripped off and this is often a useful feature of the method.

The method works best on objects that are electrically well-connected. Ideal objects for de-rusting by this method include augurs, axe heads, saws, single bow hand shears, plane bodies, cast iron pots etc. Whenever the object has multiple parts, the electrical contact between the parts will affect the results obtained. If the contact points are coated with rust, dirt or grease, little current will flow from one part to the next and de-rusting may be slow. If only a few parts are involved, it is easy to connect each part separately by providing several branches from the negative lead or use short leads to connect each part to the next using clean contact points.

Other Variants of the Electrolytic Method
1. The bath itself may be made of stainless steel and used as the anode (positive electrode while the rusty object is suspended in the middle of the solution without touching the container. This method maximizes the size of the anode and allows current to flow to the object from all directions - thus minimizing shadowing effects.

2. The opposite of the above. Hollow vessels such as iron pots are filled with the electrolyte solution and attached to the negative lead, while the anode is suspended in the middle of the solution. This allows excellent de-rusting of the inside of such pots. A similar method has been used to remove rusty encrustations from the inside of cannons found in sunken ships.

De-rusting the inside of an iron pot using a suspended anode

3. Small items may be placed on a stainless steel grid suspended in the solution and electrically connected to the negative (black) lead. The rusty items make electrical contact with the grid and do not need to be individually connected to the lead. However the de -rusting will proceed rather slowly unless the items have been cleaned where they touch the grid. Small chains and other intricate objects with connected parts may be de-rusted using this technique.

Localised de-rusting of the sole of an infill plane using an electrolyte-soaked sponge

4. To avoid immersion of objects such as wood-infill planes the bath may be replaced by a solution-soaked sponge or cloth which lies between the object and a stainless steel plate which acts as the positive electrode. The negative lead is attached to the object and current flows only through the soaked sponge. Be careful to avoid any short circuits. More solution should he added to the sponge periodically as it heats up and tends to dry out. This method can be used to de-rust small parts of artefact quite precisely without wetting the whole object.

Hydrogen Embrittlement of Steel - A Cautionary Note
Atoms of hydrogen absorbed by steel are known to enter the lattice of iron atoms and prevent the layers from sliding past each other easily. This causes the steel to become more brittle and liable to crack. The absorption of hydrogen by steel is a familiar problem in industry which arises during steel refining, heat treatment, acid pickling or electro-plating. It can also happen as a result of simple corrosion. The standard remedy is to bake the objects in ovens to drive out the absorbed hydrogen (200°C for four hours would be a typical regime in industry). The simple passage of time is also known to cause loss of hydrogen from steel. Hydrogen embrittlement may occur to some extent during electrolytic de-rusting. This may be a cause for concern with saws or other edge tools. It might be wise to wait a while before setting saw teeth after prolonged, electrolytic de-rusting. Alternatively, baking the tool in the oven for hour or so at about 150°C (300° F) should remove absorbed hydrogen. Note that this baking temperatures is low enough to leave the temper of most steels unaffected. Since hydrogen embrittlement is reversible, it should not cause too much anxiety. I believe that the advantages offered by electrolytic de-rusting justify wider experimentation by tool collectors. As more experience is gained clearer knowledge of its advantages and disadvantages will emerge.

Problems with Stainless Steel Anodes

Some thoughtful correspondents have pointed out that the use of stainless steel for the positive electrode (anode) may have some undesirable consequences. Most stainless steels contain high percentages of chromium and nickel which may be released into the bath as the anode is slowly eaten away. These are likely to be released initially as soluble cations just as the iron is released initially as ferrous ions. However, since all three cations are being released into an alkaline solution, they are likely to be immediately converted to insoluble hydroxides or oxides and form encrustations on the electrode or fall to the bottom as sediments. In this bound form the nickel and chromium are likely to be less hazardous but nevertheless waterproof gloves should always be worn when working with the bath and the bath sludge should be disposed of appropriately. It may be better to avoid the problem entirely by using simple iron electrodes and brushing the sludge off regularly.

References:
Jane and Mark Rees. Tools. A Guide for Collectors. Published by Roy Arnold, Suffolk. 1996. p22.
Nathan Lindsay. Cleaning Rusty Tools. Electrolysis Made Easy. http://rusty21.com
Ted Kinsey. The Electrolytic Rust Removal FAQ www.bhi.co.uk/hints/rust.htm
FAQ Derusting with Molasis, www.steamengine.com.au/ic/faq/mollasis.html
Metal Conservation (Marine archaeology) http://nautarch.tamu.edu/class/anth605/ File9.htm

Acknowledgements: Thanks to Kees Klep and Peter Williams for sharing their experiences of this method and to Tony Blanks, HTPAA member from Tasmania and skilled netsurfer, for providing valuable website information. Warren Hewertson and Doug McIver provided constructive review of the text.