Rust Removal Using Electrolysis

A very rusty gin trap

By Andrew Westcott

 Introduction To Electrolytic Rust Removal 

Rust electrolysis is the process of using electricity to remove rust from corroded iron and steel components.

Rust forms on ferrous metals as a result of electrolytic action, and electricity in conjunction with an electrolyte can be used to reverse this, the process being known as rust electrolysis. In badly corroded samples, not all of the rust can be converted back to metal, but the material which can't be recovered is much easier to remove after treatment. This site explains the chemistry behind rusting: Chemisty Of Rust.

The idea of using electricity to convert rust back into iron isn't new, and electrolysis has been used for metal restoration by collectors and archaeologists for many decades. The results can be impressive, with shiny metal being visible after proper treatment; however, the exact requirements are sometimes poorly understood and the equipment often crude, exacerbated by the many web pages out there offering poor or misleading advice on the process.

Why use electrolysis when there are simpler methods?
Rust removal using sand blasting or other abrasives certainly cleans the metal, but this is unsuitable for very old or valuable artifacts as it is destructive, removing good metal along with the rust, possibly obliterating fine surface details.

Rust can be dissolved using strong acids, but the acid also attacks the good metal; I required a way of removing just the rust and no more, with the hope of recovering some of the rusted metal, and electrolytic treatment offered this possibility.

Rust electrolysis is not some magical, or quick and easy way of cleaning rusty items. Setting up the apparatus and conditions for electrolysis needs space and is time consuming, and removing the loose material once treatment has been completed also takes time and is quite messy. However, if you are prepared to put in the effort, I believe the results are worth the trouble.

Note that electrolytic cleaning doesn't work for non-ferrous metals such as copper, bronze, brass, pewter, tin or aluminium. The corrosion products found on these metals aren't formed by electrolytic action and therefore the process cannot be reversed electrolytically.

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 Does Rust Electrolysis Work? 

A very rusty horseshoe before treatment with electrolysis

My test subjectAn old and very rusty horseshoe I used as my electrolysis test subject

As my test subject, I decided to try the electrolysis process on an old horseshoe which was probably well over a hundred years old and in a particularly badly rusted condition, having spent much of its time buried in the ground.

In the photo, you can see the very advanced state of corrosion. Note that even in the high resolution version of the image no surface detail can be distinguished. Attempting to clean this would represent an extreme test of the electrolytic process, but I decided to give it a try anyway just to see what could be salvaged.

The shoe was prepared for treatment by using a file to carefully remove a small area of rust on one edge in order to expose some metal, so that an electrical connection could be made using a crocodile clip. A solution of washing soda was then made up and a method of suspending the shoe devised.

A badly corroded horseshoe after treatment with electrolysis

The horseshoe after treatmentThe same horseshoe after treatment showing the shiny metal and the nail holes

Once ready, the shoe was connected as cathode, suspended in the electrolyte, a current limited to around 100mA applied and left to run for a couple of days. Once the allotted time had elapsed, the treated shoe was removed from the tub. It should be mentioned here that the solution had remained quite clear throughout, there had been no visible bubbling and no corrosion of the anode plates had occurred: exactly as it should be.

As the shoe was being rinsed under a tap to remove remnants of the solution, I found that the outer layers of rust could now be pushed off using mild finger pressure, revealing a solid black and grey metallic core.

A gentle scrub with a fine wire brush proved effective at removing the soot-like deposits, revealing shiny grey metal on which you could see the grain due to the etching effects of the rusting process. The nail holes were now clear, one having the stump of a nail still in it.

The shoe was finally given a rinse in hot water, and quickly dabbed dry using kitchen roll. The retained heat of the metal rapidly dried off any remaining damp areas, preventing re-rusting. Finally, to protect the shoe from further corrosion, it was given a coating of light oil.

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 Safety Considerations 

The electrolyte used is mildly alkaline and although not considered dangerous, prolonged contact with the skin is best avoided. Obviously, if it gets onto your eyes, wash with copious quantities of water and get medical advice.

The equipment needed for electrolytic rust removal involves the use of electricity, although the voltages usually encountered in electrolysis are not considered dangerous. However, if the hands have been immersed in electrolyte for some time and the skin has become highly conductive, even a relatively low voltage may allow significant current to flow through the body, so always switch off the supply before moving connectors. There is obviously also a concern with mains equipment being sited close to liquid, so common sense must be used as regards the relative positioning of the components.

There is also the issue of the production of explosive gasses: even when using low currents, some hydrogen and oxygen may be evolved at the electrodes, producing an explosive mixture. Ensure the procedure is conducted in a well ventilated area and avoid sparks or flames in the vicinity. Always switch off the mains transformer before adjusting the electrode connectors to avoid sparks.

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 Method 

A suitable container for the electrolyte is needed which is large enough to totally immerse the item to be cleaned, and I use one made of plastic, as this is entirely inert and non-conductive.

The equipment I use for treating rust by electrolysis

The equipment I currently use for small projects

Suitable material for the anodes needs to be obtained, and 0.5mm - 1mm steel plate as used for car repairs is an excellent choice as it can easily be cut and shaped and is inexpensive. Metal plate is often coated in a protective film of oil and this layer needs to be removed with a solvent before use, and obviously it must not be painted: anything which may inhibit the flow of electricity between the metal plates and the liquid has to be removed.

The anodes can be shaped to fit around the interior of the container ensuring part of each plate protrudes above the water level to enable a connection to be made. They should present a large surface area relative to that of the piece being cleaned and be able to 'see' most of the surface of the piece from all around to minimise areas of non-cleaning due to shadowing effects, as the current in the electrolyte tends to travel in direct lines. An anode made from a piece of steel rod as I've seen suggested is far less than satisfactory.

All the anode sections, if more than one is used, must be electrically linked to each other using clips and wire and it will be beneficial to include a plate across the bottom of the tub and ideally a piece across the top as well to ensure the item is completely surrounded by anode material.

An alkaline electrolyte can now be prepared, and I consider the best all-round option to be a solution of sodium carbonate, Na2CO3, as it is reasonably safe and also readily available at many supermarkets as washing soda. Suggested strengths for the solutions vary, but I use a 2 - 4% w/v solution of the chemical. In order to demystify quoted solution strengths, it simply indicates the weight of chemical contained in 100mL of solution, so for example a 2% solution would involve dissolving 2 grams of chemical in water and making up the final volume to 100mL. This obviously is the same as 1 litre containing 20 grams of chemical, and so on.

You may notice a slight precipitation of white calcium carbonate produced as the sodium carbonate reacts with the calcium ions present in tap water, especially in hard water regions, this having a tendency to first make the solution appear milky, then to coat the piece and electrodes with a snowy deposit as it settles out. If wished, the solution can allowed to stand for a day or two to give this precipitated chemical time to settle to the bottom and the clear solution decanted off, or the deposits left where they settle, on the bottom of the tub out of the way as it doesn't affect the outcome at all.

Diagram of how to set up the rust electrolysis equipment

This diagram illustrates the components required
and the method of connection

A low voltage DC power supply is now required, but don't just connect a battery charger to the electrodes as the low resistance of the electrolyte will allow large currents to flow causing a real risk of damage to the charger, or even fire. Even if this doesn't happen, the resulting current flow is far too high, resulting in excessive anode erosion, poor quality iron deposits at the cathode, lots of bubbling of explosive gasses and a messy electrolyte.

If a battery charger is the only current source available to you, a good work-around would be to place a low wattage automotive lamp in series with the electrolysis tank in order to reduce the current flow to a sensible value, a 12 volt 2.2 watt lamp or similar being useful for limiting the current to around 200mA or so, and for higher currents, higher wattage lamps can be used.

When connecting up, observe the correct polarity: the piece to be cleaned must be connected to the negative terminal. You need to ensure you have a good electrical connection, and this may involve removing rust from a small area to reveal shiny metal to effect a good contact. Pieces consisting of more than one component, for example a gin trap, must have all the separate components electrically linked to the cathode. This is important; don't assume the components will be electrically linked just because they are touching each other.

Once prepared and wired up, the piece can be suitably suspended in the electrolyte, ensuring it doesn't touch the steel plate anodes, and the power applied. If the current has been sensibly chosen, there should be very little or no visible gassing at the cathode.

The rust electrolysis equipment actually in use

Cleaning a pole trapA pole trap suspended in electrolyte with wires attached, being treated

The photo to the left shows my equipment in use cleaning a rusty pole trap. In the high resolution version of this picture you can see the way the trap is suspended in the solution and the separate connections to the various parts of the trap. Also visible is the large anode made up of four plates electrically connected together, and despite being in use at the time the photo was taken, the electrolyte is still clear with no bubbles being produced. This is due to the low current flowing resulting in only a small voltage appearing across the tub.

After the appropriate time has passed, the power can be switched off and the piece lifted out and given an initial clean using a scrubbing brush or something similar under running water, taking care to quickly dry it afterwards; using hot water for the washing helps the piece dry due to its retained heat. The final stage is careful removal of the remaining black conversion products using a small wire brush, and to protect against further corrosion a light film of oil can be applied. There is no risk of over-treating an iron artifact using electrolysis as bare iron is unaffected by the process and therefore the piece may be left in the electrolyte indefinitely as long as current continues to flow to give it protection from corrosion.

Certain pieces may consist of components made from a combination of metals or may have iron parts with zinc or nickel plating, or galvanising. Such plating and any brass components seem quite unaffected as long as they are given cathodic protection by being also connected to the negative terminal along with the rest of the artifact, but unfortunately any corrosion present on these non-ferrous is unaffected, and would therefore require a more conventional cleaning process.

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 Special Considerations 

Electrolysis Can Damage Paint
Be aware that electrolysis can soften and lift paint; this may be due to the original surface not being properly prepared before painting, or possibly if corrosion had crept under the paint layer. Certain paints may also be softened by the electrolyte itself. So be warned that if you have an object of value with a painted surface, and if that paint is important to its value, consider an alternative to this process.

Always Have Current Flowing
Ensure that current is flowing whilst the piece is in the electrolyte, as it is the fact that it is the cathode in the circuit that protects it from corrosion. Should the piece be allowed to remain in the electrolyte without current flowing, it is possible, depending on its exact composition, that it will start to corrode, so if the power needs to be switched off, remove the components from the electrolyte also.

Voltage And Current
The voltage is not critical, as long as it is higher than the minimum of around 2 volts and not so high as to be a health hazard, so something around 12 volts is fine. However, for good results and valuable pieces, you must limit the current. If too high a current is allowed to flow, any deposited iron will be very porous and possibly become detached from the surface and the rapid hydrogen bubble production can blast off rusted metal which could possibly have been recovered using gentler methods. If you are finding the anodes rust badly and the electrolyte bubbles a lot and becomes discoloured, the current is far too high and you may as well have just sandblasted the item.

Avoiding Rusting During Final Cleaning
The piece should be taken from the cleaning tank and promptly placed in a tub of clean hot water and scrubbed with a plastic brush to remove much of the loose rust, and well rinsed to remove the electrolyte. It can then be scrubbed and rinsed in alcohol to remove the water along with more rust, and allowed to dry. Final cleaning work can now be performed on the dry piece, and when this stage is complete, any dust can be washed off using alcohol.

Painting The Treated Item
I have received a fair bit of communication regarding whether the surface of the treated metal is suitable for painting without further treatment. I use electrolysis to reveal surface features so I don't cover that with paint. However, I would suggest a coating of 'red lead' primer followed by a conventional metal paint would be fine and that there would be little chance of rust forming under the paint.

Hydrogen Embrittlement
There is a known process by which steel can absorb hydrogen during the electrolysis process, which is known to cause springs to become brittle. This change is considered temporary and will eventually reverse if left alone, or the process can be speeded up by heating the item in an oven. The best advice is, don't operate any springs on recently cleaned items to prevent breakages. More about it here: Hydrogen embrittlement

About Cleaning Non-Ferrous Metals
I have received correspondence about cleaning of non-ferrous metals, such as copper, bronze, lead and silver. Some of these metals are often found in coins for example, so there is a requirement to clean artifacts made of such metals. The corrosion found on these metals is not formed by electrolytic action and therefore the process cannot be reversed electrolytically, requiring more conventional cleaning processes.

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 A Power Supply Suggestion 

The idea of constructing a power supply could seem a daunting task for anyone not familiar with electronics, but for those interested, capable or who know someone experienced in such matters, I'll outline the details of such a supply in order that one could be built.

A circuit diagram of a power supply for rust electrolysis

Diagram of a suitable power supply

 

C1 , C2 2,200uF electrolytics for smoothing.
Using two will half the ripple current on each.
C3 0.1uF decoupling for stability
C4 0.1uf decoupling for stability
R1 820 ohms
R2 , R3 , R4 Current limit resistors as required:
1 ohm = approx. 500mA
5 ohms = approx. 100 mA
VR1 10k variable resistor for voltage setting.
SW1 4 pole switch for selecting current.

This circuit represents a simple power supply built around the L200 voltage regulator chip available from most electronic suppliers, and many other regulator chips will do the same job. The L200 features an internal maximum current limit of 2 amps which will be more than enough for even fairly large pieces, and lower currents can be selected when required according to the values of the appropriate resistors, values for these depending on individual requirements and guided by the table above. Slower is definitely better for rust electrolysis, and my supply has a 100mA limit on its lowest setting, with 500mA and 1 amp as intermediate settings in addition to the maximum output of 2 amps.

Regarding the circuit diagram, R4 is permanently connected and when the switch is in position 1, is set to offer the lowest required current. Position 4 bypasses any limiting resistances and invokes the internal limit which is the maximum of 2 amps, whilst positions 2 and 3 are intended to offer intermediate current settings according to the resistances used, although it is entirely possible to incorporate more than the 2 intermediate settings indicated by using an appropriate switch, it is simply a matter of personal choice. A regulated voltage output can also be set using the variable resistor if needed and this will make for a more versatile piece of equipment, although being able to vary the regulated voltage is not necessary, and a fixed resistor of a value determined by experiment could be hard wired in to produce a voltage of around 10 - 15 volts.

The regulator chip itself will need to be mounted on a decent heatsink to ensure it operates at a sensible temperature. A transformer and rectifier assembly must be built in order to supply the above circuit with direct current at an appropriate voltage, and a transformer with a secondary winding of 12 to 15 volts at a rating of 50VA should be suitable, and a bridge rectifier rated at 5 amps is suggested. Obviously, high voltage components such as the mains transformer should only be fitted and connected up by a competent person, as the voltages involved are lethal. The inclusion of an ammeter is a luxury, but can give a useful indication of the actual current flowing as this may not always be at the current limit set, and such a condition may indicate poor connections to the piece.

The circuit layout is not critical and it can be constructed on a single sided copper laminate board or using a tag strip, but it is important that the decoupling capacitors C3 and C4 are fitted as close to the regulator's pins as possible to avoid instability problems, and that adequate heatsinking is employed. The case for this project can be almost anything the components will physically fit into as long as there are plenty of ventilation holes to keep the internal temperatures from climbing, and any high voltage points are shielded against being inadvertently being touched by inquisitive fingers or a probing screwdriver. If a metal enclosure is chosen, I suggest the case should be earthed.

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