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Electrolytic Rust Removal By Dr David Maddison Got something old and rusted lying about that you’d rather have old and not-so-rusty? We cannot promise miracles but you might be surprised how easy it is to restore rusted items to their (somewhat!) former glory! T here are plenty of objects around most homes that would benefit from rust removal, for example old tools, old car or engine parts for use in restorations, components of antique electrical items (eg, a rusted battery holder), garden furniture and many other items. And there are many methods and products to remove rust, which have various levels of success. Common methods include me- chanical techniques in which rust is removed by the abrasive action of sandpaper, a wire brush or high speed particles from a sandblaster; or chemical techniques in which rust is removed or converted to something more stable by a chemical agent such as an acid. All these methods have some disadvantages, such as the removal of good metal – not just the rust – or the conversion (but not removal) of the rust This rusty mint tin was found by the side of the road. It’s not in the best of condition . . . siliconchip.com.au to something more stable, thus losing information as to the original shape of the object and obliterating features such as markings in the original metal. In addition, proprietary chemicals for rust removal or conversion can be expensive and somewhat toxic. The aim is to restore the artefact to as close to original condition as possible, not just make it look pretty! Another promising method of rust removal that has increasing popularity . . . but after treatment, came out quite well. The paint was removed but that’s usual with electrolytic rust removal. October 2014  27 APPLICABILITY OF METHOD While this method has been widely tested by numerous people and in various places, as with any new procedure, before using it on an important object, you should test the process in your particular circumstances and with your own setup on a test part for which it does not matter if the process does not work to expectations. This article deals specifically with mild steel items that have rusted in air (due to the action of humidity or rain) or have been submerged in fresh water or have been buried in soil. A slightly different and more sophisticated method is required for the restoration of items that have been submerged in seawater that does not seem to affect the original metal is to use molasses; however this takes a very long time. Interested readers may wish to Google this. Electrolysis Apart from the above methods, there is another way. It involves the use of electrolysis to remove rust. This method only removes rust and leaves the base (un-rusted) metal intact. It may also reveal many details in the original metal such as patterns or stampings of numbers and letters which seem completely obliterated by the rust, which may not be preserved using other methods. This method has also been known to restore items with moving parts which were rusted solid and thought to be completely beyond hope of restoration to working condition. The method is simple and inexpensive and involves only household items, apart from a low-voltage power supply or maybe a car battery charger, which most SILICON CHIP readers would likely own anyway. The details of the method will be discussed in detail later but in essence, a container holding an aqueous solution of a common chemical is set up with two electrodes and a power supply, thus establishing an electrolytic cell. One electrode, the anode, is made of mild steel and is gradually sacrificed in the process. The other electrode is the iron/steel piece under restoration. Note that rust damage is permanent. This method will remove rust for long periods. Since such artefacts (eg, ancient cannons and anchors such as from the HMB Endeavour) are likely to be in the realm of marine archaeology, this topic will not be discussed here. Suffice to say, with such artefacts it becomes very important to remove the chloride ions from the rusted artefact which takes a long time and the chloride levels coming out of the artefact need to be monitored until there are no more. In addition, a different electrolyte chemical is used in the case of objects rusted by salt water – for example sodium hydroxide instead of sodium carbonate. but no method will return original metal that has disappeared. It is best to store and maintain tools and other items in an appropriate way to prevent them rusting in the first place. What is rust? Before discussing the electrolysis process it is necessary to first discuss the nature of rust. Put simply, rust is iron metal combined with oxygen to form chemical compounds known as various forms of iron oxide. The chemical reaction by which rusting occurs is an electrochemical one, since the flow of an electric current is involved. Iron loses electrons and both oxygen and water take up electrons and iron metal chemically combines with oxygen. The rusting of iron is said to be an oxidation reaction. In effect, this process forms a type of galvanic cell. In the process to be described, the objective is to reverse this rusting process (to the extent possible via an electrolytic process) via the opposite of the oxidation reaction, called the reduction reaction. An electrolytic process is the opposite of an electrochemical one. In an electrochemical process, reactions are spontaneous (eg, rusting) but in an electrolytic one, reactions only occur when energy in the form of an electric current is supplied. There are several forms and colours of iron oxide, depending upon the conditions of formation. To form rust, iron or steel requires contact with both oxygen and water and if salts or salt spray is present, rusting occurs much more quickly. Red rust The most destructive type of rust is the loose, poorly adherent, flaky, porous material which occurs on severely rusted items. This rusted material has a significantly greater volume than the original metal from which it is derived. When it happens in a constrained space, such as with reinforcing bars set in concrete, its expansion causes destruction of the surrounding material (a condition commonly known as “concrete cancer”). The rusting of this material is nonreversible and represents a complete and permanent loss of the original metal. This type of corrosion is often called “red rust” and generally has the chemical formula Fe2O3·nH2O (hydrated ferric oxide) when the process has occurred in the presence of oxygen and moisture. This material has minimal or no electrical conductivity. The non-hydrated form of this compound is the mineral haematite, from which iron is originally derived. Because of the limited electrical conductivity of this type of rust it is mainly unaffected by the electrolytic process in terms of reducing it back to iron. However, because it is porous the electrolyte solution will generally go through it and conduct electric This rusty nut and bolt combination was first processed as one piece as one nut had seized. After a period of electrolysis the nut could be removed and the pieces were processed separately. 28  Silicon Chip siliconchip.com.au (connected to the positive terminal). At the cathode, which is the rusty object to be restored, it is hoped that as much of the oxidised iron as possible is reduced to iron metal. As explained above, not all rust (such as most of the flaky red rust) can be reduced in this way (although some may be reduced to black rust) but the black rust, between the original metal and the red rust, mostly can be. What can be reduced back to metal will be although this will mostly be poorly adherent as will most of the This horseshoe was buried in the ground for at least 90 years. It cleaned up very rust remaining after treatment and it nicely. If the same horsehoe had been in seawater even for a relatively short time, will fall away or can be brushed away. treatment would be very much different and probably not as successful. As pure water does not conduct current to the base metal. This is sig- original metal. electricity to any significant degree, nificant and will be discussed later. It may be possible to revert this ma- an electrolyte chemical is added to When the rust is not so severe that terial back to unoxidised (unrusted) the water to make it conduct. The loose flakes of rust are generated, this metal by the electrolytic process de- electrolyte chemical is chosen on same type of red rust may still be pre- scribed here. This reaction to convert the basis that it does not itself have sent in the form of small “islands” of the oxide back to metal involves the adverse reactions that interfere with rust, or as a thin continuous coating iron atom gaining electrons and los- the process. all over an object. ing oxygen. Washing soda, also known as While not necessarily being soda ash or sodium carbonate, strongly adherent, it can still be Na2CO3 is most commonly used difficult to remove. in this process and is the basis This is possibly the most of this description. familiar form of rust and may It is safer to handle than the As a general rule, antique objects are often signifibe present on tools or other obother frequently used electrocantly devalued by attempts to clean them, including jects that have been left in the lyte, sodium hydroxide, NaOH, the removal of rust. open but not for so long that the which is often used with restoCollectors appreciate the original “patina” of an aforementioned type of rust can ration of marine artefacts. object so if you have a genuine antique, you should develop. Oxygen gas is evolved at the consult an appropriately qualified antique expert as sacrificial anode and hydrogen to whether the object should be cleaned or left as-is. Black rust gas is evolved at the work piece Our experience suggests the latter. Beneath the red rust and (cathode). above the original uncorroded As hydrogen is inflammable This black rust is electrically con- and both gases together are explosive, metal is usually a layer of “black rust” which has the chemical formula ducting and magnetic and is also the container in which electrolysis Fe3O4 (iron(II,III) oxide or more com- known as the mineral magnetite. is done must be wide-open and the monly ferrous-ferric oxide) and unprocedure must be done in a welllike red rust, this form of rust does not The electrolytic process ventilated, open area free of ignition During the electrolytic process, sources. occupy a significantly greater volume than the base metal from which it was a power supply causes a current to The formation of hydrogen gas derived. Therefore it is adherent and be passed via an electrolyte (a solu- bubbles within the rust layer also tion which will conduct electricity) helps to mechanically clean the part does not flake. This layer is somewhat protective between the item under restoration as the expanding gas bubbles within and may preserve some details such (the cathode, connected to the nega- the porous rust cause pieces of rust as decorative stamp marks from the tive terminal) and a sacrificial anode to be pushed off. Should you remove rust from antiques? (Above): A heavily rusted item (railway spike), shown at right being processed. Note the evolution of gas from the item. During electrolysis rust flakes fall away from the object as shown. This item did not start to electrolyse at first, so the voltage was increased to break whatever barrier was causing the high resistance that was preventing electrolysis and then the voltage was lowered. siliconchip.com.au October 2014  29 Washing soda (sodium carbonate) as used for the electrolyte. It is available in the laundry section of most supermarkets; swimming pool ‘pH increaser’ from pool shops or hardware stores can also be used. A plot of current versus voltage for one sample showing how the current starts to rapidly rise at a certain point corresponding to a rapid increase in the chemical reactions occurring. The reactions include the reduction of various iron oxides and gas evolution corresponding to the electrolysis of water. The current decreases as any new voltage increment is adjusted to so these currents are recorded at the moment that the new voltage is set. As the sodium carbonate is not significantly consumed in the process the solution can last indefinitely; it only being necessary to top up the water lost due to evaporation or gas formation. However, sooner or later the solution will become brown with rust and will need to be replaced. While the solution is not especially toxic if only mild steel has been used in both the cathode and the anode, it is wise to consider where to dispose of it. (Some plants might not like an iron rich soil, for example.) Experiment with the process to see what you can do. For example, for long parts like saw blades you can use PVC plumbing pipe as a container. Or for cleaning in tight places like an engine block you could try putting a wire anode inside a hose with lots of small holes in it or a permeable drip hose to get the electrode close to the work part. In such cases pay extreme care to the build up of oxygen and hydrogen in confined places and take appropriate precautions. For parts that cannot be submerged, perhaps a sponge soaked in electrolyte can be used to make contact with the part to be cleaned. The sacrificial anode The sacrificial anode should be made of mild steel and should surround (but not touch) the object under restoration to ensure that the electric field on the object is evenly applied. The sacrificial anode is eventually dissolved so make sure it is a scrap piece. Platinum (extremely expensive) or graphite can also be used for the anode and will not dissolve like mild steel. While certain grades of stainless steel can theoretically be used for the anode, they should be avoided because of the possibility of highly toxic and carcinogenic chromium ions being released into the solution. Part of the anode should be above the liquid level and this is where the electrical connection to it is made. If it is made within the liquid the connector will be destroyed. The metal of the sacrificial anode should not be coated. Commonly available materials that are suitable include scrap uncoated sheet metal from car body work and uncoated steel reinforcing bar and mesh. A scrap piece of rebar mesh was used for this story and was obtained free from a nearby building site (with permission!). The mesh (shown in photo opposite) was galvanised so the coating was removed below the electrolyte level. Most rebar you see on building sites is uncoated and has that characteristic red rust colour from being exposed to the elements for a while. SAFETY AND OTHER PRECAUTIONS Both hydrogen and oxygen gas is generated with this procedure. In combination, these gases can be explosive. Also, oxygen by itself enhances combustion. This procedure MUST be conducted in a well-ventilated area, away from sources of spark or flame (eg, don’t smoke anywhere nearby!). Turn the power supply off first before connecting or disconnecting wires to avoid sparks. For the sacrificial anode, only use mild steel (or graphite or platinum if you can afford it!). Do not use stainless steel as toxic chromium will go into the solution, even from 316 grade under certain conditions. This also creates a disposal problem. Do not 30  Silicon Chip allow the liquid solution to splash on the power supply or other electrical devices. As with all chemical processes, use safety glasses and latex or other protective gloves as per good practice. Do not use this process if you are not comfortable with using electricity or chemicals or are not confident or understanding of what you are doing. If it is a valuable or rare part you are treating, try the process on a non-valuable item first to ensure the process works to your satisfaction. Make sure the correct polarity is used as the incorrect polarity will destroy the part you are trying to treat. siliconchip.com.au The setup (left), including a look inside ‘The Bucket’ (above). Note how the sacrificial anode surrounds any object to be treated. The water becomes rust-coloured as shown. The black wire is for attachment to the object under restoration (although there is no item shown here) and the red wire goes to the sacrificial anode. To form the rebar mesh into a cylindrical form it was found to be convenient to place it diagonally across a stone gutter and strike it with a heavy club hammer in the unsupported area, gradually moving along the length of the piece. Doing a bit at a time, the flat mesh was gradually transformed into a cylindrical electrode. Of course, thin sheet metal would be much easier to shape. Coated steel can be used as a sacrificial anode but make sure to sand off the protective coating first. The anode is where the process of oxidation takes place (electrons are removed from the chemical structure). Electrical connection to the item under restoration It is important that a good electrical connection is established to the item under restoration. For a severely rusted item with a thick layer of insulating rust it may be necessary to scrape or grind away some rust to get down to original metal with which a good electrical connection can be made. Alternatively a powerful, good quality alligator/crocodile clip could be used if it has enough ‘bite’ to get through the rust layer. It may have to be moved side to side to help it ‘dig’ through the rust to establish a good connection. Another possibility is to fabricate some type of clamp with sharpened siliconchip.com.au contact points that, when tightened, will go through the rust layer and make contact with the base metal. You can test how good the connection is with a multimeter. The electrolyte The concentration of the electrolyte solution is not critical but a 5 to 10% solution by weight seems to work well. The percentage by weight for a certain volume of solution is determined by the mass of the solute (matter to be dissolved) in grams divided by the volume of solution in ml, all times 100. To have one litre of a 10% solution of washing soda (Na2CO3) it would be necessary to have 100 grams of the soda dissolved in one litre of solution. To make such a solution, one would weigh out 100g and dissolve this in slightly less than one litre of water. To facilitate the dissolving of the soda it would be best to use warm water. Once the solution is fully dissolved, or at least is dissolved as it can be (since not all the material may dis- Correct polarity is VITAL! We can’t repeat this too many times: POSITIVE goes to sacrificial anode, NEGATIVE goes to item to be restored. It is critical that you get the polarity correct, otherwise the object under restoration will be dissolved! Always test your set up with a non-valuable piece first. solve), water can be added to bring the solution to one litre. If you do not have a method of directly measuring one litre you can determine the volume of 1 litre by pouring 1 kg of water (1 litre) into a container and marking the level on the side of the container. This will then be your 1 litre measuring vessel. (Note that this procedure is NOT the same as just adding 1 litre of water to 100 grams of the soda, a common mistake). You must use protective gloves and eyewear when making and using the solution as, although washing soda is a household chemical, it is still alkaline and possibly harmful to the eyes. The power supply A 12V car battery charger is commonly used for this process but this doesn’t allow any form of current/ voltage control. For a little more convenience you could use an adjustable bench power supply. A standard car battery charger is sometimes used but has the disadvantage that very high currents may be drawn due to the highly conducting nature of the electrolyte solution. High currents may be too high for the de-rusting process and further, these chargers may not be designed to have high currents drawn for the long periods of time this process may take. For these reasons, we do not recommend the use of a car battery charger for this process. October 2014  31 REQUIRED MATERIALS +VE CONNECTION TO REBAR (ANODE) Here’s the setup we generally use for small to medium size jobs. The rebar mesh must be bright steel (ie, not zinc plated). SACRIFICIAL ANODE MADE FROM MILD STEEL (EG, REBAR MESH) A plastic bucket or container large enough to contain the item(s) to be restored. A sacrificial anode made of uncoated mild steel which preferably surrounds the part to be restored. Graphite or platinum is also suitable but platinum is extremely expensive. Washing soda powder (sodium carbonate) as distinct from baking soda. A 1kg packet costs around $4 in supermarkets. A popular brand is Lectric. What is not used will not be wasted as there are numerous household uses for this chemical (see manufacturer’s website). This chemical can also be purchased as a swimming pool pH increaser (to make the water more alkaline and less acidic) for about $10 for 2.5kg. A power supply capable of at least 12V (up to 20V or so is better) and ideally up to 2 or 3A capacity, although lower capacity will suffice. It is preferable that the supply has a variable output voltage, as is the capability to measure voltage and current. The power supply used for this article was a Manson NP-9615 with an output of 0 to 30V at 5A and built in current and voltage meters but a much less capable supply will be suitable. A car battery charger is used by some people but is not recommended. Two polarised leads with clips (ie, red and black) to connect the power supply to the electrodes. Rubber gloves and safety glasses. Scales with which to weigh out the required amount of washing soda. A way to measure the volume of water (or weigh it – remember that one litre of water weighs one kilogram). And last of all . . .   the rusty part(s) to be restored! 32  Silicon Chip PLASTIC BUCKET VOLTS DC – POWER SUPPLY ELECTROLYTE (TO COVER WORK) AMPS DC RUSTED OBJECT UNDER RESTORATION + –VE CONNECTION TO WORK (CATHODE) The procedure 1. To start, choose an appropriate size plastic vessel (such as a water bucket) and install in it an uncoated mild steel sacrificial anode. 2. Make up a 5 or 10% weight-byvolume solution of electrolyte as described above and pour into the container. Normally you would want to ensure there is enough to cover the object to be restored but in some cases it might only be necessary to treat part of an object, so the part not needing treatment can be above the solution.   Some people have reported success with much less concentrated solutions such as ‘a (US) tablespoon to a (US) gallon of water’. You may wish to try lower concentrations to see if that works with the particular artefacts that you want to restore.    The actual concentration is not likely to be critical, as long as the amount of electrolyte makes the water conduct electricity well.   Note also that any paint on an object will likely be damaged or even removed by this process as the solution softens it. Any paint remnants can be removed with paint remover if desired or even before treatment if the paint layer is particularly robust. 3. Make sure loose dirt and rust is brushed off from the item to be restored and that any oil or grease is removed with a degreaser. 4. Attach the positive wire to the sacrificial anode and the negative wire to the work piece (ie, the rusty piece being restored).   The clip attached to the work piece will not be significantly damaged but the one attached to the PLYWOOD PLATE TO HOLD REBAR IN PLACE (IF REQUIRED) sacrificial anode will likely dissolve if it is not connected above the electrolyte level. 5. The item to be restored is placed in the cell and the power supply turned on to about 12 volts. Gas evolution should be noticed on both the sacrificial anode and the item under restoration – the time for this to happen depends a lot on the voltage/current applied. 6. Periodically, items should be removed and examined. Turn the power supply off before removing the item as sparks could ignite gases present.    Each time an object is removed from the solution (wear rubber gloves and safety eyeglasses) it should be washed under cold water and loose rust brushed off with a nylon or brass brush. A sharp instrument may be needed to get material out of crevices. Notes • This article primarily refers to rust on mild steel and other low alloy steels only and was not tested or intended to be used on stainless steel (some grades of which do rust) or other metals such as copper, brass and aluminium.   If you want to use this process on other materials, do your own tests. The amount of time the process takes depends on many factors but can take as little as tens of minutes to many hours or even days for heavily rusted items with deep recesses, such as nuts on bolts. • When an item is removed and replaced it is important that the position of the electrode clip is moved siliconchip.com.au Providing a drill bit is still in reasonable state (ie, not too rusted or flaking), it can be restored to working condition. • • • • and also that the restoration object is placed in a variety of positions to ensure all areas are treated and none are shielded. As red rust is electrically insulating the number of bubble sites on heavily rusted items may be relatively few at first. However, since red rust is also porous, as the electrolyte penetrates to the more conducting layers gas evolution will start to occur. The bubbles expanding within the porous red rust help to remove it. For very heavily rusted items it was found that the current and number of bubbling sites increased and then eventually the current decreased as the process moved to completion. Sometimes, on heavily rusted objects, no significant gas evolution would happen for a long time. It was found that the process could be facilitated by increasing the voltage to a point that gas evolution became apparent.   After a while the voltage could be reduced but gas evolution would continue, as the impediment to current flow had been cleared. There is a critical voltage at which the appropriate electrochemical reactions occur. But without using a special reference electrode such as a ‘standard hydrogen electrode’ (SHE) this voltage needs to be guessed.   It is the point at which significant gas evolution occurs and the current starts to increase rapidly. The voltage should not be set so high that there is extremely vigorous gas evolution. Generally 12V is enough but as mentioned above, it is sometimes advantageous to go beyond this in order to get the process started more quickly if there is a high initial resistance.   (A DC voltage of 12 to around 30 volts is more than adequate for any conceivable cleaning operation). NEVER attempt to connect to mains • • • • • • • voltage – (a) it would be extremely dangerous and (b) being AC, it wouldn’t work! Concerning the optimal current to use (obtained by varying the voltage) it is not critical but it should be set to a level where there is a somewhat vigorous stream of bubbles coming from the artefact. In professional restoration of marine artefacts current densities in the range of anything from 0.001-0.1A per square centimetre of object area have been successfully used. Don’t let the rusted item touch the sacrificial anode or you will short out the power supply. Make sure bubbles come from all areas on the part. With multipart objects a failure for bubbles to come from one part may indicate a lack of electrical connection between the parts. Applying another connection to such parts should rectify the problem. If the object has some type of “inert” plating such as chrome, as long as it is soundly attached, it should not generally be affected. However, if the plating is not sound it may be removed. Be sure to test a small part of the piece first, leaving most of the piece out of solution. Interestingly, it was noticed how, even with the power supply turned off a small voltage was present across the cell. This is because the materials are acting as a galvanic cell. The process is quite tolerant (except if the wrong electrical polarity is used!) and there will be no harm from leaving it running for too long a time. Good metal will not be adversely affected, however due to safety considerations the process should not be left unattended. Do not turn off the power and leave the item under restoration in the electrolyte solution for an extended length of time as it will start to rust again. Whilst a current is flowing the object will not rust (as long as there is sacrificial anode still present). • To give maximum coverage it is best to suspend the articles in the solution. • Multiple parts can be done at once with multiple parallel electrical connections. • If you have a really large object to clean, be creative with your choice of container, eg, kids’ inflatable swimming pools (but don’t repurpose for use by kids after the job!), or dig a hole in the ground and line with plastic etc. After the process has finished The process may be regarded as complete at the point that no more rust can be brushed from the object and the current generally drops to a much lower level than at its peak and also remains steady. After the process is complete, remove the item (turn the power supply off first) and brush it rigorously with a plastic or brass brush under running water. Use rubber gloves and safety eyewear. It should then be immediately sprayed or soaked in methylated spirits (warning – flammable) to remove residual moisture. Methylated spirits should then be wiped off so that none remains and then the item should be put near a source of heat such as a blow heater or hair dryer to thoroughly dry it. After thorough drying the item can then be treated as required. It could be sprayed with a moisture displacing oil such as WD40 and regularly recoated, for example, or primed and painted or just left the way it is although it will again develop a surface layer of rust. There is no single answer as to the best treatment as it will vary widely according to conditions. SC Similarly, this “shifter” (which wouldn’t shift – it was rusted closed!) is now bright, shiny . . . and completely usable! siliconchip.com.au October 2014  33