Rust Removal by Electrolysis
Introduction.
The rusting of iron is an electrochemical process that requires the presence of water, oxygen and an electrolyte. Since this metal does not form a passivating coating like aluminum or copper given time it will continue to rust until a complete conversion occurs. There are 16 known iron oxides and hydroxides. Rust is primarily hydrated iron(III) oxide, or ferric oxide - Fe2O3. The process of rust formation is highly complex, dependent on the various levels of available required elements and the contaminants that might be present.
Water can be provided by moisture suspended in air, a relative humidity of 50% is enough to cause rusting to occur. A relative humidity of 80% is enough to cause severe rust to form.
Oxygen is provided by that in the air, and while a lower level available will cause a different oxide to form, iron will still rust.
The presence of an electrolyte is needed to provide a conducting pathway for current flow. This is usually iron(II) sulphate formed by atmospheric SO2 or sulfur dioxide but can also be airborne salts if near saltwater.
One way to remove rust from iron is with a reversing electrochemical process, using electrolysis in a conductive solution with a sacrificial metal present. By passing a DC current through the solution in the proper direction the ferric oxide can be converted to a different oxide and stripped from the parent metal.
Items Needed.
Plastic or other non-conductive container large enough to hold the part or parts to be cleaned. The ‘tub’ should be strong enough to hold the solution and parts without deflecting. I use a 2 pound coffee can, a 12 quart plastic tub, and the bottom third of a 40 gallon plastic barrel depending on what I would like to clean.
Water. Distilled water is preferred if the item to be cleaned is a priceless antique, but tap water works for most applications. Just be aware that very hard water can affect the process and plan accordingly.
Washing soda or sodium carbonate. This is used to make the water conductive. Other chemicals can be used, but most have their own inherent problems. Washing soda works well and adds no contaminants to the metal. Table salt could be used for instance but one byproduct would be chlorine gas, not a good thing for your health.
DC power supply with short protection on the output. High current levels are not required, a few hundred milliamps is all that is required. A battery charger can be used by adding an appropriate resistor or light bulb in series to limit the current.
Sacrificial steel anodes. Scrap steel should be used for the anode(s). Try to target at least as much surface area as the part to be cleaned, and arranged to surround the part or parts. This will give the best results although less sacrificial metal will work and the parts can be turned in the solution to provide consistent cleaning. If you have a ready source of graphite it will work nicely and leave the solution much cleaner, but buying it outright can get expensive. Remember that it is a sacrificial piece and spend your money accordingly.
Steel wire to tie the anodes together, and to provide connection to the part or parts. I use baling wire to provide a current path out of the solution and it works well. You may well have to clean a small spot by hand to get a good electrical connection to the part. Be sure to allow enough wire to make connection with the power supply clip leads above the solution, or the clips will become a part of the process and be de-plated or rusted. Using copper wire will turn the solution an interesting aqua color and proceed to crudely plate the part with copper. DO NOT use stainless wire or any stainless in the solution. Trivalent chromium is not terribly bad, but hexavalent chromium is not something you want to breath or ingest and is very difficult to get rid of safely. In most states it is illegal to improperly dispose of hexavalent chromium.
Measuring spoon or cup. You should aim for about a 1% solution, or roughly 1/2 cup of washing soda in 5 gallons of water. Adjust as needed for the size of the container. More does not help the process work faster but less will slow down results.
Rubber gloves, apron and face shield. If you are working with one small part in a two pound coffee can then things are fairly easy to control, but if the part is a car frame in a swimming pool then protection may well be advised. Remember that the solution is caustic so any splashed in the eyes requires immediate flushing and if needed medical attention. Rubber gloves will stop the irritation of the solution and leave your hands far cleaner at the end of the day.
Paper towels, cardboard, old newspapers. When removing the part from the solution it is wet and caustic. A paper towel to start drying it and something to lay it on is a good idea. I prefer to use a bucket of clear water to first rinse the part then start drying it immediately.
Hair dryer, heat lamp or sunshine. Just after rinsing the part is the time to start drying it. The part when clean is bare metal and will start to rust very quickly. A hair dryer or some form of heat will help a lot in quickly drying the part and controlling re-rusting. This is especially important on parts with intricate nooks and crannies where rust can start unseen.
Oil, grease, wax or primer. Since the metal is very clean it can start to rust in minutes. Adding some form of surface protection very quickly will help. This is especially true if you are working with multiple parts. Do not pull them all from the solution at once then try and dry and seal them at the same time. Instead pull them one at a time and after drying that one part and sealing it go to the next.
Hazards
Explosion or fire is possible if the process is carried out in an enclosed space. Oxygen and hydrogen gases are generated, so don’t seal the container and allow for some ventilation. The anode or sacrificial plate with the positive lead generates oxygen and the cathode or part with the negative lead generates hydrogen.
A shock danger exists. The solution is electrically live while the process is active so care must be taken to avoid the solution while the power is on.
Toxic gas danger. Don’t use stainless in any form in the process.
Keep children and pets away.
Do not clip or unclip the leads while power is on. The resulting sparks might start a fire. Do not smoke around the process while it is active. Be sure there are no sources of open flame or extremely hot surfaces in the immediate area.
Do not allow the parts to touch the sacrificial anodes. At this point you will test the current limiting of your power supply. If you bump the parts or anodes and the current goes up, check to see that the two poles are not touching.
Do not use small bench clip leads to attach the power supply to the devise. Be sure to use large enough wire and clips to safely carry the current.
If after running for some time the solution is hot or actually steaming you may be running too much current or have a short from anode to cathode. Be cautious of hot caustic solution splashing on you while checking it out.
Setting Up.
Choose an area to work in where normal traffic will not be impeded. Remember that this process can take days and plan accordingly.
Clean a large enough work space to safely hold the materials needed and to comfortably work in. Clutter can spell disaster if things are spilled.
Protect the area below the tub from potential splashing or spills. The solution is caustic.
Clean the sacrificial anode(s) of any dirt, grease, paint or rust. The cleaner they are the less contaminants end up in your solution and the faster the operation will progress.
Join the anodes together with the steel wire so that they can be supported in the tub with a spacing that tends to surround the part(s). One approach is to hang the parts to be cleaned from wire suspended in the tub at which point you can also have an anode in the bottom of the tub. Be sure that these anodes are secure from moving or falling. Leave enough wire lead to be able to add the positive lead outside the solution.
Clean the part or parts of dirt and grease, it will leave the solution much cleaner and hasten the process.
Add a steel wire to the parts to be cleaned, remembering that some rust might have to be sanded or scraped away to insure good electrical contact. Also if you are cleaning a part made up of more than one piece do not depend on mechanical connections to provide current flow throughout. You may have to add multiple wires to the various pieces to get them all to join in the process.
Mix the solution in the proper ratio. If you are making a large batch for your cleaning it might be easier to mix the proper amount in a small container first to insure completely dissolving the washing soda then adding the mixed batch to the container of water.
With the sacrificial anodes in place and the parts in the tub hook the positive lead from the power supply to the anode and the negative lead to the cathode. Insure that they are not touching and apply power. Be sure not to reverse these leads or you will commence to clean the sacrificial metal and only further rust the parts you wanted to clean.
With power on check to see that your voltage and current are in the range you expected, if not check for a short between cathode and anode or a loose connection. Soon bubbles will begin to form on both the anode and cathode so be sure to check to see that all parts are showing bubbles.
BE PATIENT.
A very lightly rusted part might be clean in a couple of hours, but most parts will take a day or two and very heavily rusted pieces can take a week or two.
Once the ferric oxide is broken down and the part wiped and dried you will be left with a dark layer of magnetite or Fe3O4. This layer is fairly easily removed but gloves are highly recommended. One use of magnetite is in pigments and dyes and it will leave just about everything it touches a sooty black that doesn’t want to rub off. A scotch brite pad or soft wire brush will usually remove the magnetite. If the magnetite is still strongly adhering to the iron then a while longer in the solution at low current will generally break it down.
After getting the part dry and clean remember to seal it with something to preclude rust from forming again any time soon.
NOTE:
Baking soda versus washing soda.
Baking soda will actually work to make the solution conductive, but changes the way the process works on the anodes. With washing soda the loose rust layer that forms on the anode can easily be brushed or scraped off. When using baking soda this layer formed is a fine grained tight rust that must be taken off with a wire brush or the process will slow and eventually stop.
If all you can find is baking soda then I would recommend spreading it out on a cookie tin and baking it at 300 degrees or so for about an hour. This will drive off the water and carbon dioxide and convert it to washing soda.