Electrolysis

A while ago I posted about some beat up old planes I had been given, and how I was trying out a de-rusting method that I had not used before. I said that I would write a post about it, giving more details, and so, true to my word, here is that post.

The process is Electrolytic Rust Removal, or Electrolysis and it is a bit like reverse electro-plating. You will need to look elsewhere for a thorough scientific explanation of the process, but here is the basics. The apparatus consists of a sacrificial piece of metal (anode) and your rusty workpiece (cathode) suspended in an electrolytic solution and connected to a power source. It is vitally important that the anode be connected to the positive and the workpiece to the negative. An electric current is then passed between the anode and cathode through the solution, producing an exchange of ions between them, the practical upshot of which is that the rust flakes off the workpiece and the anodes corrode.

In order to try out the process for yourself, you will need:

  • an electrolytic solution (water and washing soda)
  • sacrificial anode (iron, steel or graphite – not stainless steel)
  • a power supply (a car battery charger)

20150402_160901The first step is to set up your sacrificial anodes and electrolyte. I used a plasterers bucket from the hardware store. I drilled some holes around the top of the bucket and poked some lengths of bent iron rebar down through the holes so that they hung down the inside of the bucket, leaving a small amount poking out at the top. I used 8 anodes in all, connected to each other with copper wire stripped from 6mm twin & earth cable. While one anode would do, it helps if the workpiece is surrounded by them, so more than one is preferable.

I then filled the bucket with 5 gallons of water and mixed in 5 tablespoons of washing soda, to make my electrolytic solution. My workpiece (cathode) – the plane body in this case – is then suspended in the solution, making sure that there is no physical contact between it and the anodes. To do this, I simply wrapped some more copper wire through the mouth of the plane and then looped the wire though a stout piece of wood. The wood sat on top of the bucket and the plane body hung below. I left a small length of the copper wire protruding so that it could be connected to the power supply.

Finally, it is time to hook up the power. The sacrificial anode is connected to the positive end, and the workpiece is connected to the negative end. THIS IS VITALLY IMPORTANT FOR THE PROCESS TO WORK. If they are connected the other way round then the workpiece will become the anode and it will become further corroded.

Once the power supply is switched on, you should notice bubbles forming in the solution almost immediately. After a little white, the water will become brown and disgusting, although this will not reduce its efficacy. How long you leave the system running obviously depends on how badly corroded the workpiece is, but it might be necessary to leave things running for a few hours to get the a decent result. I left mine running for two hours and this was the result:

Not to bad. You can see that where the plane body was submersed the rust has all gone, but where it was exposed, the rust remains. When the workpiece comes out of the solution it is coated with a black film that can easily be removed with water and a scouring pad. The bare iron is left with a patina that can be left or polished out, but the rust is all gone.

The great thing about this de-rusting method is that it is not caustic or aggressive in any way. The actual metal of the workpiece is not touched by it, only the corrosion is affected. Obviously, any pitting or other damage cause by the corrosion will remain, but the process will not touch or affect sound metal.

I should point out that I am not an expert in this process – this is, after all, the first time I have tried it. There are plenty of good resources online if you want to know more about the process before trying it yourself. However, I can attest that it does indeed work, and I will be doing more of it in the future.

Some other things to remember:
  • The solution emits a small amount of hydrogen, which is explosive, so it is best not to carry out the process in a confined space.
  • The anodes will corrode and the more corroded they get the less effective the system will be. It is worth renewing them when they get too corroded.
  • Apparently the solution will last almost indefinitely, but may need topping up with water now and again to counter water loss through evaporation. If you do dispose of it, and have use a steel or iron anode, the solution is relatively harmless and can be poured onto your lawn. The iron rich solution is apparently quite good for grass!
  • Stainless steel contains chromium, which can be toxic. A stainless steel anode will last a long time but will produce toxic by-products. DO NOT USE.
  • After removing the workpiece from the solution, it is important to clean and dry it immediately, before protecting it with oil or some kind of sealant. The workpiece will be prone to corrosion until it is sealed or protected in some way.
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