Rust removal by electrolysis
I have found that mechanical methods of rust removal damage the metal and are hard work. I found that using diluted orthophosphoric acid did convert the rust but without other salts didn't offer a long term rust free life. Commercial products based on orthophosphoric acid do work better but are expensive. I have tried one of the expensive organic rust killers and wasn't impressed. On the whole I was not satisfied with these chemical methods despite being trained as a chemical engineer!
Browsing the web I found a number of sites such as;
which described rust removal using electrolysis. The basic idea is to use scrap iron or steel as the anode and the article to be de-rusted as the cathode in an alkaline bath and pass DC through it. The rust is converted into iron which form a loose powder on the surface of the article and any paint will lift off. These residues can be wire brushed off easily leaving a grey, reactive metal surface. The beauty of the method is the article is not damaged by prolonged de-rusting.
I used either a scrap cast iron clutch pressure plate or a length of 25mm steel angle as anodes and the bath was a solution of Mangers Sugar Soap which I guess is trisodium phosphate. The quantity is not critical, all that is needed is a conductive solution. I put a couple of generous tablespoonfuls into about 20 litres of water. I used a home built power supply which gave about 4 to 8 amps at 20volts DC. The current depends mainly on the surface area of the electrodes. The solution gets very dirty with red rust which will stain T shirts etc (How do I know this?). I used a couple of containers; a plastic trug borrowed from my other half and a tall container made from a length of 100mm diameter plastic sewer pipe closed at the bottom with a stopped end fitting.
I have cleaned the suspension tie rods and cups, the front hubs and the steering track rods from my 2CV.
A few caveats;
- The article must be the cathode i.e. connected to the black or negative lead of your power supply
- Don't use stainless steel as the anode because very nasty chromium salts are formed,
- Only let iron or steel parts be immersed as other metals will be eaten away rapidly
- Clean the anode when it gets covered in rust, excessive rust on the anode reduces the current.
- Care should be taken to work in a ventilated place as both hydrogen and oxygen are released. You don't want them to combine again!
- Flash rusting will occur so the item must be dried quickly. Some argue washing with cold water slows this flash rusting others say washing with hot water means the warm item will dry quickly. Take your pick!
The two photographs (sorry about the quality) are front and back views of the left and right front hubs, one before and the other after de-rusting. You can see how the paint was removed leaving a shiny surface whereas the rusted areas gave a matte finish.
David Tocher; Chassis #189
