I have had the chance now to research this a bit more and try a quick experiment.
It seems that electrochemical machining (ECM) is a well used practice that is documented in some of my old college books (so at last they come in usefull!).

The rust removal process uses the workpiece as the cathode (negative electrode) and a steel bar as the anode (positive electrode) with washing soda as the electrolyte solution. The machining process uses the reverse polarity. From this info then presumably the peice of steel used as the anode in rust removal will be eaten away as per the machining process? The process is also "line of sight" to a certain extent so I am unsure if it will free off seized screws or hinges for instance?

My quick test involved a mildly rusty peice of steel 6mm thick, 40 mm wide ,and about 3 inches long, the anode peice was of similar dimensions. Both were suspended in the washing soda solution about 6-7 inches apart and connected as per the rust removal technique (workpeice is cathode). The battery charger was then connected and a current of around 4 amps flowed with significant bubbling/fizzing from both electrodes. Time only allowed a 20 min test but the results were reasonable even after this short time. The rusty test piece turned black and the rust could be rubbed of with firm pressure from a finger, although more time would be needed for better results. The rear of the test piece was not really touched although time may improve this? The anode steel was lightened in colour but not otherwise affected so I guess the machining process would take some time and require closer proximity of the cathode and anode?

Next plan is to try a screw or hinge to gauge the effect of the "line of sight" issue?

Anyone else been trying this?

Steve

Steve

I used this process 2 years ago but with some differences.
I used sodium hydroxide or Lye as the electrolyte. Add a little dish washing liquid. The lye will cut through grease and oil a bit and the soap will help. Also the soap reduces the waters surface tension so helps stop bubbles causing a problem.

I did not find it particularily directional unless the target was a hell of a lot smaller than the other electrode in both the de-rust and machining modes. Any directionality will be caused by the electrical resistance of the bath. Not enough electrolite would cause this.

Time taken. Your 20 minutes surprised me. I found less time worked but again the electrolyte make a big difference.

By the way dont use stainless as the electrode. Steel, copper or carbon work well. If you have a septic system like I have dont throw the solution down the drain if using copper. Lye down this drain with steel or carbon is fine. The copper stops the septic working and ......

Hope this helps your experiments

Pat

Thanks Pat

I have now found some more info via the web, which agrees with your findings. It seems there are pros and cons with different electrolytes so I will maybe have ago with the Sodium Hydroxide you used which I think is also known as caustic soda?
Ref the stainless steel anode.... As you suggest dont use it! There are some quite undesirable effects that have health implications etc, although some web articles highly recomend it because it doesnt get eaten away so quickly.

You are suprised at the time? To be honest I didnt really know what to expect , but most articles online seem to indicate different times depending on what you are trying to achieve, i.e high current quick time to "blast off" rust and low current long time to "restore" delicate antiques?

I am going to try the rusty hinge I have maybe over the weekend as I reckon that this could be one of the most usefull applications of the technique, alongside perhaps cleaning out the inside of a fuel tank.

Steve

Go to Redistrip on the net , there are a few tips and adverts from commercial practitioners.

I have done this (some time ago now) and it worked like a dream. There is no gain without pain though, and it needs re-emphasizing that the current required is proportional to the surface area of the bits. An ordinary battery charger (say 10 amps) is fine for small stuff (up to the size of a brake drum)but if you want to do an A7 chassis(!) you probably need your own substation. If you use a bit of steel sheet for the electrode, you can get quite precise control of current by raising & lowering it in the solution.
John