electrolysis and resistance

[crumblebear]

Hi all, im an electronics engineer thats some across a few odd problems in the past and im wondering if you guys can help
Ive recently been using electrolysis to remove rust from some old machinery parts (renovation project) and ive noticed strange things lol. I mix a given solution of washing soda with water to increase its conductivity and it works well!! However the current draw from the power supply is massive compared to what it should be by basic ohms law. The only thing i can think of is that its due to the chemical changes in the water during electrolysis. With a standard dmm the resistance over 10mm varies wildly between 40kohm and 500kohm however when the anode and cathode are inserted the current rises to approx 9amps on a 24v supply. I understand that the electrolysis draws current but should the resistance of the water not limit this? even with small anodes and cathodes the current draw is suprisingly large!

Thanks
Harry

 

[Compositepro]

The resistance you measure will depend on the voltage applied for the measurement, so your resistance readings are somewhat meaningless. Until the applied voltage exceeds the electrolytic potential, no current will flow. Then it will increase suddenly. 24 volts is way above any eloctrolytic potential in your cleaning cell (which will be below 2 volts). Look-up "ionic mobility".

 

[crumblebear]

Hi, thanks for taking the time to reply. I understand 24v is well above the necessary voltage to run this reaction but the plating reaction is (remember im no scientist) related by i believe faradays?!? law that says it requires a given amount of amp seconds to plate a certain amount of material. As the parts are not load bearing and i wasnt worried about hydrogen embrittlement the fastest option was to increase the voltage and let the current do likewise.
So the electrolyte acts sort of like a zener diode insulating until a voltage point is reached and then allowing a much larger flow?

Thank you for dealing with a chemistry buffoon

 

[moltenmetal]

Different voltages permit different electrochemical reactions to occur at each electrode. Voltage beyond the level required to promote the reactions you want, may do other chemistry (such as generating hydrogen and OH- or oxygen from water) in addition or to the detriment of what you are trying to do. Whatever voltage is beyond that necessary for the electrochemical reactions will ultimately just become heat.

As compositepro already mentioned, resistance measurements made at low voltage in a conductive liquid with a DMM are not going to be useful to predict current flow.