[SOLVED] stainless steel electrodes oxidizing

Hi,
I'm playing around with electrolysis and i've built a small hydrogen and oxygen gas generator. At first, i filled it up with a saturated salt and water solution (i know baking soda is better but i had no more). I touched it to a 12v lead-acid battery and got quite a big spark. Even in this short time, my 20 awg wires got pretty hot but i could see some bubbles that had been generated so i knew there wasn't a short. To fix this way to low resistance, i replaced my salt and water solution with just water. This time i hooked it up to 2 12v batteries in series and measured a current of about 6amps, which dropped down to 2-3amps after a few minutes. This means that at the start, the power was 144w but despite this, i couldn't see a lot of bubbles being generated. Also, my stainless steel electrodes started oxidizing, they discoloured to black and at the water surface, some kind of a yellow-brown-greenish foam started forming..

Any ideas?

Regards,
Cecemel

Pure distilled water does not conduct electricity. Your water is probably full of salts and minerals.
My cheap Chinese solar garden lights are "stainless steel" but they oxidize. Because they are extremely cheap.
Maybe your water is extremely cheap.

Stainless steel relies on a coating of oxide to protect its self from further destruction. If you use stainless steel keel bolts on your yacht, the absence of oxygen causes the bolts not to form their protective oxide film and the bolts rot through and your keel drops off.
I would guess that the various colours are the oxides of the various metals used in the composition of your stainless steel.
Frank

Audioguru said:

Pure distilled water does not conduct electricity. .

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Pure water also conducts electricity and ionic impurities increase the electrical conductivity.

Electrolysis produces hydrogen and oxygen (mostly) but if you are using a conc salt (NaCl) solution, then your stainless steel (all varieties) will get eaten rapidly (very rapidly indeed). Use carbon electrodes (lead from pencils or the carbon rod from the C or D type cells) that will last a long time.

Who suggested baking soda? Best is dilute sulfuric acid or dilute caustic soda (or potash). If you are using caustic soda (NOT COMMON SALT) then you can use iron electrodes too (stainless steel too is also ok but Ni is still better).

Thanks for the answers everyone!

Audioguru said:

Maybe your water is extremely cheap.

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Haha! It's just tap water but maybe there was some salt left in it.

One more question tho: Does anyone have an explanation for why, if there's 144w going in there, not a lot of gas is being generated?

Cecemel

Cecemel said:

One more question tho: Does anyone have an explanation for why, if there's 144w going in there, not a lot of gas is being generated? l

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What do you mean by 144w going in there?

If you pass 96500A (yes, that is 96 KA), then you will produce 1 gram of Hydrogen gas and 8 gram of oxygen gas per second. That means to make lots of gas, you need lots (and lots) of current.

Electrolysis is not cheap.

All very true.
I was involved with all this myself a few years back, and have to agree you have to put in kilowatts of power to get enough bubbles to light a flame about as big as a candle.
Its definitely not an energy efficient process.

Running huge power through either a caustic soda solution or acid solution generates fierce heat which will easily boil the cell in a surprisingly short time.

Its rather like putting 2.5Kw through an electric jug or kettle. Tea or Coffee will be ready pretty quickly.

So you need a really big cooling system to prevent an ever increasing temperature rise up to the boiling point.

Its all rather a lot of trouble with not much to show at the end.
But its a great learning exercise and all jolly good fun.

Say you are applying 12V to the solution. Out of this, about 3V is used for electrolysis and the rest is used up for heating the solution. But if you apply just 3V, the current will be rather small. So you need to increase the electrode surface area (just the way it is done inside a battery) and decrease the spacing and make a power supply that will supply 50A at 3V. Even then you will not get the full efficiency (electrode overpotential) but that is a different story.

In practice, we make dozens of cells and connect them in series (3V at 50A - ten of then in series will be 30V at 50A) for electrolysis. Aluminum is also made by electrolysis and we need 1000A or more for each cell.

Cecemel said:

Haha! It's just tap water but maybe there was some salt left in it.

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My tap water comes from huge Lake Ontario and they do not treat it with chlorine anymore, it has no smell and no taste now but it conducts electricity like crazy. I measured distilled water and it did not conduct.

Audioguru said:

My tap water comes from huge Lake Ontario and they do not treat it with chlorine anymore, it has no smell and no taste now but it conducts electricity like crazy. I measured distilled water and it did not conduct.

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I do not know how did you measure (I have never measured) but the conductivity of pure water is about 0.5 uS and the conductivity of common water is about 1000 times more.

By the way, measuring conductivity of water and solutions is not trivial and uses AC bridges (above 1 kHz or so) with low applied potentials.

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Wikipedia says (I was quoting from memory)

High quality deionized water has a conductivity of about 5.5 μS/m, typical drinking water in the range of 5-50 mS/m, while sea water about 5 S/m[2] (i.e., sea water's conductivity is one million times higher than that of deionized water).

It is small but cannot be ignored.

My improved Plants Watering Watcher project uses AC to measure and display the conductivity of soil or water.