Stainless steel does rust!

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Stainless steel does rust!
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Just got done helping to rebuild a large York chiller that had a refrigerant (R22) leak.

We pulled the insulation off the piping to find the stainless steel refrigerant recirc tube had rusted away to practically dust.

The contractor puzzled over this, unaware that stainless steel can rust. I explained to him that in an oxygen deprived environment, stainless steel can no longer replenish its chromium oxide passivation layer, so it'll rust just as fast as plain old iron. The insulation around the recirc tube was sealed so well that not enough oxygen could get to the tube, but it wasn't sealed so well that water vapor in the air couldn't get in there and condense over a number of years, rusting the tube. This was the reason the rust was worse at the ends of the insulation where the tube connected to the machine, but there was little rust further along the tube... it doesn't matter if no oxygen can get in to replenish the chromium oxide passivation layer if no water is getting in, either.

This applies to us, as well. If you use stainless steel for your electrolyzer plates, the (-) electrode will rust (I believe that's why you see so many experimenters with brown water, despite using stainless plates). The (-) electrode is where hydrogen is produced, so it'll be an oxygen-deprived environment.

You'll also note that due to hydrogen adsorption occurring if the plate is used with AC current, and due to that hydrogen adsorption making the chromium oxide layer (a dielectric) much more conductive, stainless steel should only be used for a pure-DC or pulsed-DC system, and only for the (+) electrode. You definitely want to keep any free hydrogen atoms away from that chromium oxide passivation layer if you're relying upon the chromium oxide passivation layer as a dielectric.