Pumukli
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Stainless steel staining
Hello Everyone,
I've encountered an interesting (at least for me) phenomenon: stainless steel became stained (covered in some sort of whitish layer) after prolonged
exposure to hot, Na2CO3 containing tap water and oxygen from air.
At first I thought "ah, it is simple, just CaCO3 precipitate formed and it should be easy to remove it by acetic- or citric-acid!"
Wrong! This layer does not seem to react with these acids! It sticks to the steel very hard. Previously it was a highly glossy surface, now it is
dull, as if it was covered by a thin layer of flour.
I run a few Google searches but could not come up with anything meaningful. If it is indeed CaCO3 precipitate then what causes its inertness towards
acids?
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metalresearcher
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The only thing I might guess is Al2O3 which is stubborn.
But did you use any aluminum foil or sheet with the heating ?
Ca and Mg salts should dissolve in acids.
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Pumukli
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Thanks for the hint, metalresearcher!
Aluminum hmm. I did not check the system so thoroughly but it is something worth considering.
Maybe there is somewhere an aluminum part, a pipe or something that gets in contact with the highly alkaline water (pH is around 11, maybe reaches 12
at times) and slowly gets dissolved as hydroxo-complex. Then the complex reacts not with O2 but with CO2 from air and gets converted into Al2O3.
Anyway, do anyone know a good, reliable way to dissolve Al2O3? I may try NaF, as I remember, Al3+ makes strong complex with fluoride.
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pneumatician
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Mood: ■■■■■■■■■■ INRI ■■■■■■■■■■ ** Igne Natura Renovatur Integra **
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many inox cookware have a last layer of Al to get a wonderfull Alzehimer, thanks to the pigs. 
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