CrO4 stability in aqueous NaCl?

[TomH]

Hello,

I am trying to do some detective work to understand the cause of a failed heating element. A yellow deposit was observed and elemental analysis shows strong peaks of Cr, O, Na, and Cl. the heating element has been in water softened by ion exchange with NaCl, so that makes sense. Heating elements are often Nichrome, which forms a coating of chromate (CrO4) once heated. Does anyone know if CrO4 is stable in aqueous NaCl? Could the presence of Na and Cl ions be attacking the CrO4? 'm not sure how the chemistry of it works. There could also be an influence of galvanic action. Any help would be appreciated!

Thanks

[chenbeier]

Chromate only dissolved by acidic solutions or rduction compounds. NaCl alone is not able to do it. 

[TomH]

I have measured the pH of the water around the heaters at 10.55, so it is strongly alkali. TDS and conductivity are off-the-scale, presumably because the heating elements have  been continuously concentrating the NaCl (water evaporated leaving the salts behind). What is causing the alkali? Would that be NaOH? Does this from spontaneously from aqueous NaCL + Air + heat? Would this alkali condition then be corrosive to chromate surface of the element?

Thanks!