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Electrolytic reactions from corrosion


So I am doing a corrosion experiment, I'm getting a thick black coating on my anode while running on a sensitized piece of 304 stainless steel. I'm pretty sure its either chromium(iv) oxide or nickel(iii) oxide. Its very fragile which could be due to it being electroplated onto the anode. Both oxides are conductive and slightly magnetic. Since the steel was sensitized the chromium and nickel are preferential anodes to the iron as the two alloying elements are forced to the grain boundaries during the sanitization process. I fairly confident it is not magnetite as this did not happen when the same experiment was run with unsensitized steel (chrome is still dissolved in grains and not in the grain boundaries). How do I tell the two oxides apart? As the chunk is drying the surface is turning a white yellowish color which I'm assuming is due to the salt water solution the sample is being run under.


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