I have a stainless steel kitchen sink. When I clean it with bleach, leaving it instantly gleaming, what happens to the particles that were staining it?
Mark Dirnhuber
Bristol, UK
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Stainless steel has chromium as the key ingredient: on contact with air (or even nitric acid) this forms a thin but resistant surface layer of chromium(III) oxide (Cr2O3), which protects the steel from further corrosion. This oxide layer may well pick up stains, for example from tannins, which are polyphenols from tea.
Bleach is sodium hypochlorite (NaOCl) and is a strong oxidising agent, which is alkaline and also contains free chloride anions (Cl–).
Chloride ions disrupt the protective layer and eventually allow corrosion of the underlying metal, often forming pits: this is why stainless steel tends to corrode in seawater.
The hypochlorite can oxidise Cr(III) to Cr(VI), for example to chromate, e.g. [CrO4]2-, and also Fe(II) or (III) to Fe(VI) as in ferrate [FeO4]2-. If a little bleach is added to some ferrous sulphate or ferric chloride solution, once the precipitate has settled, the pink colour of the liquid above is visible due to the ferrate.
Hypochlorite bleaches tannins by disrupting the ring structures in the molecules, decolourising and solubilising the stain. It will also attack and dissolve protein films on the steel. By rinsing and exposing to air, you may well be refreshing the oxide layer.
Personally, my choice would be to forgo all this dubious chemistry and rely on detergent and elbow grease.
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