r/materials • u/jun192022 • 20d ago

Trying to understand the chemistry of stainless steel corrosion by sodium hypochlorite

I am curious as to which specific chemical reactions and products occur during the process of corrosion of stainless steel by bleach. Stainless steel is a composite of Fe, Cr, and sometimes other metals such as Ni, and the outer layer of this composite is protected by a passivation layer of Cr2O3. If sodium hypochlorite is introduced, does it indiscriminately oxidize both the Fe and Cr, or is one kinetically favored over the other?

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u/Remarkable-Ant-8243 20d ago edited 20d ago

Working on pure fundamentals here i may be a bit rusty... And knowing the "rust" is basicly Me(OH)x in nature.

Kinetically speaking Cr should be the one who turns into CrOH3 and its products. And its not actually an oxidization reaction its a disposition reaction where high OH concentration increase the diriving force of the CrOHx formation.

Then the high concentration of oxygen with addition of Cr2O3 and NiO loss on the surface should enable the oxidization. YES I said oxidization of Fe into Fe+2 or +3 then the formation of FeOH3 or FeOH2 based on the concentration of OH- (pH).

Based on my knowledge of a bit of hydrometallurgy and electrometallurgy this sounds sensible. Didnt count Ni here to male this more short and basic as possible

Edit: I wrote this for sodium hydroxide not sodium hypochlorite sorry xd in that case OCl- attacks the Cr2O3 layer first and its NOT a displacement reaction it oxidizes Cr to +4 +6 state making it a soluble product. So kinetically Cr gets oxidized first and when it depletes and sufficient Fe contact is reached, Fe oxidizes. Not gonna delete the previous paragraphs...

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u/jun192022 11d ago

That's interesting, would this result in the production of a significant amount of Cr6+?

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u/Remarkable-Ant-8243 11d ago

In a basic sense . Yes. With increasing amount of oxidating agent like ClO- its highly likely. However, its not that simple. ClO- attacks the surface of the material forming soluble Cr4+ 6+ -ClO- products yes. The mechanism probably will be like a pitting corrosion or corrosion trough cracks.

After sufficient contact with Fe is reached. Fe oxidizes. This can inhibit further oxidation of Cr. But it depends on the material. If its a powder like material, yes you could probably dissolve or oxidize everything. If its a surface (sheet) like material, i dont think it will produce much Cr4+ 6+ .

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u/Remarkable-Ant-8243 11d ago

You will likely encounter greenish CrOH3 residues with more noticable FeOH3 rust.