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Why is it that if the corrosion product are soluble, then the rate of corrosion increases while insoluble corrosion products cause rate of corrosion to decrease?(How exactly would insoluble corrosion product protect the material?)

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  • $\begingroup$ If the corrosion product remains on the surface, how will the corrosive reach new material to corrode? Instead, if the corrosion product floats off into the infinity of the liquid, a fresh surface is exposed for attack. $\endgroup$ – Jon Custer Sep 24 '15 at 18:19
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The process in general is called passivation. It means, that the surface of a metal is corroded and the corrosion product forms a protective layer. If the corrosion products are hardly soluble, it therefore blocks the metal beneath it from further corrosion. If the corrosion products are soluble, this protective layer can be washed away and the material is exposed to further oxidation.

In iron, for example, the initially formed iron hydroxides are somewhat soluble in water. The formed rust is flaky and friable, which basically means it is removable by little or no stress, i.e. rain can break the surface and expose more metal to the oxidising agent. It therefore usually always decomposes completely.

In another example, copper does form such a protective layer of copper oxides. This is also called patina. These layers are hard(er) to remove and protect the underlying metal from further oxidation. This is also the reason, why for example many church roofs and the Statue of Liberty are green.

This method is often used to protect metals from corrosion, i.e. the formation of the protective layer is induced chemically.

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  • $\begingroup$ If you don't mind me asking, is there a way to determine theoretically if a given metal will undergo passivation? $\endgroup$ – Tripathi Sep 25 '15 at 7:10
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    $\begingroup$ @Shubham From the top of my head I cannot think of one criterion to be able to tell that. The solubility product certainly has something to do with it, but also how good the oxide "bonds" or sticks to the surface. I think this would make a good next question for the network, you can provide a link to that question for context. $\endgroup$ – Martin - マーチン Sep 25 '15 at 7:16
  • $\begingroup$ I just posted the question, I referred to the order of passivity (from Ti to Cu) and asked how do we predict that series, and is it possible to predict if some metal would undergo passivation or not, can you suggest something else to add/edit? this is the link: chemistry.stackexchange.com/questions/37877/… $\endgroup$ – Tripathi Sep 25 '15 at 7:27
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Corrosion is nothing but actually the hydration of the metallic compound. For example if you take rusting of pure iron you have to stick water molecules around it to make it rust or corrode. And we know that solubility is greatly affected by hydration hence the substance with more solubility corresponds to more corrosion as compared to one less solubility.

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    $\begingroup$ I don't quite understand your answer here. Perhaps you could clarify how it helps to answer the question. $\endgroup$ – bon Sep 24 '15 at 16:25
  • $\begingroup$ bon is right, I don't think I understand the explanation either. $\endgroup$ – Tripathi Sep 24 '15 at 16:36
  • $\begingroup$ I guess I have interpreted the question as to the rate of corrosion of the substrate while questioner meant the corrosion agent's solubility. Can you confirm ? $\endgroup$ – Akshay Pratap Singh Sep 24 '15 at 16:43
  • $\begingroup$ Consider this example: Formation of PbSO4 in case of Pb in H2SO4 medium supresses further corrosion. $\endgroup$ – Tripathi Sep 24 '15 at 16:49
  • $\begingroup$ Sorry I don't have an answer to this. Maybe it's just a fact or a experimental analysis. $\endgroup$ – Akshay Pratap Singh Sep 24 '15 at 17:24

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