I am attaching a Pourbaix diagram that combines Ag, Cl and S in the same plot. This is based on published Eh-pH diagram by McNiel and Little Article Corrosion Mechanisms for Copper and Silver Objects in Near-S...
I don't understand why solid Ag2SO4 does nor appear in the plot. It is only very sparingly soluble. It is likely that in a corroded (tarnished) silver object there is a thin film of the sulphate sitting on top of the sulphide .
Can anybody suggest changes to my plot?
You have said there could be 'likely' formation of thin sulphate film on the tarnish (sulfide).
It would be great to dig into this.
In my opinion there is no full agreement about the composition of the tarnish layer forming on silver surface since this depends on the type and composition of the exposed environments. Some studies concludes that the tarnish layer consisting of Ag2S as major constituent and Ag2SO4 in a negligible while other concludes that Ag2SO4 as major constituent. However, if you have a look on the diagram you see clearly a zone for Ag(OH)2 formation as follow:
Ag→Ag++e−
O2+2H2O+4e−→4OH−
Ag++2OH−→Ag(OH)2
If you consider another route for silver tarnnishing , Ag2O is formed instead of Ag(OH)2 and Ag2SO4 should appear as follow:
Ag→Ag++e−
O2+2H2O+4e−→4OH−
2Ag++2OH−→Ag2O+H2O
This oxide is important for the formation of silver sulfate
OH−+SO2→HSO−3
Ag2O+HSO−3→Ag2SO3+OH−
Ag2SO3→Ag2SO4
Best regards
https://www.researchgate.net/publication/281293527_The_Role_of_Sulfur_in_the_Atmospheric_Corrosion_of_Silver
https://www.scirp.org/Journal/PaperInformation.aspx?paperID=80544&
Good answer by Mohammed. This definitely shows the pourbaix diagram needs redrafting once the Eh-pH stability regions are determined. Good question posed there by Gordon.
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