Why lowering the pH increases the corrosion rates? lowering the pH increases the rates of cathodic reduction, so how this increases the rates of anodic reactions?
Metals typically develop a passivation layer in moderately alkaline (high pH) solutions, which lowers the corrosion rate as compared to acidic (low pH) solutions. The passivation layer provides a measure of immunity to further corrosion (e.g. Ti, Ta).
However, the corrosion rates can be expected to be comparable in the transpassive region (i.e. highly alkaline versus highly acidic)
The question is complex enough to give answer with general validity, but typically, metal oxides/hydroxides covers the surface to save it, and these can be formed in alkaline but dissolved in acidic environment. Furthermore, the corrosion is an electrochemical process, and H+-concn. has role in it (Nernst eq.).
Pourbaix diagrams is only thermodynamic
kinetics during th e course of the reaction surface corrosion products will hinder corrosion - see the surface composition with time using esca
Dear Faysal: Say for example that the attacking species are the protons in the aqueous solution of acidic nature, they have a higher diffusion rate (due to the small size of the protons) towards the surface of the electrode compared to ions in alkaline solutions. However, this is only general, some metals which are amphoteric in nature such as aluminum, showing corrosion in both alkaline and acidic media. Thermodynamically, the alkaline media can destroy the passive Al2O3 faster. However, what you did mention is correct in general with few execptions as I mentioned.
Some of the metals alloys can be affected by the H+ which can redeem the properties of the materials due to hydrogen embrittlement.
dear all
please see my review on" Corrosion of aluminum alloys in aqueous media" with Dr M Jayalkshmi appeared in" Corrosion reviews" Freund publishers
Corrosion the field i am working for the past 46 years is a beautiful field of science - no body understood it..A fine branch of electrokinetics
in high pH , the passive layer formed and prevent corrosion as compared to low pH
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