From the metallurgical understanding, each reaction (anodic and cathodic) usually takes place preferentially on a specific spot of the metal microstructure. For carbon steels, for instance, ferrite is consumed preferentially over cementite. Then, we can say that the anodic reaction occurs on ferrite, whereas the cathodic occurs on cementite. However, for pure iron, considering a high-purity metal, there is only one phase (ferrite). From discussions with my colleagues, the main understanding is that the cathodic reactions would occur on the grain boundaries, whereas ferrite would again be the anodic site. Unfortunately, I could not find any literature addressing this mechanism in detail.
Hi Bernardo, hope all is well with you. I think it is not just the purity of the material, 99.99% pure iron steel has some sites sensitive to dissolution and some other spots with higher affinity to HER. Any physical irregularity of the surface, e.g., crystal imperfections such as dislocations and surface steps, different phases, and roughnesses due to machining, grinding, scratches, etc tend to have a higher surface activity and can create local anodic spots (more sensitive to dissolution). Assuming the main cathodic reaction is HER, the cathodic reaction on the other side occurs in those sites with the lowest adsorption-free energy of hydrogen and (highest affinity for HER). Please check this ref.: https://www.google.com/books/edition/Fundamentals_of_Electrochemical_Corrosio/baHwfLpWpP8C?hl=en&gbpv=0 chapter 7.
I hope you find it helpful
Good luck
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