Pourbaix Diagram and Galvanic Cell

Question

I am trying to understand how a galvanic cell where copper and mild steel are linked occurs. It seems that on most galvanic series, Fe and Cu are far enough apart (>.2V difference) that Fe would preferentially ionize to Fe+2, while copper is "plated" onto the copper electrode from solution.

I understand how to calculate the voltage difference given by the galvanic cell (roughly .45 V difference). What I do not understand is why the iron electrode even corrodes at all. On Pourbaix diagrams of Fe around pH of 7, it seems that either the cell potential of -0.45V or the galvanic series potential of -1.1V for iron would both put it near/within the stable Fe area of the Pourbaix diagram.

My questions is:

1. Which potential/voltage do I use to determine stability on a Pourbaix: overall cell potential or galvanic series potential?
2. If the pH and potential on a Pourbaix put it in the stable metal range, does that mean that no reaction will occur (since Cu needs to "get" e- from Fe turning into Fe+2)?

Answer 1

I believe you might not be completely sure on how to read a Pourbaix diagram. The potential you use in the Pourbaix diagram is the potential at your electrode, this might be the mixed potential at your electrode, applied potential, potential formed when you connect the electrochemical circuit, etc.

A Pourbaix diagram is constructed using the Nernst potential, which is not a potential difference, but an equilibrium potential (relative to some reference). Your system will not be thermodynamically stable so long there is a net driving force, in your case until the concentrations of Fe2+ ions and Cu2+ ions are so that the cell potential (difference in Nernst potentials) is zero. If the Pourbaix diagram indicates that you have the Fe species, then Fe2+ ions will be plated on your electrode until the cell potential is 0. If there are no Fe (or Cu) ions anymore you might notice that the Nernst equation blows to infinity. In reality no reaction will indeed occur, or other species (for example the solvent) might start donating/accepting electrons.

In short, current will always flow (although possibly negligible amounts) in the case that there is a non-zero cell potential.