Much of the calculated EMF-pH (Pourbaix) diagram for iron originally derived by Pourbaix is consistent with actual experimental observations of immunity, passivity, and corrosion versus potential and pH.1 An example of this original diagram at 298 K, 10−6 ionic activity is shown in Figure 1. The compounds originally considered by Pourbaix are shown in Table 1.
However, one portion of the diagram that disagrees with experimental observation is the prediction that the area of stability of metallic iron lies adjacent to the area of stability of Fe3O4 under mildly alkaline conditions. This discrepancy was pointed out by Pourbaix in his discussion of the diagram.1 He noted that, whereas Figure 1 predicts that in the absence of oxygen iron may be taken directly from immunity to passivity (Fe to Fe3O4) by increasing potential, experimental results have indicated that an...
