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An example of a Pourbaix diagram is shown in Figure 2.4.

Pourbaix diagrams have two axes, one for corrosion potential as measured in hydrogen potential, and one for measuring acidity of the environment shown as pH. For a given set of potential‐pH‐environment, some “domains” will be created. These domains can be used to predict, under the given conditions for the three parameters mentioned above, if it is possible to expect corrosion or immunity to corrosion (in other words, passivity). Existence of some corrosive ions such as, but not limited to, chlorides, can somehow change the domains and thus shift the potentials in which corrosion or passivation can be expected.

One very important aspect of Pourbaix diagrams in addition to enabling us to predict safe and unsafe values of potential and pH with regards to corrosion for a given environment is to put emphasis on what is almost forgotten by a majority of non‐corrosion expert professionals; it is normally assumed that as long as we know about the pH of the environment, it is safe to say if it is corrosive or not! The rule of thumb for these self‐acclaimed corrosionists is that if pH is below 7 the environment is acidic and thus corrosive, if pH is 7 it is neutral, and if it is larger than 7 the environment is basic. This is wrong! In order to interpret correctly, one has to know both corrosion potential and pH. As an example, take the Pourbaix diagram in Figure 2.4 when chloride is present; at an acidic pH = 6 and potential = −0.6 V (red dashed line intersection in Figure 2.4), there is immunity against corrosion, whereas in the same system, but this time for a neutral pH = 7 and potential = −0.4 V, corrosion is highly likely to happen (black dashed line intersection in Figure 2.4). This alone can serve to show how powerful Pourbaix diagrams are in dealing with corrosion and predicting it. However, as noticed by our readers, the restrictions due to temperature do still remain.

Figure 2.4 A typical Pourbaix diagram (simplified) for an Fe–water system at iron concentration of 10–6 mol/l and 25 °C. The region between the two dashed lines is the water stability zone. The two dashed lines, upper and lower, define the domains for oxygen and hydrogen stability; above the upper line water is oxidized to O₂, so oxygen is evolved above the upper line. Below the lower line, water decomposes to H₂ and thus, hydrogen will be liberated below the hydrogen line. The zones shown indicate the chemical species formed. Red and black dashed lines, show immunity under acidic and corrosion under neutral pH values, respectively, emphasizing the necessity for specifying and studying pH–voltage pair to decide about possible corrosivity scenarios. Hematite: Fe₂O₃; Magnetite: Fe₃O₄; Ferric ion: Fe⁺³; Ferrous ion: Fe⁺².