How many anodes should I place? At what voltage should I set the ICCP system? It all comes down to the question of what potential is necessary to protect the object. This, of course, depends on the type of material. Alloy steels require a different protection potential than pure iron. The pourbaix diagram is an important tool for determining which protection potential is required. Each material and/or allow has its own pourbaix diagram. In it, the acidity (pH) is plotted against the potential (voltage).
For a given pH value and potential, the diagram can also show in which area the metal is located. These areas are indicated by the dotted lines. The lower dotted line is the first crossing to take into consideration. Above the lower dotted line, corrosion or passivation will take place. Under the line, the material is relatively immune (will not change). Exactly on the dotted line is the reaction in equilibrium.
Please note: This is a simplification of reality; in addition to the pourbaix diagram, there are other influences that we will not discuss intensively in this article.
In this diagram it can be seen that iron will not corrode at a potential more negative than ca. -950 mV against a Cu/ CuSO4 reference electrode (read more about electrodes in this article), independent from the acidity (pH). In practice however, the corrosion rate of steel will already be very low at potentials more negative than –850 mV. This value of –850 mV against a Cu/CuSO4 reference electrode is under normal conditions considered as the protection criterion. A criterion of –950 mV against a Cu/CuSO4 reference electrode is used under oxygen free conditions (anaerobe) because under such circumstances, bacterial corrosion could occur: Sulphate Reducing Bacteria (SRB) can produce hydrogen sulphide (H2S) which leads to a lowering of pH and a direct loss of electrons. This can create a significant loss of metal ions. Usually, very locally. Often less negative potentials are used as protection criteria under circumstances with higher soil resistivity.