Pourbaix diagram electrochemical stability, water

Figure 1.9 is the Pourbaix diagram for iron and some of its compounds in an aqueous system at 25°C. The equilibrium potential of the reaction Fe° = Fe2+ + 2e falls outside the stability region of water represented by dashed lines. Hence, measurement of the equilibrium electrode potential is complicated by the solvent undergoing a reduction reaction, while the iron is undergoing electrochemical oxidation. This is the basis of the mixed potential model of corrosion. 

Figure 4 is the Pourbaix Diagram for iron and some of its ionic species and compounds in contact with water at 25 C. The equilibrium potential of the iron-ferrous ion reaction falls outside the region of stability of water (dashed lines). Therefore, any attempt to measure the equilibrium potential will fail since the solvent will undergo electrochemical reduc-... 

Fig. 13M PotentiallpH diagrams for water. Only the equilibria for water electrolysis and for self ionization are shown. The partial pressures of oxygen and hydrogen are taken as unity. The shaded area is the region of thermodynamic stability of water. Data from Pourbaix in "Atlas of Electrochemical Equilibria in Aqueous Solutions", Pergamon Press, 1966.

Thermodynamic calculations are extremely important in the field of corrosion because they can be used to predict the tendency for a metal to corrode in a given environment. Details of thermodynamic principles can be found in a number of textbooks [1-3]. The Atlas of Electrochemical Equilibria in Aqueous Solutions by Marcel Pourbaix provides a comprehensive summary of the application of thermodynamics to corrosion as well as a compendium of stability diagrams for all elements in water [4). 

FIGURE 4.6.2. Potential-pH diagram for the Fe—H2O system at 25°C [3]. The region between the lines (a) and (b) represents the domain of Are thermodynamic stability of water. The numbers on the lines represent the logarithm of the activity of the species. (M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Publishers NACE and CEBELCOR.)... 

The Pourbaix (phase) diagram is a plot of redox potential (ordinate) as a function of pH (abcissa) for a given metal under standard, thermodynamic conditions (usually water at 25° C). The diagram takes account of electrochemical and chemical equilibria and defines the domain of stability for the electrolyte (normally water), the metal and selected compounds, e.g. oxides, hydroxides and hydrides. The following criteria are often adopted regarding dissolved metal-ion levels ... 

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