Pourbaix diagram for a metal

This reaction involves electron transfer only. Thus, the Nemst equation, eq. (2.32), with [M+ ] = 10 mol/l and T = 298 K becomes 

The value for the standard potential is given in Table 2.2, provided 

This reaction involves a fixed hydrogen concentration only, the equilibrium constant K for this reaction must be known so that 

Notice that this reaction involves both hydrogen and electron transfer. Then, the Nemst equation becomes 

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Pourbaix diagram for a metal showing two valence states and... 

The thermodynamic information is normally summarized in a Pourbaix diagram7. These diagrams are constructed from the relevant standard electrode potential values and equilibrium constants and show, for a given metal and as a function of pH, which is the most stable species at a particular potential and pH value. The ionic activity in solution affects the position of the boundaries between immunity, corrosion, and passivation zones. Normally ionic activity values of 10 6 are employed for boundary definition above this value corrosion is assumed to occur. Pourbaix diagrams for many metals are to be found in Ref. 7. 

A schematic Pourbaix diagram for a typical transition metal such as iron, showing two valence states, M + and M +, is drawn in Figure 9.27(a). Remember that the boundaries are concentration dependent, and the figure is representative of a typical concentration. [The method of construction of the diagram for the real iron-water-air system is given in Section S3.6.]... 

In order to illustrate how the Nemst equations for the various reactions involving a metal in an aqueous solution can be combined to create a Pourbaix diagram for that metal, it is instructive to create a real diagram. A portion of the Pourbaix diagram for Fe will be developed in this section. 

Fig, I0j6 Pourbaix diagrams for pure metals in water at 25 0. (a) Tin. (b) Copper. Potential axes are with respect to NHE. 

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

The thermodynamic data pertinent to the corrosion of metals in aqueous media have been systematically assembled in a form that has become known as Pourbaix diagrams (11). The data include the potential and pH dependence of metal, metal oxide, and metal hydroxide reactions and, in some cases, complex ions. The potential and pH dependence of the hydrogen and oxygen reactions are also suppHed because these are the common corrosion cathodic reactions. The Pourbaix diagram for the iron—water system is given as Figure 1. 

A comprehensive list of standard potentials is found in Ref. 7. Table 2-3 gives a few values for redox reactions. Since most metal ions react with OH ions to form solid corrosion products giving protective surface films, it is appropriate to represent the corrosion behavior of metals in aqueous solutions in terms of pH and Ufj. Figure 2-2 shows a Pourbaix diagram for the system Fe/HjO. The boundary lines correspond to the equilibria ... 

Pourbaix (16) has prepared theoretical stability diagrams of potential vs. pH for many common metals and nonmetalloids. A review of these results indicates that semiconductor compounds of Au, Ir, Pt, Rd, Ru, Zr, Si, Pd, Fe, Sn, W, Ta, Nb, or Ti should serve as relatively acid-stable photoanodes for the electrolysis of water. Indeed, all of the stable photo-assisted anode materials reported in the literature, as of March, 1980 (see Table III) contain at least one element from this stability list, with the exception of CdO. Rung and co-workers (18) observed that the CdO photoanode was stable at a bulk pH of 13.3. The Pourbaix diagram for Cd (16) shows that an oxide film passivates Cd over the concentration range 10.0 < pH < 13.5. Hence the desorption of the product H+ ion for the particular case of CdO must be exceptionally facile without producing an effective surface pH lower than 10.0. This anamolous behavior for CdO is not well understood. 

There is a second group of metals like Fe, Cr, Ni and their alloys, which do not follow all predictions of their potential-pH diagrams. As an example, the Pourbaix Diagram for iron of Fig. 3 predicts corrosion for all potentials in strongly acidic electrolytes. However, experiments show that it is passive for potentials above a potential of Ep = 0.58 — 0.059 pH. For these conditions the passive layer is far from any dissolution equilibrium and its protecting properties have to be related to its slow dissolution kinetics. The same arguments hold for the passivation of Cr, Ni and their alloys. 

Reference to the Pourbaix diagrams for water and copper shows the stable species to be Cu2+, H+ and H2O. Corrosion is possible under these conditions. Let us consider the case of iron metal at —0.750 V vs SCE in a solution of pH 5.0 and a ferrous ion activity of 10 6 at 25°C. The electrode potential with respect to SHE is 0.509 V and reference to Pourbaix diagrams for water and iron shows the stable species to be Fe2+ and H2, and that corrosion is possible under these conditions. 

The Pourbaix diagram for platinum [109] is shown in Fig. 9. The domain of stability of water itself lies between the lower broken diagonal line a (below which water can be reduced to hydrogen) and the upper broken diagonal line b (above which water may be oxidised to oxygen). Platinum metal is stable below the lowest full diagonal line which stands for the reaction... 

Next we consider the Pourbaix diagram for iron, which is, of course, of paramount importance for the understanding of corrosion of ferrous alloys such as the many types of steel and stainless steel. This is a rather complex diagram, since two oxidation states of iron exist both in the liquid and the solid state and the metal is amphoteric to some extent. Figure 16M is a simplified version of the diagrams shown in the original work of Pourbaix. The two soluble species in acid solutions are Fe and Fe. The relevant equilibria are... 

A somewhat simplified Pourbaix diagram for the iron/water system is shown in Fig. 2.11. In this case, the possible solid phases are restricted to metallic iron, Fe304, and Fe203. A more detailed diagram and a diagram with Fe(OH)2 and Fe(OH)3 are shown subsequently. 

Using Eh-pH diagram for particular metal and measured soil pH and redox (Eh) condition, we can obtain an estimate of the metal species present under a specific set of soil or sediment conditions. Some of the most comprehensive data detailing Eh-pH diagram for elements can be obtained in the studies by Pourbaix (1966), Garrels and Christ (1965), Krauskopf (1979), Berner (1971), and Stumm and Morgan (1981). 

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). 

Pourbaix diagrams for metals in aqueous solutions can be generated in order to visualize the stabiKty regions for the metal and its various corrosion products. In order to construct a metal Pourbaix diagram, the possible reaction products in an aqueous solution must be known. In general, a metal will oxidize to form a soluble cation, a soluble anion, or a metal oxide or hydroxide. For a generic metallic element M, the electrochemical half reactions that form these various products are ... 

Cathodic protection can also be applied to prevent pitting. Regarding aluminium, strong cathodic polarization should be avoided because this can lead to a large increase of pH close to the metal surface, which can cause so-called alkaline corrosion (compare with the Pourbaix diagram for aluminium in Figure 3.11, Section 3.8). Use of sacrificial anodes of Zn or A1 alloys is therefore safer than impressed current. 

This means that reducing environments are compatible with relatively noble metals or alloys (copper, lead, niekel and alloys based upon these metals). When metallie materials are to be used in oxidizing environment, on the other hand, their corrosion resistance must be based upon passivity (e.g. titanium and alloys that contain sufiRcient amounts of chromium). These relationships are easy to understand when the Pourbaix diagrams for metals such as Cu and Ti (Figure 10.1) are considered, and it is kept in mind that reducing environments lower the corrosion potential and oxidizing environments lift it. Irrespective of the mentioned rule, a metal is usually most corrosion resistant when it contains the smallest possible amounts of impurities. Some natural combinations of environment and material are listed in Table 10.1. 

Thus the stability of the passive fihn depends on two parameters, the electrode potential and the pH value. Pourbaix developed special diagrams of stabUity regions of oxides on metal surfaces as function of electrode potential and pH value. The diagrams were calculated from thermodynamic equilibrium values for selected reactions between the metal and aqueous electrolyte. A Pourbaix diagram for iron is shown as example in Figure 10.11 (Kaesche ). 

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