Pourbaix diagrams iron-water system

Figure C2.8.1. Simplified /pH diagram (Pourbaix diagram) for the iron-water system at 25°C. The diagram is...

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. 

Fig. 1. Pourbaix diagram for the iron—water system at 25°C, considering as solid substances only Fe, Fe(OH)2, and Fe(OH)2, where (-), line a, represents...

Figure 15.3 Fh-pH (Pourbaix) diagram for the iron-water system at 25 °C for a maximum concentration of 0.001 mol L-1 dissolved iron. The shaded area shows the Fh-pH range of natural waters.

Fe2C>3 film to soluble FeC>42, requires a high impressed EMF and is only feasible in alkaline conditions (cf. the Pourbaix diagram of the iron-water system, Fig. 15.3). 

FIGURE 21.9 The Pourbaix diagram for the iron-water system at an ambient temperature (from Ref. 20). 

A thermodynamic analysis was conducted for corrosion of iron alloys in supercritical water. A general method was used for calculation of chemical potentials at elevated conditions. The calculation procedure was used to develop a computer program for display of pH-potential diagrams (Pourbaix diagrams). A thermodynamic analysis of the iron/water system indicates that hematite (Fe203> is stable in water at its critical pressure and temperature. At the same conditions, the analysis indicates that the passivation effect of chromium is lost. For experimental evaluations of the predictions, see the next paper in the symposium proceedings. 

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. 

In 1966, Pourbaix published his Atlas of Electrochemical Equilibria in Aqueous Solutions, which contains electrode-potential/pH diagrams for many elements and a critical analysis of the data on which the diagrams are based (Ref 9). Figures 2.13 and 2.14 are from this publication and represent the iron/water system, assuming the solid phases to be iron and iron oxides in the first case and iron and iron hydroxides in the second case. It should be noted that the two diagrams differ only in relatively small detail, which results from the relatively small difference between the GFEs of a hydroxide and the oxide related to it. This can be demonstrated by writing ... 

Fig. 2.16 Pourbaix diagrams for the iron/water system, (a) Reproduction of Fig. 2.11 showing regions of corrosion, immunity, and possible passivation, (b) Form of the diagram frequently employed. Source Ref 9, 10...

To illustrate the significance of the Pourbaix diagram to passivity, consider the iron-water system at point A in Fig. 5.2 (Ref 2). At this point, iron at a potential of -620 mV (SHE) is in equilibrium with a so-... 

Figure 4 The Pourbaix diagram for the iron-water system. The solid line is drawn for [Fe +] = lO M. The dashed line shows the limits of stability of water with respect to oxidation and reduction.

Figure 10.11 Pourbaix diagram of the iron-water system at 25 °C (after Kaesche "). The parameters mean the activities of the dissolved ions in mol dm . (Reproduced with permission from Ref. [2], 2003, Springer-Verlag.)...

Figure 2.19 Simplified Pourbaix diagram for iron/water system...

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. 

The Pourbaix diagram for the copper/water system is shown in Fig. 2.15. The more positive standard electrode potential of copper (+337 mV (SHE)) as compared to iron (-440 mV (SHE)) is evident. This greater nobility results in copper being thermodynamically stable in water that is, line 14 (-6) representing aCu2+ = 10-6 lies above line a. 

Fig. 5.2 Pourbaix diagram for the system iron-water. Encircled numbers ° identify phase boundaries as identified by Pourbaix. Numbers, 0to...

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.]... 

Figure S3.4 Simplified Pourbaix diagram showing the stable species in the iron-water-oxygen system (a) oxidation-reduction boundaries that are not pH-dependent (b) acid-base boundaries that are not oxidation-dependent (c) as part (b), including a sloping boundary that is dependent on pH and oxidation potential (d) complete diagram, showing further sloping boundaries that delineate regions that are dependent on pH and oxidation potential...

A Pourbaix diagram for copper/water system is shown in Fig. 2.22. Copper (E° = 0.337 V) is more noble than iron ( p = —0.444V), however, it is more stable in water (SHE) than iron. Copper is not passive in acid electrolytes. The oxide of copper, Cu+ and Cu " " are only... 

Fig. 1.16 Poiential-pH diagram for the Fe H20 system in which results obtained for the behaviour of iron in Brussels water have been inserted see Table 1.12) (after Pourbaix )...

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