Potential-pH diagrams of iron

Figure 2.19 Potential-pH diagram of iron T = 25 °C). The concentrations of the dissolved ionic species, Fe and are equal to 10 mol...

Table 1-1. Equilibria for the potential-pH diagram of iron shown in Fig. 1-2.

The aqueous corrosion and passivation of metal surfaces involve electrochemical processes at the electrode-electrolyte interface. Like for all chemical reactions, the two aspects of equilibrium and kinetics have to be treated. Thermodynamic data give an important first insight into layer formation. They have been used to compose potential-plT diagrams for all elements and thus for all metals [26,27]. In Chapter 1 of this book on the fundamentals of corrosion, passivity of iron has been mentioned and the calculation of some of the lines based on thermodynamic data has been described. Here a more detailed description of foe potential-pH diagrams of iron and copper are presented. 

Electrochemical and Chemical Equilibria for the Potential-pH Diagram of Iron at 298 K... 

Fig. 12.11. The potential-pH diagram for iron immersed in a ferrous ion solution of unit activity.

The corrosion of an iron vessel has been treated here only to make the discussion less abstract. A similar approach can be used to inspect the potential-pH diagrams of other metals and decide whether they tend to corrode spontaneously in solutions of a given pH. 

Fig. 16M Simplified potential/pH diagram for iron. Vertical lines represent chemical equilibria, in which the state of oxidation does not change. Horizontal lines correspond to electrochemical equilibria in which and OH ions do not participate.

FIG. 4—Potential-pH diagram for iron in water, providing a iink between the chemistry of aqueous environment, material, and corrosion [34. ... 

Figure 1-2. a) Potential-pH diagram for iron [after Pourbaix (1963)], including the experimental passivation potential in acidic electrolytes (central dashed line), b) Simplified poten-tial-pH diagram for iron indicating the domains of immunity, corrosion, and passivity (Pourbaix, 1963), including the experimental passivation potential (bold dashed line). 

The conditions for stability of chemisorbed sulfur on the surface of metals can be predicted using a formalism that was developed for the calculation of potential-pH diagrams of adsorbed species. Such diagrams have been calculated for Sads on iron (Marcus and Protopopoff, 1990), nickel... 

Figure 4. Potential-pH diagram for iron (A) and nickel (B) in supercritical aqueous solutions at 400"C and 450°C, respectively, P = 500 bar. The hydrogen equilibrium line for unit hydrogen fugacity is coincident with the Ni/NiO equilibrium line. The diagram for iron shows the approximate regions in potential-pH space for the operation of SCWO reactors and supercritical thermal power plants (SCTPPs). Reprinted from Ref. 5, Copyright (1997) with permission from Elsevier.

It should be emphasised that potential-pH diagrams can also be constructed from experimental E -I curves, where E is the polarised potential and / the current. These diagrams, which are of more direct practical significance than the equilibrium potential-pH equilibrium diagrams constructed from thermodynamic data, show how a metal in a natural environment (e.g. iron in water of given chloride ion concentration) may give rise... 

Fig. 1.56(a) E-i curves and experimental potential-pH diagram for Armco iron in chloride-free solutions of different pHs (A is the unpolarised potential and P the passivation potential) and (b) E—i curves and experimental potential-pH diagram for Armco iron in solutions of different pHs containing 10 mol dm of chloride ion (r is the rupture potential and p the protection potential). (After Pourbaix )... 

Before considering the principles of this method, it is useful to distinguish between anodic protection and cathodic protection (when the latter is produced by an external e.m.f.). Both these techniques, which may be used to reduce the corrosion of metals in contact with electrolytes, depend upon the electrochemical mechanisms that result from changing the potential of a metal. The appropriate potential-pH diagram for the Fe-H20 system (Section 1.4) indicates the magnitude and direction of the changes in the potential of iron immersed in water (pH about 7) necessary to make it either passive or immune in the former case the stability of the metal depends on the formation of a protective film of metal oxide (passivation), whereas in the latter the metal itself is thermodynamically stable and egress of metal ions from the lattice into the solution is thus prevented. 

Figure 6 Potential-pH diagram the possible passivity, immunity and corrosion areas for iron in the presence of Cr042 under ambient conditions.

Fig. 12.12. Example of potential-pH diagram for a system with a solid phase as a dissolution product. Iron has a tendency to corrode at all pHs, but at pH > 9 it forms Fe(OH)2.

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. 

Figure 4.4 Representative Pourbaix diagram of iron. The small gray section represents the region of cell potential and pH where Fe3C>4 can exist.

In the discussion of E the vs pH diagram for iron in water depicted in Figure 1.70, we noted that, with application of high positive potentials, the system moves into a region of passivity and results in a reduced corrosion rate. The passive film formed should be coherent and insulating to withstand corrosion and mechanical breakdown. Upon formation of the passive state the corrosion rate is reduced. Thus by polarization and applying more positive potentials than corrosion potentials the metal attains passivity and is protected. This is the principle of anodic protection. It is necessary that the potential of passivation be maintained at all times, since deviations outside the range would result in severe corrosion. 

In terms of the Pourbaix potential/pH diagrams, the theoretical scale compares the potentials of immunity of the different metals, while the practical scale compares the potentials of passivation. But this is not enough either. The real scale depends on the environment with which the structure will be in contact during service. Passivity, as we have seen, depends on pH. It also depends on the ionic composition of the electrolyte, particularly the concentration of chloride ions or other species that are detrimental to passivity. Finally, one must remember that construction materials are always alloys, never the pure metals. The tendency of a metal to be passivated spontaneously can depend dramatically on alloying elements. For example, an alloy of iron with 8% nickel and 18% chromium (known as 304 stainless steel) is commonly used for kitchen utensils. This alloy passivates spontaneously and should be ranked, on the practical scale of potentials, near copper. If... 

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. 1.16 Potential-pH diagram for the Fe HjO 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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