Potential-pH diagram for iron

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

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.

FIGURE 14.27. Potential-pH diagrams for Iron-H20 system at elevated temperatures. Lines marked a and b represent reactions 8 and 9 respectively [31]. (With permission from Elsevier.)... 

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). 

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.

Fig. 2-2 Simplified potential-pH diagram for an iron/aqueous electrolyte system at 25°C c(Fe ) + c(Fe ) = 10" mol L (explanation in the text).

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. 

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.

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. 

Sketch the potential-pH diagram for the system O2-H2-H2O, and superimpose it on Figure 12-1 to illustrate the expected reactions for iron in this solvent. ... 

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 )...

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

In the case of many metals showing the ability to be passive, and of importance for engineering alloys, e.g. iron, nickel and chromium, the passive film, in acidic solution, does not result from thermodynamic equilibrium, but from the fact that the dissolution rate of the oxide in the acidic solution is slow. Figure 3-2 b shows a schematic potential-pH diagram for metal-water... 

Figure 10.11 Potential-pH diagram for the Fe-H20 system at 25°C, for a concentration of iron ionic species of 0.1M. The approximate regimes for fog and rain are indicated...

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... 

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

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. 

The principle of the cathodic protection may be elucidated for the case of carbon steel. The Pourbaix diagram for iron in water consisting of the plot E (potential) vs pH is shown in Figure 1.68. The regions of passivity, immunity and corrosion are seen in the figure. 

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... 

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