Iron stability diagram

Fig. 4.2. Eh/pH iron stability diagram showing the natural domains of the main groups of the iron bacteria.

In recent work phase stability diagrams were used to evaluate the effect of molten Na2S04 on the kinetics of corrosion of pure iron between 600° C and 800° C by drawing a series of superimposed stability diagrams for Na-O-S and Fe-O-S at 600°C, 700° C and 800°C and thus to account for the differences in the corrosion behaviour as a function of temperature. 

Stability diagrams are able to condense a great amount of information into a compact representation, and are widely employed in geochemistry and corrosion engineering. The Pour-baix diagram for iron is one of the more commonly seen examples. 

On the basis of thermodynamic constants obtained for hydroxide compounds of iron with different aging time and also of experimental data, the physicochemical character of the diagenetic transformations of iron sediments of various compositions (oxide, silicate, carbonate, sulfide) can be traced. The results obtained are represented graphically in the form of stability diagrams of iron compounds as a function of variations in the main parameters governing the physicochemical character of the environment of diagenesis—pH, Eh, activity of iron and dissolved forms of sulfur and carbon dioxide. 

Diagenesis of iron cherts does not lead to any changes in principle in the relationships between minerals and mineral associations. As a result of diagenesis, no new compounds arise which could not have been formed by chemical precipitation. From a comparison of the stability diagrams of the sediments and minerals it is seen that only the numerical values of the... 

Fig. 5.3. Eh-pH stability diagrams for manganese and iron at 25°C and activity of 10 5 M (redrawn from Morgan and Stumm, 1965b).

Stability diagrams developed using thermodynamic data for iron and manganese are shown in Figures 10.3 and 10.4. These diagrams indicate the dominant stable species of iron and manganese... 

The redox potential-pH stability diagram (Figure 12.11) indicates that between pH 7 and 8, zinc carbonate (ZnCOj) is formed when the concentration of dissolved carbon dioxide (CO2) is 10 mol L . At low redox values, zinc sulfide is the most stable combination. Zinc precipitation by the hydrous metal oxides of manganese and iron is the principal control mechanism for zinc in wetland soils and freshwater sediments. The occurrence of these oxides as coatings on clay and silt enhances their chemical activity in excess of their total concentration. The uptake and release of the metals is governed by the concentration of other heavy metals, pH, organic and inorganic compounds, clays, and carbonates. 

Figure 7.11 The iron-sulphur-oxygen stability diagram at 900 °C showing reaction paths for duplex scale formation. The reaction path starting at X corresponds to the case for which Fe304 is stable at the bulk gas composition and the one starting at X corresponds to a bulk gas in which FeS is stable.

There is a need to develop the concept of stability diagrams to complex systems such as real alloys in concentrated acids or organic solvents. In such systems, it is critical to accurately represent the standard state properties as well as the activity coefficients. Recently, approaches have been developed and applied to a range of problems such as the formation of iron sulfide scales [7]. 

By combination of thermodynamic stability diagrams of the salt phase to be investigated and the oxide phase under consideration, phase stability diagrams can be constructed to predict the behavior of any oxide in any molten salt. Figure 5 shows the Na-Fe-S-O-phase diagram [11,12] for prediction of corrosion of iron in a Na2S04 melt at 1200 K. This diagram is constructed from... 

Comparison of the conventional stability diagram (left), the "static" quasi-stability diagram (middle), and the "dynamic" quasi-stability diagram (right) for iron at 800 C (for explanation see text). 

Over the years, Pourbaix and his co-workers in the CEBELCOR Institute, founded under his direction, extended these diagrams by including lines for metastable compounds. Figure 7.66 illustrates such a presentation for the Fe-O system over the temperature range 830-2200 K. Pourbaix used these diagrams as a basis for a discussion of the stability of metallic iron (solid, liquid and vapour phases), the oxides of iron as a function of oxygen pressure and temperature from which he explained the protection of iron at high temperature by immunity and passivation. He also pointed out the... 

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. 12.2. Redox-pH diagram for the Fe-S-H20 system at 100 °C, showing speciation of sulfur (dashed line) and the stability fields of iron minerals (solid lines). Diagram is drawn assuming sulfur and iron species activities, respectively, of 10-3 and 10-4. Broken line at bottom of diagram is the water stability limit at 100 atm total pressure. At pH 4, there are two oxidation states (points A and B) in equilibrium with pyrite under these conditions.

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