Chromium potential diagram

Fig. 2. Potential diagram of the redox couples of chromium. According to Csanyi . References pp, 577-580...

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 computer program was developed for electrochemical equilibria calculations and graphical pH-potential diagram presentation of a one-metal/one-nonmetal/water system. The program can be used for temperatures and pressures exceeding the supercritical point of water. The calculations show that hematite (Fe203) is the oxidation product of iron in supercritical water, and the oxidation product of chromium in supercritical water is an ionic species, Cr04 . Passivation effect of chromium is lost in supercritical water. 

Fig. 21.9 Potential diagram for chromium at pH 0. A Frost-Ebsworth diagram for Cr is shown in Figure 7.4a.

Fig. 5.18—cont d (a) Tafel polarization curves for chromium corrosion diagram showing the corrosion potential and the corrosion current, (b) The corrosion current and the potential if mass limitations for hydrogen limit the maximum current to 10 A/cm, and (c) to 10 A/cm, respectively. 

FIGURE 3 Oxidation state-potential diagrams for vanadium and chromium [1. OM H+(aq)]. 

Use data from the text to construct a standard electrode potential diagram relating the following chromium species in acidic solution. 

Latimer diagrams were invented by W. M. Latimer and consist of lines of text of the various oxidation states of an element arranged in descending order from left to right, with the appropriate standard reduction potentials (in volts) placed between each pair of states. The diagram for chromium in acid solution is written as ... 

Fig. 1. Estimated Latimer diagrams for the reduction of aqueous dioxygen calculated on the thermodynamic assumption of [02] = 1 M at pH = 0, 25°C. The overall four-electron process has the same potential (1.27 V) for both the chromium free and chromium mediated processes (14).

The production of corrosion-resistant materials hy alloying is well established, hut the mechanisms are noi lull) understood. It is known, of course, that elements like chromium, mckcl. titanium, and aluminum depend for their corrosion resistance upon a tenacious surface oxide layer (passive film). Alloying elements added for the purpose of passivation must be in solid solution. The potential of ion implantation is promising because restrictions deriving from equilibrium phase diagrams frequently do not applv li e., concentrations of elements beyond tile limits of equilibrium solid solubility might he incorporated). This can lead to heretofore unknown alloyed surfact-s which are very corrosion resistant... 

FIGURE 21.21 Potential-pH equilibrium diagram for the chromium-water system at 25 °C in solutions not containing chloride. [Figure established considering Cr(OH)3.] (from Ref. 33). 

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... 

Passivation of iron under critical conditions is predicted. Hematite (Fe203) may still be the main corrosion product in the neutral water pH (pH 7.2) region, but the passivation potential range is narrower and shifts to negative potentials, compared with regions on the diagram for ambient conditions. For chromium, no solid chromium oxide stable species is predicted within the stable region of neutral water. This indicates chromium oxidation without any passivation oxide film formation. 

FIGURE 10.5 Potential energy diagram (enthalpy of reaction in kcal/mol) for oxidative addition of thiols. The first step involves the formation of a reversible 19 e coordinated thiol adduct that has an estimated enthalpy of binding of —3 to —6 kcal/mol. The transition state occurs when this is attacked by a second mole of chromium radical. The measured enthalpy of activation is near 0 kcal/mol since the exothermic enthalpy of binding in the first step is canceled by the activation enthalpy of the second step. Adapted with permission from reference 79. Copyright 1996, American Chemical Society. 

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. 

Figure 10.18 Current-potential plots of iron-chromium alloys in 0.5 mol-dm" H2SO4. (The figures in the diagram are weight percent chromium.) (Reproduced with permission from Ref. [32], 1989, Elsevier.)...

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