Phase stability diagram

The thermodynamic phase stability diagrams appear to be preferred by corrosion scientists and technologists for the evaluation of gas-metal systems where the chemical composition of the gaseous phase consisting of a single gas or mixture of gases has a critical influence on the formation of surface reaction products which, in turn, may either stifle or accelerate the rate of corrosion. Also, they are used to analyse or predict the reason for the sequence of formation of the phases in a multi-layered surface reaction product on a metal or alloy. 

A detailed explanation of the construction of thermodynamic phase stability diagrams may be found in References 22-25. In this section the basic principles of construction and interpretation for the specific situation of gas-metal equilibria will be addressed using a hypothetical system. 

Fig. 7.67 Phase stability diagram for a metal-sulphur-oxygen (M-S-O) system at I 000 K. (For the thermodynamic data AC 000 for the various across-boundary reactions, see Table 7.37)...

Table 7.37 Data for the construction of thermodynamic phase stability diagram M-S-0 at 1 000 K...

Therefore, the calculated coordinates of the triple point for the coexistence of MO, MS and A/SO4 are logPso2 = +2 and logpo = - 12 and the slope of the MO/MSO4 boundary is - y. The straight line from point B having slope — y gives the boundary line (i) between the stability areas of MO and A/SO4. This completes the construction of the phase stability diagram forM-S-O at 1000 K. 

Fig. 7.70 Phase stability diagram for the Cr-O-S system on Incoloy 800H at 1 023 K showing thermodynamic and kinetic boundaries (after Natason )...

Phase Stability Diagrams with a Liquid Phase... 

Fig. 7.72 A thermodynamic phase stability diagram for Al-O-S species in the eqilibrium with liquid Na2S04 at 1 000°C as a function of the oxygen activity and the acidity of the salt (after...

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. 

Phase Stability Diagrams and the Formation of Volatile Halides... 

Fig. 7.79 Five possible reaction paths on a schematic thermodynamic phase stability diagram, and the corresponding distribution of phases in the reaction systems (after Stringer...

Fig. 7.80 A schematic thermodynamic phase stability diagram for the A-C-O system, showing three reaction paths. Paths 2 and 3 are only possible if gaseous diffusion in pores in the oxide product results in a carbon activity increase through the scale, as shown in Fig. 7.81 (after...

Because of the financial importance of this process to steel producers (about one-third of all the steel produced in the world is subsequently galvanised) a great deal of research has been carried out throughout the world to establish the true equilibrium phase boundaries in the Fe,, -Zn( system and the critical temperature of stability of the f phase. Since the AG -7 diagrams or the phase stability diagrams could not account for these discrepancies in this system, AG ,-concentration curves were used for... 

This concept has also been demonstrated at ambient temperature in the case of the Li-Sn-Cd system [47,48]. The composition-de-pendences of the potentials in the two binary systems at ambient temperatures are shown in Fig. 15, and the calculated phase stability diagram for this ternary system is shown in Fig. 16. It was shown that the phase Li4 4Sn, which has fast chemical diffusion for lithium, is stable at the potentials of two of the Li-Cd reconstitution reaction plateaus, and therefore can be used as a matrix phase. 

Figure 16. Calculated isothermal Li-Cd-Sn ternary phase stability diagram at ambient temperature [48],...

Fig. 12. Schematic representation of variations in dehydration rates (ft) with prevailing water vapour pressure (Ph2o) These examples include Smith—Topley behaviour and indicate correlations with phase stability diagrams. (After Bertrand et al. [596], reproduced with permission, from Journal of Inorganic and Nuclear Clemistry.)...

This is a rather useful computer program that allows the user to make several types of calculations, including reaction enthalpies, heat balances, equilibrium compositions, and phase stability diagrams. The noncritically evaluated database contains more than 11,000 compounds. Updated versions are now available. 

Fig. 26. Phase stability diagram as a function of pQl and Psio/Pn2 at T = 1350°C and ac = 1. After Wang and Wada [134]. Reproduced with permission of the American Ceramic Society, Westerville.

Figure 9. Reaction phase stability diagram of pure formic acid. Water melt curve is from F. Datchi et al., Phvs. Rev. B, 10 (2001) 6535.

P. L. Hemmings and R. A. Perkins, Thermodynamic phase stability diagrams for the analysis of corrosion reactions in coal gasification/combusion atmospheres. Research Project 716-1, Interim Report, Palo Alto, CA, Lockheed Palo Alto Research Laboratories for Electric Power Research Insitute, December, 1977. 

Erom the phase stability diagram of a metal-sulphur-oxygen system, as shown in Eigure 7.6, it should be possible to predict which phase will be present in the scale, given the atmosphere composition. 

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... 

The phase stability diagrams for natural source hydrates can become very complicated, especially when there are several guest components. This poses a major problem when assessing the quantity of gas, stability zones, structural types, etc. Also, the delicate physical balances determining structural types means that very small changes in the relative amount of a third component may trigger a structural transformation. 

In the phase-stability diagrams of Figure 14.7, contours of constant energy are shown that result from a numerical integration of Equation 14.4 for OH radicals in the 7 = 3/2, Af = - 9/4 state. The computations were carried out with the parameters of a Stark decelerator operated in our laboratory, at the values of the synchronous... 

FIGURE 14.7 Phase-stability diagram for OH(7 = 3/2,Mfi = —9/4) radicals when the decelerator is operated at a synchronous phase angle ())o = 0 ° (upper panel) or 4)o = 70 ° (lower panel), with Vi the velocity of the nonsynchronous molecule along the longitudinal coordinate z and 4) its phase. The value = 0 corresponds to the velocity of the synchronous molecule. The positions of the electrodes of the decelerator are indicated by dashed lines. (From van de Meerakker, S.Y.T. et., Annu. Rev. Phys. Chem., 57,159-190, 2006. Copyright (2006) by Annual Reviews www.annualreviews.org. With permission.)... 

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