I—curves

I (curve D). Thus the micropores had been able to enhance the adsorbent-adsorbate interaction sufficiently to replace monolayer-multilayer formation by micropore filling and thereby change the isotherm from being convex to being concave to the pressure axis. 

The Tafel potential is given by a bend in the U ff (log I) curve. According to criterion No. 4 in Table 3-3, under the conditions given in Section 3.3.3.1, it corresponds to the protection potential. 

Vibrio (i) Curved, rod-shaped bacterial cell, (ii) Bacterium of the genus Vibrio. Virion Virus particle the virus nucleic acid surrounded by protein coat and in some cases other material. 

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... 

Although the zones of corrosion, immunity and passivity are clearly of fundamental importance in corrosion science it must be emphasised again that they have serious limitations in the solution of practical problems, and can lead to unfortunate misconceptions unless they are interpreted with caution. Nevertheless, Pourbaix and his co-workers, and others, have shown that these diagrams used in conjunction with E-i curves for the systems under consideration can provide diagrams that are of direct practical use to the corrosion engineer. It is therefore relevant to consider the advantages and limitations of the equilibrium potential-pH diagrams. 

Fig, 1.24 Tafel lines for a single exchange process. The following should be noted (a) linear f-log I curves are obtained only at overpotentials greater than 0-052 V (at less than 0-052 V E vs. i is linear) b) the extrapolated anodic and cathodic -log / curves intersect at tg the equilibrium exchange current density and (c) /, and the anodic and cathodic current densities... 

A typical Evans diagrams for the corrosion of a single metal is illustrated in Fig. 1.26a (compare with Fig. 1.23 for two separable electrodes), and it can be seen that the E -I and E -I curves are drawn as straight lines that intersect at a point that defines and (it is assumed that the resistance for the solution is negligible). E can of course be determined by means of a reference electrode, but since the anodic and cathodic sites are inseparable direct determination of /co by means of an ammeter is not... 

Over the years the original Evans diagrams have been modified by various workers who have replaced the linear E-I curves by curves that provide a more fundamental representation of the electrode kinetics of the anodic and cathodic processes constituting a corrosion reaction (see Fig. 1.26). This has been possible partly by the application of electrochemical theory and partly by the development of newer experimental techniques. Thus the cathodic curve is plotted so that it shows whether activation-controlled charge transfer (equation 1.70) or mass transfer (equation 1.74) is rate determining. In addition, the potentiostat (see Section 20.2) has provided... 

Fig. 1.30 Corrosion of a metal in an acid in which both metal dissolution and hydrogen evolution are under activation control so that the .log i curves are linear, (a) Effect of pH on and I o Hi increase in pH (decrease in an + ) lowers E and decreases / o (b) Effect of...

Fig. 1.32 E-i curve for the simultaneous cathodic reduction of HjO (curve ImqpH) and dissolved oxygen (ABCG) which give the combined curve/4flC > E, Fis the anodic curve for...

Figure 1.32 shows the E-i curves for a metal corroding in an acid in which both dissolved oxygen and ions act as cathode reactants (note that... 

It is now appropriate to, consider the kinetics of the, hnodic reaction with particular refer ce to the phenomenon of passivity, but since the ml h anism is dealt with in detail in Seetion 1.5 this discussion will, place the emphasis on the anodic i curves. 

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 )... 

Potential-current density (E-i) curves, which have been determined for a number of the austenitic cast irons and also for the nickel-free ferritic irons, indicate that in general the austenitic cast irons show more favourable corrosion characteristics than the ferritic irons in both the active and passive states. 

Table 3.47 Breakdown potentials evaluated from E-i curves in 3% NaCl at 60°C...

Anodic E-i curves for nickel obtained by potentiostatic, potentiokinetic or, in earlier days, galvanostatic techniques, have been published by many workers. Unfortunately, good agreement is not always found between data from different sources. The principal reasons for the discrepancies appear to lie in the nature and amount of impurities in the metaP or in the solution -both of which may have a profound effect on the shape of the curve, and in variations in experimental procedure" . 

Fig. 4.20 Potentiostatic f-log i curve for nickel, anodically polarised in 0-05 M H2SO4 saturated with Nj at 25°C (after Sato and Okamoto )...

Skold and Larson" in studies of the corrosion of steel and cast iron in natural water found that a linear relationship existed between potential and the applied anodic and cathodic current densities, providing the values of the latter were low. However, the recognition of the importance of these observations is due to Stern and his co-workerswho used the term linear polarisation to describe the linearity of the rj — i curve in the region of E o , the corrosion potential. The slope of this linear curve, AE — AJ or Af - A/, is termed the polarisation resistance, / p, since it has dimensions of ohms, and this term is synonymous with linear polarisation in... 

From the magnitude of the Tafel slope 3tj/3 log / ( 0-12V), the magnitude of dr)/d In ( 0-24 V) and the linearity of the J vs. /i curves for pure iron in H2SO4 and NaOH at various temperatures in the range 18-80°C, Bockris, et al. concluded that the mechanism conformed to the reaction sequence shown in equation 20.107. 

FIGURE 14.6 Influence of surface-active ions [N(C4H9)4]+ (curve 2) and I (curve 3) on the polarization curve for hydrogen evolution at a mercury electrode in acidic solutions (curve 1 is for the base electrolyte). 

FIGURE 16.4 Galvanostatic vs. i curve for the anodic polarization of lead in 5 M H2SO4... 

Arai et al. (1997) used EQCM to study iodide absorption on polycrystalline gold in IM NaC104 containing different concentrations of Nal. Eigure 27.21 shows dynamic frequency change-potential A/-i curves for 0.1M NaC104 containing different concentrations of Nal. The frequency at potentials more positive than -0.8 V was less than that found without Nal. The frequency increased with... 

The above considerations concern a reversible electrodic process, ox + ne red as instead of 20-100 hz in the sinusoidal technique a fixed frequency of 225 Hz is normally used in the square-wave mode, the chance of irreversibility in the latter becomes greater, which then appears as asymmetry of the bellshaped I curve. Such a phenomenon may occur more especially when the complete i versus E curve is recorded on a single drop, a technique which has appeared useful51 in cases of sufficient reversibility. 

Experimental AE - I curves can be used for comparison of the air electrodes with respect to the transport hindrances. In order to illustrate this possibility in Fig. 9 are presented the AE - I curves for air electrodes with identical catalytic layers and gas layers differing in their thickness only. Apparently the hindrances in the transport of molecular oxygen will be higher in the electrodes with thicker gas layers. 

In Figure 11, we have presented the AE - I curves of air electrodes with catalysts from active carbon and active carbon promoted with different amounts of silver (from Figure 5). It is seen that the transport hindrances in the electrodes with catalysts from pure active carbon and with active carbon promoted with 5% of silver are near to each other. The transport hindrances in the air electrodes with catalyst containing 30% of silver are much higher. That s why catalysts containing large amount of silver are suitable to be used in air electrodes operating at comparatively low current densities. 

Figures 5 and h show how the shape of the creep curve is modified by changes in the constants of the model. The values of the constants are given in Table I. Curve I is the same as shown in Figure 4, curve II shows onlv a small amount of viscous creep, and in curve 111, viscous flow is a prominent part of the total creep. The same data were used in Figures 5 and 6, but notice the dramatic, change in the shapes of the curves when a linear time scale is replaced by a logarithmic time scale. In the model, most of the recoverable creep occurs "Within about one decade of the retardation time.

Use the solar Fe I curve of growth in Fig. 3.12 to deduce the solar iron abundance, using 9i0n = 0.9 and given that Fe I has an ionization potential of 7.87 eV and that the partition function of Fe II is 42. Compare the result with the one in Table 3.4. 

---