Polarization curve predicting change

Even with an established anodic polarization behavior, the performance of a material can vary greatly with relatively minor changes in the corrodent. This is also illustrated in Fig. 3. Frame 1 illustrates the case where the anodic and cathodic polarization curves intersect much as in materials with no active-passive behavior. The anode is actively corroding at a high, but predictable, rate. 

An additional test of the theory is given by the comparison of the data for different electrode materials. Experimentally the reduction of the same anion at different metals leads to quite different patterns for the polarization curve, in particular, a drastic variation of the interval in which the diffuse-layer minimum is observed. According to the theory (24), if the reacting species do not enter the compact layer and the possible change of the electron-tunneling factor does not influence the reaction rate (e.g. for adiabatic electrochemical reactions [58]) the corrected Tafel plots must be independent of the electrode material. This prediction has also been confirmed experimentally for the persulfate reduction at Hg (amalgams), Bi, Sn,... 

To verify these predictions the measurements of DMFC polarization curves in conditions -C A , J 1 were performed (Kulikovsky et al., 2005b). The results are shown in Figure 4.32(b). Comparing Figures 4.32(a) and 4.32(b), we see that the experimental polarization curves reproduce, remarkably well, the two main features of the model curves the drastic change in the slope at a current density J, where the jumper forms (large diamonds in Figure 4.32(b)) and the constancy of the product A°J J s for A° = 8,4 and 2 are related nearly as 1 2 4. 

Chang et al. [2, 3] provided an extended model with Butler-Volmer electrochemical reaction kinetics and the capabihty of predicting complete polarization curves. The results obtained for Y-shaped [2] andF-shaped [3] formic acid/dissolved oxygen-based cells were in good agreement with previous experimental studies [4, 5] and confirmed the cathodic activity and mass transport limitation of these cells. Consequently, the predicted cell performance was essentially independent of anodic fonnic add concentration. The numerical results also recommended high aspect... 

This effect is seen even more cleariy when the pH value is iowered below9.0. The anodic polarization curves (Figure 6) predict that passivation is not possible below a pH of 8.6. However, the passivation of steel at pH values as low as 8.0 has been demonstrated. Hausler inferred that EDTA forms an interphase inhibitor layer on steel composed of some sort of insoluble FeEDTA complex. Such a complex layer would change the Iron dissolution kinetics and also possibly influence the passivation behavior. As the free-EDTA concentration increased, this layer would tend to be less stable. The present data confirm such a trend. 

Example 4.13 Predicting Change in Polarization Curve Based on onr nnderstanding of... 

A possible reason for the lack of agreement between the experimental and theoretical curves is the large scatter in depth of traps for e. This scatter may be due to both the different initial depth of the traps and its change (increase) with time, e.g. as a result of polarizing the medium with the electron charge. The data indicating the presence of scatter in depth of traps for etr in MTHF were already reported in Sect. 1. Beitz and Miller [96] also present data on the possibility of changes in the depth of traps with time. Under the conditions of scatter of traps for et" in depth the electrons stabilized in shallower traps will decay faster than those stabilized in deeper ones. As a result, the kinetic curves for etr decay in MTHF turn out to be more "flat than is predicted by eqn. (7) of Chap. 5. 

Therefore we mention only two examples. The nonrelativistic 3 parameter for s-electrons is equal to 2, while the inclusion of both relativistic and correlational corrections leads in the case of 5s-electrons in Xe to a very complicated curve >, given in Fig. 2 together with recent experimental data. The action of both 5p and 4d excitations upon 5s-electrons is very essential. The inclusion of many-electron correlations leads to a very complex behaviour of D(a)) as a function of O), D((jo) even changing its sign several times. The detailed information on D(a)) may be obtained in a so called complete experiment For photoionization it includes measurement of partial subshell cross section and angular distribution, and of electron spin direction, i.e. its polarization. The polarization follows all variations of D(o)), Recently, extensive calculations of polarization parameters for noble gases were performedand the RPAE predictions are now confirmed expe-... 

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