Polarization curves hysteresis

FIGURE 8-3. Fuel cell polarization curve hysteresis cathode drying (temperature. 80 C, Hi/Air humidification 80/60°C, Hi/Air stoichiometry 1.5/5.0). 

Fig. 2 distinguishes the domains of immunity, corrosion and passivity. At low pH corrosion is postulated due to an increased solubility of Cu oxides, whereas at high pH protective oxides should form due to their insolubility. These predictions are confirmed by the electrochemical investigations. The potentials of oxide formation as taken from potentiodynamic polarization curves [10] fit well to the predictions of the thermodynamic data if one takes the average value of the corresponding anodic and cathodic peaks, which show a certain hysteresis or irreversibility due to kinetic effects. There are also other metals that obey the predictions of potential-pH diagrams like e.g. Ag, Al, Zn. 

Figure 23 Schematic polarization curve for metal that spontaneously passivates but pits upon anodic polarization. A hysteresis loop, which can appear during a reverse scan, is shown ending at Erp. One dotted line shows behavior for anodizing conditions, while the other shows transpassive dissolution.

Figure 24 Schematic Evans diagram and polarization curve illustrating the origin of the negative hysteresis observed upon cyclic polarization for materials that do not pit. Line a represents the (unchanging) cathodic Evans line. Line b represents the anodic Evans line during the anodically directed polarization, while line c represents the anodic Evans line for the material after its passive film has thickened because of the anodic polarization. The higher corrosion potential observed for the return scan (E (back)) is due to the slowing of the anodic dissolution kinetics.

Potentiodynamic polarization determines Eap for positive scans, whereas negative scans yield E, . If they are different, the polarization curve shows a hysteresis. In many cases, this difference gets smaller with decreasing scan rates, indicating that the critical potentials are influenced by the composition of the pit electrolyte as well as kinetic factors like pit nucleation and pit growth. The ASTM standard G61 applies 10mVmin [11]. Potentiostatic tests depend less on the experimental conditions and thus are more reliable but time consuming. Usually a potential is applied and the current density is followed for some time. If the current decreases continuously, E < np will hold, whereas it increases when E exceeds np-If pits are formed at > np and then the potential is stepped to less positive values, the current density will drop continuously when E < rp is reached. For some systems, both critical potentials are... 

A correlation has been between pitting potential and pitting in the field the tendency to form pits naturally at open circuit increases as the experimentally determined pitting potential decreases [51]. The difference between E-p and E, which is related to the extent of hysteresis in a cyclic potentiodynamic polarization curve, has also been considered to be a measure of the susceptibility to localized corrosion [3,52]. Issues associated with this interpretation have been reviewed [53]. 

The polarization curves in Figure 3.10 show a hysteresis loop, where the steady state current and voltage depend on the direction of approach. It was possible to go around the hysteresis loop shown in Figure 3.10 reproducibly many times. When the load resistance was between 5 and 110, the steady state current and voltage were dependent on the direction of approach. These multi-valued steady states were stable the current and voltage at any point on the steady state polarization curve was steady for periods of >24 h. 

FIG. 7—Cyclic potentiodynamic polarization curve for type 316L SS showing hysteresis and crevice corrosion. 

Determination of pitting potential. In analyzing polarization curves, the appearance of a hysteresis (or loop) between the forward and reverse scans is often thought to denote the presence of locahzed corrosion (pitting or crevice corrosion). This observation is particularly valid when the corrosion potential is higher or more noble than the pitting potential. 

Figure 23. Cyclic polarization curves recorded near the corrosion potential under potentiostatic conditions for a polycrystalline (A) and a single-crystal Johnson Matthey iron rotating disk electrode (B) in 0.5M H2SO4 solution (pH 0.3), displaying hysteresis only for the single-crystal electrode. (From Ref. 33.)...

However, it was found by Drazic and Drazic that polycrystalline Armco iron in 0.5M H2SO4 also exhibited hysteresis, the Tafel-like slope being 67 mV dec upon reversal of the polarization whereas the value on the first scan in the anodic direction was = 30-36 mV dec" (see Fig. 24). The hysteresis effect was greater when the anodic cyclic polarization was applied after a previous cathodic cyclic polarization. The slope of the anodic polarization curve after the anodic cleaning of the surface was higher and depended on the highest current density used and the time taken for the measurements, during which the readsorption process proceeds to a certain extent. 

This treatment also provides a theoretical support for the earlier ideas of Epelboin and co-workers concerning the role of hydrogen as responsible for the hysteresis observed near the corrosion potential, as well as those of Allgaier concerning the shift of the anodic polarization curves toward lower current densities for cathodically prepolarized iron. 

COMMENTS This calculation assumes aU water generated goes directly to liquid and none to vapor, which is unlikely to be true, so that the time scale required for liquid water accumulation will be even greater than calculated. In a PEFC, this slow time scale, the slow time scales of evaporation or drainage from the porous media and electrolyte, coupled with the nearly instantaneous electrochemical and gas-phase response times lead to hysteresis in a polarization curve and the fuel cell memory effect discussed. 

Hysteresis of the fuel cell polarization curve is caused by ... 

Later, a more detailed investigation of the evolution of hydrogen at tungsten[177] showed that the shape of the polarization curve described above, i.e. the existence of two Tafel regions, is unstable and corresponds to a surface on which an indefinite quantity of oxide is retained. A cleaner surface, on which stable values of overpotential can be obtained almost without any hysteresis for forward and backward runs, is characterized by the absence of straight semi-logarithmic sections and by a continuous increase in the slope. In acidic and alkaline solutions, the overpotential is found to be almost independent of pH both in salts and in pure alkalis. Besides, it is also found to be independent of the nature of the cation (Li" ", Na, K", Cs" ", Ba, N(Ct H9)J). The electrode has a large adsorption capacity. All these facts indicate the predominance of a slow recombination mechanism. 

For perm anen t-m agnet materials where the coercivity is large, the demagaetizatioa curve, which correspoads to the secoad quadrant of the hysteresis loop, sometimes is plotted as the polarization J(= B — vs H(B — H vs H) to show the intrinsic characteristics of the material. The value of... 

Hysteresis curve of a ferroelectric crystal, v = initial (virginal) curve, Pr = remanent polarization, Ps = spontaneous polarization, Ec = coercive field... 

---