Cyclic polarization potential

It has been demonstrated by Arvia and his co-workers that surfaces of preferred crystallographic orientation can be obtained by fast repetitive potential perturbations. After a very fast cyclic polarization in the range 0.04 to 1.50 V, peaks characteristic of Pt(lll) and Pt(lOO) become more pronounced, whereas a peak typical for Pt(llO) disappeared. Voltam-metric curves indicating the change of the Pt polycrystalline electrode structure are shown in Fig. 1. 

The more densely packed reconstmcted surface has a higher work function and a more positive pzc than the unreconstructed one. During cyclic polarization, the shape of voltammograms changes markedly if the scan enters higher positive potentials. The current charge associated with the removal of the reconstruction must be accounted for in the electrochemical studies on reconstructing surfaces. 

Fig. 1 A typical response of Ni to cyclic polarization in 0.1 mol dm NaOH. Potential measured against Hg HgO electrode. Scan rate 60 mV. Explanation see in the text [91].

Fig. 11 Cyclic current-potential curve forAu(lOO) in 0.1 M H2SO4 solution beginning of polarization —0.2 V (versus SCE) using a freshly prepared reconstructed surface. Scan rate ...

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.

From polarization curves of the type shown in case 3, three important parameters can be determined ECOSI, Ebth and In the literature there exists a nearly infinite number of variations of nomenclature, many of which are shown in Table 2. The interpretation of cyclic polarization curves has been and continues to be a subject of great controversy. The classic interpretation of case 3 would be that the potential of a material must exceed EM for new pits (or localized corrosion sites) to nucleate, but that at potentials between EM and En existing pits can propagate. At potentials below En all localized corrosion sites repassivate. Thus, from a design or material selection perspective, a material will perform well if its Econ is kept below This criterion can be met by environment... 

Controversy concerning the interpretation of cyclic polarization curves has raged for many years. Of particular interest is which (if either) of the two potentials can be used for material selection and mitigation strategy decisions. The classic interpretation is that a material s potential must exceed Ehl[ in order to initiate pits, but if flaws were introduced into the surface in any way, they could propagate at all potentials above Ew. Thus Eq, could be used in design as a protection potential. 

Figure 42 Cyclic polarization curve for Type 302 stainless steel in 1,000 ppm NaCl. Note the definition of the breakdown and repassivation potentials, the vertex current density, and the appearance of metastable pits.

These tests focused on the determination of a materials resistance to localized (pitting) corrosion. To accomplish this goal, three types of electrochemical experiments were conducted (cyclic polarization, electrochemical scratch, and potenti-ostatic holds) to measure several key parameters associated with pitting corrosion. These parameters were the breakdown potential, EM, the repassivation potential, Etp, and the passive current density, tpass. 

Apparatus for electrochemical measurements during corrosion fatigue. CF tests can be done using an apparatus designed by the Continental Oil Company, as shown in Figure 6.52.110,111 The polarization potential and current can be controlled for the four samples tests at the same time. The apparatus consists of a Monel tank in which four specimens are subjected to cyclic bending. The preliminary step in the experiment is to determine the displacement caused by the desired applied load. The exact stresses are then determined with the use of strain gages. 

For alloys showing high susceptibility to crevice corrosion, measurements of the pitting potentials are of limited value since failure in service by crevice corrosion would predominate. Polarization measurements can be useful in showing relative susceptibility of alloys to crevice corrosion. Figure 7.46 shows results from cyclic polarization measurements on specimens of three alloys containing O-rings to produce crevices (Ref 66). The environment was aerated water with 3.5... 

Cyclic nucleotide-modulated channels consist of two groups the cyclic nucleotide—gated (CNG) channels, which play key roles in sensory transduction for olfactory and photoreceptors, and the hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels. HCN channels are cation channels that open with hyperpolarization and close with depolarization upon direct binding of cyclic AMP or cyclic GMP, the activation curves for the channels are shifted to more hyper-polarized potentials. These channels play essential roles in cardiac pacemaker cells and presumably in rhythmically discharging neurons. 

Fig. 7.1 Typical cyclic polarization plot for stainless steel that shows the corrosion potential, c, critical pitting potential, Epn, protection potential, Eprot, and metastable pitting region.

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