Polarization curves, noble metals

The region of an anodic polarization curve, noble to and above the passive potential range, in which there is a significant increase in current density (increased metal dissolution) as the potential becomes more positive (noble). 

In the polarization curve for anodic dissolution of iron in a phosphoric acid solution without CP ions, as shown in Fig. 3, we can see three different states of metal dissolution. The first is the active state at the potential region of the less noble metal where the metal dissolves actively, and the second is the passive state at the more noble region where metal dissolution barely proceeds. In the passive state, an extremely thin oxide film called a passive film is formed on the metal surface, so that metal dissolution is restricted. In the active state, on the contrary, the absence of the passive film leads to the dissolution from the bare metal surface. The difference of the dissolution current between the active and passive states is quite large for a system of an iron electrode in 1 mol m"3 sulfuric acid, the latter value is about 1/10,000 of the former value.6... 

Figure 11. Schematic diagram of anodic polarization curve of passive-metal electrode when sweeping electrode potential in the noble direction. The dotted line indicates the polarization curve in the absence of Cl-ions, whereas the solid line is the polarization curve in the presence of Cl ions.7 Ep, passivation potential Eb, breakdown potential Epit> the critical pitting potential ETP, transpassive potential. (From N. Sato, J, Electrochem. Soc. 129, 255, 1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

The anodic evolution of oxygen takes place at platinum and other noble metal electrodes at high overpotentials. The polarization curve obeys the Tafel equation in the potential range from 1.2 to 2.0 V with a b value between 0.10 and 0.13. Under these conditions, the rate-controlling process is probably the oxidation of hydroxide ions or water molecules on the surface of the electrode covered with surface oxide ... 

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

The individual polarization curves for the metals are often modified as a result of interactions resulting from codeposition. If the alloy deposition occurs at low polarization, the nobler metal will be deposited preferentially (Cu in Example 11.1). All factors, however, that increase polarization during electrodeposition, such as high current density, low temperature, and quiescent solution—factors that increase concentration polarization—will favor the deposition of the less noble metal (Zn in Example 11.1). 

One example of the application of polarization curves in a predictive manner involves their use in galvanic corrosion. Galvanic corrosion occurs when two dissimilar metals are in electrical and ionic contact as is schematically shown in Fig. 29. Galvanic corrosion is used to advantage in sacrificial anodes of zinc in seawater and magnesium in home water heaters. It slows corrosion of millions of tons of structural materials. The darker side of galvanic corrosion is that it also causes major failures by the accelerated dissolution of materials that are accidentally linked electrically to more noble materials. 

T. P. Hoar (38a) has suggested that the different behavior of ferric and ceric ions in silver dissolution may be considered from a purely electrochemical viewpoint. The anodic and cathodic potentials are very close in the ferric system their polarization curves meet at rather small values of the corrosion current, which does not require that all ferric ion near the metal surface be reduced to ferrous. On the other hand the potential of the ceric-cerous couple is about 0.8 V more noble, and complete reduction at the interface (or complete concentration polarization with respect to ceric ion) is necessary to lower this potential to the value of the Ag-Ag+ couple. 

Oxygen Electrocatalytic Properties Oxygen Reduction. Figure 8 compares steady-state polarization curves for the electroreduction of Op on a typical pyrochlore catalyst, Pb2(Rui.42Pbo.53)06.5 15 w/o platinum on carbon. The latter was considered representative of conventional supported noble metal electrocatalysts. The activities of both catalysts are quite comparable. While the electrodes were not further optimized, their performance was close to the state of the art, considering that currents of 1000 ma/cm could be recorded, at a relatively moderate temperature (75 C) and alkali concentration (3M KOH). Also, the voltages were not corrected for electrolyte resistance. The particle size of the platinum on the carbon support was of the order of 2 nanometers, as measured by transmission electron microscopy. 

Fig. 4.21 Schematic representation of polarization curves for the analysis of galvanic coupling when one metal is significantly more noble. Tafel polarization is represented.

The isolated H2 molecule possesses an occupied lOg, bonding level well below the bottom of most metal bands and a luu, antibonding level above Ep. At large distances from the metal surface, the electronic stmcture of H2 is little affected by the presence of the surface. The physisorption well is determined by the dynamic polarization properties of H2 (van der Waals attraction) and the steep rise in energy due to Pauli repulsion as the separation is reduced. H2 acts as a neutral but polarizable adsorbate. A physisorbed state of that nature is expected on all simple and noble metal surfaces. The corresponding potential energy curves were calculated for the simple metals Al, Mg, Li, Na and K and for the noble metals Cu, Ag and Au [101-106]. They compare weU with the few available experimental results [107,108]. 

Potentiodynamic anodic polarization curves of binary noble metal alloys exhibit... 

Figure 3.27. Tafel plots of the polarization curve for the best non-noble metal catalyst obtained on Norit compared to a catalyst from E-Tek loaded with Pt at 10 wt%. The GDE currents are expressed in A/mg metal. Dark circle reported state-of-the-art activity at 900 mV for H2/saturated O2 at 65°C, 100 kPa, for a cathode with 0.4 mg Pt/cm. The nonnoble metal catalyst was made by adsorbing iron acetate (0.2 wt% Fe) on pretreated Norit. This material was heat treated at 900°C in H2 Ar NH3 (1 1 2). The pretreated Norit was obtained by refluxing Norit in HNO3 (according to Figure 10 in ref. [113] reproduced with permission of The Electrochemical Society).

CPPs of several metals in 0.1 A NaCl are listed in Table 6.1, values of which were derived from anodic polarization curves. Most of the data were obtained by allowing 5 minutes or more at a given potential and observing whether the resultant current increases or decreases with time. The CPP is the most noble potential for which the current decreases or remains constant it is usually confirmed by holding the potential at the critical value for 12 hours or more and observing absence of pits under a low-power microscope. 

Fig. 7.2 Polarization curves for the electrodeposition of more noble metal (A) and less noble metal (B) /l(A) diffusion limiting current density for the electrodeposition of metal (A), M(B) current density for the electrodeposition of metal (B), /d(all) current density for the electro-deposition of alloy (Reprinted from Ref. [5] with kind permission from Springer)...

Fig. 7.1b the oveipotential for electrodeposition of metal A is slightly lower than that for metal B, i.e., the polarization curves are almost parallel. Hence, the electrodeposition of alloy commences at the potential r(B)> while the alloy contains more metal A than B. If the difference between r(A) and r(B) is high and the overpotential for electrodeposition of the more noble metal A is lower than that for the less noble metal B, the third case presented in Fig. 7.1c applies in such a case, alloy electrodeposition is impossible. The difference between the reversible potentials of two metals could be changed (lowered) by the change of metal ion concentration (activity), and in most cases, this is achieved by the complexation. 

In the 1920 s, E. MQller and his co-workers made a series of studies on the anodic oxidation of methanol, formaldehyde, and formic acid which represent the first extensive mechanistic investigation of these compounds, although the principles of electrode kinetics had not yet been formulated. Muller did not establish mechanisms for these reactions however, many of his observations have been later confirmed and his studies were among the first with a comparison of polarization curves on several noble metals including platinum, palladium, rhodium, iridium, osmium, rubidium, gold, and silver (cf. Figure 1). As was usual at that time, Muller discussed his results in terms of polarization, rather than in terms of current or reaction rate. 

The behavior is illustrated by the anodic polarization curves of two gold-copper alloys and the pure metals shown in Figure 7.29, which were measured in a concentrated chloride electrolyte capable of dissolving copper as well as gold. The value of the critical potential increases with the noble-metal content in the alloy. The current in the subcritical potential region is lower at higher gold concentration. 

Cathodic control protection protects the substrate by coating with a less noble metal, for which the slopes of the cathodic polarization curves are steep. The cathodic overpotential of the surface is increased by the coating therefore, the corrosion potential becomes more negative than that of the substrate. Coating materials used for this purpose are zinc, aluminum, manganese, cadmium, and their alloys. The electrode potential of these metals are more negative than those of iron and steel. When exposed to the environment, these coatings act as sacrificial anodes for the iron and steel substrates. 

A cathodic polarization curve of the metal with a noble corrosion potential and an anodic polarization curve of the metal with a negative corrosion potential are plotted. The two polarization curves are overlapped. The intersection of two polarization curves is the amount of galvanic current that will be produced when these two metals are in contact with each other. One important point to mention is that, due to the different surface areas of two metals, they receive the same galvanic current (instead of the same current density) during galvanic corrosion. To determine the galvanic current when the two polarization curves are overlapped, they should be modified to use current, not current density, as the x-axis. 

The oxygen reversible potential of 1.23 V was confirmed indirectly by extrapolation of anodic and cathodic polarization curves on various noble metal electrodes. The interception of the polarization curves at the potential close to 1.23 V was observed with highly oxidized Pt electrode [1,8]. These results were unusual as the extrapolations were made from high cathodic and high anodic overpotentials where the surface ccmditions were different and it was expected that the mechanisms for reduction and oxidation of oxygen might be different. 

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