Steady anodic polarizations

Anode Polarization-the difference between the potential of an anode passing current and the steady-state or equilibrium potential of the electrode with the same electrode reaction. 

D.Y. Wang, and A.S. Nowick, Cathodic and anodic polarization phenomena at platinum electrodes with doped Ce02 as electrolyte. I. Steady-state overpotential, J. Electrochem. Soc. 126(7), 1155-1165 (1979). 

Fig. 12. Steady-state anodic polarization curves (a), and potentiostatic transient curves (b), of a mild steel hemisphere in neutral Na2S04 solution. From [15].

Assuming negligible anode polarization, the steady-state polarization curves can be described by a semi-empirical equation ... 

Figure 68. The exchange current density as a function of oxygen partial pressure for different temperatures confirming the electrode kinetical model given in the text.256 (Reprinted from D. Y. Wang, A. S. Nowick, Cathodic and Anodic Polarization Phenomena at Platinum Electrodes with Doped CeC>2 as Electrolyte. I. Steady-State Overpotential. , J. Electrochem. Soc., 126, 1155-1165. Copyright 1979 with permission from The Electrochemical Society, Inc.)...

In an earlier study we had reported the XPS analysis of tungsten oxides formed during anodic polarization experiments. It was determined that even at high applied potentials, the oxide thickness values are less than the mean free path of electrons in the oxides (generally assumed to be between 30 to 50 A ). Clearly the oxide growth in tungsten is a slow process. However, despite the relatively small thickness vsilues, the steady state current density during anodic polarization is restricted to a few tens of microamperes. 

Figure 1 Effect of pH on steady state current density during anodic polarization at 3 V. 

Figure 11 (A) Stripping voltammetry (20 m Vs at 55 °C) of CO layers on humidified PEM fuel-cell anodes (1) platinum catalyst (2) platinum/molybdenum catalyst. Voltammetry in the absence of adsorbed CO on the platinum/molybdenum catalyst is shown in (3). Molybdenum-mediated electro-oxidation of adsorbed CO takes place on the alloy catalyst in the peak at 0.45 V and at lower overpotentials [79]. (B) Steady-state polarization curves of PEM fuel-cell anode at 85 °C for platinum (squares) and platinum/molybdenum catalysts in the presence of 100 ppm CO (filled points) and pure H2 (unfilled points). (From Ref 79.)... 

A representative anodic polarization curve for iron in a buffered environment of pH = 7 is shown in Fig. 5.4. The solid curve is representative of experimental observations the dashed curve is an extrapolation of the Tafel region to the equilibrium half-cell potential of -620 mV (SHE) and aFg2- = 10 6. This extrapolation allows estimation of an exchange current density of 0.03 mA/m2. The essentially steady minimum current density of the passive state is ip = 1 mA/m2. 

Fig. 5.6 Anodic polarization curves for iron dissolution (solid curves) and for total current density of iron plus oxygen evolution (dashed curves) after 1 h at steady state in deaerated 0.15 M Na3P04 solution. Indicated pH obtained by use of acid and base buffers and additions of H2S04 or NaOH. Redrawn from Ref 5...

The potentiostat can be set to polarize the WE either anodically, in which case the net reaction at the WE surface is oxidation (electrons removed from the WE), or cathodically, in which case the net reaction at the WE surface is reduction (electrons consumed at the WE). With reference to the potentiostatic circuit in Fig. 6.1, determination of a polarization curve is usually initiated by measuring the open-circuit corrosion potential, Ecorr, until a steady-state value is achieved (e g., less than 1.0 mV change over a five-minute period). Next, the potentiostat is set to control at Ecorr and connected to the polarization cell. Then, the set-point potential is reset continuously or stepwise to control the potential-time history of the WE while Iex is measured. If the set-point potential is continuously increased (above Ecorr), an anodic polarization curve is generated conversely, if the potential is continuously decreased (below Ecorr), a cathodic polarization curve is produced. 

Figure 14. Plot of the total steady-state reaction rate increase, r-s, -Vo, as a function of the anodic polarizing current, I. Reprinted from J. Ekctroanal. Chem., 0. F6ti, V. Stankovic, 1. Bolzonella, and Ch. Comninellis, Transient Behavior of Electrochemical Promotion of Gas-Phase Catalytic Reactions, (2002), in press, with permission from Elsevier Science.

Other research in the field of simultaneous dissolution has focused on the active dissolution of Fe—Cr alloys, which was shown to proceed in the simultaneous mode at quasi-steady state conditions [40]. Applying y-spectroscopic methods, Kolo-tyrkin [41] measured the partial anodic polarization curves of the components Fe and Cr and was able to show that the dissolution rate of Cr from the alloy is more decreased than would have been expected on the basis of its bulk mole fraction (that is, Cr becomes the slow-dissolving component), and the contrary is true for the dissolution of Fe. This implies an enrichment of the Cr in the corroding alloy surface that may promote its subsequent passivation [34]. Also, with increasing Cr concentration of the alloy, the Tafel slope of the partial polarization curves of the components was shown to change from values that are typical for pure Fe to values that are typical for pure Cr [40, 41]. It appears, therefore, that for Fe—Cr alloys, the dissolution of the alloy components occurs in an interdependent... 

Partial anodic polarization curves of fast-dissolving alloy components under quasi-steady state conditions are of considerable interest for both practical and theoretical reasons. Experimental methods for their evaluation have been reviewed in the literature [71]. The most thoroughly investigated alloy/electrolyte system appears to be the anodic dissolution of Cu from binary Cu—Au alloys in acidic solutions. 

Instationary passive corrosion occurs for example at changing pH or changing potential. Here we refer to the work of Vetter and Gom [139]. Figure 22 shows their classical diagram for passivation of Fe. It was obtained from slow, instationary measurements of U(i) with analytical detection of i corr- In pure acid solution, the solubility product of Fc203 is not reached. Hence, the oxide dissolves slowly, but in the steady state, due to anodic polarization. 

Figure 4.4 shows an anodic polarization curve obtained under potentiostatic conditions. The current is allowed to reach a steady state value at each potential. 

---