Anodic polarization curves

Iron has a tendency to oxidation at potentials more positive than the equihbrium potential of the reaction Fe — Fe + 2e, which is about —1 V SCE. Therefore, below IV steel is in a condition of immunity. 

In the range of potentials between —800 mV and +600 mV, the anodic current is very low (0.1 mA/m ) because the steel is covered by a very thin fihn of iron oxide that protects it completely (passive fihn). Thus in this interval of potentials 

Schematic anodic polarization curve of steel in non-carbonated concrete without 

In the interval of potentials between equilibrium and about —800 mV, the protective film does not form spontaneously. In this condition, called activity, steel can theoretically corrode. Nevertheless, given the proximity to equilibrium conditions, the rate of the anodic process is still negligible. To emphasize the fact that these conditions of activity are characterized by low corrosion rates because of this proximity to equihbrium, they are also called quasi-immunity conditions. 

Above the passivity range, that is for potentials above about +600 mV, the steel is brought to conditions known as transpassivity. oxygen may be produced on its surface according to the anodic reaction of oxygen evolution 2H2O — O2 + 4H + 4e, which produces acidity. Steel reaches these conditions only in the presence of an external polarization (for example in the presence of stray currents). Since the anodic reaction is oxygen evolution, dissolution of iron and consequent corrosion of the steel does not take place (i. e. the passive film is not destroyed). Nevertheless, if these conditions persist until the quantity of acidity produced is sufficient to neutralize the aLkaHnity in the concrete in contact with the steel, the passive film will be destroyed and corrosion will initiate. This aspect wiU be dealt with in Chapter 9. 

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Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

It is worth emphasising too, that the position of those lines representing equilibria with the dissolved species, M, depend critically on the solubility of the ion, which is a continuous function of pH. For example, iron in moderately alkaline solution is expected to be very passive and so it is in borate solutions (in the absence of aggressive ions). However, the anodic polarization curve still shows a small active loop at low potential. 

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.)...

FIGURE 15.9 Anodic polarization curves recorded at a platinum electrode in the region of high anodic potentials in the presence of acetate ions (1) total current (2) partial current of oxygen evolution (3) partial current of oxidation of adsorbed species. 

This is the equation of a reversible polarization curve. The anodic polarization curve of the reduced form obeys an identical equation... 

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

In research laboratories, potential is often used as the critical value for comparison purposes. A pitting potential is determined at a given suitable constant temperature, recording the anodic polarization curve while keeping other factors constant (Fig. 11). 

Figure II. Schematic anodic polarization curves at a fixed temperature. Determination of either transpassive potential ( ,) or pitting potential ( p and repassivation potential ( ,) at the critical current density (/ ). t rcvis the current density at which the scan is reversed. ...

For decades a common way to characterize the resistance of specimens to pitting corrosion has been determination of the pitting potential, . 64,69.75-76 recotding an anodic polarization curve at a... 

Fig. 8-7. Cathodic and anodic polarization curves observed for a transfer reaction of redox electrons of hydrated Ti /Ti particles at a mercury electrode in 1 M H28O4 solution containing 0.17 M and 0.03 M Ti 4 at 25°C electrode surface area = 0.15 cm. [From Vetter, 1967.]...

In the state of band edge level pinning where all the change in electrode potential occurs in the space diarge layer, Mec, the anodic polarization curve of the oxidative dissolution follows Eqn. 9-53. As anodic polarization increases, the electrode interface enters a state of Fermi level pinning, in which all the change in electrode potential occurs in the compact layer, A ir, and the concentration of surface cations in Eqns. 9-54 then decreases with increasrng anodic polarization. 

An example of the effect of photon irradiation on the flat band potential is shown in Fig. 10-18 this figure compares a Mott-Schott plot with the anodic polarization curve of the dissolution reaction of a semiconductor anode of n-type molybdeniun selenide in an acidic solution in the dark and in the photoexcited conditions. In this example photoe dtation shifts the flat band potential from Em in the dark to pii) in the photoexcited state is about 0.75 V more positive than Em. This photo-shift of the flat band potential, Emi )-Em, corresponds to the change in the potential, of the compact layer due to photoexcitation as defined in Eqn. 10-23 ... 

Fig. 10-28. Polarization curves for cell reactions of photoelectrolytic decomposition of water at a photoezcited n-type anode and at a metal cathode solid curve M = cathodic polarization curve of hydrogen evolution at metal cathode solid curve n-SC = anodic polarization curve of oxygen evolution at photoezcited n-type anode (Fermi level versus current curve) dashed curve p-SC = quasi-Fermi level of interfadal holes as a ftmction of anodic reaction current at photoezcited n-type anode (anodic polarization curve r re-sented by interfacial hole level) = electrode potential of two operating electrodes in a photoelectrolytic cell p. sc = inverse overvoltage of generation and transport ofphotoezcited holes in an n-type anode.

It then follows that the anodic polarization curve intersects the cathodic polarization curve (i = i ) at the corrosion potential as illustrated in Fig. 11-5. 

Fig. 11-9. Anodic polarization curve of a metallic electrode for active dissolution, passivation, and transpassivation in aqueous acidic solution > u = anodic current of metal dissolution = passivation potential = transpassivation potential = maximum metal...

Fig. 11-10. Anodic polarization curves observed for metallic iron, nickel, and chromium electrodes in a sulfuric acid solution (0.5 M H 2SO 4) at 25°C solid curve = anodic metal dissolution current dot-dash curve s anodic oxygen evolution current [Sato-Okamoto, 1981.]...

Fig. 11-13. Anodic polarization curve of a metallic nickel electrode in a sulfuric add solution transpassivation arises at a potential relatively dose to the flat band potential because of p-type nature of the passive oxide film. [From Sato, 1982.]...

As described in Sec. 11.3, the spontaneous corrosion potential of a corroding metal is represented by the intersection of the anodic polarization curve of metal dissolution with the cathodic polarization curve of oxidant reduction (Figs. 11—5 and 11-6). Then, whether a metal electrode is in the active or in the passive state is determined by the intersection of the anodic and cathodic polarization curves. 

The intersection of the anodic polarization curve of iron dissolution with the cathodic polarization ctuve of nitric add reduction occurs in the range of potential of the active state in dilute nitric acid, but it occurs in the range of potential of... 

A mixed polarization diagram (where the polarization behavior of the two different electrodes is represented) for the sphalerite-hypersteel combination is given in Fig. 1.10 (Vathsala and Natarajan, 1989), in which the cathodic polarization curves for the sphalerite and the anodic polarization curves for the hypersteel ball material are seen to overlap. The active nature of the ball material is evident. The current values were observed to be lower in the absence of oxygen which indicated a lower anodic dissolution of the hypersteel grinding medium in the absence of oxygen. 

Figure 10, Cathodic polarization curve of p-type GaP and anodic polarization curve of n-type TiOg in 0.5M HgSO with illumination by an ultrahigh-pressure...

The anodic polarization curves of a Pt powder modified by adsorbed Bi are shown in Figure 1. The ionization of adsorbed hydrogen on unmodified Pt (curve a) ranges between -0.8 and -0.4 V ... 

Figure 4. Anodic polarization curve of a Bi-Pt catalyst (Bi/Pts=0.39) and the conversion - catalyst potential relationship in the oxidation of 1-phenylethanol, in an aqueous Na2C03 solution a - Bi-Pt/alumina, Bi/Pts=0.20, b - unsupported Bi-Pt, Bi/Pt=0.39.

Figure 16.9 Anodic polarization curves for typical stainless and ordinary steels. At low Eh, stainless steel may become active, in which case it corrodes more rapidly than ordinary iron or steels.

Figures 16.8 and 16.9 show only the anodic polarization curves for corrosion cells. The important question is, where do these curves intersect with the polarization curves for likely cathodic reactions, such as hydrogen evolution or oxygen absorption The intersection point defines the corrosion current density icorr and hence the corrosion rate per unit surface area. As an example, let us consider the corrosion of titanium (which passivates at negative Eh) by aqueous acid. In Fig. 16.10, the polarization curves for H2 evolution on Ti and for the Ti/Ti3+ couple intersect in the active region of the Ti anode. To make the intersection occur in the passive region (as in Fig. 16.11), we must either move the H+/H2 polarization curve bodily...

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