Metal dissolution Tafel slope

Figure 11-7 shows the polarization curve of an iron electrode in an acidic solution in which the anodic reaction is the anodic transfer of iron ions for metal dissolution (Tafel slope 40 mV/decade) the cathodic reaction is the cathodic transfer of electrons for reduction of hydrogen ions (Tafel slope 120 mV /decade) across the interface of iron electrode. 

Participation in the electrode reactions The electrode reactions of corrosion involve the formation of adsorbed intermediate species with surface metal atoms, e.g. adsorbed hydrogen atoms in the hydrogen evolution reaction adsorbed (FeOH) in the anodic dissolution of iron . The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterised by a change in the Tafel slope observed for the process. Studies of the anodic dissolution of iron in the presence of some inhibitors, e.g. halide ions , aniline and its derivatives , the benzoate ion and the furoate ion , have indicated that the adsorbed inhibitor I participates in the reaction, probably in the form of a complex of the type (Fe-/), or (Fe-OH-/), . The dissolution reaction proceeds less readily via the adsorbed inhibitor complexes than via (Fe-OH),js, and so anodic dissolution is inhibited and an increase in Tafel slope is observed for the reaction. 

Adsorbed species may also accelerate the rate of anodic dissolution of metals, as indicated by a decrease in Tafel slope for the reaction. Thus the presence of hydrogen sulphide in acid solutions stimulates the corrosion of iron, and decreases the Tafel slope The reaction path through... 

From Eqs. (1222) and (12.23), it is clear that the corrosion current depends upon the exchange currents (i.e., available areas and exchange-current densities), Tafel slopes, and equilibrium potentials for both the metal-dissolution and electronation reactions. To obtain an explicit expression for the corrosion current [cf. Eq. (12.22)], one has first to solve Eqs. (12.22) and (12.23) for A0corr. If, however, simplifying assumptions are not made, the algebra becomes unwieldy and leads to highly cumbersome equations. 

Fig. 12.19. The effect of (a) the Tafel slope, (b) the equilibrium potential of the metal dissolution, or (c) the transport difficulties of the electron acceptor are reflected in the Evans diagram.

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

Competing reactions (e.g., metal dissolution occurring along with hydrogen ionization) may make the determination complex or even impossible. In a few simple cases, a determination of the Tafel slope alone may be sufficient to elucidate the mechanism (cf. Section IV,A). However, in most cases other information is also necessary. 

The difference between the anodic external current and the independently determined anodic partial current (dissolved Fe) is the cathodic partial current density. The results as obtained by Hoar and Holiday are shown in Fig.6. The dashed curve represents the external polarization behavior in the absence of inhibitor and the black lines are the Tafel slopes for the anodic partial current density (the metal dissolution) for different inhibitor concentrations. The cathodic partial current density (hydrogen eveolution) is found for all values of the inhibitor concentrations in the shaded area. Therefore, it is obvious that the inhibitor in this case acts exclusively by reducing the anodic reaction rate but not the cathodic one. 

It can be seen that the corrosion current and potential depend on both the equilibrium potentials for the hydrogen evolution reaction and metal dissolution calculated from the Nernst equation, and the kinetic parameters, the exchange currents and the Tafel slopes. Table 9.1 shows the corrosion currents calculated for some typical values of these parameters it is also important to note that even a... 

Adsorbed species may also accelerate the rate of anodic dissolution of metals, as indicated by a decrease in Tafel slope for the reaction. Thus the presence of hydrogen sulphide in acid solutions stimulates the corrosion of iron, and decreases the Tafel slope - - . The reaction path through (Fe-HS-W has been postulated to lead to easier anodic dissolution than that through (Fe-OH) s.. This effect of hydrogen sulphide is thought to be responsible for the acceleration of corrosion of iron observed with some inhibitive sulphur compounds, e.g. thioureas , at low concentrations, since hydrogen sulphide has been identified as a reduction product. However, the effects of hydrogen sulphide are complex, since in the presence of inhibitors such as amines , quaternary ammonium cations , thioureas . ... 

Tafel slopes for the hydrogen evolution, metal dissolution reaction respectively... 

The presence of adsorbed inhibitors will interfere with the formation of these adsorbed intermediates, but the electrode processes may then proceed by alternative paths through intermediates containing the inhibitor. In these processes the inhibitor species act in a catalytic manner and remain unchanged. Such participation by the inhibitor is generally characterized by an increase in the Tafel slope of the anodic dissolution of the metal. 

Tafel slope and a decrease in the reaction order with respect to OH have been observed and mark the start of processes specific to the transition range of the overall active range of iron dissolution among them, the formation of crystallized ferrous and ferric solid species including anions and their blocking effect on the metal dissolution superimpose and change the mechanism and the kinetics. 

Figure 39. Anodic polarization curves for metal dissolution in one step concomitantly with the lateral attack of laterally vulnerable, superficial blocked atoms, calculated for particular values of rate constants, fitting the oscillatory behavior of the Tafel slope for iron dissolution in chloride electrolytes. (From Ref. 180.)...

It is worth noting that the error is so small at small 6 /6 values that, for many practical corrosion measurement situations, it can be considered negligible. The maximum error is only 20% at Z /6 = 0.25, and 33% at 6 /6 = 0.5. While a complete generalization cannot be made, the Tafel slope of the anodic reaction in a corroding system is very often considerably smaller than the Tafel slope of the cathodic reaction. Typically, the Tafel slope of an anodic metal dissolution reaction is in the range of 0.03 to 0.06 while common cathodic reactions occurring during... 

Wire samples were also tested in a standard migration cell, and l-V curves were measure to determine the Tafel slope during anodic dissolution in various aqueous media. The results of the migration cell tests in Oj + HjO and 350 ppm HCL showed no dendrite formation however, the anode was roughened (dissolution) and there was Cu in solution. No silver was detected. The Tafel slope was identical to that of Cu, though there was a slight shilft in the noble metal direction. These results support the observations in the water-drop experiment that only Cu dendrites form and that the Cu-15%Ag-2.5%P wire behaves like pure Cu. 

If the oxygen reduction is the counter reaction of metal dissolution, its kinetics is often determined by diffusion due to the small solubility of O2 gas and long diffusion distances especially in unstirred solution. In this case, the cathodic reaction gets potential independent at negative potentials with a diffusion-limited cathodic current density Ido2 the vicinity of the rest potential (Figure 1.42). In this situation, the corrosion current density ic is equal to the limiting cathodic current density ip,02/ and Equation 1.159 simplifies to Equation 1.164, which becomes Equation 1.165 for n -> 0 (E —> Er). Hence ic can be calculated from Rp and the anodic Tafel slope (Equation 1.166). 

Figure 6.15 Tafel plots for a metal ion transfer reaction, dissolution and deposition of Cd/Cd ". From the slopes the charge transfer coefficient is determined, a z = 1.09 and (1 — a )z = 0.91. With z = 2 one obtains = 0.55 and (1 — aj = 0.45. The exchange current density is Ig = —2.8.

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