Anodic dissolution reaction

Anodic dissolution reactions of metals typically have rates that depend strongly on solution composition, particularly on the anion type and concentration (Kolotyrkin, 1959). The rates increase upon addition of surface-active anions. It follows that the first step in anodic metal dissolution reactions is that of adsorption of an anion and chemical bond formation with a metal atom. This bonding facilitates subsequent steps in which the metal atom (ion) is tom from the lattice and solvated. The adsorption step may be associated with simultaneous surface migration of the dissolving atom to a more favorable position (e.g., from position 3 to position 1 in Fig. 14.1 la), where the formation of adsorption and solvation bonds is facilitated. 

Corrosion measurements by titration (CMT) were first introduced in 1990. The method is based on the following considerations When a metal corrodes with no applied current in an aqueous solution, the anodic dissolution reaction... 

The anodic dissolution reaction may produce an ion of a lower valence a, which is subsequently oxidized in the solution by oxygen or by hydrogen ions to a valence of b. Electrochemical measurements will then reflect oxidation to the valence a, but CMT measurements will correspond with the final valence b. The ratio of the two measurements will be a/b. 

These results suggest that the GaP surface with backward scanning develops an oxidized structure, which is acting as a precursor, or precursors, to the anodic dissolution reactions. 

However, the linear method (i.e., a small displacement of potential) is better. Thus, going out far from Vcorr on the anodic side may run into a region in which the metal forms oxide fdms and on the cathodic side, the evolution of Hj could interfere with the anodic dissolution current, which could confusingly lead to an erroneous contribution (via H2 - 2H+ + 2e) to the anodic dissolution reaction. 

The Tafel slope for the anodic dissolution reaction, i>anodit is drjld In i = 2RT/3F,... 

For dibenzyl sulfoxide on iron, it turns out that indeed it is the hydrogen evolution reaction, the partner reaction to the anodic dissolution reaction, which controls the corrosion rate. Because the inhibitor acts cathodically, it must interfere with and slow down the rds of this reaction, i.e., make it more difficult for the H to be desorbed. 

Consider an Evans diagram in a general way. The anodic dissolution reaction is to be represented in the Tafel region the same applies for the cathodic partner reaction, (a) Draw the two Tafel lines and show the region of intersection Oanodic= Cathodic)- Indicate on the graph the corrosion rate and corrosion potential. 

Zinc-Manganese Dioxide. In 1866 Leclanche invented a galvanic cell in which the reduction of Mn02 is the cathodic reaction in the cell s discharge. The corresponding anodic dissolution reaction is the oxidation of zinc. The Leclanche cell is a (so-called) dry cell, i.e., the ammonium electrolyte is immobilized in the form of a paste. There are three forms of the zinc-manganese dioxide batteries ... 

ANODIC DISSOLUTION REACTIONS at semiconductor electrodes require electron holes. 

The /- F behavior demonstrates that holes are involved in the anodic dissolution reactions the question arises, however, whether charge transfer occurs exclusively over the valence band. A straightforward way to investigate this problem is by comparing the number of photons absorbed in an n-type photoanode to the number of electrons flowing through the external circuit. This procedure has been used by Kohl et al. [33], and its results indicate a non-negligible contribution of the conduction... 

The current densities in KIO3 are lower than in K3pe(CN)6 solutions. The pH of minimum potential is also shifted to somewhat higher pH in KIO3 solution. The difference in their behavior may be attributed to ionic radii of IO3 and Fe(CN)6 ions. IO3 ions have much larger size and are thus may provide a hindrance to the anodic dissolution reactions. The value of 5ln(Iss)/8pH is about 1.45. This is lower than the value of 2.303 obtained by Macdonald et al. This study was carried out in static environment, unlike the rotating disk electrode set-up used by MacDonald et al. The differences in the value of this slope may probably be attributed to the differences in mass transfer properties in solution. 

Tsuru et al. [129-131] applied the double channel electrode transient technique to the investigation of the anodic dissolution of iron in sulphate and chloride aqueous media, in the pH range 1-3. The upstream electrode served as the generator electrode at which the anodic dissolution reaction occurred. Overall this involves... 

A final constraint on selection of electrode materials is frequently imposed by the problem of their corrosion, or oxidation. Thus accepting that a reduction such as that of ethene occurs over the potential range + 0.1 V to — 0.1 V, a number of metals such as Fe, Co, and Ni will not be stable, at least in more acidic solutions, but will corrode by means of the anodic dissolution reactions-... 

Cathodic Reactions For materials at their open circuit potential (OCP), the overall rate of the cathodic reaction(s) must balance the anodic dissolution reactions. In crevice corrosion, the catliodic reactions occur predominantly outside the... 

A system in which the reduction process is diffiision-controUed is illustrated in Fig. 5.13. In this case, the metal follows a typical anodic dissolution reaction under activation control. The reduction process follows the following equation ... 

Galvanized steel is a common example of galvanic coupling where steel (Fe), with a standard electrode potential of —0.440 V vs. SHE, is cathodicaUy protected by zinc, which has a more active standard electrode potential of —0.763 V. Obviously, zinc is not a corrosion-resistant metal and cannot be classified as a barrier coating. It protects steel from corrosion through its sacrificial properties. Because zinc is less noble than iron in terms of the standard electrode potentials, it acts as an anode. The sacrificial anode (zinc) is continuously consumed by anodic dissolution reaction and protects the underlying metal (iron in steel) from corrosion. In practice, sacrificial anodes are comprised of zinc, magnesium alloys, or aluminum. 

Fig. 14.10 Effect of nitrite anion on reducing the rate of anodic dissolution reaction on steel [61]. NACE International 1984.

The most typical concentration polarization occurs when fliere is a lack of reactants, and (in corroding systems) therefore most often for reduction reactions. This is the case because reduction usually implies that ions or molecules are transported from the bulk of the liquid to the electrode surface, while for the anodic (dissolution) reaction, mass is transported from the metal, where there is a large reservoir of the actual reactant. 

For metals such as chromium and alloys such as stainless steel, the plot of potential versus corrosion rate above the range is shown in Figure 20.67. Figure 20.68 shows a sudden sharp drop in corrosion above some critical potential. Despite a high level of anode polarization above V, the corrosion rate drops precipitously due to the formation of a thin, protective oxide film as a barrier to the anodic dissolution reaction. Resistance to corrosion above is termed passivity. The drop in corrosion rate above can be as much as 10 to 10 times below the maximum rate in the active state. With increasing corrosion potential, the low corrosion rate remains constant until at a relatively high potential the passive film break down, and the normal increase in corrosion rate resumes in a transpassive region. 

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