Zinc dissolution

Fig. 3. Evans-diagram for the cementation of Cu2+ and Pb2 with zinc amalgam of different zinc content. If the zinc concentration in the mercury employed for this special extraction technique is low, the anodic zinc-dissolution current density may be diffusion controlled and below the limiting cathodic current density for the copper reduction. The resulting mixed potential will lie near the halfwave potential for the reaction Cu2+ + 2e j Cu°(Hg) and only Cu2 ions are cemented into the mercury.

The "classical" Leclanche cell uses zinc sheet formed into a cylindrical can serving simultaneously as the anode and as the cell container (AB1C1). The cathode is a mixture of Mn02 and graphite wrapped into a piece of separator and contacted by a central carbon rod. The can dissolves slowly when the cell is not in use and faster when the cell delivers electrical energy. The reaction following the primary electrochemical zinc dissolution [Eq. (19)] leads, in the case of an ammonium chloride electrolyte, to a zinc diammine cation ... 

Zinc crystallizes in the hexagonal close-packed system its electronic structure is 4s2 and the melting point is 693 K. Since the zinc dissolution takes place at potentials very close to ffa0 the differential capacitance curves in the region of Ea=c in pure surface-inactive electrolyte solutions (KC1, pH = 3.7) can be determined directly for the Zn(llJO) face only... 

Fig. 5.54 Mixed potential. (A) Zinc dissolution in acid medium. The partial processes are indicated at the corresponding voltammograms. (B) Dissolution of mercury in nitric acid solution. The original dissolution rate characterized by (1) the corrosion current y a is enhanced by (2) stirring which causes an...

Kim and Jorne [37] have used a rotating zinc hemisphere to study the kinetics of zinc dissolution and deposition reactions in concentrated zinc chloride solutions. The electrodeposition reaction of cadmium on mercury was used by Mortko and Cover [43] in their investigation of a rotating dropping mercury electrode their data behaved according to Eqs. (74)-(76). 

A different pattern of dissolution was seen with a Zn-Sn alloy containing 26% zinc. In this case the stable dissolution situation established after ca. 90 min showed a ratio of EC to CMT measurements of 1 4. As seen in Fig. 3, this remained fairly constant, though the corrosion potential increased by more than 50 mV. Only selective zinc dissolution took place, and analysis by atomic absorption spectroscopy of the amount of dissolved zinc agreed within 10% with the value according to the titration. This pattern is still difficult to understand. The ratio of ca. 1 4 between EC and CMT measurements could be interpreted in terms of formation of the low-valent zinc species ZnJ, which seems unlikely, or in terms of dissolution of divalent zinc ions accompanied by loss of chunks consisting of precisely three zinc atoms, each time a zinc ion is dissolved. The latter alternative seems to require a more discrete mechanism of dissolution than... 

The influence of Pb + ions on the kinetics of zinc electrodeposition on Zn electrode in acidic sulfate electrolyte was discussed [217] in terms of a reaction model involving hydrogen adsorption and evolution, a multistep mechanism for zinc deposition and the overall reaction for zinc dissolution. The strongly adsorbed Pbads inhibited all the reactions taking place on the zinc electrode. 

Zinc dissolution was also investigated in phosphate solutions over a wide pH range of 4.5-11.7 [258], in aerated neutral perchlorate [259], and in sulfate solutions [260, 261]. In the phosphate solutions [258], zinc phosphates were present in a passive layer of zinc electrode, while for sulfate solutions a kinetic model of spontaneous zinc passivation was proposed [261]. 

Fig. 3.8 Mechanism of zinc dissolution by the formation of a local corrosion couple...

Such displacements result in the formation of local couples where hydrogen evolution and zinc dissolution can occur at a greatly accelerated rate (Fig. 3.8). (Where the concentration of such impurities is very high, displacement reactions have been known to produce such extensive dendritic growths that the cells have become internally short-circuited.)... 

The anodic reaction is an oxidation reaction producing electrons in the anode, while the cathodic reaction is a reduction reaction consuming electrodic electrons at the cathode interface. We shall consider, as an example, an electrochemical cell consisting of a metallic zinc electrode and a metallic copper electrode, in which the anodic reaction of zinc ion transfer (zinc dissolution) is coupled with the cathodic reaction of copper ion transfer (copper deposition) as shown in the following processes ... 

The next step in the thought experiment consists in taking a large number of strips of copper and zinc and joining them so that there are alternate strips of the two metals, a multiband arrangement. If this assembly is immersed in solution containing cupric ions, the copper strips will be the sites for copper deposition and the zinc strips will be the sites for zinc deelectronation. Once again, the net result is copper deposition and zinc dissolution. 

Contrary to reversible cells, this reaction proceeds, though at a very limited rate, even when the electrodes are not connected and no current is flowing through the cell. If its EMF is compensated in a potentiometric connection by an equal potential of opposite direction, the zinc dissolution continues. Should the compensating potential exceed the EMF of the cell, the above mentioned reaction will not proceed in the opposite direction but a new process appears instead, during which the copper is dissolved and the hydrogen is evolved on the zinc electrode ... 

The nascent hydrogen is produced in reaction of zinc with HCl (3-4 M). The zinc used should be free of traces of As, and suitably comminuted to provide high rate of dissolving and liberation of hydrogen. Nickel(II) ions catalyse the zinc dissolution. SnCh and f facilitate the reduction of As traces. 

It is shown that the reaction of tin cementation onto zinc is reversible. Zinc ions in the solution affect the rate of tin deposition and tin film microstructure. The repeated processes of tin/zinc dissolution/deposition provide fabrication of rather thick films (2-5 pm), facilitate grain recrystallization and are responsible for the formation of narrow deep channels with a width not more than 30-50 nm. 

This requirement is shown graphically in Fig. 34.12, which shows the z versus t] curves for the zinc dissolution reaction and for the hydrogen evolution reaction. At the potential 0 M 5 the current densities sum to zero. At this point, z = z corr the corrosion current density. The potential of the Zn-Pt composite is a mixed potential, 0m- Since is determined by... 

The passive film that forms on zinc not only reduces the rate of the anodic process (zinc dissolution), but even hinders cathodic reactions of oxygen reduction and hydrogen development. In conditions of passivity, the corrosion potential of galvanized steel is therefore much lower than that of carbon steel. Values typically measured are between -600 and -500 mV SCE compared to values above -200 mV usually found for passive carbon-steel reinforcement. 

According to X-ray diffraction and light optical investigations, the anodic dissolution of -brass results in a e — y — a phase transformation with porous product phases [22, 23]. As revealed by a more detailed investigation of the corrosion morphology, the extent of this transformation depends on the overpotential of the zinc dissolution reaction [24]. At a low overpotential of Eh = —0.75 V, the only product phase is y. Similar to the scheme of... 

In Fig. 19(a), a defect in a polymer coating on a metal substrate is shown. The anodic dissolution in the defect leads to a positive current peak. The intact polymer-coated area shows zero current. A second important situation, a cut edge of coil-coated galvanized steel, is shown in Fig. 19(b). The zinc dissolution leads to a positive peak while the area of oxygen reduction on the cathodically protected steel surface is characterized by a broad negative current peak. The activation, distribution, and passivation of these local anodes and cathodes can now be studied by the SVET as a function of coating compositions. 

It is obvious that in case of a defect down to steel, which leads to the enhanced anodic dissolution of zinc, the delamination of the purely alkaline cleaned galvanized steel surface is not faster than that of a phosphated surface. Such a behavior can be explained by an anodic delamination process. If the corrosion conditions are such that no formation of a cathode is possible in front of the anode, then just the kinetics of zinc dissolution determine the degradation of the polymer-metal composite. 

The situation changes when the defect is prepared just down to zinc and the kinehcs of zinc dissolution are rather slow. In this case, the cathodic delamination determines the kinetics of undermining. The delaminated area of the phosphated sample is now smaller than for the defect down to steel, whereas the just alkaline cleaned sample shows delamination that is much faster than in the case of the defect down to steel. This example shows how complex the corrosion mechanisms are and that no generally accepted mechanism can be found in the literature. 

Determine the corrosion potential, the corrosion rate, and the protective current of zinc in 1 N hydrochloric acid. Assume that the entire zinc surfece acts as a cathode, Tafel slopes are b = 0.1 V/decade and b = — 0.1 V/decade, and the exchange current densities for zinc dissolution and for hydrogen evolution on zinc are 10 and 10 A/cm, respectively. Additional information ... 

A further example, which will be discussed, is zinc dissolution. Heusler investigated this system, and two Tafel lines with a slope of 40 mV and 118 mV were observed (Figure 10.7). 

Figure 10 Time dependence of the partial current densities of Zn and Cu of a-brass (CuZujq) in a chloride electrolyte, (1) partial current density of zinc dissolution, (2) partial current density of copper dissolution, and (3) total current density. (Reproduced with permission from Ref. [17], 1980, Elsevier.)...

In the beginning the zinc dissolution dominates and Cu dissolution is zero. But with time Zn dissolution rate decreases, Cu dissolution rate increases, and finally a stationary ratio equal to the alloy composition develops. In this situation an intermediate layer exist on the surface that regulates the zinc dissolution by the Zn diffusion through this layer. Otherwise, the great differences in the dissolution rates can also lead to a dezin-cation leaving a porous spongy Cu matrix. 

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