Iron electrodes

The cleaning or depassivation eflect is of great importance in sonoelectrochemistry, as it can be employed to wash off surface-adsorbed species and reduce blocking of the electrode by adsorption of reaction products. This eflect has been reported, for example, for the depassivation of iron electrodes and for the removal of deposits and in the presence of polymer films on the electrode surface. However, damage of the electrode surface, especially for materials of low hardness such as lead or copper, can also occur under harsh experimental conditions and applied intensities [70, Tf, 80]. 

Tubular Cells. Although the tubular nickel electrode invented by Edison is ahnost always combined with an iron negatwe electrode, a small quantity of cells is produced in wliich nickel in the tubular fomi is used with a pocket cadniium electrode. Tliis type of cell construction is used for low operating temperature environments, where iron electrodes do not perfomi well or where charging current must be limited. 

Fig. 9. Discharge and charging curves for a sintered iron electrode at a constant current of 0.2 A where the apparent geometrical surface area is 36 cm and porosity is 65%. A and B represent the discharging and charging regions, respectively. Overall electrode reactions, midpoint potentials, and, in parentheses, theoretical potentials at pH 15 ate Al, n-Fe + 2 OH Fe(OH)2 + 2, 0.88 V (1.03 V) B, Fe(OH)2 FeOOH + H+ +, 0.63 V (0.72 V) C,...

Silver—Iron Cells. The silver—iron battery system combines the advantages of the high rate capabiUty of the silver electrode and the cycling characteristics of the iron electrode. Commercial development has been undertaken (70) to solve problems associated with deep cycling of high power batteries for ocean systems operations. 

Cells consist of porous sintered silver electrodes and high rate iron electrodes. The latter are enclosed with a seven-layered, controUed-porosity polypropylene bag which serves as the separator. The electrolyte contains 30% KOH and 1.5% LiOH. 

Iron—Air Cells. The iron—air system is a potentially low cost, high energy system being considered mainly for mobile appHcations. The iron electrode, similar to that employed in the nickel—iron cell, exhibits long life and therefore this system could be more cost effective than the ziac-air cell. Reactions iaclude ... 

In the experimental systems studied the iron electrode has been of the siatered type and the oxygen —air electrodes have been of the bifunctional type. 

E. R. Bowerman, "Sintered Iron Electrode," Proc. 22 Power Sources Conf. (1968). 

Cast iron body, feet or ribs etc. found broken or cracked during transit or otherwise. Replacement of the motor in such cases may not be practical. However, using the motor may not be advisable in view of a weaker foundation and insufficient cooling. In such cases the broken parts can be welded using cast iron electrodes. Cracks, however, cannot be remedied. Unless the cracks are wide and may cause extensive damage during operation, the body may still not require replacement. Minor cracks, however, which do not impair the motor s perfor-mance or cause development of further cracks, may be compromised. 

Electrolytic method This procedure is also known as the Williams Corfleld test d. It is based on loss of metal from iron electrodes buried in a water-saturated soil through which current from a 6-V battery is passed. It does not reflect field conditions and depends upon soil conductance under saturated conditions. 

Combination electrical methods Tomashov and Mikhailovsky describe a method developed in the Soviet Union. This test is essentially a combination of resistivity measurement and polarisation rates on iron electrodes in soil in situ. The usefulness and value of this procedure has not as yet been determined by practical application by corrosion engineers. The development of this combination test does, however, represent an attempt to integrate some of the complex factors controlling corrosion rates in soil. Much more research on these factors and methods of measurement should in the future enable the corrosion engineer to evaluate soil properties with respect to application of corrosion-alleviating operations. 

In the polarization curve for anodic dissolution of iron in a phosphoric acid solution without CP ions, as shown in Fig. 3, we can see three different states of metal dissolution. The first is the active state at the potential region of the less noble metal where the metal dissolves actively, and the second is the passive state at the more noble region where metal dissolution barely proceeds. In the passive state, an extremely thin oxide film called a passive film is formed on the metal surface, so that metal dissolution is restricted. In the active state, on the contrary, the absence of the passive film leads to the dissolution from the bare metal surface. The difference of the dissolution current between the active and passive states is quite large for a system of an iron electrode in 1 mol m"3 sulfuric acid, the latter value is about 1/10,000 of the former value.6... 

Figure 5 shows the relationship between the passive film thickness of an iron electrode and the electrode potential in an anodic phosphate solution and a neutral borate solution.6,9 A passive film on an iron electrode in acidic solution is made up of an oxide barrier layer that increases its thickness approximately linearly with increasing electrode potential, whereas in a neutral solution, there is a precipitated hydroxide layer with a constant thickness outside the oxide barrier layer. 

The film-destructive anions are CF, T, CIO4, SO4, etc. for iron electrodes and in addition to these ions, NOJ, SCN , etc. for aluminum electrodes.19 In many cases, the film-destructive anions are the anions of strong acids the most well-known and studied anion is the chloride anion. 

A student was given a standard Fe(s) Fe2+(aq) half-cell and another half-cell containing an unknown metal M immersed in 1.00 M MNO,(aq). When these two half-cells were connected at 25°C, the complete cell functioned as a galvanic cell with E = +1.24 V. The reaction was allowed to continue overnight and the two electrodes were weighed. The iron electrode was found to be lighter and the unknown metal electrode was heavier. What is the standard potential of the unknown MT/M couple ... 

A galvanic cell can be constmcted from a zinc electrode immersed in a solution of zinc sulfate and an iron electrode immersed in a solution of iron(II) sulfate. What is the standard potential of this cell, and what is its spontaneous direction under standard conditions ... 

A galvanic cell can be constructed from a silver-silver chloride electrode in contact with a solution containing chloride anions and an iron electrode in contact with a solution containing iron(IIt) cations. 

Several electrode reactions occurring simultaneously. The following reactions can occur at an iron electrode in HCl solution containing FeClj while a hydrogen atmosphere is maintained ... 

Different reactions (anodic and cathodic) can occur simultaneously at an electrode, even when there is no net current flow. In Section 2.5.1 we mentioned the example of an iron electrode in HCl + FeCl2 solution where anodic iron dissolution (2.24) and cathodic hydrogen evolution (2.25) occur simultaneously these are the reactions of spontaneous dissolution of iron not requiring a net current. 

Similar effects may exist at other metals. For instance, when the surface of an iron electrode is thermally reduced in hydrogen and then anodically polarized at a current density of 0.01 mA/cm in 0.1 M NaOH solution, passivation sets in after 1 to 2 min (i.e., after a charge flow of about 100 mC/cm ). This amount of charge is much smaller than that required for formation of even a thin phase film. Since prior to the experiment, oxygen had been stripped from the surface, passivation can only be due to the adsorbed layer formed as a result of polarization. 

V, and Tpe= -0.44 V. If the zinc and the iron electrodes connected by an electrical conductor are dipped into standard electrolytes in this cell, then iron would serve as the cathode (Fe2+ + 2 e —> Fe) and zinc as the anode (Zn —> Zn2+ + 2 e ). The result would be a tendency for zinc to dissolve in the electrolyte, and this process is known as the galvanic corrosion of a less noble metal (zinc) in comparison with the more noble metal iron in this system. The reversible emf of the corrosion cell would be... 

Mossbauer spectroscopy can be used for in situ study of electrodes containing nuclei capable of resonance absorption of y radiation for practical systems, primarily the 57Fe isotope is used (passivation layers on iron electrodes, adsorbed iron complexes, etc.). It yields valuable information on the electron density on the iron atom, on the composition and symmetry of the coordination sphere around the iron atom and on its oxidation state. 

Fig. 5.53 Anodic processes at an iron electrode. (According to K. J. Vetter)...

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