Iron anode

In Jiltemative 3 (Fig. 3), the electrolysis may be operated on a semicontinuous basis with the cadmium eventually being stripped completely from the electrolyte, which is then discarded after suitable treatment. Instead of the usual silver—lead anodes, high siUcon-iron anodes, such as Duriron, are commonly used. 

Galvanic anodes of cast iron were already in use in 1824 for protecting the copper cladding on wooden ships (see Section 1.3). Even today iron anodes are still used for objects with a relatively positive protection potential, especially if only a small reduction in potential is desired, e.g., by the presence of limiting values U" (see Section 2.4). In such cases, anodes of pure iron (Armco iron) are mostly used. The most important data are shown in Table 6-1. 

In total, three high-silicon iron anodes of 3 kg each were installed at points a, aj and as shown in Fig. 11-3. The anodes were bedded vertically in fine-grained coke in boreholes about 2.3 m deep and J = 0.2 m so that the length of the coke backfill was about 1 m. Each anode was connected by a separate cable to the anode bus bar of the transformer-rectifier to allow the current of individual anodes to be monitored. Three cathode cables 2x4 mm were installed for the return path of the protection current and attached on the tank end to the connecting clamps of the dome support. 

As an example, a tank farm that is to be cathodically protected by this method is shown schematically in Fig. 11-4. As can be seen in the figure, injection of the protection current occurs with two current circuits of a total of about 9 A, via 16 vertically installed high-silicon iron anodes embedded in coke. These are distributed over several locations in the tank farm to achieve an approximately uniform potential drop. The details of the transformer-rectifier as well as the individual anode currents are included in Fig. 11-4. Anodes 4, 5 and 6 have been placed at areas where corrosion damage previously occurred. Since off potentials for 7/ -free potential measurements cannot be used, external measuring probes should be installed for accurate assessment (see Section 3.3.3.2 and Chapter 12). 

In the first reconstruction [27] of road slabs contaminated with CL, silicon iron anodes were embedded in a layer of coke breeze as shown in Fig. 19-4a or the current connection was achieved with noble metal wires in a conducting mineral bedding material. Slots were ground into the concrete surface for this purpose at spacings of about 0.3 m (see Fig. 19-4b). This system is not suitable for vertical structures. 

Fig. 19-4 Older anode system for roadway plates (a) Silicon iron anodes in coke breeze bed, (b) noble metal lead in conducting bedding in a trench.

In addition, with high solid content of the cooling water and at high flow velocities, severe corrosive conditions exist which continuously destroy surface films. Cathodic protection alone is not sufficient. Additional measures must be undertaken to promote the formation of a surface film. This is possible with iron anodes because the anodically produced hydrated iron oxide promotes surface film formation on copper. 

Six iron anodes are required for corrosion protection of each condenser, each weighing 13 kg. Every outflow chamber contains 14 titanium rod anodes, with a platinum coating 5 /tm thick and weighing 0.73 g. The mass loss rate for the anodes is 10 kg A a for Fe (see Table 7-1) and 10 mg A a for Pt (see Table 7-3). A protection current density of 0.1 A m is assumed for the coated condenser surfaces and 1 A m for the copper alloy tubes. This corresponds to a protection current of 27 A. An automatic potential-control transformer-rectifier with a capacity of 125 A/10 V is installed for each main condenser. Potential control and monitoring are provided by fixed zinc reference electrodes. Figure 21-2 shows the anode arrangement in the inlet chamber [9]. 

The effective costs of a cathodic protection station depend very much on the local conditions, particularly on the costs of the power supply and the extent of the anode bed. Table 22-2 contains the costs of a protection station with four silicon iron anodes. 

The transition resistance between the surface of the metal and the electrolyte with uncoated iron anodes in coke backfill, the transition resistance is usually low. With metals in soil, it can be increased by films of grease, paint, rust or deposits. It contains in addition an electrochemical polarization resistance that depends on the current [see Eq. (2-35)]. 

In recent years it has been regarded as somewhat passe to refer to Sir Humphrey Davy in a text on cathodic protection. However, his role in the application of cathodic protection should not be ignored. In 1824 Davy presented a series of papers to the Royal Society in London in which he described how zinc and iron anodes could be used to prevent the corrosion of copper sheathing on the wooden hulls of British naval vessels. His paper shows a considerable intuitive awareness of what are now accepted as the principles of cathodic protection. Several practical tests were made on vessels in harbour and on sea-going ships, including the effect of various current densities on the level of protection of the copper. Davy also considered the use of an impressed current device based on a battery, but did not consider the method to be practicable. 

In practice the loss for an iron anode is approximately 9 kg/Ay. Thus consumable anodes must be replaced at intervals or be of sufficient size to remain as a current source for the design life of the protected structure. This poses some problems in design because, as the anode dissolves, the resistance it presents to the circuit increases. More important, it is difficult to ensure continuous electrical connection to the dissolving anode. 

The high cost of platinised materials for use in borehole groundbeds as opposed to conventional silicon-iron anodes may also be offset by the reduction in required borehole diameter, hence lower installation cost, with the relative economics between the different systems dependent upon a combination of both material and installation costs. 

Graphite anodes when used in soils are invariably placed in a carbonaceous backfill. This helps to compensate for the lower electrical resistivity of graphite when compared with silicon iron. In such an environment, no build-up of a film of high resistance between the anode and backfill occurs, unlike silicon-iron anodes where the resistance can increase with... 

Voltage drop caused by groundbed resistance, as previously explained. Back voltage polarisation between groundbed and pipeline. In the case of both graphite and silicon-iron anodes, an allowance of 2 V is normally used. This back voltage is that which exists between the anodes and the structure in opposition to the applied voltage. 

DcMilIc Campbell, E., High Silicon Iron Anodes for Cathodic Protection , Corrosion, 27 No. 4, 141 (1971)... 

Probably the best-known battery system using an iron anode is called the nickel/iron battery. It should be written (-) Fe / KOH / NiO(OH) (+), having its merits as a heavy-duty accumulator [7], By... 

In a similar way, 2-arylpyrimidines and 2-arylpyrazines have been prepared from 2-chloropyrimidine and 2-chloropyrazine and various functionalized aryl halides. An iron anode is used in order to generate iron salts that allow the desired couphng reaction (Scheme 150) [267]. 

The following two half-reactions take place in an electrolytic cell with an iron anode and a chromium cathode. 

During the process, the mass of the iron anode decreases hy 1.75 g. 

When the technique of electroplating is used to deposit a layer of iron on another metal surface, it is usual to employ a sacrificial iron anode and a solution of a ferrous salt e. g. ferrous sulphate. (This is the usual technique for silver plating.) The typical anodic (Eqs. 6.7 and 6.8) and cathodic reactions (Eqs. 6.9-6.12) are given below. 

N-Methylpyridinium and N-methylquinolinium ions are oxidised in water to the pyridone and quinolone respectively. A two-compartment cell with an iron anode is used with a catalytic amount of ferricyanide present. The electrolyte is main-... 

Since Fe3+ is a reactant in the cathode half-reaction, Fe(N03)3 would be a good electrolyte for the cathode compartment. The cathode can be any electrical conductor that doesn t react with the ions in the solution. A platinum wire is a common inert electrode. (Iron metal can t be used because it would react directly with Fe3+, thus short-circuiting the cell.) The salt bridge contains NaN03/ but any inert electrolyte would do. Electrons flow through the wire from the iron anode (—) to the platinum cathode ( + ). Anions move from the cathode compartment toward the anode while cations migrate from the anode compartment toward the cathode. 

The nickel-iron battery has an iron anode, an NiO(OH) cathode, and a KOH electrolyte. This battery uses the following half-reactions and has an E° value of 1.37 V at 25°C ... 

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