Arrangement of Anodes and Reference Electrodes

Anodes are not attached to the rudder but are situated between the rudder shaft and the ship s wall and connected via a copper strip. The propeller is protected via a slip ring on the shaft. To achieve a low-resistance contact, the divided copper or bronze ring has a rolled silver-bearing surface on which metal graphite brushes slide. The transmission voltage should be below 40 mV. 

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Fig. 17-9 Example of the arrangement of anodes and reference electrodes from Ref. 13.

Figure 21-1 shows the object to be protected and the arrangement of impressed current anodes and reference electrodes. A central anode and two ring anodes of platinized titanium wire 3 mm in diameter provided with additional copper wire conductors are installed here. It is worth noting that the central anode is suspended from a float, whereas the ring anode is mounted on plastic supports. The zinc reference electrodes are also on floats near the inner side of the bell, while the 17 reference electrodes are mounted on plastic rods on the bottom of the cup and in the ring... 

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]. 

With the arrangement shown above, the reaction proceeds spontaneously, in which electrons move from left to right and X ions from right to left so that the electroneutrality is maintained. This type of reactions which take place in an electrochemical manner is called electrochemical reaction. A device like the one shown above, which permits a spontaneous electrochemical reaction to produce a detectable electric current, is termed a galvanic cell. As shown in the above figure, oxidation occurs in one half-cell and reduction occurs in the other half-cell. The electrode at which oxidation occurs is referred to as the anode, while the electrode at which reduction occurs is termed cathode. 

It is also possible to scan a pair of reference or pseudoreference electrodes separated by a small, fixed distance of a few micrometers to measure the local potential field gradient, dvldl, and estimate the local current density from Eq. (48) (128). This is a slightly more sophisticated measurement because the anodic or cathodic character of local sites can be determined from the polarity of the current, and the intensity of the attack can be estimated from the current density flowing in solution. The difficulty with this arrangement is that the potential difference between two closely spaced reference electrodes in a conductive solution is usually less than 1 microvolt. The stability of reference electrodes is on the order of microvolts, and thus it often exceeds the magnitude of the potential difference signal. This imposes a fundamental limitation on the usefulness of this technique. 

Polarization experiments on a corrosion system are carried out by using a potentiostat. The experimental arrangement of the cell consists of a working electrode, reference electrode and a counter-electrode. The counter-electrode is used to apply a potential on the working electrode both in the anodic and the cathodic direction, and measure the resulting currents. The electrochemical cell is depicted in Figure 1.26. 

FIGURE 1.11 Schematic of the electrochemical cell and circuit arrangements for anodic oxidation of a titanium metal plate under hydrothermal conditions. (A) Counter electrode (platinum plate), cathode (B) thermocouple (C) stirrer (D) reference electrode (platinum plate) (E) working electrode (titanium plate), anode. 

The relationship between electrode potential and current is determined by the electrochemical reaction taking place at the working electrode. Measurements are done usually in a three-electrode arrangement with a reference electrode to control the potential of the working electrode (typically no current is in practice allowed to flow through the reference electrode and its potential is constant) and a counter (auxiliary) electrode where a counterbalancing but not rate-deterrnmirig electrode process takes place. In cyclic voltammetry for a reversible electrode reaction, the cathodic, pc and anodic, pa peak potentials depend on the formal potential, E ... 

The technician on board relies entirely on the information transmitted by the diver, which is not always reliable and reproducible. Therefore, such measurements are carried out with the additional aid of a television camera so that the technician on board can record the position and measurement on videotape. The arrangement shown in Fig. 16-12A is advantageous because the reference electrode can be coupled with the TV camera. The state of the anodes, their possible passivation and material loss can be investigated at the same time as the potential measurements, and the marine growths can be removed if they threaten to smother the anodes. Platforms are given an annual visual examination. Potential measurements are also carried out on these occasions. Impressed current installations are also subject to continual monitoring so that defects can be detected at an early stage and measures to repair them instituted. 

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