Impressed Current Anodes and Reference Electrodes

Attached anodes provide the particular advantage of proportionately low grounding resistance for the protection current because the actual anode body is set into both sloping sides of the plastic body as long narrow strips. The disadvantage is that the body is exposed on the ship s side and can easily be damaged mechanically. 

The size of the protection shield depends on the protection current, the voltage and the shape of the anode [3]. 

Measuring electrodes for impressed current protection are robust reference electrodes (see Section 3.2 and Table 3-1) which are permanently exposed to seawater and remain unpolarized when a small control current is taken. The otherwise usual silver-silver chloride and calomel reference electrodes are used only for checking (see Section 16.7). All reference electrodes with electrolytes and diaphragms are unsuitable as long-term electrodes for potential-controlled rectifiers. Only metal-medium electrodes which have a sufficiently constant potential can be considered as measuring electrodes. The silver-silver chloride electrode has a potential that depends on the chloride content of the water [see Eq. (2-29)]. This potential deviation can usually be tolerated [3]. The most reliable electrodes are those of pure zinc [3]. They have a constant rest potential, are slightly polarizable and in case of film formation can be regenerated by an anodic current pulse. They last at least 5 years. 

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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... 

In the cathodic protection of storage tanks, potentials should be measured in at least three places, i.e., at each end and at the top of the cover [16]. Widely different polarized areas arise due to the small distance which is normally the case between the impressed current anodes and the tank. Since such tanks are often buried under asphalt, it is recommended that permanent reference electrodes or fixed measuring points (plastic tubes under valve boxes) be installed. These should be located in areas not easily accessible to the cathodic protection current, for example between two tanks or between the tank wall and foundations. Since storage tanks usually have several anodes located near the tank, equalizing currents can flow between the differently loaded anodes on switching off the protection system and thus falsify the potential measurement. In such cases the anodes should be separated. 

As an example. Fig. 20-7 shows potential and protection currents of two parallel-connected 750-liter tanks as a function of service life. The protection equipment consists of a potential-controlled protection current rectifier, a 0.4-m long impressed current anode built into the manhole cover, and an Ag-AgCl electrode built into the same manhole [10,11]. A second reference electrode serves to control the tank potential this is attached separately to the opposite wall of the tank. During the whole of the control period, cathodic protection is ensured on the basis of the potential measurement. The sharp decrease in protection current in the first few months is due to the formation of calcareous deposits. 

Partially platinized titanium impressed current anodes were chosen because contamination of the feed water by anodic decomposition products had to be avoided. Four pure zinc reference electrodes were installed in the tank to control and regulate the potential. The supports for the anodes were of polypropylene, which can operate for short periods up to 100°C, in contrast to the usual PVC supports used in cold water. 

Fig. 1DJ5 The application of (impressed current) anodic protection, (a) Protection of a steel tank storing add using a reference electrode and potentiosiatic control (b) Pro tection of a tube-and-shell heat-exchanger, (after West (1980).)...

The impressed current method with metal oxide-coated niobium anodes is usually employed for internal protection (see Section 7.2.3). In smaller tanks, galvanic anodes of zinc can also be used. Potential control should be provided to avoid unacceptably negative potentials. Pure zinc electrodes serve as monitoring and control electrodes in exposed areas which have to be anodically cleaned in the course of operation. Ag-AgCl electrodes are used to check these reference electrodes. 

Chronopotentiometry has also been used to determine chloride ions in seawater [31]. The chloride in the solution containing an inert electrolyte was deposited on a silver electrode (1.1 cm2) by the passage of an anodic current. The cell comprised a silver disc as working electrode, a symmetrical platinum-disc counter-electrode and a Ag-AgCl reference electrode to monitor the potential of the working electrode. This potential was displayed on one channel of a two-channel recorder, and its derivative was displayed on the other channel. The chronopotentiometric constant was determined over the chloride concentration range 0.5 to 10 mM, and the concentration of the unknown solution was determined by altering the value of the impressed current until the observed transition time was about equal to that used for the standard solution. 

An optional anode and an Ag/AgCl reference electrode for potential measurement are placed in the seawater filled in a tank to be inspected as shown in Figure 1. The potential changes at several location in the tank with reference electrode are measured in the two cases. The first case is that the prescribed current is impressed with an optional zinc anode and the second case is that no current is impressed. Each case is represented with subscript ON and OFF respectively in the following. The differential potential 8on - 4>off) is calculated from the results. 

Impressed current cathodic protection requires (i) DC power supply (rectifier) (ii) an inert anode such as catalyzed titanium anode mesh (iii) wiring conduit (iv) an embedded silver/silver chloride reference electrode. A schematic diagram of an impressed current cathodic system is shown in Figure 5.26. By an impressed current, the potential of the steel is adjusted to values greater than -850 mV, thus making the steel bar cathodic and prevent the corrosion (25). 

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. 

The most frequently used materials for buried metal structures are the carbon steels. For prevention of their corrosion the most recommended, economical, and effective method is cathodic protection (CP). The use of CP is now standard procedm-e for long-term corrosion protection of imderground pipelines, oil and gasoline tanks, and other structures. With a shift of the metal potential to more of a negative value of -0.85 V versus a C11/CUSO4 reference electrode, it is possible to make the metal surface a cathode, which ensures an immune (no corrosion) state of the carbon steel. Cathodic polarization is achieved by direct current, which can be supplied either by sacrificial anodes in galvanic contact with the steel structure, or by impressed current from a rectifier. 

Fig. 10.29 Cathodic protection using impressed current, (a) A circuit showing the principles the signal from the reference electrode is passed to a power-unit control where it is compared with a preset level. The resultant error signal is amplified and used to control semiconductor power devices which allow a controlled current to pass through the anodes, (b) A typical layout of components in various types of vessel, (c) Transformer/rectifier power units for marine use. (d) Platinized titanium or lead-silver alloy anodes being installed on a ship s hull. The anodes are insulated from the hull and have special insulating, backing shields which help to improve potential distribution and prevent over-protection, i.e. too negative a potential (Photographs courtesy Corrintec (UK) Ltd.)...

One-electrode potentiometry involves the measurement of the potential of an indicator electrode with respect to a reference (nonpolarizable) electrode either at open circuit or with a small anodic or cathodic current applied to the indicator electrode. These three possibilities are shown in Figure 11.5.2 for the Fe -Ce titration, and the resulting titration curves are shown in Figure 11.5.3. The i = 0 curve, a), is the usual potentiometric titration curve, showing the equilibrium potential of the solution (F gq) as a function of/ When a small anodic current is impressed on the indicator electrode, the measured potential at a given/will be somewhat more positive than Fgq [curve (c)]. When a small cathodic cur-... 

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