Power systems, cathodic protection

Cathodic potection stations (rectifiers) are made up of a device powering the cathodic protection system with a current and/ or voltage (potential) adjustment equipped... 

In impressed current systems cathodic protection is applied by means of an external power current source (Fig. 11.7). In contrast to the sacrificial anode systems, the anode consumption rate is usually much lower. Unless a consumable scrap anode is used, a negligible anode consumption rate is actually a key requirement for long system hfe. Impressed current systems typically are favored under high-current requirements and/or high-resistance electrolytes. The following advantages can be cited for impressed current systems ... 

In an impressed-current cathodic protection system the power source has a substantial capacity to deliver current and it is possible to change the state of polarisation of the structure by altering that current. Thus effective control of the system depends on credible potential measurements. Since the current output from any given anode is substantial, the possibility of an IR error which may reach many hundreds of millivolts in any potential measurements made is high. Thus the instant-off technique (or some other means of avoiding IR error) is essential to effective system management. 

An impressed-current cathodic protection system circuit comprises an anode, the power source, the structure and the environment in which it... 

In recent years, there has been interest in using zinc as a power-impressed anode for the cathodic protection of steel in concrete. The zinc is flame sprayed onto a grit blasted concrete surface to a final film thickness of approximately 250 m. A primary anode is necessary. Early systems used brass plates as the primary anode, but more recent systems used platinised titanium or niobium wire anodes as the primary current conductor. 

Stray current schemes are relatively rare in occurrence in the UK as few localities now have widespread d.c. transport systems. Such systems are extensively used in overseas countries where d.c. transport systems are in use, i.e. Australia and South Africa. Where stray current can be employed it is normally the most economical method of applying cathodic protection since the power required is supplied gratis by the transport system. 

Manually Controlled System A manually controlled system comprises one or more transformer-rectifiers each with its associated control panels which supply the d.c. to the various anodes installed in the water box spaces. Each transformer-rectifier is provided with its own control panel where each anode is provided with a fuse, shunt and variable resistor. These enable the current to each anode to be adjusted as required. Reference cells should be provided in order to monitor the cathodic protection system. In the case of a major power station, one transformer-rectifier and associated control panel should be provided for separate protection of screens, circulating water pumps and for each main condenser and associated equipment. 

Solar Power With improved technology and production methods considerable use is being made of solar power in remote locations. The output of photovoltaic arrays is used to maintain conventional storage batteries in a state of charge. The cathodic protection system is in turn energised from the batteries. It is usual to incorporate sufficient battery storage to accommodate a number of no-sun days. Whilst in theory the capacity of equipment is unlimited, a practical maximum would be ca. SOO W. 

Cathodic protection has many applications, e.g. in refineries, power stations, gas, water, and oil utilities on marine structures, e.g. jetties, piers, locks, offshore platforms, pipelines, ships hulls, etc. and on land structures, e.g. buried pipeline, storage tanks, cables, etc. For each use, the cathodic protection system requires careful design, either impressed current, sacrificial anodes, or a combination of both may be chosen. There may also be other protection systems, e.g. paint, the nature of which will affect the design parameters and must be taken into consideration. 

Cathodic protection of reinforced-concrete structures exposed to the atmosphere was apphed for the first time to bridge decks contaminated by de-icing salts by R. F. StratfuU in California in 1973 [1,2]. In the years following, design and protection criteria were elaborated, as well as power supply and monitoring systems completely different from those used for cathodic protection of buried steel structures or structures operating in seawater. Above all, it was proved that cathodic protection was a rehable repair technique even in the presence of high chloride contents, where traditional systems of rehabilitation are inefficient or very costly. 

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). 

For the application of cathodic protection to structures to be protected, the initial considerations are best made at the early design and preconstruction phase of the structure. For underground structures, it may be necessary to visit the proposed site, or for pipelines the proposed route, to obtain additional information on low-resistivity areas, availability of electric power, and the existence of stray dc current or other possible interactions. Other considerations will include fundamental design decisions to select the type of system and the most suitable type of anode appropriate to that system. In addition, it will be a requirement to determine the size and number of the power sources or sacrificial anodes and their distribution on the structure. Other factors that must be considered to ensure that cathodic protection is applied in the most economic and reliable manner are given as follows. 

Cathodic protection (CP) is an electrical method of mitigating corrosion on metallic structures that are exposed to electrolytes such as soils and waters. Corrosion control is achieved by forcing a defined quantity of direct current to flow from auxiliary anodes through the electrolyte and onto the metal structure to be protected. Theoretically, corrosion of the structure is completely eliminated when the open-circuit potentials of the cathodic sites are polarized to the open-circuit potentials of the anodic sites. The entire protected structure becomes cathodic relative to the auxiliary anodes. Therefore, corrosion of the metal structure will cease when the applied cathodic current equals the corrosion current. There are two basic methods of corrosion control by cathodic protection. One involves the use of current that is produced when two electrochemically dissimilar metals or alloys (Table 19.1) are metallically connected and exposed to the electrolyte. This is commonly referred to as a sacrificial or galvanic cathodic protection system. The other method of cathodic protection involves the use of a direct current power source and auxiliary anodes, which is commonly referred to as an impressed-current cathodic protection system. Then cathodic protection is a technique to reduce the corrosion rate of a metal surface by making it the cathode of an electrochemical cell [3]. 

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