Galvanic systems

In poorly conducting media or soils, however, the low driving voltage can limit the use of galvanic anodes. Raising the current delivery, which becomes necessary in service, is only practically possible with the help of an additional external voltage. In special cases this is used if an installed galvanic system is overstretched or if the reaction products take over additional functions (see Section 7.1). 

Galvanic systems Barrier systems Inhibiting systems... 

Some advantages of galvanic system are (i) no external power required (ii) little maintenance (iii) ease of installation (iv) little cathodic interference and (v) infrequent inspection. 

Common uses of the impressed current method of protection include long transmission pipelines, complex underground structures, marine structures, ship hulls, and replacement for dissipated galvanic systems, large condenser water boxes, reinforcing steel in concrete, bare or poorly coated structures, unisolated structures and water storage tank interiors. 

One particular photoredox system, that involving Thionine (245 X = S) and ferrous ions, originally subjected to investigation by the flash-photolysis technique by Hatchard and Parker, is of current interest for photo-galvanic systems. ... 

Design of cathodic protection for marine structures in both fresh and salt water require special techniques. Galvanic systems usually employ zinc or aluminum alloy anodes. Impressed current systems frequently use high silicon, chromium bearing iron, platinized niobium, or mixed-metal oxide/titanium anodes. The structure being protected affects the design. Stationary facihties such as bulkheads and support piles require different techniques from ship hulls [55]. 

The advantages of galvanic system over its impressed-current counterpart can be enumerated as follows ... 

On the other hand, galvanic systems are more favored for small, well-coated, low-current-demand structures or for localized protection. However, in some offshore environments, it has been found to be cost-effective to employ galvanic protection systems to large and uncoated structures where the initial cost of coating applications and maintenance is very high. 

The earliest galvanic systems for reinforced concrete were installed in the United States on bridge decks. The first was in 1978 using aluminium, and from 1976 to 1980 a study was carried out on a zinc system in Illinois. 

Galvanic cathodic protection systems have been used extensively since the early 1990s in Florida on prestressed concrete bridge support piles in the sea. One of the reasons the galvanic system is used there is because concrete resistivity is low due to the marine exposure conditions. The Florida systems frequently incorporate a distributed anode of zinc fixed on the atmospherically exposed concrete and bulk zinc anodes in the water which pass current through the low resistance sea water to protect the submerged area as shown in Figure 7.4. 

Since the maximum voltage that can be generated with zinc anodes is extremely unlikely to generate hydrogen embritdement, galvanic systems have been used to protect prestressed concrete members. They are also used on fusion bonded epoxy coated steel reinforced piles as the effects of electrical discontinuity between bars is unlikely to lead to significant stray current induced corrosion as the currents and potentials are low. 

Anodes have a limited life and need replacement at intervals that can be difficult to define. Most galvanic systems aim to provide at least ten... 

Although well behind impressed current cathodic protection which has protected over 2 million m of concrete structures, galvanic systems are progressing rapidly with at least 200,000 m in less than five years of serious commercial exploitation. 

The principles of galvanic anode cathodic protection were discovered by Sir Humphrey Davy in 1824. His results were used over the next century or so to protect the submerged metallic parts of ships from corrosion. In the early decades of the 20th century the technology was applied to underground pipelines. Impressed current cathodic protection was developed when it was found that the electrolytes like soils had too high electrical resistance for galvanic systems to be effective. 

The electrolyte is the medium through which the ionic current flows from the anode to the cathode. It can be soil or sea water for pipelines or ships. In the case of atmospherically exposed reinforced concrete structures it is the concrete pore water. In the case of buried or submerged concrete it will be firstly water or soil and then the concrete pore water as shown in Figure 7.4 for a galvanic system. Our discussion will concentrate on atmospherically exposed concrete structures. 

Figure 3.5 (a) A typical potentiodynamic polarization plot for a Co electrode, graphically analyzed to determine E on and icon- (b) and (c) display coupled Tafel plots recorded in selected slurry solutions for the Co—A1 and Cu—Ta bimetallic systems, respectively. The juncture point (p) of the two plots in (h) corresponds to the galvanic parameters g and ig for the Co—A1 galvanic system. The plots in (c) represent a reversal of conventional galvanic polarities, where there are two crossover points (pi and P2). In all cases of (a), (b), and (c), the potential was scanned at a speed of 5 mV/s. 

Ruling out an aU-galvanized system, the next best scheme uses galvanized wire (guy-wire material) to tie the guy wires together. Just above the soil, Cadweld the galvanized wire to a copper conductor that... 

Cathodic protection is an electrochemical polarization process that is widely and effectively used to limit corrosion. Simply stated, it is an electrical system whose energy operates in opposition to the natural electrochemical decomposition process of corrosion. All cathodic protection systems require the artificial development of an alternative corrosion cell with (-) electrons flowing finm the artificially installed anode to the structure in the metallic path. It also requires the flow of (+) ions (atoms or molecules carrying electrical charge) from the anode to the structure by the electrolyte path and/or (-) ions in the opposite direction. For a constant current, the level of protection depends on the polarization slope of the cathodic reaction on the structure. Current can be supplied by a galvanic or impressed current system. In a galvanic system, the electrons flow because of the difference in half-cell potential between the metal of the structure and the cathodic protection anode metal, given that the anode metal is more reactive than the metal of concern. In an impressed current system, an... 

The PG effect is described by Rabinowitch as the change in the electrode potential of a galvanic system, produced by UluminatirHi and traceable to a photochemical process in the body of the electrolyte [9, 10]. Cells exhibiting a PG effect have a higher storage capacity than PV cells, but a lower conversion efficiency (theoretically 18 % but observed values are much lower) [5, 11]. Consequently, extensive research has been carried out concerning the manipulation of substances used in PG cells (i.e., reductants and photosensitizers) in order to maximize electrical output [5]. 

In contrast to the galvanic anode system, the flow of current from the anode to the cathode is forced from a DC source in the impressed current system. Thus, whereas the current is provided by the corrosion of the electrode in the anodic galvanic system, the electrode acts as a conductor and hardly corrodes in the impressed system and the AC input is transformed and rectified to a varying DC voltage. A transformer rectifier is the most important component of the system. 

Backfill The special soil placed around the anodes to provide uniform resistivity. The material used as backfill in an impressed current system is generally coke breeze whereas in the galvanic system the backfill is composed of a mixture of gypsum, bentonite and clay, the composition being dependent on soil resistivity. Bond An electrical connection between two metallic structures. 

Therefore, in a galvanic system, it is a normal phenomenon that the real galvanic corrosion is more severe than that estimated by a measured anodic polarization or galvanic current, particularly in the zone relatively far away from the cathode where anodic polarization has become weak and the cathodic current is not negligible. 

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