External anodic

Paint stripping by water is most likely to occur from cathodic areas, the phenomenon of cathodic disbonding sometimes observed on steel protected by external anodes or impressed current being a particularly... 

The potentiostat has supplied an experimental tool for the study of anodic protection. The elucidation of passive behaviour made possible by poten-tiostatic anode polarisation curves allowed investigators to determine the conditions necessary for maintaining a metal in a stable passive condition by provision of a suitable environment, addition of cathodic alloying elementsand/or maintenance of the required potential by means of external anodic polarisation - . ... 

Thus, when 2T0X > EIred, Iex will be positive and identified as Iex ox (i.e., net oxidation occurs at the electrode surface and produces an external anodic current). Conversely, when ZIred > ZI0X, Iex will be negative and... 

A similar analysis for an external anodic (net oxidation) current, Iex ox, leads to the following Tafel-based equations under conditions that the E is sufficiently greater than Ecorr to make Ired x negligible ... 

Consequently, under these conditions and with reference to Fig. 4.15, the solid curve for the external anodic current, Iex ox, becomes linear and establishes the individual Iox M Tafel line. In this case, extrapolation of the linear section to E M gives an experimentally determined value for I0 M, the slope is 30X M, and the intersection of the extrapolated line with E = Ecorr gives the same experimental value for Icorr. 

Instead of measuring the attenuation of a beam, one may also count the ions produced with very high efficiency by the use of channelplates or a hot-wire detector [387], an approach which has mainly been applied in laser spectroscopy, where high sensitivity can be achieved by space charge amplification. The principle of the thermionic diode is that the atomic vapour under study is formed within the detector, and a current limited by the space charge is obtained by appropriately biasing a diode, consisting of an external anode (often the outer wall of the vacuum system, formed by a metal tube) and a heated cathode made of a suitable material to emit many electrons (thoriated W is suitable in many cases). A sketch of... 

Transferred arcs with water-cooled non-consumable cathodes are illustrated in Fig. 4-44. The generation of electrons on the inner walls of the hollow cathodes is provided by field emission, which permits operation of the transferred arcs at the multi-megawatt power for thousands of hours. The electric circuit is completed by transferring the arc to an external anode, which is a conducting material, where the arc is to be applied. The arc root can move over the cathode surface, which further increases its lifetime. 

F re 7.n Schematic representation of the influence of external anodic or cathodic polarization... 

The chloride ion itself is negative and will be repelled by the negatively charged cathode (reinforcing steel). It will move towards the (new external) anode. With the carbon-based anodes it may then combine to form chlorine... 

We have already determined that the chloride ion is a catalyst to corrosion (Section 3.2.3). As it is negatively charged we can use the electrochemical process to repel the chloride ion from the steel surface and move it towards an external anode. This process, called electrochemical chloride extraction (ECE), desalination or chloride removal, uses a temporary anode and a higher electrical power density than CP, but is otherwise similar (Figure 7.1). Preparation in terms of concrete repair, power supplies etc are similar to those for impressed current cathodic protection except that the power supply is temporary and may be from a temporary source such as a generator. The output is larger, up to 50 V and 2 A-m. ... 

With the two electrodes shortened together, the maximum Fermi level possible in the cell is the flat-band potential. In Figure 5, Ufb is below the H /H2 potential. Hence, even with illumination intensity sufficient to completely flatten the semiconductor bands, H2 could not be evolved at the counter electrode because the Fermi level is below the Vi /Wz potential. In order to raise the Fermi level in the metal counter electrode above the H7H2 potential an external anodic bias, E, must be applied, as shown in Figure 5d. This bias also provides the overvoltage at the metal cathode, tiq, required to sustain the current flow, and it increases the band bending in the semiconductor to maintain the required charge separation rate. 

The recovery of the passive state of SSh, characterized by disrupted passive film, in our opinion, is brought about also by some other reasons. It is caused by the flow of internal cathodic current (instead of external anodic current), which is determined by the occurring... 

Passivity is an electrochemical reduction mechanism that occurs on a metal-electrolyte interface. The resultant reduction product is a solid film of a metal oxide compound having a stoichiometric reaction (a reaction that goes to completion) based on the corroding metal. Thus, passivity occurs on metals that are active-passive by natural passivation without the influence of external potentials or artificial passivation due an applied external anodic potential. 

Crete. Also, external anodes are used and placed on the surface of the concrete, but the chloride extraction process is carried out using higher current parameters. Titanium anodes coated with mixed oxides (MMOs) are most frequently used as the anodic material. The current density is high and reaches 20 A m on the reinforced concrete surface. The system is aided by an electrolyte, frequently sodium carbonate with different additives. In the formed electric field, similar to cathodic protection, the migration of anions (chlorides) occurs to the external anode. On the surface of the anode, chlorine liberation reactions take place (Reaction 8-3) along with the formation of hypochlorites (Reaction 8-25)... 

In all electrochemical restoration techniques a direct current is applied between the reinforcement (cathode) and an external anode in electrolytic contact with the concrete (Fig. 8-22). Cathodic protection (CP) is a permanent installation with design currents below 10 mA m , electrochemical chloride removal (ECR) and electrochemical realkalization (ER) are applied only on a temporary basis and use currents up to 2 A m. Both the electrochemical reactions at the rebars and ion migration in concrete are important for the treatments. 

In contrast to cathodic protection, anodic protection is relatively new. The feasibility of anodic protection was first demonstrated in 1954 and tested on a small-scale stainless steel boiler designed to handle sulfuric acid [23]. Anodic protection refers to the corrosion protection achieved by maintaining an active-passive metal or alloy in its passive state by applying an external anodic current. The basic principle for this type of protection is explained by the behavior shown in Fig. 5.40. 

The main target here is to reduce corrosion by lowering the potential either (1) by connection to an external anode (sacrificial anode) which is a metal more active than the corroding metal, or (2) by adjusting the potential of the material by application of an external current (impressed currait). Referring to Figure 2.1, it is seen that by reducing the potential of the metal from Ei to E2 and further down to E3, the corrosion current is reduced (note how Ij is reduced to I2). It follows that... 

The above relationship suggests that enough current must be provided by an external anode to flow through the cathode resistance Rq to make the cathode potential ( c) equal to the anode potential (E ). When this condition, Ea = (Ec + Rc h)> is achieved, no corrosion would occur and the structure would be cathodically protected. Figure 5.12 illustrates the complete and incomplete protection. In part(b) current is not flowing to all anodic areas, whereas in part(c) current is supplied to all anodic areas. Figure 5.12A shows... 

It has been shown above that there must be a source of current to supply electrons to the areas of the metal which is corroding. In a metal buried in ground, anodic areas corrode by release of electrons and if an equal number of electrons are not introduced from an external source, the metal would continue to corrode. An external anode which supplies such current is called auxiliary anode in the electrochemical cell and referred to as anode in a cathodic protection system. Electrodes of graphite, cast iron, platinum and titanium act as conductors of electricity and supply the desired current to the structure to be protected. The conductors are energized by a DC source. The rate of consumption of anode electrodes... 

Electrochemical chloride removal Electrochemical extraction to reduce chloride extraction near reinforcement. It is achieved by using an external anode powered by a DC source. The negatively charged chloride ions... 

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