Cathodic protection continued cathode potentials

Electrochemical protection of bleach washers has permitted the continued use of lower alloyed stainless steels, while washing conditions have become more corrosive. Electrochemical protection was applied successfully in an early chlorine dioxide washer [101], Electrochemical protection at slighdy cathodic potentials was commercially appUed for C- and D-stage washers [51,102-106], The effectiveness of electrochemical protection has been assessed by means of coupons [104],... 

Theoretically, it would be possible to hold the potential at a value where the metal is thermodynamically immune from corrosion. In practice, however, this Is likely to be too expensive. The continuous passage of current requires the consumption of power (j cell 0 tid, particulary if the medium between the protected surface and the anode is not highly conducting, the use of anything but a very low current will entail an unacceptably high energy consumption, The voltage required of the power supply also depends upon the active area of Che structure, the nature of the electrode reactions and resistance of the anode. The cathode potential is kept within the desired limits by ... 

If a continuous metallic structure is immersed in an electrolyte, e.g. placed in the sea or sea-bed or buried in the soil, stray direct currents from nearby electric installations of which parts are not insulated from the soil may flow to and from the structure. At points where the stray current enters the immersed structure the potential will be lowered and electrical protection (cathodic protection) or partial electrical protection will occur. At points where the stray current leaves the immersed structure the potential will become more positive and corrosion may occur with serious consequences. 

Interaction tests should be made on all unprotected structures in the vicinity of a proposed cathodic protection installation, and should be repeated annually or at some other suitable interval to ensure that alterations in the layout of plant or in the electrical conditions are taken into account. It is most convenient if the tests on all unprotected pipes or cables are made at the same time, the potential measurements being synchronised with the regular switching on and off of the protection current. It may then be convenient to continue with further tests to confirm that any remedial measures applied to one installation do not adversely affect other installations. 

In order to make the potential of iron more negative, the iron must receive a continuous supply of electrons. As has already been pointed out, polymer films do not contain free electrons there remains the possibility of obtaining these from a pigment. The only pigments which contain free electrons are metallic ones, and such pigments will protect iron cathodically if the following conditions are fulfilled ... 

The cell potential for such a process is shown in Fig. 15.28 (Dandapani, 1987). In fact, at about 80 °C and pH 14, polysulfides are first produced, but pure sulfur precipitates on cooling. The process has been developed to an engineering stage (Petrov and Srinivasan, 1996). It is necessary to protect the cathode against catalytic inactivation by polysulfide (i.e., a membrane must be used). For continuous use, the pH must be controlled. The cooling and production of S may be best carried out outside the cell. 

Adsorption-induced brittle fracture. This model is based on the hypothesis that adsorption of environmental species lowers the interatomic bond strength and the stress required for cleavage. This model of chemical adsorption can explain the fact that a certain alloy is susceptible to specific ions. An important factor in support of this mechanism is the existence of a critical potential below which the SCC does not occur in some systems, and this model underlines the relation between the potential value and the capacity of adsorption of the aggressive ion. It also explains the preventive action of SCC for some systems by cathodic protection. This model may interpret the rupture of plastic materials or glass. It is referred to as the stress-sorption model, and similar mechanisms have been proposed for HE and LME. In this model, the crack should propagate in a continuous way at a rate determined by the arrival of the embrittling species at the crack tip. The model does not explain how the crack maintains a sharp tip in a normally ductile material.156... 

Cathodic protection is one of the methods to mitigate the corrosion of steel in concrete Figure 7.24. Some factors to be considered in this connection are remaining service life of the structure should be more than lOyr delamination and spalls should be less than 50% by weight of concrete half-cell potential should be less than —200 mV (indicating breakdown of passive film) the structure should be sound the reinforcing bars should be electrically continuous AC power should be available. 

In general terms, the systems for protection of steel in concrete are generally full wave rectifiers with smoothing to minimize interference and any possible adverse effects on the anode. A continuously variable output is usually specified. Most cathodic protection systems are run under constant current control, although constant voltage (or an option for both methods) is sometimes specified. Control by constant half cell potential against an embedded reference electrode is rarely specified for steel in atmospherically exposed concrete but may be applied to buried or submerged parts of structures. 

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