Cathodic protection continued systems

A continuous polymer anode system has been developed specifically for the cathodic protection of buried pipelines and tanks. The anode, marketed under the trade name Anodeflex , consists of a continuous stranded copper conductor (6AWG) which is encased in a thick jacket of carbon-loaded polymer, overall diameter 12-5 mm. To prevent unintentional short circuits an insulating braid is sometimes applied to the outer surface of the conductive polymer. 

The continued effectiveness of a cathodic protection system shall be monitored in accordance with para. GR-5.3.1.1(c). 

Examinations shall be made as required to maintain continuous and effective operation of the cathodic protection system. 

As a means of maintaining the integrity of its pipeline system, each operating company shall establish and implement procedures for continuing surveillance of its facilities. Studies shall be initiated and action shall be taken where unusual operating and maintenance conditions occur, such as failures, leakage history, drop in flow efficiency due to internal corrosion, or substantial changes in cathodic protection requirements. 

Where control of corrosion is dependent on the concentration of contaminants or moisture, the operator should demonstrate that procedures and the necessary controls are in place to maintain a safe operating condition. Similarly, where inhibitors are added or systems such as cathodic protection are used, the operator should demonstrate that these systems are inspected and adequately maintained to ensure continued protection of the process. 

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... 

Anodes have been developed in the form of conductive coatings, metals embedded in concrete overlays, conductive concrete overlays and probes drilled into the concrete. Anodes continue to be developed, applied in new configurations and to new structures. In the next section we will discuss the major components of the cathodic protection system, and particularly the anode systems that are available as these are the most prominent part of the cathodic protection system. Judicious choice of cathodic protection anode can maximize the cost effectiveness of the system. 

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. 

It is rare for a cathodic protection system to consist of one continuous anode passing current from a single power supply. It is normal to divide the structure or elements to be protected into zones that are powered and controlled separately, and electrically separated by a gap of typically 25 mm. 

Rebar continuity is essential to avoid stray currents that can accelerate corrosion. Figure 7.19 shows how an isolated rebar between the anode and the cathode will be cathodic where the current enters the steel and anodic where it exits. This will accelerate corrosion at the anodic site. Although there are few serious cases identified in cathodic protection systems, this is a greater concern for realkalization and desalination systems where the charge density is higher. 

Street railways have now in large part been replaced by other forms of transportation, but the problems of stray-current corrosion originating from metropolitan railway transit systems continue [6]. Also, cathodically protected structures requiring high currents, when located in the neighborhood of an unprotected pipeline, can produce damage similar to that by the railway illustrated in Fig. 12.1. 

An important aspect, when considering cathodic protection or other electrochemical techniques, is zero cover or tie wire touching the surface. If there is a direct metallic connection between the anode and the rebar it will short out the electrochemical protection. Excessive. shorts have caused cathodic protection systems to be abandoned in the USA, On the other hand, the lack of electrical continuity between reinforcing steel that concerns many engineers is rarely a problem and is usually straightforward to overcome. These issues are discussed further in Section 6.6 on cathodic protection. 

A cathodic protection system requires steel continuity, no shorts to the surface, a po ver supply for the life of the system and continuous monitoring and maintenance. 

Continuity and short circuits can be remedied prior to installation if necessary. At present all applications of cathodic protection to structures with prestressing are experimental with the exception of the cathodic prevention applied to new bridges in Italy to keep corrosion from initiating. This uses very low level currents and voltages with. special control system. to prevent hydrogen embrittlement,... 

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