Cathodic protection continued electrical continuity

The danger of corrosion is in general greater for pipelines in industrial installations than in long-distance pipelines because in most cases cell formation occurs with steel-reinforced concrete foundations (see Section 4.3). This danger of corrosion can be overcome by local cathodic protection in areas of distinct industrial installations. The method resembles that of local cathodic protection [1]. The protected area is not limited, i.e., the pipelines are not electrically isolated from continuing and branching pipelines. 

The cathodic protection of reinforcing steel and stray current protection measures assume an extended electrical continuity through the reinforcing steel. This is mostly the case with rod-reinforced concrete structures however it should be verified by resistance measurements of the reinforcing network. To accomplish this, measuring cables should be connected to the reinforcing steel after removal of the concrete at different points widely separated from each other. To avoid contact resistances, the steel must be completely cleaned of rust at the contact points. 

The cost and economics of cathodic protection depend on a variety of parameters so that general statements on costs are not really possible. In particular, the protection current requirement and the specific electrical resistance of the electrolyte in the surroundings of the object to be protected and the anodes can vary considerably and thus affect the costs. Usually electrochemical protection is particularly economical if the structure can be ensured a long service life, maintained in continuous operation, and if repair costs are very high. As a rough estimate, the installation costs of cathodic protection of uncoated metal structures are about 1 to 2% of the construction costs of the structure, and are 0.1 to 0.2% for coated surfaces. 

Although iron pipes suffer from the same corrosion risk as steel pipelines, associated with the generation of a galvanic cell with a small anode and a large cathode, the risk is mitigated for iron pipelines because the electrical continuity is broken at every pipe joint. For this reason long-line currents are uncommon in iron lines and cathodic protection is rarely necessary. It also accounts for the ability to protect iron lines by the application of nonadherent polyethylene sleeving . 

Scrap steel In some fortunate instances a disused pipeline or other metal structure in close proximity to the project requiring cathodic protection may be used. However, it is essential in cases of scrap steel or iron groundbeds to ensure that the steelwork is completely electrically continuous, and multiple cable connections to various parts of the groundbed must be used to ensure a sufficient life. Preferential corrosion can take place in the vicinity of cable connections resulting in early electrical disconnection, hence the necessity for multiple connections. 

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. 

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. 

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. 

Chloride removal cannot be applied to prestressed structures due to the risk of hydrogen embrittlement. The use of lithium-based electrolytes suggests that ASR can be controlled. As stated earlier for impressed current cathodic protection, there must be electrical continuity within the reinforcement network for any of the electrochemical techniques to be applied. We do not know how long the treatment process will last but a range of 5-20 years is likely, depending upon conditions. 

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. 

In order to install cathodic protection, electrical continuity can be established by welding in extra rebars. However, at Florida DoT one approach has been to expo.se bars in damaged areas, grit blast them... 

There are problems with applying cathodic protection, chloride removal and re alkalization to FBECR because the rebars are electrically isolated, Special care must be taken to connect each rebar together, This problem does not arise with the welded cages where each cage is continuous. 

In recent years, continuous zinc ribbon anodes have been used in a variety of underground applications (Kurr, 1973 Peabody, 1976 O Connell, 1977). This type of product has broadened the applications for zinc anodes, for it provides small increments of current continuously along the entire length of a cathode. Its uses are generally considered to lie in specialty applications, where other methods of cathodic protection are either impractical or extremely costly (see later section on induced ac on pipelines). Bagnulo (1973, 1984) has developed a tape with an electrically conducting adhesive as described in the Mechanical Coatings part of Chapter 1. 

In the process droplets of semimolten zinc are sprayed from a special gun that is fed with either wire or powder onto a grit-blasted surface. The semimolten droplets coalesce with some zinc oxide present at each interface between droplets. Electrical continuity is maintained both throughout the coating and with the iron substrate so that full cathodic protection can be obtained since the zinc oxide forms only a small percentage of the coating. 

Has cathodic protection, electrical continuity and electronic grounding been adequately addressed ... 

Low resistivities and high chloride concentrations in the soil may lead to corrosion of buried steel pipehnes or structures. Cathodic protection should be considered for all buried steel pipehnes or structures. Where cathodic protection is not provided, corrosion monitoring equipment should be incorporated into the design to allow the operating staff to monitor the condition of the pipelines or structures. Nonwelded joints should be bonded for electrical continuity. In addition, coatings should also be considered. Coatings may be used alone or in con-jimction with cathodic protection. 

The durability and protective ability depends, in addition to environmental factors, on the relationship between the permeability of the film during the first stage of exposure and the cathodic protection that takes place (Xiyan et al., 2010). The protection of iron and steel continues with available zinc in the film and effective electrical contact therefore, particularly in outdoor exposure, the time of satisfactory inhibitory action may be more prolonged due to the polarizing effect of the corrosion products of zinc (Thorslund Pedersen et. al, 2009). 

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... 

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