Cathodic protection distribution

Fig 9-18 Current distribution and voltage cone Af/ at a defect in the pipe coating of a cathodically protected pipeline and the variation in the pipe/ soil potential of a pipeline subjected to interference. 

Buried steel pipelines for the transport of gases (at pressures >4 bars) and of crude oil, brine and chemical products must be cathodically protected against corrosion according to technical regulations [1-4], The cathodic protection process is also used to improve the operational safety and economics of gas distribution networks and in long-distance steel pipelines for water and heat distribution. Special measures are necessary in the region of insulated connections in pipelines that transport electrolytically conducting media. 

Cathodic protection with magnesium anodes can be just as economical as impressed current anode assemblies for pipelines only a few kilometers in length and with protection current densities below 10 xA m" e.g., in isolated stretches of new pipeline in old networks and steel distribution or service pipes. In this case, several anodes would be connected to the pipeline in a group at test points. The distance from the pipeline is about 1 to 3 m. The measurement of the off potential... 

The requirements derived in Eq. (10-5) are relevant in the cathodic protection of distribution networks for low and as uniform as possible values of resistance and leakage loading. The second requirement is often not fulfilled with old pipeline networks on account of their different ages and the type of pipe coating. When setting up cathodic protection, a distinction must be made between old and new steel distribution networks. 

Insulating connections or joints are used in new distribution networks for gas service pipes. Their installation in these pipes has been compulsory in Germany since 1972 [28]. Therefore, use of cathodic protection here is no problem and is... 

A check on the cathodic protection of the pipeline should be carried out annually according to Section 10.4, where, of course, only the on potential should be measured. This value should also be compared with the values of the measurements in Section 10.4. If there are no changes in the on potentials and the protection current densities for the individual sections of the pipeline, it can be concluded that the off potential has not changed. The values can easily be compared using computers and represented in plots. If the protection current and potential distribution have changed, or in any case every 3 years, the off potentials as well as the on potentials should be measured. 

As an example, a tank farm that is to be cathodically protected by this method is shown schematically in Fig. 11-4. As can be seen in the figure, injection of the protection current occurs with two current circuits of a total of about 9 A, via 16 vertically installed high-silicon iron anodes embedded in coke. These are distributed over several locations in the tank farm to achieve an approximately uniform potential drop. The details of the transformer-rectifier as well as the individual anode currents are included in Fig. 11-4. Anodes 4, 5 and 6 have been placed at areas where corrosion damage previously occurred. Since off potentials for 7/ -free potential measurements cannot be used, external measuring probes should be installed for accurate assessment (see Section 3.3.3.2 and Chapter 12). 

Structures or pits for water lines are mostly of steel-reinforced concrete. At the wall entrance, contact can easily arise between the pipeline and the reinforcement. In the immediate vicinity of the pit, insufficient lowering of the potential occurs despite the cathodic protection of the pipeline. Figure 12-7 shows that voltage cones caused by equalizing currents are present up to a few meters from the shaft. With protection current densities of 5 mA mr for the concrete surfaces, even for a small pit of 150 m surface area, 0.75 A is necessary. A larger distribution pit of 500 m requires 2.5 A. Such large protection currents can only be obtained with additional impressed current anodes which are installed in the immediate vicinity of the pipe entry into the concrete. The local cathodic protection is a necessary completion of the conventional protection of the pipeline, which would otherwise be lacking in the pit. 

Fig. 12-6 Local cathodic protection of a tank farm in high-resistance soil using the anodic voltage cones of distributed anodes the lines indicate soil potential values for an increase of 0.5 V relative to a remote ground numerical pairs volts.

Cathodic protection of an uncoated ship is practically not possible or is uneconomic due to the protection current requirement and current distribution. In addition, there must be an electrically insulating layer between the steel wall and the antifouling coating in order to stifle the electrochemical reduction of toxic metal compounds. Products of cathodic electrolysis cannot prevent marine growths. On the contrary, in free corrosion, growths on inert copper can occur if cathodic protection is applied [23]. 

Cathodic protection, complete or partial (stem and bow), is arranged by the distribution of the anodes so that the desired current distribution is maintained correctly in the relevant areas. Galvanic anodes, depending on their dimensions and current output, deliver a certain maximum current which depends on the conductivity. The calculated maximum current from Eq. (6-12) based on the driving voltage and grounding resistance is reduced in practice on working anodes due to film for-... 

Nonuniform current and potential distribution is usually to be expected with uncoated objects to be protected. The distribution can be considerably improved by coatings (see Section 20.1.3). In enamelled tanks, the current and potential distribution of cathodic protection is very good. By arranging the anode centrally, IR errors from equalizing currents in the switching-off phase can be ignored. The anode potential in the switching-off phase can be evaluated from the information... 

Cathodic protection of enamelled tanks with Mg anodes has long been the state of the art, with potential-controlled equipment being used with increasing frequency in recent years. A high-resistance coating with limited defects according to Ref. 4 enables uniform current distribution to be maintained over the whole tank. 

As an example of potential distribution, Fig. 20-8 shows the potential on the vertical axis in a 300-liter electric storage reservoir. The water had an extremely low conductivity of x (20°C) = 30 fiS cm l A Mg rod anode served for cathodic protection it reached to just above the built-in heating element to give uniform current distribution. This was confirmed by the measurements. 

The arrangement and distribution of anodes in gravel and activated charcoal filters is different. Cathodic protection of activated charcoal filters is basically feasible but requires a large number of electrodes and high protection current densities that are twice those for gravel bed filters, so that an electrically insulating layer can be deposited on the steel wall. 

Cathodic protection of uncoated objects in the soil is technically possible however, the high current requirement, as well as measures for the necessary uniform current distribution and for //f-free potential measurement, result in high costs. In determining the costs of cathodic protection of pipelines, it has to be remembered that costs will increase with increases in the following factors ... 

Magnesium anodes suspended inside a galvanised hot-water tank and in electrical connection with it afford cathodic protection to the zinc, the alloy layer and the steel, at high temperatures as well as in the cold. The magnesium is eventually consumed but it is probable that in the interim a good protective scale will have formed on the inside of the tank, so that the magnesium anode will then no longer be necessary. One of the difficulties of this method, however, is the maintenance of a sufficiently even current distribution over the inside of a tank to protect the whole surface, especially in waters of low conductivity. The method is therefore unlikely to be applicable to soft waters. 

The anode may be installed in conventional groundbeds or be laid in close proximity to the cathode, e.g. parallel to a pipeline route. The anode may be buried either directly in soil or in carbonaceous backfill. The major applications for this material are tank protection, internal protection, mitigation of poor current distribution and hot spot protection, i.e. to supplement conventional cathodic protection systems and provide increased levels of cathodic protection in areas that exhibit low levels of protection. 

The anode is fixed to the concrete using non-metallic fixings and may be supplied as a prefabricated mesh or more often as a continuous anode strand which is laid over the surface of the structure to be protected. The spacing between the anode strands may be adjusted to give the required current distribution and current density per unit area of concrete necessary to provide cathodic protection to a particular structure. 

Advantages No external source of power is required installation is relatively simple the danger of cathodic protection interaction is minimised more economic for small schemes the danger of over protection is alleviated even current distribution can be easily achieved maintenance is not required apart from routine potential checks and replacement of anodes at the end of their useful life no running costs. 

The tines of flow of the protection current and the interaction current when cathodic protection is applied to a pipeline buried in the soil parallel to a buried secondary pipeline, are shown in Fig. 10.38. The distribution of... 

One must be wary of the use of anodic protection, in that any area that is not polarized completely into the passive region will dissolve at a high rate. The optimum protection range is shown in Fig. 16. Therefore anodic protection is more susceptible to the presence of crevices, deposits, or poor placement of polarizing electrodes than is cathodic protection. If a component is cathodically under protected, the maximum rate at which the unprotected area corrodes is the normal open circuit corrosion rate in anodic protection, underprotection results in high rate dissolution of the unprotected area and can therefore can lead to unexpected career changes. Understanding the manner in which current from an anodic protection system is distributed across a surface is important in such installations. The issues involved in current distribution are discussed in detail in Chapter 4. 

Isopotential lines may vary with electrode position for the secondary and tertiary current and potential distributions, where interfacial polarization of various types is considered. The variation of local true potential across the electrochemical interface with electrode position is of great interest in galvanic corrosion, cathodic protection, etc., since this true potential drives electrochemical reactions. 

Recall that the Wagner number depends on the solution conductivity, characteristic length, as well as the interfacial electrode characteristics. A solution has been given for the primary current distribution where the entire interior pipe surface (radius r0) is uniformly cathodically protected to / and the pipe interface is considered to be nonpolarizable (16). The IR drop down the pipe to a distance, L, can be calculated so that the maximum tolerable potential drop from the entrance to the far end is known ... 

Therefore let us instead consider the more practical case of the tertiary current distribution. Based on the dependency of the Wagner number on polarization slope, we would predict that a pipe cathodically protected to a current density near its mass transport limited cathodic current density would have a more uniform current distribution than a pipe operating under charge transfer control. Of course the cathodic current density cannot exceed the mass transport limited value at any location on the pipe, as said in Chapter 4. Consider a tube that is cathodically protected at its entrance with a zinc anode in neutral seawater (4). Since the oxygen reduction reaction is mass transport limited, the Wagner number is large for the cathodically protected pipe (Fig. 12a), and a relatively uniform current distribution is predicted. However, if the solution conductivity is lowered, the current distribution will become less uniform. Finite element calculations and experimental confirmations (Fig. 12b) confirm the qualitative results of the Wagner number (4). 

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