Cathodic protection points

Corrosion due to stray current—the metal is attacked at the point where the current leaves. Typically, this kind of damage can be observed in buried stmctures in the vicinity of cathodic protection systems or the DC stray current can stem from railway traction sources. 

The basic standard for cathodic protection was laid down for the first time in DIN 30676 to which all the application areas of the different branches of protection can be referred. In this the most important point is the technique for accurately measuring the object/soil potential [58]. The usual off-potential measurement method for underground installations has been slowly implemented and enforced in Europe since the 1960s [59]. 

In analyzing the results on a cathodically protected pipeline, the protection current density and coating resistances should be calculated for individual sections of the pipeline in addition to the on and off potentials, the pipe current, and the resistances at insulating points and between the casing and the pipeline. The results should be shown by potential plots to give a good summary [15] (see Fig. 3-20). 

In the cathodic protection of storage tanks, potentials should be measured in at least three places, i.e., at each end and at the top of the cover [16]. Widely different polarized areas arise due to the small distance which is normally the case between the impressed current anodes and the tank. Since such tanks are often buried under asphalt, it is recommended that permanent reference electrodes or fixed measuring points (plastic tubes under valve boxes) be installed. These should be located in areas not easily accessible to the cathodic protection current, for example between two tanks or between the tank wall and foundations. Since storage tanks usually have several anodes located near the tank, equalizing currents can flow between the differently loaded anodes on switching off the protection system and thus falsify the potential measurement. In such cases the anodes should be separated. 

The variation in the on and off potentials or the potential difference along the pipeline will usually indicate faults that prevent the attainment of complete cathodic protection. The protection current requirement of the pipeline may be estimated from experience if the age and type of pipeline is known (see Fig. 5-3). Figure 3-20 shows the variation in the on and off potentials of a 9-km pipeline section DN 800 with 10-mm wall thickness. At the end of the pipeline, at 31.84 km, an insulating unit is built in. The cathodic protection station is situated at 22.99 km. Between this and the end of the pipeline there are four pipe current measuring points. The applied protection current densities and coating resistances of individual pipeline sections are calculated from Eqs. (3-40) and (3-41). In the upper diagram the values of... 

In Section 24.4.3, the cathodic protection length (2 L) of a pipeline from a current drainage point is derived for locally constant values of R and G [see Eq. (24-75)] ... 

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... 

Fig. 10-8 Pipe/soil potentials and protection currents for a pipeline. Drainage test x-x after 1 year o-o. P = potential test point R = pipe current test point LA = cathodic protection station / = insulating joint SP = pipe casing potential test point.

At the relatively low protection current density of 200 llA m and with the anode positioned on one side, it is to be expected that with this storage tank sufficient reduction in potential would be achieved on the other side of the tank from the anode. The off potential was measured using a measurement point at a depth of about 2 m as f/cu-cuso4 = -0.88 V at the tank. At the other side of the tank as well as above it, off potentials of-0.90 to -0.94 V were found. These potentials were measured with a protection current of 10 mA (anode 1 6 mA, anode 2 4 mA) with an additional resistance of 8 Q in the protection current circuit (see Fig. 11-2). With a direct connection between the tank and the group of magnesium anodes, the initial current was about 16 mA, which after 1 h of polarization decreased to about 14 mA. The reserve current, based on a long-term current of 10 mA, amounted to ca. 40% in the operation of the cathodic protection installation. 

Cathodic protection installations must be tested when commissioned and at least annually. The potentials should be measured at several points on the bottom of the tank with special probes under the oil, and the height of the electrolyte solution should be checked. The off potential and the protection potential as in Section 2.4 are the means for checking the protection criterion according to Eq. (2-39). 

Determination of tank/soil potentials is only possible at the outer edge of the tank. For monitoring the local cathodic protection, the distance between test points should not exceed 2 m [3]. The measurement of the tank/soil potential by the current... 

For several years now, cable ducts have been manufactured from plastic pipes, which are watertight and form a continuous run of piping. In laying the ducts, low points can occur in which condensed water or water penetrating from the ends can collect. In many cases this water has led to corrosion damage in lead-sheathed cables. Lead-sheathed cables must therefore only be used in such ducts with an additional PE sheath of type A-PM2Y. Cathodic protection of these cables is not possible because of their complete insulation by the plastic pipe. 

Cathodic protection with impressed current anodes is used predominantly with cables or steel casing in which the cable is inserted, outside built-up areas where it is possible to build large anode installations without damaging interference with other lines. In densely populated areas, protection with impressed current anodes is often only possible with deep anodes, with surface anodes or locally at individual problem points (local cathodic protection, see Chapter 12). 

For adjustment and for monitoring the cathodic protection, test points are necessary along the cable (see Section 10.3.2). It is convenient to install these test points at the cable sleeves. This gives distances between test points of about 0.5 km. The installation of test points to measure conduit current is also convenient for locating accidental contacts. 

With buried pipelines, the degree of corrosion danger from cell formation and the effectiveness of cathodic protection can be determined by pipe/soil potential measurements along the pipeline (see Sections 3.6.2 and 3.7). This is not possible with well casings since the only point available for a measuring point is at the well head. Therefore, other methods are required to identify any corrosion risk or the effectiveness of corrosion protection. 

After measuring the zero profile, AU measurements are carried out with the injection of a cathodic protection current. In contrast to the zero profile measurements, the distance between the individual measurements is 25 to 50 m. Shorter distances between the measuring points are used only at depths where there are unusual AU profiles. Current should be injected at at least three different levels. The protection current density of about 12 mA m obtained from experience should be the basis for determining the maximum required protection current. As shown by the results in Fig. 18-3, the AU profiles are greater with increasing protection current. The action of local cells is suppressed when the AU values no longer decrease in the direction of the well head. This is the case in Fig. 18-3 with a protection current I = 4A. 

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. 

As the measurements show, the small heater without an electrical separation (from the boiler) is not detrimental to cathodic protection. However, with the uninsulated built-in Cu heat exchanger without an electrical separation, cathodic protection was not achieved. As expected, the polarization increased with increasing conductivity of the water. It should be pointed out that the Cu tube was tinned and that the tin could act as a weak cathodic component. Apart from the unknown long-term stability of such a coating, the apparent raising of the cathodic polarization resistance of tin is not sufficient to provide cathodic protection with such a large fixture. This applies also to other metal coatings (e.g., nickel). 

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